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

Abstract

System and method for determining the position of an object by entangled photons. The invention relates to a system (1) for determining the position of a target (10) 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 beam splitter (5) disposed on the first optical path and configured to separate incident photons towards a third optical path (6) or a fourth optical path (7) depending on their polarization state; - a polarization detector (26) disposed on the second optical path, the fourth optical path being intended to encounter the target. Figure for the abstract: Fig. 1

Description

Title of the invention: System and method for determining the position of an object using entangled photons technical field

[0001] The present invention relates to the field of quantum detection and quantum imaging.

[0002] It relates more particularly 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 implementing such a system. Previous technique

[0003] It is known to implement radar-type devices to detect the presence of objects and to measure their position and speed.

[0004] 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.

[0005] This proven technique, however, has the drawback of allowing radar localization, being energy-intensive, requiring large receiving antennas, or being vulnerable to radar glare.

[0006] 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.

[0007] However, this method does not allow observation of 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.

[0008] There is therefore a need to propose a system that remedies the disadvantages of the prior art.

[0009] The object of the invention is to meet at least partially this need. Summary of the invention

[0010] To this end, the invention relates, in one of its aspects, to a system for determining the position of a target, comprising:

[0011] - an entangled photon pair generator, 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;

[0012] - a beam splitter disposed on the first optical path and configured for separate incident photons towards a third optical path or a fourth optical path depending on their polarization state;

[0013] - a polarization detector disposed on the second optical path,

[0014] the fourth optical path being intended to meet the target.

[0015] Optionally, the determination system can determine the position of a second target, the third optical path being intended to meet the second target.

[0016] Since the third and fourth optical paths are different, the invention makes it possible to measure the distance to one or more distant objects which absorb the radiation made up of the first photons used to illuminate it.

[0017] For this purpose, a pair of entangled photons is created. One of the photons of the pair is sent to a beam splitter having two optical output paths.

[0018] The other photon of the pair is sent to a polarization detector. The polarization of the second photon is measured after a determined travel time of the second photon.

[0019] 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 said second photon is measured by the polarization detector. It may also be absorbed after a longer time or possibly not be absorbed at all.

[0020] 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.

[0021] In a 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 has passed between said beam splitter and the target, through a medium that has modified its polarization. The entanglement of the two photons before the absorption of the first of the two photons then causes a modification of the polarization of the second photon.

[0022] 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 photon generator In entangled photons, 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.

[0023] Detecting a change in the polarization of the second photon relative to a reference polarization allows the presence of a target to be detected on one of the second or third optical paths, closer to the entangled photon generator, in terms of optical path length, than the optical path distance separating the entangled photon generator from 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 said detection.

[0024] 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 45° polarization rotation 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 45° polarization rotation of its entangled photon in the trigonometric direction.

[0025] The PI polarization of the first photon exiting the entangled photon generator is modified during its passage through the beam splitter and becomes a Pib polarization if the first photon is directed towards the third optical path or a Pic polarization, different from Pib, if the first photon is directed on the fourth optical path.

[0026] The polarization of the second photon upon arrival at the polarization detector is a P'2a polarization if the photon with which it is entangled has not already been absorbed, a P'2b polarization if the photon with which it is entangled has been absorbed on the third optical path, or a P'2c polarization if the latter has been absorbed on the fourth optical path. The three polarizations P'1a, P'1b, and P'1e are all different from each other, particularly if the photons do not pass through systems that modify their polarization before absorption.

[0027] The detection of the polarization state of the second photon of the entangled photon pair in the P'2c polarization corresponds to the absorption of the second photon by a target on the fourth optical path. This detection allows us to conclude that the target is present 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. The length of an optical path takes into account the refractive indices of the media traversed.

[0028] The detection of the polarization state of the second photon of the entangled photon pair in the P'2b polarization corresponds to the absorption of the second photon by a target on the third optical path. This detection allows us to conclude that there is a target on the third optical path, at a distance corresponding to an optical path shorter than the length of the optical path between the entangled photon generator and the polarization detector.

[0029] An inaccurate detection of the polarization of the second photon may correspond to a target located at an equal distance from the entangled photon generator and the photon amplifier.

[0030] If the medium in which the photons propagate along the third optical path and the fourth optical path is not likely to modify the polarization of the photons propagating therein, 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, makes it possible to conclude respectively that there is no target on the second and third optical paths, that there is a target on one of the second or third 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.

[0031] By varying the travel time of the second photons, it is possible to determine the presence of a target object on the fourth optical path at different travel times of the photons on the third and fourth optical paths, and therefore at different distances from the beam splitter.

[0032] The system according to the invention may include one or more of the following optional features:

[0033] - the system comprises an optical assembly disposed on the third optical path and configured to modify the trajectory of incident photons, the optical assembly being intended to be positioned upstream of the second target;

[0034] - the system comprises an optical assembly disposed on the fourth optical path and configured to modify the trajectory of incident photons, the optical assembly being intended to be positioned upstream of the target;

[0035] - the system comprises an absorber disposed on the third optical path and configured to absorb incident photons;

[0036] - 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;

[0037] - the adjustment device includes at least one first configured switch to guide an incident photon towards a specific segment among a plurality of optical fiber segments of different lengths,

[0038] - the adjustment device includes at least one second configured switch to direct an incident photon from a specific segment among the plurality of optical fiber segments towards a single optical output path;

[0039] - the first and / or second switch comprises at least one cell of Pockels and / or liquid crystals;

[0040] - the system comprises a photon amplifier disposed on the second optical path between the entangled photon pair generator and the polarization detector;

[0041] - the beam splitter comprises a birefringent crystal configured to separate the light depending on its polarization.

[0042] 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 with erbium-doped fibers.

[0043] 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.

[0044] 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 third optical path between the beam splitter and the polarization detector. Such an adjustment device may, for example, comprise a plurality of switches configured to direct the photons to optical fiber segments of different lengths. Such a switch may, in particular, comprise one or more high-frequency actuated Pockels cells and / or liquid crystals and / or materials, such as, for example, mirrors that move under the action 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.

[0045] According to an advantageous feature, the entangled photon pair generator comprises a laser diode, for example emitting light centered on a wavelength of 405 nm, illuminating a [3-barium borate (BBO) target.

[0046] 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).

[0047] Advantageously, the optical assembly is configured to scan a portion of space, with the photons of the fourth optical path being successively directed in a plurality of directions.

[0048] The invention further relates to a method for determining the position of a target implementing a system as described above, comprising:

[0049] a / the generation of a plurality of entangled photon pairs by means of the entangled photon pair generator;

[0050] b / for each pair of entangled photons, measure the polarization state of the second photon on the second optical path and compare this measured state to a polarization state of a reference photon reaching the polarization detector by propagating along the second optical path without a first photon entangled with the reference photon having been absorbed.

[0051] Preferably, the method comprises, after the polarization state measurement and the comparison b / , the determination c / of the probability of photon absorption by the target on the fourth optical path.

[0052] According to an advantageous feature, the 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.

[0053] 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 second photons polarized differently from the polarization of a photon reaching the polarization measurement system is performed, having 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 less than the length of the second optical path.

[0054] We can also then measure, modulo 2ir, the polarization rotation imparted to photons traveling for example the third optical path and having traveled an optical path length between two reference optical path lengths.This measurement is carried out for example by absorbing photons on the fourth optical path at the output of the beam splitter and by measuring for each slice of rotation angle of the polarization of the second photon that has traveled the longer of the two reference optical paths a first ratio Ra of photons that have undergone a polarization rotation of an angle included in this slice of rotation angles before the polarization of their entangled photon is measured, similarly calculating a second ratio Rb for the shorter of the two reference paths, then subtracting, for each of these slices of angles the ratio Rb from the ratio Ra and finding the slice of angle for which this difference of ratios is the greatest. Brief description of the drawings

[0055] [Fig-1] Fig. 1 represents a system for determining the position of a target according to the invention.

[0056] [Fig.2] La [Fig.2] représente un ensemble comprenant un dispositif d'ajustement de la longueur du deuxième chemin optique et un détecteur de polarisation.

[0057] [Fig.3] La [Fig.3] représente un détecteur de polarisation pouvant être utilisé dans le framework of the invention. Detailed description

[0058] On a illustré à la [Fig.l] un système 1 de détermination de la position d'une cible 10 selon l'invention.

[0059] The system 1 comprises 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.

[0060] Entangled photons can be generated, for example, by using a spontaneous parametric down-conversion (SPDC) process in which a laser beam is emitted towards a birefringent crystal, such as beta-barium borate or lithium niobate, and entangled photons emerge from it. Pairs of entangled photons 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”, Hojo et al., published on September 9, 2021 in the journal Nature, Scientific Reports.

[0061] A beam splitter 5 is arranged on the first optical path 3. The first photons emitted by the generator 2 therefore pass through the beam splitter 5. The latter is configured to separate the incident photons according to their polarization state. The photon splitter 5 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.

[0062] At the output of the beam splitter 5, the photons are therefore sent on a third optical path 6 or on a fourth optical path 7 depending on their polarization state.

[0063] A photon absorber 8 can be arranged on the third optical path 6. This photon absorber 8 is preferably arranged sufficiently far from the photon splitter so that a photon propagating on the third optical path is absorbed by the photon absorber 8 after the polarization of the second photon has been measured. Alternatively, it is ensured that no obstacle is located on the third optical path for a sufficient distance.

[0064] The target 10 is located on the fourth optical path 7. Advantageously, an optical assembly 9 is located between the beam splitter 5 and the target 10. The optical assembly 9 is configured to change the direction of the fourth optical path so as to scan the target 10.

[0065] The optical assembly 9 includes, for example, a plane mirror oriented using piezoelectric materials.

[0066] 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.

[0067] An example of such an adjustment set 11 is shown in [Fig.2].

[0068] In this example, the adjustment device 12 comprises a photon amplifier 20 and a plurality of switching sets 24. The photon amplifier 20 is arranged downstream of the switching sets 24.

[0069] 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 in order to to make the optical path more precise in assembly 11. A switching assembly can for example form optical paths between 0 and 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).

[0070] 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.

[0071] Fig. 3 illustrates an example of a polarization detector 26 that can be implemented within the framework of the invention.

[0072] As illustrated, the polarization detector 26 comprises 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. Functioning

[0073] The operation of a system 1 according to the invention results directly from the preceding description.

[0074] Generator 2 allows the generation of entangled photon pairs. Each photon in a pair is sent along a different optical path.

[0075] 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 the assembly 11, which includes for this purpose the adjustment device 12 for the length of the second optical path. It is thus possible to measure the polarization state of the second photon after a predetermined and adjustable travel time.

[0076] The other photon of the pair, called the first photon, is sent on the first optical path 3. The beam splitter 5 allows the first photon to be directed onto one of the optical paths 6, 7.

[0077] A photon emitted on the third optical path 6 is absorbed by the absorber 8 and a photon emitted on the fourth optical path 7 interacts with the target 10. The assembly optics 9 allows the direction of a photon on the fourth optical path to be changed and thus the target 10 to be scanned.

[0078] The probability of absorption of a photon by the target on the fourth 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 fourth 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 third optical path.

[0079] 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 fourth optical path over different distances from the beam splitter.

[0080] Therefore, the invention makes it possible to measure the distance to a distant object which absorbs the radiation used to illuminate it.

[0081] 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.

Claims

Demands

1. System (1) for determining the position of a target (10), 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 beam splitter (5) disposed on the first optical path and configured to separate incident photons towards a third optical path (6) or a fourth optical path (7) depending on their polarization state; - a polarization detector (26) disposed on the second optical path, the fourth optical path being intended to meet the target.

2. System according to claim 1, comprising an optical assembly (9) disposed on the fourth optical path and configured to modify the trajectory of incident photons, the optical assembly being intended to be disposed upstream of the target.

3. System according to any one of the preceding claims, comprising an absorber (8) disposed on the third optical path and configured to absorb incident photons.

4. System according to any one of the preceding claims, 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.

5. 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 optical output path (25).

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

7. System according to any one of the preceding claims, comprising a photon amplifier (20) disposed on the second optical path between the entangled photon pair generator and the polarization detector.

8. System according to any one of the preceding claims, the beam splitter comprising a birefringent crystal configured to separate light according to its polarization.

9. A method for measuring the position of a target (10) implemented by means of a system (1) according to any one of the preceding claims, comprising: a / the generation of a plurality of entangled photon pairs by means of the entangled photon pair generator; b / for each pair of entangled photons, measuring the polarization state of the second photon on the second optical path and comparing this measured state to a polarization state of a reference photon reaching the polarization detector by propagating along the second optical path without a first photon entangled with the reference photon having been absorbed.

10. A method according to the preceding claim implementing a system according to claim 2, the optical assembly modifying the direction of propagation of photons on the fourth optical path so as to scan the target.

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