GB2537081A

GB2537081A - Imaging system

Info

Publication number: GB2537081A
Application number: GB1412199.0A
Authority: GB
Inventors: Giacomo Robert Colosimo Nicholas
Original assignee: BAE Systems PLC
Current assignee: BAE Systems PLC
Priority date: 2014-07-09
Filing date: 2014-07-09
Publication date: 2016-10-12
Also published as: WO2016005737A1; GB201412199D0

Abstract

An imaging system comprising a photon source, such as a burst illumination laser 1, fires photon pulses 2 into an optical device, such as a non-linear crystal 3, which causes each source photon to be split into two or no more entangled photons 4a & 4b. At least one of the entangled photons 4a is directed toward a first gated camera 7, which is synchronised with the start of each photon pulse. The remaining entangled photons 4b are directed toward a target 5 which may be partially or completely concealed by a barrier 6 or other scattering medium, such as poor weather, foliage or simply significant distance. Photons reflected from the target are received by a second gated camera 8, and a quantum correlation circuit 9 compares photons received at the second camera with those received at the first to determine if they originate from the source. If so, they are used to generate an image of the target; if not, the photons are discarded as noise.

Description

IMAGING SYSTEM

This invention relates to an imaging system and, more particularly, to a laser imaging system such as a Burst Illumination Laser (BIL) imaging system for enabling long-range imaging of targets.

Burst illumination laser (BIL) imaging is a technique combining active laser illumination with time gating (or range gating). It can image targets up to several kilometres away in limited or even no-visibility conditions, for example, at night. For these reasons, BIL imaging has become increasingly important in defence and security applications.

BIL imaging consists of a laser source, illuminating the scene with a single pulse, and a camera, that is synchronised with the start of the pulse. If the pulse starts at, say, time to (seconds) and the camera is switched on at time tR (seconds), where 1,=i0+41, the depth R (metres) corresponding to the return registered by the image is given by R= c(Al 2), where c is the speed of light (3 x 108 m/s). This procedure eliminates the pulse returns from unwanted depths and atmospheric backscatter.

Burst illumination laser techniques have many advantages compared with techniques using, for example, passive infrared radiation. One of the most significant of these advantages is an increased target identification range of 20 to 40%. However, a significant disadvantage of known techniques is that they cannot be used to image completely concealed targets. Furthermore, it would be desirable to further increase the target range, i.e. the distance at which targets can be imaged.

The present invention seeks to address at least some of these issues, and provides an imaging system comprising a photon source for generating a photon beam, a device located within the photon beam and configured to split photons from said beam into two or more respective entangled photons, the system being configured to direct at least one of said entangled photons to a first detector and to direct at least another of said entangled photons toward a target, the system further comprising a second detector for receiving reflected photons from said target, a quantum correlation circuit for comparing photons -2 -received by said second detector with photons received by said first detector to determine if said photons received by said second detector originated from said photon source, and an image generation circuit for generating an image of said target using photons received by said second detector determined to have originated from said photon source.

The photons determined not to have originated from the photon source may be discarded.

In an exemplary embodiment of the present invention, the photon source is a laser, and in one preferred embodiment, the photon source is a burst illumination laser configured to transmit light pulses. In this case, the first and/or second detectors comprise gated cameras, synchronised with the start of each light pulse.

In one exemplary embodiment of the invention, the device configured to split photons from said source into two or more respective entangled may comprise a nonlinear crystal, quantum dots or one or more non-linear optical fibres.

Also in accordance with the present invention, there is provided a method of generating an image of a target, the method comprising generating a photon beam, splitting photons from said photon beam into two or more respective entangled photons, directing at least one of said entangled photons to a first detector, directing at least another of said entangled photons toward a target, receiving reflected photons at a second detector and comparing said received photons with the photons received by said first detector to determine if they originate from said photon beam, and generating an image of said target using photons received by said second detector determined to have originated from said photon beam.

In an exemplary embodiment, photons determined not to have originated from said photon beam are discarded.

In one embodiment, the wavelength of said photon beam is selected 30 such that the number of photons returned from the target, which may be a barrier, or other scattering medium, such as foliage or even a significant -3 -distance, is maximised. In other words, the wavelength of the photon beams is ideally selected so as to maximise or otherwise optimise the number of classically transmitted and quantum tunnelled photons returned to the second detector, and this is dependent on the barrier or other scattering medium in the path of the target. Thus, the method may comprise the further step of determining the nature of a scattering medium located between said photon beam and said target, and selecting the wavelength of said photon beam according to the nature of said scattering medium so as to optimise (e.g. maximise) the number of reflected photons receivable by said second detector.

The present invention extends to a computer-implemented method of generating an image of a target using source photons which have been split into two or more respective entangled photons, the method comprising the steps of receiving data representative of a first, unobstructed entangled photon, receiving data representative of a second, reflected entangled photon, comparing said second entangled photon with said first entangled photon to determine if said second entangled photon originated from a source photon, and, if so, using said second entangled photon to generate said image.

The present invention extends further to a computer program or programs which, when installed on a computing device, is/are configured to 20 perform the method defined above.

These and other aspects of the present invention will become apparent from the following description of embodiments of the present invention, which will be described by way of examples only and with reference to the accompanying drawings, in which: Figure 1 is a schematic diagram illustrating a method of producing entangled photons using a non-linear crystal; and Figure 2 is a schematic block diagram of the principal elements of an imaging system according to an exemplary embodiment of the present invention.

The present invention utilises the concept of Quantum Entanglement which means that multiple particles are linked together in a way such that the -4 -measurement of one particle's quantum state determines the possible quantum states of the other particles. Measurements of physical properties such as position, momentum, spin, polarization, etc. performed on entangled particles are found to be appropriately correlated. For example, if a pair of particles is generated in such a way that their total spin is known to be zero, and one particle is found to have spin up on a certain axis, then the other particle measured on the same axis, will be found to have spin down, irrespective of the physical distance between the particles.

A light beam is composed of a stream of photons. The direction of lights electric field is its direction of polarization. The polarization direction of a photon can be at any particular angle, for example "vertical" or "horizontal". It is possible to generate a pair of entangled photons if, for example, a laser is shone at a crystal. In that case, a single photon can be split to become two photons. Each photon produced in this way will always have a polarization is correlated to the other photon. Thus, if two entangled photons are received and measured at two different respective locations, this apparent connection between the two photons will always be present, irrespective of the physical distance between them.

Referring to Figure 1 of the drawings, the abovementioned concept of splitting a single photon into two entangled photons is illustrated schematically.

A photon source, such as a laser 1, fires a photon 2 into a nonlinear crystal 3, causing the source photon to be split into two, entangled photons 4a, 4b. Nonlinear optic (NLO) crystals for generating entangled photons are known, and examples include beta Barium Borate (B130), Silver Gallium Sulfide (AgGaS2) and Silver Gallium Selenide (AgGaSe2). However, other examples of nonlinear crystals will be known to a person skilled in the art, and the present invention is not intended to be in any way limited in this regard. It will be further appreciated by a person skilled in the art, that the source photon can be split into more than two entangled photons, as required.

Referring to Figure 2 of the drawings, in an exemplary embodiment of the present invention, a photon source, such as a burst illumination laser 1, fires photon pulses 2 into a nonlinear crystal 3, which causes each source photon to -5 -be split into two (or more), entangled photons 4a, 4b, as described above. At least one of the entangled photons 4a is directed (by the crystal or other optical elements, not shown) toward a first gated camera 7, which is synchronised with the start of each pulse. The remaining entangled photon(s) 4b is/are directed toward a target 5 which may be partially or completely concealed by a barrier 6, such as poor weather, a jungle canopy, a very long distance, and the like.

Many of the photons may be scattered or reflected by the barrier before reaching the target. However, some of the photons 4b will reach the target 5, through the barrier or other scattering medium, such as foliage or simply a to significant distance, 6 by either classical transmission or a phenomenon known as "quantum tunnelling". Quantum tunnelling is a process described in quantum mechanics by which particles can, with a very small probability, cross the barrier or other scattering medium 6 to the other side and strike the target 5. This phenomenon arises from the treatment in quantum mechanics as having properties of waves and particles. One interpretation of this duality involves the Heisenberg uncertainty principle, which defines a limit on how precisely the position and the momentum of a particle can be known at the same time. This implies that there are no solutions with a probability of precisely zero (or one). Hence, the probability of a given particle's existence on the opposite side of an intervening barrier is non-zero, and such particles will "appear" on the other side of the barrier or other scattering medium with a frequency which is proportional to this probability.

The probability of an entangled photon tunnelling through the barrier or other scattering medium 6 is a well-established function of its wavelength.

Thus, it will be appreciated by a person skilled in the art that the source wavelength and the nonlinear crystal selected for use in embodiments of the present invention can be dependent on the level of quantum tunnelling required to be achieved by the application and the environment in which it is required to be used. The aim is to use a source wavelength which maximises the number of photons which are returned to the second gated camera 8, whether by classical transmission or quantum tunnelling, and it will be appreciated by a person skilled in the art that this will be dependent on the barrier or scattering -6 -medium required to be traversed. In one exemplary embodiment, the source wavelength may be selected or set by a user, according to the barrier or scattering medium required to be traversed. However, in other exemplary embodiments, it is envisaged that a control system may initially determine the nature of the scattering medium by, for example, transmitting a photon beam or other exploratory signal toward the scattering medium and identifying the nature thereof according to properties of the signal returned thereby, and then select and set the source wavelength accordingly.

A quantum correlation circuit 9 is provided to receive data representative of photons received by the first and second gated cameras 7, 8 and generate an image of the target 5. The quantum correlation circuit includes a quantum detector, which compares each photon received by the second gated camera 8 against photons received from the first gated camera 7 to determine if it is a photon which originated from the source 1. If it is determined not to have originated from the source 1, the photon is discarded as noise. If the photon is determined to have originated from the source 1, it is used to generate an image. Many of the photons, which have originated from the source,received by the second gated camera 8 will simply have been reflected back to the second gated camera 8 by the barrier 6. In addition, some of the photons will have been classically transmitted or have "tunnelled" through the barrier 6 to the target 5 before being reflected back (either by classical transmission or quantum tunnelling) to the second gated camera 8. Thus, even if the target 5 is completely concealed by the barrier or other scattering medium 6, some reflected photons relating to the target 5 are received at the second gated camera 8.

The intensities of the photons not discarded by the quantum detector are then used by an image generation module to generate a sequence of intensity frames. In practice, each frame actually returns not from a single depth, but from a small range of depths. Therefore, the scene space actually imaged in each frame is a narrow fronto-parallel volume (cuboid) or target "window", not a plane. The thickness of this cuboid depends on the time for which the cameras 7, 8 are on (gate width), and the burst illumination laser and gated camera -7 -elements serve to further ensure that, of the photons which could potentially be received by the second detector 8, only those from the desired target "window" are detected.

Embodiments of the present invention have been described above by way of examples only, and it will be apparent to a person skilled in the art that modifications and variations can be made to the described embodiments without departing from the scope of the invention as claimed. For example, in the exemplary embodiment described above, polarisation is used as the photon property used correlate photons. However, in alternative exemplary embodiments, one or more other correlated photon properties, such as spin, may be used, and the present invention is not intended to be limited in this regard. In addition, the exemplary embodiment described above uses a nonlinear crystal to produce entangled photon pairs. However, in alternative exemplary embodiments, other entangled photon sources known to those skilled in the art (such as quantum dots, non-linear optical fibres, and so on) may be used, and the present invention is not intended to be limited in this regard.

Claims

-8 -CLAIMS 1. An imaging system comprising a photon source for generating a photon beam, a device located within the photon beam and configured to split photons from said beam into two or more respective entangled photons, the system being configured to direct at least one of said entangled photons to a first detector and to direct at least another of said entangled photons toward a target, the system further comprising a second detector for receiving reflected photons from said target, a quantum correlation circuit for comparing photons received by said second detector with photons received by said first detector to determine if said photons received by said second detector originated from said photon source, and an image generation circuit for generating an image of said target using photons received by said second detector determined to have originated from said photon source.
2. A system, according to claim 1, wherein photons determined not to have originated from the photon source are discarded.
3. A system according to claim 1 or claim 2, wherein the photon source is a laser.
4. A system according to claim 3, wherein the photon source is a burst illumination laser configured to transmit light pulses.
5. A system according to claim 4, wherein the first and/or second detectors comprise gated cameras, synchronised with the start of each light pulse.
6. A system according to any preceding claim, wherein the device configured to split photons from said source into two or more respective entangled comprises a nonlinear crystal, quantum dots or one or more non-linear optical fibres.
7. A method of generating an image of a target, the method comprising generating a photon beam, splitting photons from said photon beam into two or more respective entangled photons, directing at least one of said entangled photons to a first detector, directing at least another of said -9 -entangled photons toward a target, receiving reflected photons at a second detector and comparing said received photons with the photons received by said first detector to determine if they originate from said photon beam, and generating an image of said target using photons received by said second detector determined to have originated from said photon beam.
8. A method according to claim 7, wherein photons determined not to have originated from said photon beam are discarded.
9. A method according to claim 7 or claim 8, further comprising the step of to determining the nature of a scattering medium located between said photon beam and said target, and selecting the wavelength of said photon beam according to the nature of said scattering medium so as to optimise the number of reflected photons receivable by said second detector.
10.A computer-implemented method of generating an image of a target using source photons which have been split into two or more respective entangled photons, the method comprising the steps of receiving data representative of a first, unobstructed entangled photon, receiving data representative of a second, reflected entangled photon, comparing said second entangled photon with said first entangled photon to determine if said second entangled photon originated from a source photon, and, if so, using said second entangled photon to generate said image.
11.A computer program or programs which, when installed on a computing device, is/are configured to perform the method according to claim 10.
12.An imaging system substantially as herein described with reference to Figure 2 of the accompanying drawings.