System and method for determining the position of an object via entangled photons

Abstract

The invention relates to a method for measuring the position of a target (10), comprising the following successive steps: a) generating a plurality of pairs of entangled photons, each pair comprising a first photon transmitted over a first optical path (3) and a second photon, entangled with the first photon, transmitted over a second optical path (4); b) measuring the polarisation state of the second photons on the second optical path and comparing this measured polarisation state with a reference state corresponding to the polarisation state of a reference photon having propagated over the second optical path without a first photon entangled with the reference photon having been absorbed; c) determining the probability that the first photons have been absorbed by the target.

Description

[0001] Description

[0002] Title: System and method for determining the position of an object using entangled photons

[0003] technical field

[0004] The present invention relates to the field of quantum detection and quantum imaging.

[0005] It relates more specifically to a quantum system using entangled photon pairs to determine the distance to a target object. It also relates to a method for determining the position of an object using such a system.

[0006] Previous technique

[0007] It is known to implement radar-type devices to detect the presence of objects as well as measure their position and speed.

[0008] Radar systems operate by emitting electromagnetic radiation towards reflective objects. Detecting the reflected radiation and measuring its travel time allows the position of the observed object to be determined.

[0009] This proven technique, however, has the drawback of allowing radar localization, being energy-intensive, requiring large receiving antennas, and being vulnerable to radar glare.

[0010] A process called quantum image distillation, for example as described in the article Quantum image distillation, Hugo Defienne et al., Sci. Adv.5, eaax0307 (2019). D01: 10.1126 / sciadv.aax0307, ​​makes it possible to filter the light coming from an object to extract quantum information from an image that contains both quantum and classical information.

[0011] However, this method cannot be used to observe objects whose surfaces absorb the photons used to illuminate the source. It requires strong illumination of the object so that it can reflect photons back to the imaging device.

[0012] Therefore, there is a need to propose a system that addresses the drawbacks of the previous art.

[0013] The aim of the invention is to at least partially meet this need. Summary of the invention

[0014] To this end, the invention relates, in one of its aspects, to a method for measuring the position of a target, comprising the following successive steps: a) generation of a plurality of entangled photon pairs, each pair comprising a first photon emitted on a first optical path and a second photon, entangled with the first photon, emitted on a second optical path; b) measurement of the polarization state of the second photons on the second optical path and comparison of this measured polarization state with a reference state corresponding to the polarization state of a reference photon that has propagated on the second optical path without a first photon entangled with the reference photon having been absorbed; c) determination of the probability that the first photons have been absorbed by the target.

[0015] Thus, the inventor thought of using the effect of quantum entanglement between two photons on their respective polarization in order to determine whether the second photons have been entangled with first photons absorbed or not by the target.

[0016] When a first photon, whose polarization can be predetermined, is absorbed by the target, the polarization of the second photon of the same pair is modified by quantum entanglement. When the polarization of a second photon is detected before the first photon of the same pair has been absorbed, then the polarization of said second photon has not been affected by its quantum entanglement with said first photon.

[0017] Therefore, by comparing the polarization state of the second photons measured with a reference state corresponding to the polarization state of the second photon, it is possible to determine the probability that the first photons were absorbed by the target.

[0018] Step b) can be performed after the second photons have traveled a predetermined optical distance. Step c) thus determines whether the target is at an optical distance less than this predetermined optical distance. A probability that the first photons were absorbed by the target greater than the probability that they were absorbed by a medium separating the target from the photon generation indicates a high probability that the target is at an optical distance, relative to the generation point of the entangled photon pairs, less than the predetermined optical distance.Conversely, a probability that the first photons were absorbed by the target that is less than or equal to the probability that they were absorbed by a medium separating the target from the photon generation indicates a high probability that the target is at an optical distance, relative to the generation point of entangled photon pairs, greater than the predetermined optical distance. The probability that first photons were absorbed by a medium separating the target from the photon generation can be extrapolated from at least one measurement of first-photon absorption upstream of the target.

[0019] If, during step c), it is determined that the first photons were not absorbed by the target, then steps a) through c) can be repeated by increasing the predetermined optical distance. If, during step c), it is determined that the first photons were absorbed by the target, then steps a) through c) can be repeated by decreasing the predetermined optical distance. It is thus possible to determine an optical distance range within which the target lies.

[0020] The method may include a step between steps a) and b) in which a portion of the electromagnetic wave associated with each of the first photons is directed along a third optical path, and another portion of the electromagnetic wave associated with each of the first photons is directed along a fourth optical path. This is referred to as preparing the first photons before their interaction with the target. Advantageously, by preparing the first photons, the method according to the present invention does not require the individual transmission of entangled photon pairs but is suitable for the simultaneous transmission of a multitude of entangled photon pairs.

[0021] The preparation of the first photons takes place before they enter a region where the target might be present. Thus, the preparation of the first photons is carried out before any possible irradiation of the target by the first photons.

[0022] The method may include a step between steps a) and b), and after the second photons have traveled the predetermined optical distance, during which the second photons are multiplied while preserving their polarization. Advantageously, the multiplication of the second photons allows the polarization of single photons to be measured. This multiplication is particularly advantageous when there is no preparation of the first photons, or when the flux of second photons is too low to allow an average measurement of their polarization.

[0023] The method may include a step between steps a) and b), during which the propagation direction of the first photons entering an area where the target may be present is modified by an optical system so as to scan the target if it is present. The propagation direction of the photons in space may, for example, be modified as is done by LiDARs. Alternatively, the first photons may be sent after passing through an optical system to be diffracted towards an area to be covered. The probability of the first photons being absorbed by a target is proportional to the ratio of the solid angles of said target as seen from said optical system to the solid angle covered by the first photons.

[0024] According to another aspect, the present invention also relates to a system for determining the position of a target, the system being adapted to implement the method according to the present invention.

[0025] According to a first embodiment, the system for determining the position of a target may include:

[0026] - a generator of entangled photon pairs, the first photons of a pair being emitted on a first optical path and the second photons of the pair being emitted on a second optical path;

[0027] - a photon preparer disposed on the first optical path and configured to direct a portion of the electromagnetic wave associated with each of the first photons is directed onto a third optical path, and another portion of the electromagnetic wave associated with each of the first photons is directed onto a fourth optical path;

[0028] - a polarization detector placed on the second optical path, the third optical path being intended to meet the target.

[0029] Preferably, the photon preparer includes a beam splitter configured to separate the electromagnetic wave of incident photons to the third optical path or the fourth optical path depending on the electric field component of the electromagnetic wave.

[0030] Preferably, the beam splitter includes a birefringent crystal configured to separate light according to its polarization.

[0031] The generator can be configured so that the polarization of the first and second photons on the first optical path, respectively the second optical path, is linear.

[0032] The photon preparer may include on the fourth optical path, downstream of the beam splitter, a delay plate, preferably a quarter-wave plate, configured to transform the linear polarization of the first photons into a circular polarization.

[0033] Optionally, the determination system can determine the position of a second target, with the fourth optical path intended to meet the second target.

[0034] Since the third and fourth optical paths are different, the invention makes it possible to measure the distance to one or more distant objects that absorb the radiation made up of the first photons used to illuminate it.

[0035] For this purpose, a pair of entangled photons is created. One of the photons from the pair is sent to a beam splitter having two optical output paths.

[0036] The other photon of the pair is sent to a polarization detector. The polarization of the second photon is measured after a predetermined travel time.

[0037] A photon from the first optical path that has passed through the beam splitter may be absorbed after traveling a shorter optical path than the optical path traveled by the second photon before the polarization of that second photon is measured by the polarization detector. It may also be absorbed after a longer time or possibly not at all.

[0038] Preferably, the system includes a photon amplifier arranged on the second optical path between the generator and the detector, the photon amplifier being configured to multiply the second photons while preserving their polarization.

[0039] Preferably, the photon amplifier comprises a source configured to emit amplification light with a wavelength shorter than that of the second photons, and at least one optical fiber configured to multiply the second photons, preserving their polarization, when simultaneously traversed by said second photons and the amplification light. Preferably, the second photons are multiplied by a photon amplifier arranged upstream of the polarization detector on the second optical path. At the moment of multiplication of a second photon, the associated first photon travels either through the third optical path, or through the fourth, or through both simultaneously if it has not already been absorbed.

[0040] In the first case, where the optical path between the target and the entangled photon generator is shorter than that between the polarization detector and the entangled photon generator, the first photon is absorbed polarized according to the polarization it had upon exiting the beam splitter, or, if it passed between the beam splitter and the target, through a medium that modified its polarization. The entanglement of the two photons before the absorption of the first photon then results in a change in the polarization of the second photon.

[0041] In the second case where the optical path between the target and the photon emitter is longer than that between the polarization detector and the entangled photon generator, the path taken by the first photon is still undetermined at the time of the multiplication of the second photon and its polarization is not modified before its arrival in the photon amplifier other than if the photon had not been emitted without entanglement.

[0042] Detecting a change in the polarization of the second photon relative to a reference polarization allows us to detect the presence of a target on one of the second or fourth optical paths, which is closer to the entangled photon generator, in terms of optical path length, than the optical path distance between the entangled photon generator and the polarization detector. The predetermined reference polarization corresponds to the polarization of a second photon arriving without any of the first photons having encountered a target at the time of detection.

[0043] The entangled photons are preferably generated with linear polarization, and the first photons are preferably split in two by a birefringent crystal, separating linearly polarized photons along a first direction into two linearly polarized half-photons along two perpendicular directions at 45° to said first direction. Preferably, one of the half-photons has a polarization oriented at 45° in the trigonometric direction as observed in the direction of propagation of the first photon relative to the first photon entering said crystal, the other half-photon having a polarization oriented at 45° in the clockwise direction.Preferably, the absorption of the half-photon oriented at 45° in the trigonometric direction causes a polarization rotation of 45° of the photon with which it is entangled in the clockwise direction, and conversely, the absorption of the photon oriented at 45° in the clockwise direction causes a polarization rotation of 45° of its entangled photon in the trigonometric direction.

[0044] The PI polarization of the first photon exiting the entangled photon generator is modified when it passes through the beam splitter and becomes a Pib polarization for its part directed towards the fourth optical path or a Pic polarization, different from Pib, for its part directed on the third optical path.

[0045] The polarization of the second photon upon arrival at the polarization detector is P'2a if the photon with which it is entangled has not already been absorbed, P'2b if the photon with which it is entangled has been absorbed on the fourth optical path, or P'2c if it has been absorbed on the third optical path. The three polarizations P'1a, P'1b, and P'1e are all distinct from each other, particularly if the photons do not pass through systems that alter their polarization before absorption.

[0046] The detection of the polarization state of the second photon in the entangled photon pair in the P'2c polarization corresponds to the absorption of the second photon by a target on the third optical path. This detection allows us to conclude that the target is present on the third optical path, at a distance corresponding to an optical path shorter than the optical path length between the entangled photon generator and the polarization detector. The length of an optical path takes into account the refractive indices of the media traversed.

[0047] The detection of the polarization state of the second photon in the entangled photon pair with P'2b polarization corresponds to the absorption of the second photon by a target on the fourth optical path. This detection allows us to conclude the presence of a target on the fourth optical path, at a distance corresponding to an optical path shorter than the optical path length between the entangled photon generator and the polarization detector.

[0048] An inaccurate detection of the polarization of the second photon may correspond to a target located equidistant from the entangled photon generator and the photon amplifier.

[0049] If the medium in which the photons propagate along the fourth optical path and the third optical path is not likely to modify the polarization of the photons propagating there, a detection of the polarization of the second photon either in the reference polarization, or in a polarization at 45° in the trigonometric direction of the reference polarization, or in a polarization at 45° in the clockwise direction of the reference polarization, allows us to conclude respectively that there is no target on the third and fourth optical paths, that there is a target on one of the third or fourth optical paths, or that there is a target on the other of the optical paths downstream of the first of the first optical path, the target being closer to the entangled photon generator, in terms of optical path, than the optical path distance separating the entangled photon generator from the polarization detector.

[0050] By varying the travel time of the second photons, it is possible to determine the presence of a target object on the third and fourth optical paths at different travel times of the photons on the third and fourth optical paths, and therefore at different distances from the beam splitter.

[0051] The system may include an optical assembly arranged on the fourth optical path and configured to modify the trajectory of incident photons, the optical assembly being intended to be arranged upstream of the second target.

[0052] The system may include an optical assembly arranged on the third optical path and configured to modify the trajectory of incident photons, the optical assembly being intended to be arranged upstream of the target.

[0053] The system may include an absorber arranged on the fourth optical path and configured to absorb incident photons.

[0054] According to an advantageous feature, optical assemblies modify the direction of propagation of photons on the third and fourth optical paths so as to scan, advantageously in two different directions, the medium in which the target is located, or even to scan the target.

[0055] The determination system can also be used when the third and / or fourth optical paths pass through semi-transparent media, that is, media that absorb a portion of the light passing through them. A statistical count of the second photons polarized differently from the polarization of a photon reaching the polarization measurement system that has passed through the same second optical path, without the first photon entangled with said second photon having been absorbed, then shows the proportion of light absorbed in said third and fourth optical paths over third or fourth optical path lengths shorter than the length of the second optical path.

[0056] According to a second embodiment, the system for determining the position of a target may include: - a generator of entangled photon pairs, the first photons of a pair being emitted on a first optical path and the second photons of the pair being emitted on a second optical path;

[0057] - a polarization detector arranged on the second optical path;

[0058] - a photon amplifier arranged on the second optical path between the generator and the detector, the photon amplifier being configured to multiply the second photons while preserving their polarization.

[0059] Preferably, the photon amplifier comprises a source, configured to emit an amplification light of a wavelength shorter than the wavelength of the second photons, and at least one optical fiber, said optical fiber being configured to multiply the second photons, preserving their polarization, when it is simultaneously traversed by said second photons and by the amplification light, the photon amplifier being configured to direct the amplification light so that it flows with the second photons in the optical fibers.

[0060] The system according to this second embodiment may include an optical assembly disposed on the first optical path and configured to modify the trajectory of the first photons or the wave plane along which the first photons propagate, the optical assembly being intended to be disposed upstream of the target.

[0061] The system according to the invention may include, for each of the first and second embodiments, one or more of the following optional features:

[0062] - the system includes an optical path length adjustment device configured to allow modification of the length of the second optical path between the generator and the polarization detector;

[0063] - the adjustment device includes at least one first switch configured to guide an incident photon towards a specific segment among a plurality of optical fiber segments of different lengths,

[0064] - the adjustment device includes at least one second switch configured to direct an incident photon from a determined segment among the plurality of optical fiber segments to a single optical output path;

[0065] - The first and / or second switch comprises at least one Pockels cell and / or liquid crystals. According to an advantageous feature, the polarization of the second photons can be measured by measuring the polarization of a luminous flux resulting from the multiplication of the second photon by a photon amplifier, the luminous flux preserving the polarization of the second photon. This could, in particular, be an amplifier using erbium-doped fibers.

[0066] The measurement of the polarization of a light flux can be carried out by a polarization detector, for example comprising a birefringent crystal defining two optical output paths separating the light flux along two perpendicular polarization directions and two light detectors each disposed on one of the optical output paths of the birefringent crystal.

[0067] In one embodiment, the travel time of the second photons of the pair can be adjusted by means of an optical path length adjustment device configured to modulate the length of the second optical path between the beam splitter and the polarization detector. Such an adjustment device may, for example, include a plurality of switches configured to direct photons to optical fiber segments of different lengths. Such a switch may, in particular, include one or more high-frequency actuated Pockels cells and / or liquid crystals and / or materials such as, for example, mirrors that move under the influence of piezoelectric materials, i.e., materials that expand under the influence of an electric field. According to an advantageous feature, the beam splitter comprises a birefringent crystal configured to separate photons according to their polarization.One of the optical paths exiting the birefringent crystal is directed towards the target object while the other optical path is directed towards a photon absorption device and / or the other optical path has no obstruction near the birefringent crystal.

[0068] According to an advantageous feature, the entangled photon pair generator includes a laser diode, for example emitting light centered on a wavelength of 405 nm, illuminating a P-barium borate (BBO) target.

[0069] The second optical path may include at least one optical guide through which the second photons travel. The optical guide may be a waveguide, a bundle of waveguides, or a bundle of optical fibers, preferably a polarization-conserving optical fiber bundle. The first and second photons of each entangled photon pair may have the same or different wavelengths. The wavelengths of the first and second photons may be in the X-ray, ultraviolet, visible, infrared, or microwave ranges.

[0070] Alternatively or in combination, the entangled photon pair generator includes quantum dots, for example as described in the publication "Highly-efficient extraction of entangled photons from quantum dots using a broadband optical antenna", Yan Chen et al., Nature Communications vol. 9:2994 (2018).

[0071] Brief description of the drawings

[0072] [Fig 1] Figure 1 represents a system for determining the position of a target according to a first variant of the first embodiment of the invention.

[0073] [Fig 2] Figure 2 represents an assembly comprising a device for adjusting the length of the second optical path and a polarization detector.

[0074] [Fig 3] Figure 3 represents a polarization detector that can be used in the context of the invention.

[0075] [Fig 4] Figure 4 represents a system for determining the position of a target according to a second variant of the first embodiment of the invention.

[0076] [Fig 5] Figure 5 represents a system for determining the position of a target according to a third variant of the first embodiment of the invention.

[0077] [Fig 6] Figure 6 represents a system for determining the position of a target according to the second embodiment of the invention.

[0078] Detailed description

[0079] Figure 1 illustrates a system 1 for determining the position of a target 10 according to the invention. The invention is not limited to this system 1.

[0080] System 1 includes a generator 2 of entangled photon pairs. The first photons of a pair are emitted on a first optical path 3 and the second photons of the pair are emitted on a second optical path 4.

[0081] Entangled photons can be generated, for example, using a spontaneous parametric downconversion (SPDC) process in which a laser beam is emitted towards a birefringent crystal, such as beta-barium borate or lithium niobate, from which entangled photons emerge. Entangled photon pairs can thus be produced, for example, with wavelengths between 2 pm and 5 pm, as described in the article "Broadband infrared light source by simultaneous parametric down-conversion" by Hojo et al., published on September 9, 2021, in the journal Nature, Scientific Reports. It is also possible to generate entangled photon pairs in X-rays, as described in the article by Goodrich C. Justin & Mahon, et al. : “Quantum Imaging with X-rays”, 2024, 10.48550 / arXiv.2412.09833, or by obtaining a birefringent crystal suitable for X-rays as described in BA's articlePalmer, et al. : “X-ray Birefringence from a Model Anisotropic Crystal”, The Journal of Physical Chemistry Letters, 2011, 2(18):2346-2351.

[0082] It is also possible to generate X-ray photons entangled with visible photons, as described in the ESRF experiment of September 20, 2016, in the article by Marco Moretti and Blanka Detlefs entitled "Parametric Down-Conversion of X-Rays into Visible Wavelengths." The X-ray photons are then preferentially directed along the first optical path, and then, in whole or in part, to a potential target, for example, located in a biological body or underground, while the visible photons are directed along the second optical path and then to a polarization detector.

[0083] A photon preparer 5 is arranged on the first optical path 3 such that the first photons emitted by the generator 2 pass through said photon preparer 5 and are, at least in part, directed onto a third optical path 7. The photon preparer 5 is configured to direct a portion of the electromagnetic wave associated with each of the first photons onto the third optical path 7, and another portion of the electromagnetic wave associated with each of the first photons is directed onto a fourth optical path 6.

[0084] In the embodiment shown in Figure 1, the photon preparation unit 5 includes a beam splitter 50 arranged on the first optical path 3. The first photons emitted by the generator 2 therefore pass through the beam splitter 50. The latter is configured to separate the incident photons according to their polarization state. The beam splitter 50 comprises, for example, a birefringent crystal that separates photons linearly polarized along one initial direction into photons linearly polarized along two directions, each at 45° to the initial polarization. At the output of the beam splitter 50, the photons are thus sent onto a fourth optical path 6 or onto the third optical path 7, depending on their polarization state.

[0085] A photon absorber 8 can be placed on the fourth optical path 6. This photon absorber 8 is preferably placed sufficiently far from the photon splitter so that a photon propagating along the fourth optical path is absorbed by the photon absorber 8 after the polarization of the second photon has been measured. Alternatively, care is taken to ensure that no obstacle is located on the fourth optical path for a sufficient distance.

[0086] The photon preparer 5 may include a quarter-wave plate 51 arranged on the fourth optical path 6. The quarter-wave plate 51 is configured to transform the linear polarization of the first photons into a circular polarization.

[0087] Target 10 is located on the third optical path 7. Advantageously, an optical assembly 9 is located between the beam splitter 50 and target 10. Optical assembly 9 is configured to change the direction of the third optical path so as to scan target 10.

[0088] Optical assembly 9 includes, for example, a plane mirror oriented using piezoelectric materials.

[0089] An adjustment assembly 11, which includes an adjustment device 12 for the length of the second optical path and a polarization detector 26, is disposed on the second optical path 4.

[0090] An example of such an adjustment set 11 is shown in Figure 2.

[0091] In this example, the adjustment device 12 includes a photon amplifier 20 and a plurality of switching sets 24. The photon amplifier 20 is arranged downstream of the switching sets 24.

[0092] A switching assembly 24 comprises a first switch 21 that directs a photon to one of a plurality of optical fibers 22 of different lengths, forming optical paths of different lengths. Each optical fiber is connected to a second switch 23 that directs a photon from one of the optical fibers 22 to the optical output of the switching assembly 24. The adjustment device 12 may comprise a single switching assembly 24 or a plurality of such assemblies arranged in series to allow for more precise optical pathing within the assembly 11. A switching assembly can, for example, form optical paths ranging from 0 to 30 km.In particular, the adjustment device 12 may include a first, a second and a third switching set, the first switching set forming optical paths of lengths between 0 and 24 km, in particular in steps of 3 km (for example optical paths of 0 km, 3 km, 6 km, 9 km, ..., 24 km) while the second switching set may form optical paths of lengths between 0 and 2700 m, in particular in steps of 300 m (for example optical paths of 0 m, 300 m, 600 m, ..., 2700 m) and the third switching set may form optical paths of lengths between 0 m and 270 m, in particular in steps of 30 m (0 m, 30 m, 60 m, ..., 270 m).

[0093] A polarization detector 26 is disposed on the optical path 25 of the output of the last switching assembly 24. The polarization detector 26 disposed immediately after the photon amplifier is configured to measure the polarization of the second incident photons.

[0094] Figure 3 illustrates an example of a polarization detector 26 that can be implemented within the framework of the invention.

[0095] As illustrated, the polarization detector 26 includes a beam splitter 27, such as a birefringent crystal, which defines two optical paths 28, 29 at the optical output as a function of the polarization of the incident photon. A light intensity detector 30, 31 is arranged on each of the optical paths 28, 29.

[0096] Functioning

[0097] The operation of a system 1 according to the invention results directly from the preceding description.

[0098] Generator 2 allows the generation of entangled photon pairs. Each photon in a pair is sent along a different optical path.

[0099] One of the photons, called the second photon, is sent along the second optical path 4 to the polarization detector 26. Its travel time along the second optical path 4 can be modified by means of assembly 11, which includes a second optical path length adjustment device 12. This makes it possible to measure the polarization state of the second photon after a predetermined and adjustable travel time. The other photon of the pair, called the first photon, is sent along the first optical path 3. The beam splitter 50 directs the first photon onto one of the optical paths 6, 7.

[0100] A photon emitted on the fourth optical path 6 is absorbed by the absorber 8 and a photon emitted on the third optical path 7 interacts with the target 10. The optical assembly 9 allows the direction of a photon on the third optical path to be changed and thus the target 10 to be scanned.

[0101] The probability of absorption of a photon by the target on the third optical path can be calculated by observing the probability of arrival of the second photon polarized in the polarization corresponding to the absorption of the first photon on the third optical path, to which is subtracted the probability of the second photon being observed in this polarization because of the absorption of the first photon on the fourth optical path.

[0102] By adjusting the travel time of the second photons on the second optical path 4, it is thus possible to determine the probability of absorption of the photons by the target 10 on the third optical path over different distances from the beam splitter 50.

[0103] Therefore, the invention makes it possible to measure the distance to a distant object that absorbs the radiation used to illuminate it.

[0104] Other variations and improvements may be envisaged without departing from the scope of the invention. In particular, the presence of the optical assembly 9 or that of the absorber 8 is not mandatory.

[0105] For example, according to a variant of the first embodiment, the system 1 for determining the position of a target 10 according to the invention may include the absorber 8, the photon preparer being free of the quarter-wave plate 51 on the fourth optical path 6. A system 1 according to such a variant is illustrated in Figure 4. Preferably, the generator 2 is then configured to generate entangled photon pairs at a high rate so that the polarization detector 26 can average the polarizations of several second photons incident directly on said detector 26 or entering the amplifier 20 if such an amplifier is present.

[0106] According to another variant of the first embodiment, the system 1 for determining the position of a target 10 according to the invention can be free of absorber 8 on the fourth optical path 6. A system 1 according to this other variant is illustrated in Figure 5.

[0107] According to a second embodiment, the system 1 for determining the position of a target 10 according to the invention may be free of a photon preparation device 5 and may, if necessary, include the photon amplifier 20. A system 1 according to this second embodiment is illustrated in Figure 6. When a first photon is absorbed by the target 10, the polarization of the second photon entangled with said first photon becomes circular or elliptical, with the direction of rotation depending on the spin of the first photon at the time of its absorption. It is thus possible to determine which of the second photons have become entangled with first photons absorbed by the target 10 by measuring their polarization, and conversely, to determine which of the second photons have become entangled with first photons not absorbed by the target object by measuring their polarization.However, measuring the polarization of the second photons is more complicated here and requires multiplying them. In the second embodiment, the generator 2 is preferably configured to emit entangled photon pairs at a rate such that the polarization detector 26 combined with the photon amplifier 20 can individually measure the polarization of each second photon entering said amplifier 20.

Claims

Demands 1. A method for measuring the position of a target (10), comprising the following successive steps: a) generating a plurality of entangled photon pairs, each pair comprising a first photon emitted on a first optical path (3) and a second photon, entangled with the first photon, emitted on a second optical path (4); b) measuring the polarization state of the second photons on the second optical path and comparing this measured polarization state with a reference state corresponding to the polarization state of a reference photon that has propagated along the second optical path without a first photon entangled with the reference photon having been absorbed; c) determining the probability that the first photons have been absorbed by the target.

2. A method according to the preceding claim, comprising a step between steps a) and b) during which a portion of the electromagnetic wave associated with each of the first photons is directed onto a third optical path (7), and another portion of the electromagnetic wave associated with each of the first photons is directed onto a fourth optical path (6).

3. A method according to any one of the preceding claims, comprising a step between steps a) and b), and after the second photons have traveled a predetermined optical distance, during which the second photons are multiplied while preserving their polarization.

4. System (1) for determining the position of a target (10), the system being adapted to implement the method according to claim 2, or claim 3 depending on claim 2, the system comprising: - a generator (2) of entangled photon pairs, the first photons of a pair being emitted on a first optical path (3) and the second photons of the pair being emitted on a second optical path (4); - a photon preparer (5) disposed on the first optical path and configured to direct a part of the electromagnetic wave associated with each of the first photons is directed on a third optical path (7), and another part of the electromagnetic wave associated with each of the first photons is directed on a fourth optical path (6); - a polarization detector (26) disposed on the second optical path, the third optical path being intended to meet the target.

5. System according to the preceding claim, comprising an optical assembly (9) disposed on the third optical path and configured to modify the trajectory of incident photons, the optical assembly being intended to be disposed upstream of the target.

6. System according to any one of claims 4 and 5, the photon preparer comprising a beam splitter (50) configured to separate the electromagnetic wave of incident photons to the third optical path or the fourth optical path (6) depending on the electric field component of the electromagnetic wave.

7. System according to the preceding claim, comprising an absorber (8) disposed on the fourth optical path and configured to absorb incident photons.

8. System according to claim 6 or 7, the beam splitter comprising a birefringent crystal configured to separate light according to its polarization.

9. System according to any one of claims 4 to 8, the generator being configured such that the polarization of the first photons and second photons on the first optical path, respectively the second optical path, is linear.

10. System according to the preceding claim, the photon preparer comprising on the fourth optical path, downstream of the beam splitter, a delay plate (51), preferably a quarter-wave plate, configured to transform the linear polarization of the first photons into a circular polarization.

11. System according to any one of claims 4 to 10, comprising a photon amplifier (20) arranged on the second optical path between the generator and the detector, the photon amplifier being configured to multiply the second photons while preserving their polarization.

12. System (1) for determining the position of a target (10), the system being adapted to implement the method according to claim 3, the system comprising: - a generator (2) of entangled photon pairs, the first photons of a pair being emitted on a first optical path (3) and the second photons of the pair being emitted on a second optical path (4); - a polarization detector (26) arranged on the second optical path; - a photon amplifier (20) arranged on the second optical path between the generator and the detector, the photon amplifier being configured to multiply the second photons while preserving their polarization.

13. System according to the preceding claim, comprising an optical assembly (9) disposed on the first optical path and configured to modify the trajectory of the first photons or the wave plane along which the first photons propagate, the optical assembly being intended to be disposed upstream of the target.

14. A system according to any one of claims 4 to 13, comprising an optical path length adjustment device (12) configured to allow modification of the length of the second optical path between the generator and the polarization detector.

15. A system according to the preceding claim, the adjustment device comprising at least a first switch (21) configured to guide an incident photon to a specified segment among a plurality of optical fiber segments (22) of different lengths, and / or at least a second switch (23) configured to direct an incident photon from a specified segment among the plurality of optical fiber segments to a single output optical path (25).

16. System according to the preceding claim, the first and / or second switch comprising at least one Pockels cell and / or liquid crystals.

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