Description:FIELD OF THE INVENTION
[0001] Embodiments of the present disclosure relate to a Quantum Lidar system for detection of objects with low reflectivity in noisy backgrounds, and in particular it relates to detection and ranging of object using polarization-path entangled single photons.

BACKGROUND OF THE INVENTION
[0002] Generally, quantum correlations in the form of entanglement are a salient feature of quantum mechanics and is central to many quantum information processing protocols and quantum technology developments. However, these are generally known to be highly sensitive to environmental noise and can be easily destroyed thereby affecting advantages gained by nonclassical correlations. Quantum Illumination (QI) using quantum correlations between pair of entangled photons for object detection in a noisy environment is probably a known exception for object detection.
[0003] Usually, approaches that are in use for QI rely upon two entangled pair of beams in the form of signal photons and idler photons, which form a probe for object detection, wherein signal photons are directed to a region of space containing the object merged in background noise and the idler photon is stored locally until the signal reflects from the object. The enhancement of performance of QI over classical analog stands as a challenge and is made possible by using detection and joint measurement techniques which capture the nonclassical correlations between the stored idler photons and the reflected signal photons by isolating background noise. Typically, QI measurements focus on reducing uncertainty in unknown parameter estimation and false signal using quantum correlation. Thus, QI extends principles of target detection accuracy, ranging sensitivity, and degree of resilience towards preponderant noise from conventional lidar technology, radar technology to quantum metrology, and there is a need in the art for a better, efficient, and highly accurate method of detection of objects of very low reflectivity especially in noisy background with very low signal to noise ratio, especially by using quantum resources like entanglement and other forms of quantum correlations.

SUMMARY OF THE INVENTION
[0004] Embodiments of the present disclosure relate to a method and system using quantum states of light, having single photons in polarization-path entangled state for determining a low reflectivity object located within a noisy background with very low signal to noise ratio. An embodiment includes computing a quantum correlation parameter value (S) between the polarization and path degree of freedom of a single photon by detecting a coincidence measurement between of photon pairs in multiple paths, wherein the pair of photons is simultaneously generated at an instant of time T1 from a heralded single photons source generated from process such as spontaneous parametric down conversion (SPDC). In a further embodiment, the pair of photons generated simultaneously from SPDC process, which need not be entangled, includes a signal photon and an idler photon, the signal photon configured to take at least one of a signal path or a reference path, and the idler photon configured to take an idle path. A further embodiment includes adjusting the path length of the idler photon for maximum coincidence for receiving the signal photon and the idler photon simultaneously at a receiver.
[0005] A further embodiment includes determining, based on the coincidence measurement, if the computed quantum correlation parameter (S) is above a known threshold value. In a further embodiment, if the quantum correlation parameter (S) is greater than a first threshold value for the quantum correlation, conclusively denoting a presence of the object within the noisy background. In a further embodiment, if the quantum correlation parameter is greater than a second threshold value for a residual of quantum correlation, then conclusively denoting a presence of the object within the noisy background. Various other embodiments are also disclosed. A further embodiment includes ranging the object distance from an optimized length of the idler photon distance for maximum coincidence making the system a compete quantum lidar using polarization-path entangled single photons. In an embodiment, compared to the usual approach that uses two entangled photons or entangled beam of light for QI, embodiments of the present disclosure also include a first of its kind methodology that uses only single photon entangled in polarization and path degree of freedom for QI.

BRIEF DESCRIPTION OF THE DRAWINGS
[0006] The detailed description is described with reference to the accompanying figures. Features, aspects, and advantages of the subject matter of the present disclosure will be better understood with regard to the following description and the accompanying drawings. The figures are intended to be illustrative, not limiting, and are generally described in context of the embodiments, and it should be understood that it is not intended to limit the scope of the disclosure to these particular embodiments. In the figures, the same numbers may be used throughout the drawings to reference features and components. In order that the present disclosure may be readily understood and put into practical effect, reference will now be made to exemplary embodiments as illustrated with reference to the accompanying figures. The figures together with detailed description below, are incorporated in and form part of the specification, and serve to further illustrate the embodiments and explain various principles and advantages.
[0007] Figure 1 is an illustration of an exemplary set-up disclosing the method of using polarization-path entangled single photons to detect objects with low reflectivity in a noisy background with very low signal to noise ratio disclosed as an embodiment of the present disclosure.
[0008] Figure 2A illustrates a schematic of an exemplary experimental setup for QI in general and quantum lidar in particular using polarization-path entangled single-photon using an interferometric approach for measurement where photons from both the paths are made to interfere, using the principles described in Figure 1 in accordance with an embodiment of the present disclosure.
[0009] Figure 2B illustrates a schematic of an exemplary experimental setup for QI in general and quantum lidar in particular using polarization-path entangled single-photon using a non-interferometric approach for measurement where photons from both the paths do not interference, using the principles described in Figure 1 in accordance with an embodiment of the present disclosure.
[0010] Figure 3A illustrates an exemplary method for producing a signal photon and an idler photon in accordance with the embodiments of the present disclosure.
[0011] Figure 3B illustrates an exemplary method for determining the presence or absence of an object in accordance with the embodiments of the present disclosure.
[0012] Figure 3C illustrates an exemplary method for determining the presence or absence of an object in accordance with the embodiments of the present disclosure.
[0013] Figure 3D illustrates an exemplary method for determining whether the received signal is noise.
[0014] Figure 3E illustrates an exemplary method for determining whether the received signal is noise.
[0015] Figure 4A illustrates the exemplary theoretically calculated quantum correlation (CHSH) parameter Smax when an object of different reflectivity η is placed along the signal path using polarization-path entangled single photons as probe and different measurement schemes at the receiving unit in accordance with embodiments of the present disclosure.
[0016] Figure 4B is an exemplary illustration of experimentally obtained maximum value of the CHSH parameter Smax for an object with different reflectivity in accordance with embodiments of the present disclosure.
[0017] Figure 5 illustrates the exemplary experimentally obtained maximum value of CHSH parameter Smax for different percentage of signal (P) and noise (100 – P) in presence of an object of reflectivity η = 0.7 in accordance with embodiments of the present disclosure.
[0018] Figure 6 illustrates a Maximum value of CHSH parameter Smax expected and experimentally obtained with change in polarization visibility for object reflectivity η = 1 and η = 0.7 in accordance with embodiments of the present disclosure.
[0019] Throughout the drawings, identical reference numbers designate similar, but not necessarily identical elements. The figures as disclosed herein are not necessarily to scale, and the size of some parts may be exaggerated to more clearly illustrate the example shown. Moreover, the drawings are meant to only be provided as examples and/or implementations consistent with the description, and the description may not be limited to the examples and/or implementations provided in the drawings.

DETAILED DESCRIPTION
[0020] The following describes technical solutions in exemplary embodiments of the subject matter of the present disclosure with reference to the accompanying drawings. In this application as disclosed herein, "at least one" means one or more, and "a plurality of" means two or more. The term "and/or" describes an association relationship for describing associated objects and represents that three relationships may exist. For example, A and/or B may represent the following cases: Only A exists, both A and B exist, and only B exists, where A and B may be singular or plural. The character "/" usually indicates an "or" relationship between the associated objects. "At least one item (piece) of the following" or a similar expression thereof means any combination of the items, including any combination of singular items (piece) or plural items (pieces). For example, at least one item (piece) of a, b, or c may represent a, b, c, a and b, a and c, b and c, or a, b, and c, where a, b, and c each may be singular or plural.
[0021] It should be noted that in this application articles “a”, “an” and “the” are used to refer to one or to more than one (i.e., to at least one) of the grammatical object of the article. The terms “comprise” and “comprising” are used in the inclusive, open sense, meaning that additional elements may be included. It is not intended to be construed as “consists of only”. Throughout this specification defined above, unless the context requires otherwise the word “comprise”, and variations such as “comprises” and “comprising”, will be understood to imply the inclusion of a stated element or step or group of elements or steps but not the exclusion of any other element or step or group of elements or steps. The term “including” is used to mean “including but not limited to”. “Including” and “including but not limited to” are used interchangeably. In the structural formulae given herein and throughout the present disclosure, the following terms have been indicated meaning, unless specifically stated otherwise.
[0022] Unless otherwise defined, all terms used in the disclosure, including technical and scientific terms, have meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. By means of further guidance, term definitions are included for better understanding of the present disclosure. The term ‘about’ as used herein when referring to a measurable value such as a parameter, an amount, a temporal duration, and the like, is meant to encompass variations of ±10% or less, preferably ±5% or less, more preferably ±1% or less and still more preferably ±0.1% or less of and from the specified value, insofar such variations are appropriate to perform the present disclosure. It is to be understood that the value to which the modifier ‘about’ refers is itself also specifically, and preferably disclosed.
[0023] It should be noted that in this application, the term such as "example" or "for example" or “exemplary” is used to represent giving an example, an illustration, or descriptions. Any embodiment or design scheme described as an "example" or "for example" in this application should not be explained as being more preferable or having more advantages than another embodiment or design scheme. Exactly, use of the word such as "example" or "for example" is intended to present a related concept in only a specific manner.
[0024] It should be understood that in the embodiments of the present subject matter that "B corresponding to A" indicates that B is associated with A, and B can be determined based on A. However, it should be further understood that determining B based on A does not mean that B is determined based on only A. B may alternatively be determined based on A and/or other information.
[0025] In the embodiments of this application, "a plurality of" means two or more than two. Descriptions such as "first", "second" in the embodiments of this application are merely used for indicating and distinguishing between described objects, do not show a sequence, do not indicate a specific limitation on a quantity of devices in the embodiments of this application, and do not constitute any limitation on the embodiments of this application.
[0026] Exemplary embodiments of the present disclosure relate to a method and system using quantum states of light, having polarization-path entangled single photons for determining a low reflectivity object located within a noisy background. An embodiment includes computing a quantum correlation parameter value (S) by detecting a coincidence measurement between of a pair of photons, wherein the pair of photons is simultaneously generated at an instant of time T1 from a spontaneous parametric down conversion (SPDC) process. In a further embodiment, the pair of photons, includes a signal photon and an idler photon which are referred as heralded single photons source. In an embodiment, the signal photons are entangled in polarization and path degree of freedom by configuring them to take signal path and a reference path in superposition, and the idler photon configured to take an idle path. A further embodiment includes receiving the signal photon and the idler photon simultaneously at a receiver. A further embodiment includes determining, based on the coincidence measurement, if the computed quantum correlation parameter (S) is above a known threshold value. In a further embodiment, if the quantum correlation parameter is greater than a first threshold value for the quantum correlation, conclusively denoting a presence of the object within the noisy background. In a further embodiment, if the quantum correlation parameter is greater than a second threshold value for a residual of quantum correlation, then conclusively denoting a presence of the object within the noisy background.
[0027] In a further embodiment the signal photon is in an entangled state of a polarization and a path degree of freedom. A further embodiment includes a coincidence measurement being performed at a receiver, of the signal photon and the idler photon, primarily based on a non-interferometry technique. In a further embodiment the coincidence measurement also includes information extracted from an alternate method of interference of superimposed signal photon and idler photon. It should be obvious to a person of ordinary skill in the art that various other techniques and methodologies may be available to perform such coincidence measurement and all such techniques and methodologies fall within the scope of the present disclosure. In a further embodiment, the non-interferometry technique is implemented at the receiver where noise from the signal photon received from the object is isolated to a maximum extend.
[0028] In a further embodiment, the coincidence measurement for the pair of photons includes simultaneously detecting the signal photon travelling along signal path and the idler photon travelling along the idle path. In a further embodiment, the coincidence measurement further includes simultaneously detecting the signal photon travelling along reference path and idler photon travelling along the idle path. In a further embodiment, simultaneous detecting the signal photon along the signal path and along the reference path includes conclusively indicating the signal photon received at the receiver along signal path is a noise signal. In a further embodiment, simultaneously detecting the signal photon along the signal path and along the reference path without a simultaneous detection of idler photon includes conclusively indicating the signal photons along the signal path is a noise signal.
[0029] A further embodiment includes, receiving pair of single photons of wavelength F2 generated from a spontaneous parametric down conversion (SPDC) process using a coherent LASER source that is operating at wavelength F1 = F2/2, for example a LASER source operating at 405 nm or 775 nm, when down converted will generate pair of photons at 810 nm or at 1550 nm, and conserves energy and momentum. A further exemplary embodiment includes down converting the coherent photon, i.e., down converting the source photon of 405 nm to two photons of 810 nm each, wherein one of the down-converted photon is referred to as the signal photon and the other down-converted photon is referred to as the idler photon. In a further embodiment, the signal photon and the idler photon are generated at a simultaneously instant of time, for example T1, wherein if the photon source has a wavelength F1 (405nm), then each of the down-converted signal photon and the idler photon has a wavelength of F2 (810nm). It should be obvious that photons of other wavelength may be used, and all such variations of photons input at a wavelength F1, being down converted to wavelength F2 fall within the scope of the present disclosure. In a further embodiment down converting the coherent photon is performed by a non-linear crystal. It should be obvious to a person of ordinary skill in the art that other techniques may be used to down convert the source photons into a signal photon and an idler photon, and all such techniques fall within the scope of the present disclosure. It should also be obvious to a person of ordinary skill in the art that non-linear crystals are most commonly used to down-convert photons, and embodiments of the present disclosure are not limiting in that sense. It should also be obvious to a person of ordinary skill in the art that any single photon source or on demand single photon source, for example quantum dots, may be used in place of down converted single photons to generate polarization-path entangled single photons and calculate quantum correlation, all such techniques fall within scope of the present disclosure.
[0030] A further embodiment includes transmitting the signal photon, wherein the signal photon is entangled into a polarization and a path degree of freedom, at least along one of a signal path or a reference path. In a further embodiment, the signal photon that is travelling along the signal path is primarily directed to detect the object, the object may be having low reflectivity and the object being embedded within a noisy background, wherein it becomes difficult to detect such objects with conventional techniques when signal to noise ratio is very low. A further embodiment includes determining the quantum correlation parameter, wherein the computed quantum correlation parameter (S) is above a value of 2 for quantum correlation and a between a range of between 1.44 and 2 for residual of quantum correlation. In an embodiment, Bells inequality violation in the form of Clauser, Horne, Shimony and Holts (CHSH) parameter (S) is employed to quantify the quantum correlation and detect the presence or absence of an object in the presence of background noise.
[0031] In a further embodiment, an optimized coincidence count of the idler photons and the signal photons is obtained by varying the path length of the idler photon to match the path length of the signal photon. In a further embodiment, a system comprising a source for generating photons, down converting the photons, transmitting the down-converted photons in the direction of an object and receiving the signal photons from the objects via a signal path and/or a reference path and the idler photons from an idler path by varying the path length and determining a coincidence measurement of the photons at the received and computing a quantum correlation parameter above to detect the presence or absence of an object in a noisy background may be devised and the method discussed above employed therein.
[0032] Reference is now made to Figure 1, which is an illustration of an exemplary set-up disclosing the method of using polarization-path entangled photons to detect objects with low reflectivity in a noisy background disclosed as an embodiment of the present disclosure. Source photons 110 is received as input from a coherent light source (not shown in the Figure). Source photon 110 may be for example a coherent source of photons generated by a LASER source, and in an exemplary case source photons have a wavelength of F1. In an exemplary case the wavelength of the source photon may be about 405nm. Source photon is passed through down converting device 120, wherein down converting device consists of a waveplate, lenses and a non-linear crystal. Down converting device 120 down converts source photons 110 into a signal photon 132 and idler photon 134, wherein the signal photon and the idler photon have a wavelength of 2 times F1. In the exemplary case, when source photon 110 has a wavelength of 405 nm, the down converted signal photon 132 has a wavelength of 810 nm and idler photon 134 has a wavelength of 810 nm. If should be obvious to a person of ordinary skill in the art that source photons with 410 nm is only exemplary in nature and in general any coherent source can be used, and the use of any coherent source falls within the scope of the present disclosure. In this disclosure, the terminology signal photon and idler photon has been used for convenience, and it should be obvious to a person of ordinary skill that these down converted photons may be referred by other names and all such variations fall within the scope of the present disclosure. Again, it should be obvious to a person of ordinary skill in the art that down converting device 120 as indicated herein is a non-linear crystal and all other devices and methods for down converting a source photon into a signal photon and idler photon fall within the scope of the present disclosure.
[0033] Idler photon 134 takes an idle path from the down converting device 120 to receiver 150. The time and path of receiving idler photon 134 from down converting device 120 may be varied, such that it may match with the time of arrival of signal photon 132 taking a different path from down converting device 120. Both photons, signal photon 132 and idler photon 134 are generated at a similar instant of time T1, from the coherent source of photons received as input at down converting device 120. Signal photon 132 passes through a polarizer [R()] and then directed to a polarizer beam splitter 145 (hereinafter also referred to as PBS). At PBS depending on the polarization of signal photon 132 takes either signal path 104 or reference path 106 before being received at receiver 150. Signal photon 132 is thus entangled in a polarization-path degree of freedom and the quantum correlation between pairs of quantum states of light, which are highly sensitive to background noise and losses, offer tremendous advantages over traditional illumination methods for detecting objects within a noisy background, and especially objects having low reflectivity which get embedded within the noisy background. Therefore, instead of using correlated photon pairs which are more sensitive to noise and object detection, embodiments of the present disclosure advantageously use the concept of heralding single-photons entangled in polarization and path degree of freedom for quantum illumination.
[0034] Signal photon 132 as discussed previously, either take the reference path 106 which may be detected at receiver 150. Signal photon 132 travelling along reference path 106 and idler photon 134 travelling along the idle path 102 are simultaneously detected at receiver 150 or alternatively signal photon 132 travelling along reference path 104 and idler photon 134 travelling along the idle path 102 are simultaneously detected at receiver 150. Signal photon travelling along path 104 may be directed towards detection of object 170, wherein object 170 may be having low reflectivity and may be within noisy environment 135, and signal photon 132 being reflected from object 170 present within noisy background 150. In an exemplary case noisy background 135 may be considered to be a dense could mass encompassing a flying object such as a drone. It should be obvious to a person of ordinary skill in the art the as an exemplary case drone has been mentioned as an object and clouds as noisy background, but there could be several other variations for the objects and for the noisy background and all such variation fall within the scope of the present disclosure.
[0035] Signal photon 132 received at receiver 150 taking either one of signal path 104 or reference path 106, along with idler photon 134 taking idler path 102 are received at receiver 150 by varying the path length 102 of idler photon 134 to match the path length 104 or path length 106 of signal photon 132. In an exemplary case, based on the coincidence measurement of signal photon 132 and idler photon 134 at receiver 150 for signal photon 132 and idler photon 134 generated at a given instant of time T1, a quantum correlation parameter (S) may be computed for the pair of photons (signal photon and idler photon) which are generated at the instant of time T1. In an exemplary case, the computed quantum correlation parameter S from the coincidence measurement is greater than a threshold value of 2 for quantum correlation and the quantum correlation parameter S is within the range of 1.44 to 2 for residual quantum correlation. In accordance with the exemplary embodiment of the present disclosure, if the quantum correlation parameter is above a value of 2 for quantum correlation and between a range of 1.44 to 2 for residual correlation, conclusively the presence of object 170 may be determined in noisy background 135.
[0036] In an exemplary embodiment, simultaneous detecting signal photon 132 along signal path 104 and along reference path 106 conclusively indicated that signal photon 132 received at receiver 150 along signal path 104 is a noise signal. Again, simultaneously detecting signal photon 132 along signal path 104 and along reference path 106 without a simultaneous detection of idler photon 134 conclusively indicates signal photons 134 along signal path 104 is a noise signal.
[0037] In an exemplary case, object 170 of different reflectivity may be placed along signal path 104 in a variable background noise, for example thermal noise, and computing the quantum correlation. In an exemplary case, receiver 150 advantageously uses non-interferometric measurements along the multiple paths for the photon received at receiver 150 to isolate the signal from the object from the background noise and outperform in detecting and ranging low reflectivity objects even when the signal-to-noise ratio is as relatively low in the range of about 0.03. In another exemplary case, a decrease in visibility of polarization along the signal path also results in similar observations, which may have a direct relevance to the development of single photon based quantum LiDAR and quantum imaging.
[0038] In an exemplary case, quantum correlations in the form of entanglement becomes a feature of quantum mechanics that is central to many quantum information processing protocols. In an exemplary case, however, such techniques are highly sensitive to environmental noise and can be easily destroyed affecting advantages gained especially by nonclassical correlations. In an exemplary case, QI using quantum correlations between pair of photons for object detection in a noisy environment provides much better detection of objects in noisy background environments. In an exemplary case, currently, known approaches for QI rely mainly on two entangled pair of beams in the form of signal photon and idler photon as probe for object detection, wherein signal beam is sent to a region of space containing object merged in background noise and the idler photon beam is stored locally until the signal reflects from the object, for example by varying the path length. In an exemplary case, enhancement of performance of QI over classical analog is made possible by using detection and joint measurement techniques which capture the nonclassical correlations between the stored idler photon and the reflected signal photon by isolating background noise. In an exemplary case, QI measurements primarily focus on reducing uncertainty in unknown parameter estimation using quantum correlation. In an exemplary case therefore, QI extends principles of target detection accuracy, ranging sensitivity, and degree of resilience towards preponderant noise from conventional radar technology to quantum metrology.
[0039] In an exemplary case, even though several QI protocols have been suggested, experimental realization has been challenging mainly due to the unavailability of quantum optimal receivers which involves the difficulty in devising perfect mode-matching for joint phase-sensitive measurements between the reflected signal beams and the stored reference beams. In an exemplary case, single photon entangled in internal degree of freedom like path and polarization provide a natural representation of quantum bits. In an exemplary case an experimentally laboratory setup was made to demonstrate the advantage of using single photons entangled in polarization and path degree of freedom for QI. In an exemplary case, single photons may be heralded from the photon pairs generated using a spontaneous parametric down conversion (SPDC) process, where one of the photon from the pair is retained as an idler photon and used for heralding, whereas the other signal photon is entangled in the polarization and path degree of freedom and used along two paths, the signal path and the reference path.
[0040] In the exemplary case, photon pairs generated from SPDC process were used and not the entangled photon pairs. In the exemplary case, three pathways are employed for the two photons. In the exemplary case, one of the pathways is used for heralding the polarization-path entangled photon and the other two pathways are used as a signal path and reference path, where the signal photons are sent in superposition towards the object and directly to the receiver. In the exemplary case, an object of different reflectivity (𝜂) may be placed along the signal path of the signal photons and controlled noise in the form of thermal background may be introduced along the signal path of the signal photon before taking joint measurements of the signal photon and the idler photon at the receiver and computing the quantum correlations. Bell’s inequality violation in the form of Clauser, Horne, Shimony, and Holt (CHSH) parameter S > 2 is conclusive evidence to quantify quantum correlation and detect the presence or absence of object in presence of background noise.
[0041] In the exemplary case Figure 1 illustrates a schematic of the quantum illumination protocol using polarization-path entangled single photons. In the exemplary case, spontaneous parametric down conversion process may be used to generate photons pairs and one of them, signal photon along path 132 is entangled in polarization and path degree of freedom and sent along signal path,104 and reference path 106. Another photon from the pair along path 102 is used as idler for heralding signal photons. In an exemplary case, object 170 of different reflectivity is placed in the signal path 104 with variable background noise 135. In the exemplary case, entanglement between polarization and path degree of freedom using coincidence counts of photons along reference path 104 and signal path 104 (reflected from the object within the background noise) is calculated with photons along the idler path 102. In an exemplary case, Bell’s inequality violation in the form of CHSH parameter S is used to quantify quantum correlation and object detection.
[0042] In exemplary Figure 1, the laboratory setup specifically was limiting and it should be obvious that the disclosure made herein only illustrates the experimental setup and is not limiting in any manner and a skilled person may replicate the same methodology to detect objects in an open environment using the principles implemented in the present disclosure. In the exemplary case, the protocol for QI using polarization-path entangled single photons includes source photons at 405 nm down converted to 810 nm using the SPDC process. In the exemplary case, From the pair of down converted photon, signal photon 132 is entangled in polarization and path degree of freedom and the idler photon 134 is used as a reference photon for heralding. In the exemplary case, one of the two paths of the polarization-path entangled single photon is sent towards the object along signal path 104 and other path is used as reference path 106. In an exemplary case, using the coincidence detection of photons along the three paths, CHSH parameter is computed to quantify quantum correlation. In the exemplary case, to calculate CHSH parameter using coincidence counts along multiple paths, interferometric and non-interferometric techniques may be used at receiver 150. In the exemplary case, and in accordance with the present disclosure, both the approaches, interferometric and non-interferometric techniques, work efficiently when objects 170 of different reflectivity are placed in the signal path 104 and in absence of background noise 135. In an exemplary case, in the presence of background noise 135, significant advantages are noticed for only non-interferometric measurements which are advantageously used for detecting object 170. In the exemplary case, the demonstrated protocol using non-interferometric techniques isolates the background thermal noise from the signal photons 132 at the receiver 150 and object 170 can be easily detected by returning a quantum correlation value of S > 2 even when signal photons 132 received at receiver 150 are buried under the noise with SNR as low as 0.03.
[0043] In an exemplary case, even when quantum correlation S < 2 cannot be recorded, for a range 1.44 < S < 2, showing the classical correlation as residual of quantum correlation may still be used to detect object 170, where the object has low reflectivity, and is immersed in noisy background 135, with SNR as low as 0.03 corresponding to -15 dB. In the exemplary case, use of CHSH parameter illustrates a significant suppression of background noise using the embodiments presented in the present disclosure. In an exemplary case, the effect of decrease in polarization visibility which models the polarization scattering by the object has also been studied in the laboratory setup and even when the polarization visibility is as low are 0.2, the presence of object 170 could be easily detected in accordance with the embodiments of the present disclosure. In the exemplary case in accordance with the embodiments of the present disclosure using heralded single photons of 810 nm wavelength indicated a direct relevance to development of quantum LiDAR. It should be obvious that the exemplary case discussed above was under laboratory conditions to illustrated the detection of the presence of objects, especially with low reflectivity, in a noisy background and the embodiments disclosed herein can be adopted and easily implemented to other wavelengths and at a larger scale, for example detecting objects with low reflectivity in the atmosphere embedded within the clouds, which would otherwise be practically impossible to detect.
[0044] Reference is now made to Figure 2A, which illustrates a schematic of an exemplary experimental setup for QI using polarization-path entangled single-photon using an interferometric approach where photons from both the paths are made to interfere. Reference is also made to Figure 2B, which illustrates a schematic of an exemplary experimental setup for QI using polarization-path entangled single-photon using a non-interferometric approach where photons from both the paths do not interference. It should be noted here that the illustrations in Figure 2A and Figure 2B were essentially laboratory setup replating the principles of Figure 1 to illustrate the working/enablement of the embodiments in accordance with the present disclosure, and hence no reference numerals are used in Figure 2A and Figure 2B.
[0045] As illustrated in Figure 2A and Figure 2B, a schematic of the experimental setup for QI using polarization-path entangled single-photon in the laboratory environment to illustrated enablement of the technique is provided. In the exemplary case, the heralded single-photon entangled state may be sent across two paths where one path acts as reference and reaches the receiving unit (also referred to as a receiver in Figure 1) directly and the other path intercepts with the object and reflects back to the receiver. In the exemplary laboratory setup, two different measurement procedures are shown to calculate quantum correlation. In an exemplary case, with respect to Figure 2A, an interferometric approach is illustrated where photons from both the paths are made to interfere, and in Figure 2B, a non-interferometric approach is illustrated where photons from both the paths do not interference. In the exemplary case, the PBS and polarization rotator R(.) in the scheme may be used to control the splitting ratio along the paths by varying θ and δ. In the exemplary case, using the coincidence counts of photons along different paths with the idler photons for different combination of (θ, δ), and (θ′, δ′) the CHSH parameter, S, may be computed. When the object reflectivity, η = 1 and in absence of background noise, both the procedure using interferometric and non-interferometric techniques are equivalent and result in the same CHSH parameter.
[0046] In the exemplary case, the state of the single photon in equal superposition of two linearly polarized states, |h⟩ and |v⟩ when passed through the polarizing beam splitter (PBS) may be represented by the equation |Ψ0⟩=(1/√2) [|ℎ⟩|0⟩−|𝑣⟩|1⟩]. In the exemplary case, the states |0⟩ and |1⟩ are the two polarization dependent paths for the photons, which may be referred to as the reference path and signal path. In the exemplary case, the preceding state is maximally entangled in polarization and path degree of freedom. When the photon passes through the object with reflectivity 0 ≤ η ≤ 1 along the signal path, the effect of the object on the photons state may be represented in the form of a controlled operator causing loss along the path of the signal by ,
T(η) = |h⟩⟨h| ⊗ |0⟩⟨0| +(√η)|v⟩⟨v| ⊗ |1⟩⟨1|. The density matrix of the polarization-path entangled photon at the receiving end of the signal and reference path will be in the form ρ(η) = T(η) (|Ψ0⟩⟨Ψ0|) T(η)†.
[0047] In an exemplary case, in accordance with the embodiments of the present disclosure, under laboratory conditions, two measurement procedure to analyse
the photons arriving along both, signal path and reference path being of identical length is considered. In an exemplary case, an ideal method to calculate quantum correlations from single photons in path degree of freedom involves interference of the two paths and probabilities of output states. To quantify quantum correlation, the CSHS parameter S is computed using the formula, S = |E(θ, δ) − E(θ, δ′) + E(θ′, δ) + E(θ′, δ′)|, where E(θ, δ) = Ph0(θ, δ) + Pv1(θ, δ) − Ph1(θ, δ) −
Pv0(θ, δ).
[0048] In the exemplary case, Pij’s represent the probabilities of different basis states of the photon in polarization and path composition. In the exemplary case, parameter θ and δ are the angles that that represent the polarization of photons. In the exemplary case, different basis states of the photon in polarization and path degree of freedom may be obtained using the combination of the polarization rotator R(θ)(R(δ)) and polarizing beam splitter (PBS) along the paths of the photons.
[0049] Referencing back to Figure 2A, the schematic illustrates a combination of polarization rotator R(・) and PBS along the interfering paths of the photons in accordance with the embodiment of the present disclosure. The effect of the polarization rotator along both the paths before they interfere at PBS and after they interfere on the state of polarization-path entangled photon may be represented by ρ(θ, δ)I=(1⊗R(δ)) (R(θ)⊗1)ρ(η)(R(θ)⊗ 1)†( 1⊗R(δ))† where polarization rotator, R(θ) (R(δ)) are given by, .
[0050] In an exemplary CASE, using the half-wave plate (HWP) rotated by an angle κ/2, H(κ) = , where R(θ) ≡ H(κ/2). The probabilities, Ph0 = ρ(θ, δ)44, Pv1 = ρ(θ, δ)11, Ph1 = ρ(θ, δ)22 and Pv0 = ρ(θ, δ)33 are the diagonal elements of the density matrix. In the exemplary case, coherence of single photon along multiple paths is extremely hard to achieve experimentally for longer path lengths. In the exemplary case, therefore, an equivalent non-interferometric method may be effectively used to calculate the quantum correlations in single photon states. In the exemplary case, the equivalent density matrix and the probabilities of the basis state of the composite system may be obtained by performing an identical rotation independently along both the paths.
[0051] Referring now to Figure 2B, the schematic of the combination of polarization rotator and PBS along both the non-interfering paths are illustrated and the state can be represented by ρ(θ, δ)NI = (R(θ+δ)⊗1)ρ(η)(R(θ+δ)⊗1)†. In the exemplary case, for combinations of θ and δ the maximum value of CHSH parameter can be recorded, S = 2√2, when the object reflectivity, η = 1 for both the interferometric and the non-interferometric approach.
[0052] Reference is now made to Figure 3A, which illustrates an exemplary method for producing a signal photon and an idler photon in accordance with the embodiments of the present disclosure. In step 310, coherent photons are received from a photon generating source, for example a LASER, wherein the source photons are at a wavelength F1. In an exemplary case, photons source photons at 405 nm were used for enablement of the present disclosure. It should be obvious to a person of ordinary skill in the art that in general any coherent source of photons may be used, and all such variations fall within the scope of the present disclosure. In step 312, the source photons which are at a wavelength F1 are down converted using a non-linear crystal, into two photons, a signal photon and an idler photon, both the signal photon and the idler photon have a wavelength F2, and the signal photon is entangled in a polarization-path degree of freedom.
[0053] In step 314, the signal photon is transmitted at least along one of two available paths, the signal path or the reference path depending on the polarization of the signal photon and the idler photon is transmitted along the idle path. Both the signal photon and the idler photon are generated at the same instant of time. In step 316, the signal photon and the corresponding idler photon generated at a given instant of time are collected at a receiver, wherein the path length of the idler photon is varied such that the idler photon and the signal photon coincide at the receiving unit (also referred to as receiver in the present disclosure). All other embodiments have been described previously or with respect to Figure 1.
[0054] Reference is now made to Figure 3B, which illustrates an exemplary method for determining the presence or absence of an object in accordance with the embodiments of the present disclosure. In step 320, the signal photon and the idler photon are received at the receiver. Based on the time of arrival at the receiver, and the coincidence measurement of the signal photon and the idler photon, in step 322, a quantum correlation parameter is computed. If the computed quantum correlation value is greater than 2, then the measurement indicating the presence of an object, wherein the object can be in a noisy background as disclosed in the present application. All other embodiments have been described previously or with respect to Figure 1, and computing the correlation parameter will be described later in the disclosure.
[0055] Reference is now made to Figure 3C, which illustrates an exemplary method for determining the presence or absence of an object in accordance with the embodiments of the present disclosure. In step 330, the signal photon and the idler photon are received at the receiver. Based on the time of arrival at the receiver, and the coincidence measurement of the signal photon and the idler photon, in step 332, a quantum correlation parameter is computed by performing a coincidence detection between the pair of photons. In step 3, if the residual of quantum correlation is determined to be between the range of 1.44 and 2, i.e., 1.44 < S < 2, the presence of an object in the noisy background may be established. All other embodiments have been described previously or with respect to Figure 1, and computing the correlation parameter will be described later in the disclosure.
[0056] Reference is now made to Figure 3D, which illustrates an exemplary method for determining whether the received signal is noise. In step 340 a coincidence measurement is performed at the receiver, wherein the signal photon and the idler photon are received at the receiver. In step 342, detection of signal photon along signal path or coincidence detection of photon along signal path and reference path with no detection of idler photon is checked. In step 344, no detection of idler photon with coincidence detection of photon along signal path and reference path or detection of the photon along signal path only, conclusively indicates that the photons received along signal path of the receiver is noise, the noise being associated with the background noise. In an exemplary case, mention may be made here that the computed correlation parameter S < 1.44 indicates that the signal received is only noise and no object is detected. All other embodiments have been described previously or with respect to Figure 1, and computing the correlation parameter will be described later in the disclosure.
[0057] Reference is now made to Figure 3E, which illustrates an exemplary method for determining whether the received signal is noise. In step 350 a coincidence measurement is performed at the receiver, wherein the signal photon and the idler photon are received at the receiver. In step 354, coincidence detection of photon along signal and reference path along with detection of idler photon is checked. In step 358, when the idler photon is detection along with coincidence detection of photon along signal and reference path conclusively indicates that the photons received along the signal path of the receiver is noise, the noise being associated with the background noise. In an exemplary case, it may be mentioned that the computed correlation parameter S < 1.44 indicates that the signal received is only noise and no object is detected. All other embodiments have been described previously or with respect to Figure 1, and computing the correlation parameter will be described later in the disclosure.
[0058] As discussed previously, if the signal photon is travelling along the signal path and the idler photon is travelling along the idle path, then conclusively indicating the signal photon received at the receiver only along signal path is a noise signal. Further, if the signal photon is travelling along the signal path and along the reference path without a simultaneous detection of idler photon, then again conclusively indicating the signal photons along the signal path is a noise signal.
[0059] Reference is now made to Figure 4A, which illustrates the theoretically calculated CHSH parameter Smax when an object of different reflectivity η is placed along the signal path using polarization-path entangled single photons as probe and different measurement schemes at the receiving unit. Non-interference approach shows advantage at low reflectivity regime. In the exemplary case, to show the classical regime, Smax when single photons in superposition state is sent only across signal path is shown and the value Smax < 1.44. In the exemplary case, this allows the use classical correlation between polarization and path degree of freedom in the range 2 > S > 1.44 as residual of quantum correlation to identify the object of very low reflectivity using non-interferometric measurement scheme. In the exemplary case of Figure 3, Smax as a function of object reflectivity η is shown for both, interferometric and non-interferometric measurement schemes. The dotted line for a Smax value of 0 to 1.44 represent the classical regime for object reflectivity η, which is almost a linear line obtained when source is not entangled in polarization and path entangled state. The dotted line for a Smax value of 1 to 2.8 represent the interference regime where object reflectivity η, which increases from 1.0 when object reflectivity is 0 to about 2.5 when object reflectivity is about 0.6 and then remains linear going to About 2.8 when object reflectivity is 1. The solid line for a Smax value of 1.44 and above following a linear pattern represent the non-interference regime where object reflectivity η, which is almost a linear line starting at 1.44 when the object reflectivity is 0 and is around 2.8 when object reflectivity is about 1.
[0060] In the exemplary case, for one set of parameters a maximum value is obtained when the initial state is of the form |Ψ0⟩=(1/√2) [|ℎ⟩|0⟩−|𝑣⟩|1⟩] and sent across signal and reference paths θ = 0, δ = π/16, θ′ = 3π/16 and δ′ = 5π/16. In the exemplary case, the non-interference approach provides advantage at low reflectivity region by returning higher S value. In the exemplary case, this may be attribute the difference in value of E(θ, δ), wherein a straight forward calculation of combination of probabilities of finding photons along four paths in both the interference and non-interference scheme. In the exemplary case, only two of the probabilities varies with varying reflectivity in non-interference scheme and all the four varies in interference scheme affecting the overall value of S. IN the embodiments of the present disclosure, difference in value of S at low reflectivity regime is noticed.
[0061] In the exemplary case, the same scheme can be turned to a classical illumination scheme by replacing the initial polarization-path entangled single photon state with the single photon in state |Ψ⟩=(1/√2) [|ℎ⟩−|𝑣⟩]m, only along the signal path. In the exemplary case, for the choice of parameters given above, the measuring unit will not record any correlation in polarization and path degree of freedom. This is illustrated in Figure 3 for a value of Smax < 1.44 when measurement configuration as illustrated in Figure 2B is used. In the exemplary case, this provides an upper bound on the value of maximum value of S when single photon without being in correlation with its internal degree of freedom is used. Therefore, in the exemplary case, even when S < 2, in absence of violation of Bell’s inequality, the classical correlation 1.44 < S < 2 as the residual of the quantum correlation to identify the presence of object with low reflectivity may be advantageously used.
[0062] In the exemplary case, the value of S > 1.44 is not observed when source is not in correlated state, the residual of quantum correlation, 1.44 < S < 2 can be attributed to the presence of correlation in the initial state of photons source used for illumination. Notably, the exemplary framework of single photon in signal path and reference path guided into the receiving unit is in the form of two step discrete-time quantum walk where the half wave plate (HWP) and polarization beam splitter (PBS) are quantum coin operation and polarization dependent shift operators. Thus, in the exemplary case, it should be obvious to a person of ordinary skill in the art that various other configuration of parameters may be adopted to control and measure correlation between the polarization and path degree of freedom in presence of noise, and all such configurations fall within the scope of the present disclosure.
[0063] In the exemplary case, when thermal noise is introduced in the form of white light along the path of signal, the noisy photons also get detected along with the photons from the signal path but their random polarization will only result in the increase in offset of the photons detected in the detectors and ideally only the photons from signal will contribute to change in the polarization when rotated using the HWP. Therefore, in the exemplary case, until the fluctuation in thermal noise supersedes the change in signal photon counts in detectors with HWP, a reliable S value may be obtained which in turn helps in detecting the presence of an object. In an exemplary case, however, in the interferometric measurement scheme, since all the four detectors receive noisy photons, there may be a contribution to false coincidence counts resulting in decrease in S parameter. In an exemplary case, in the non-interferometric scheme, the reference path which does not receive any noisy photons will reduce the false coincidence counts contributing robustness against noisy photons.
[0064] In an exemplary case, since the polarization degree of freedom is used in the QI scheme, scattering of polarization state of photons may affect the value of S affecting the detection of object. In an exemplary case, the depolarizing noise on signal photons can be modelled using a path dependent depolarizing channel and the final state may be represented by , where fi = 1⊗|0⟩⟨0| + σi⊗|1⟩⟨1| and σi are referred to as the Pauli operators. In the exemplary case, by subjecting D[ρ(η)] to the HWP and PBS as illustrated in Figure 2A and Figure 2B, for different values of θ and δ the CHSH parameter may be computed.
[0065] Reference is again made to Figure 2B, which a non-interferometric approach is used for Oi suing heralded single photon entangled in polarization and path degree of freedom. In an exemplary laboratory setup, a 10-mm long periodically poled potassium titanyl phosphate (PPKTP) nonlinear crystal with poling period Λ = 10 μm and aperture size of 1x2 mm2 may be deployed to generate the heralded single photons using a type-II SPDC process. In the exemplary case, the crystal is pumped using continuous-wave diode laser at 405 nm with 5 MHz a linewidth. In the exemplary case, a half-wave plate is used to set the polarization of the laser and a plano-convex lens of 300 mm is used to focus the pump beam into the centre of the crystal with beam waist w0 = 42.5 μm. In the exemplary case, a PPKTP crystal is housed in an oven and its temperature is maintained at 23oC to obtain degenerate photon pairs at 810 nm. In the exemplary case, a bandpass interference filter at 810 nm centre wavelength with a bandwidth of 10 nm FWHM is used for collecting the SPDC photons from the residual pump light. In the exemplary case, the wavelength of the down-converted photons is confirmed using a spectrometer. In the exemplary case, the generated orthogonally polarized photon pairs (|h⟩, |v⟩) are collimated using a plano-convex lens of 35 mm and separated using a polarization beam splitter. In the exemplary case, the idler photons |h⟩ which is used as reference for heralding are coupled to single-mode optical fiber using appropriate collection optics and sent directly to the receiving unit.
[0066] In the exemplary case, the signal photons in free space is passed through a halfwave plate at π/8 and a polarizing beam splitter (PBS) to generate a polarization-path entangled state. In the exemplary case, from the two pathways of the polarization-path entangled photons, the reference path may be coupled to single-mode fiber and sent to the receiving unit whereas the signal path is sent towards the object. In the exemplary case, a non-polarizing beam splitter (BS) with variable reflectivity is used as an object in the signal path. In the exemplary case, a broadband thermal light source is used to add noise into the system through another input port of the object BS and the photons from the signal path are collected at the receiving unit. In the exemplary case, at the receiving unit, HWP and PBS are placed along both, the signal path and reference path. In the exemplary case, the path length of the idler photon used for heralding is adjusted to the path length of the signal path by using the maximum coincidence counts for a fixed time window as reference point. In the exemplary case, similar path length is set to reference path. In the exemplary case, output from the both PBS are connected to four fiber coupled detectors, single photon counting modules, SPCMj and the idler photons is also connected to another SPCM5. In the exemplary case, all five detectors are connected to time-correlated single-photon counter. In the exemplary case, by taking the coincidence counts of photons from the four detectors with idler photon, probabilities of the basis states of the polarization-path composition of the photon are measured using the formula:

where Cj,5(θ, δ) are the number of coincidence detection of photons in SPCMj and SPCM5. In the exemplary case, using these probabilities for different combination of (θ, δ) we can calculate CHSH parameter S. In the exemplary case, for the set of angles (θ, δ, θ′, δ′) = (0, π/16, 3π/16, 5π/16) realised using HWPs angles (0, π/32, 3π/32, 5π/32) the maximum S value is obtained.
[0067] Reference is now made to Figure 4B, which is an exemplary illustration of experimentally obtained maximum value of the CHSH parameter Smax for an object with different reflectivity. In the exemplary plot Smax was calculated using coincidence counts of four signal detectors with one heralding detector. In the exemplary case, line 410 is the theoretical plot using a non-interference scheme. Line 420 represents data points are for a pump power of 2 mW, line 430 represents data point for a pump power of 5mW, line 440 represents data points for a pump power of 10 mW and line 450 represents data points for a pump power of 15 mW, respectively. In the exemplary case, the error bars are for the standard deviation of the measurements. It may be noted here that the experimental results obtained for different pump powers are in close agreement with the theoretical value.
[0068] As illustrated in exemplary Figure 4B, the maximum value of CHSH parameter, Smax experimentally obtained when object of different reflectivity η is illuminated using polarization-path entangled photon for the non-interferometric scheme. In the exemplary case, the curve 410 without any error bars is the theoretical plot using a non-interference scheme. Lines 420, 430, 440 and 450 represent data points are for a pump power of 2, 5, 10, and 15 mW, respectively, as disclosed previously. In the exemplary case, the corresponding signal photon counts are measured to be around 0.95×105, 2.64×105, 4.45×105, and 6.93×105 counts/s, respectively. In the exemplary case, the experimental value for different object reflectivity and for different pump power are all in close agreement with the theoretical expectation. In an exemplary case, for an object with reflectivity η = 0.3 the estimated Smax = 1.6±0.05. In the exemplary case, even though the violation of Bell’s inequality is not noticed, here as presented in the theoretical description, for the value of 1.44 < S < 2, the presence of object with low reflectivity can easily be inferred.
[0069] Reference is now made to Figure 5, which illustrates the exemplary experimentally obtained maximum value of CHSH parameter Smax for different percentage of signal (P) in presence of an object of reflectivity η = 0.7. the solid line indicated a measurement at pump power of 10mW and the dotted line is measurement at a pump power of 15mW, with an object reflectivity of 0.7 in the thermal background of (100-P)%. In the exemplary case, the background noise is increased such that the percentage of signal varied from 100 to 2. In the exemplary case, the solid line data points are for a pump power of 10 and the dotted line data points are for a pump power of 15 mW. In the exemplary case, the error bars as illustrated are for the standard deviation of the measurements. In the exemplary case, the inset illustrates a zoomed region of the percentage of signal from 9 to 2, where it is noticed that Smax > 2 even when SNR=0.03.
[0070] Reference is now made to exemplary Figure 5, where Smax for object reflectivity η = 0.7 when background noise was introduced. In the exemplary case, data points with the solid and dashed lines are for a pump power of 10 and 15 mW. In the exemplary case, the number of signal photons was kept fixed by fixing the pump power. In the exemplary case, the background noise was increased such that the percentage of noise is varied from 0% to 98 % and then the Smax computed. In the exemplary case, when the signal is only 10% and noise is 90%, SNR is about 0.11. In the exemplary case, by increasing background noise, the Smax value remains almost a constant till signal is only 3%, and SNR is about 0.07, and with further increase in noise percentage it significantly reduces from 2.36±0.04 to 1.97±0.04. In the exemplary case, even when SNR is about 0.02 which corresponds to -15 dB, the value of S is approximately around 1.9. Using these result and the results illustrated in Figure 4, even when η = 0.2 and SNR is about 0.11, Smax > 1.44, and hence embodiments of the present disclosure as disclosed herein can effectively be used as an indicator of presence of object with very low reflectivity in the presence of background noise.
[0071] In the exemplary case, the object illuminated using polarization-path entangled photons can isolate the noise to a significant extent and register photons correlated only in photon correlated in polarization and path degree of freedom. In the exemplary case, from the inset in Figure 5 it may be noticed that the value of S > 2.2 even when signal level is only 5% of the background noise level entering the detector. In the exemplary case, when background noise is > 95% a significant and sudden dip in the value of S may be noticed, and beyond that point the photon number fluctuations from the noise starts coinciding with the SPDC photons contributing to the change in photon counts due to the change in (θ, δ) in the receiving unit. In the exemplary case, this makes polarization and path entangled QI scheme a highly robust even in high noise regime.
[0072] Reference is now made to Figure 6, which illustrates a maximum value of CHSH parameter Smax expected and experimentally obtained with change in polarization visibility for object reflectivity η = 1 and η = 0.7. In the exemplary case, the maximum value of S as a function of depolarization in the form of polarization visibility of photons from signal path when received from the object with η = 1) and η = 0.7. In the illustration of Figure 6, lines 610 and 620 indicated the theoretical plots and line 615 and 625 indicate the experimental plots for reflectivity of 1m and 0.7. In the exemplary case, theoretical expectation may be obtained using a depolarizing channel along the signal path. For the experimental value, the polarization visibility is used to mimic depolarizing channel. In the exemplary case, by changing the combination of waveplates along the signal path. the experimental results (615) obtained for polarization visibility in the range of 0.3 to 1 is lower than the theoretical value but they follow a similar trend. In the exemplary case, by collaborating the observations, even for the combination of depolarization effect, reflectivity and thermal noise the value of S > 1.5 can be used as an indicator of presence of object.
[0073] As illustrated in accordance with the embodiments of the present disclosure, the scheme will be highly effective when all the three path lengths match. In an exemplary case, in real time scenario path lengths can be estimated by looking for the consistent match of the coincidence and single photon detection of the idler photons with the signal photons in schemes using SPDC process. In an exemplary embodiment, when an on-demand photon source is used, the time of arrival and the time difference assist in the ranging.
[0074] In an exemplary case, embodiments of the present disclosure implement the use of polarization and path entangled single photons for QI that illustrate that low reflectivity object in noisy background can be easily detected by computing the quantum correlation parameters, even where SNR is very low. In the exemplary case, using heralded single photons from SPDC process (but not limiting to this process) a maximally entangled state in polarization and path degree of freedom of a single photon as the optimum probe state to illuminate the object was prepared. The quantum advantage of using polarization-path entangled single photons over only single photons for QI can be clearly envisioned from the embodiments disclosed in the present disclosure. The embodiments disclosure herein, also provide an advantage over the fragility of two photon entangled state. In the embodiments disclosed herein for an object reflectivity, η > 0.5 violation of Bell’s inequality, S ≥ 2 confirms the presence of object even when the SNR is as low as 0.03.
[0075] In the exemplary embodiment of the present disclosure, additionally, it was shown that in the regime of classical correlation that are residual of quantum correlation to identifying object with η < 0.5, using non-interferometric measurement scheme background noise has been isolated from the signal and only signal photons contributes towards calculating S value. In the exemplary case, only when SNR > 0.02 noise was seen to take prominence. Exemplary embodiment of the present disclosure show the ability to detect object with reflectivity as low as η = 0.2 in the background noise with SNR = 0.05, and hence demonstrate the robustness of the embodiments of the present disclosure. Accordingly, embodiments also suggests the possibility of estimation of noisy environment by analysing the photon measured and the deviation from the expected value at the receiver.
[0076] Although the present disclosure has been described with reference to several preferred embodiments, it should be understood that the present disclosure is not limited to the preferred embodiments disclosed here. Embodiments of the present disclosure are intended to cover various modifications and equivalent arrangements within the spirit and scope of the appended claims. Although the foregoing disclosure has been described in some detail for purposes of clarity of understanding, it will be apparent that certain changes and modifications may be practised within the scope of the appended claims. Examples of the present disclosure have been described in language specific to structural features and/or methods. It should be noted that there are many alternative ways of implementing both the process and apparatus of the present invention. Accordingly, embodiments of the present disclosure are to be considered illustrative and not restrictive, and the invention is not to be limited to the details given herein but may be modified within the scope and equivalents of the appended claims. It should be understood that the appended claims are not necessarily limited to the specific features or methods described. Rather, the specific features and methods are disclosed and explained as examples of the present disclosure.

, Claims:1. A method for determining a low reflectivity object 170 located within a noisy background 135 using quantum states of light, the method comprising:
- computing a quantum correlation parameter value (S) between a polarization and path degree of freedom of a single photon using a coincidence measurement between a pair of photons 132, 134, wherein the pair of photons 132, 134 is simultaneously generated at an instant of time T1, and the pair of photons 132, 134 comprises:
a signal photon 132 and an idler photon 134, the signal photon 132 configured to take at least one of a signal path 104 or a reference path 106, and the idler photon 134 configured to take an idle path 102;
- receiving the signal photon 132 and the idler photon 134 simultaneously at a receiver 150; and
- conclusively denoting a presence of the object 170 within the noisy background 135 when the computed quantum correlation parameter is greater than the first threshold value for quantum correlation and greater than a second threshold value for a residual of quantum correlation.

2. The method as claimed in claim 1, wherein the coincidence measurement is performed at a receiver 150 based on a non-interferometry technique.

3. The method as claimed in claim 2, wherein the coincidence measurement comprises extracting information from the coincidence of the signal photon along single path and idler photon along the idle path, and signal photon along reference path and idler photon along the idle path.

4. The method as claimed in claim 2, wherein the non-interferometry technique is implemented at the receiver 150 comprises:
- isolating noise 135 from the signal photon 132 along signal path 104 received from the object 170.

5. The method as claimed in claim 3, wherein the coincidence measurement for the pair of photons comprises:
simultaneously detecting
- the signal photon 132 travelling along the signal path 104 and the idler photon 134 travelling along the idle path 104; and
- the signal photon 132 travelling along reference path 106 and idler photon 134 travelling along the idle path 102.

6. The method as claimed in claim 5, wherein simultaneous detecting the signal photon 132 along the signal path 104 and along the reference path 106 comprises:
conclusively indicating the signal photon 132 received at the receiver 150 along signal path 104 is a noise signal.

7. The method as claimed in claim 5, wherein simultaneously detecting the signal photon 132 along the signal path 104 and along the reference path 106 without a simultaneous detection of idler photon 134 comprises:
conclusively indicating the signal photons 134 along the signal path 104 is a noise signal.

8. The method as claimed in claim 1, the method comprising:
receiving coherent photons 110 produced at a pre-determined input wavelength F1 to a parametric down conversion unit 120, wherein the coherent photons 110 oscillate at a same frequency and a wavelength of the photons are in phase; and
down converting the coherent photon 110 into the signal photon 132 and the idler photon 134, wherein the signal photon 132 and the idler photon 134 are generated at a simultaneously instant of time, wherein the signal photon 132 and the idler photon has a wavelength of F2, wherein the wavelength F2 = 2xF1.

9. The method as claimed in claim 1, wherein down converting the coherent photon is performed by a down converting unit 120, the down converting unit 120 comprising of a combination of a waveplate, a plurality of lenses and a non-linear crystal.

10. The method as claimed in claim 1, the method comprising:
transmitting the signal photon 132 wherein the signal photon 132 is entangled into a polarization and a path degree of freedom, along the signal path 104 and the reference path 106 in superposition state.

11. The method as claimed in claim 10, wherein signal photon 132 is directed to detect the object 170 along the signal path 104.

12. The method as claimed in claim 11, wherein the object 170 is embedded in a noisy background 135 and/or the object 170 has low reflectivity and/or is a combination thereof.

13. The method as claimed in claim 1, wherein the computed quantum correlation parameter (S)
- for the first threshold is 2 for the quantum correlation; and
- for the second threshold is between 1.44 and 2 for the residual of quantum correlation.

14. The method as claimed in claim 1, wherein an optimized coincidence count of the idler photons and the signal photons is obtained by varying the path length 102 of the idler photon 134 to match the path length 104 of the signal photon 132 to range the position and distance of the object.

15. A method as claimed in claim 1, wherein the source of the single photon includes an on-demand single photons source and/or by entanglement between other internal degrees of freedom of single photons, wherein single photons being entangled in other internal degree of freedom for quantum illumination.

16. The method as claimed in claim 15, wherein the time of generation of the on-demand single photons is recorded.

17. The method as claimed in claim 15, wherein the on-demand single photons source only has the signal photons in a superposition of signal and reference path with time of generation of photon is used in place of idler photon.

18. The method as claimed in claim 17, wherein the measurement of the single photon in signal and reference path conclusively indicating the presence of the object if the computed quantum correlation parameter (S)
- for the first threshold is 2 for the quantum correlation; and
- for the second threshold is between 1.44 and 2 for the residual of quantum correlation

19. A system configured to perform the method as claimed in claims 1 to 18