Entangled quantum communications and quantum imaging

• US 7,536,012 B1
• Filed: 07/28/2004
• Issued: 05/19/2009
• Est. Priority Date: 08/06/2003
• Status: Expired due to Fees

• Current Assignee: United States Of America As Represented By The Secretary Of The Army (Government of the United States of America)
• Original Assignee: The United States Of America As Represented By The Secretary Of The Army
• Inventors: Meyers, Ronald E., Deacon, Keith S., Rosen, David L.
• Primary Examiner(s): Zand; Kambiz
• Assistant Examiner(s): Powers; William S
• Application Number:
  US10/900,351
• Time in Patent Office: 1,756 Days
• Field of Search: 380/44
• US Class Current: 380/44
• CPC Class Codes: H04L 9/0858 Details about key distillation or coding, e.g. reconciliation, error correction, privacy amplification, polarisation coding or phase coding

Abstract

An apparatus for generating a shared quantum key between a sender and a receiver comprises a sending apparatus which generates entangled photon pairs, and a receiving apparatus. A shared quantum key can be generated using temporal coincidences between photon detection events. For example, coincidences may be determined between sender and receiver photon detection events using detection data shared through a non-secure communications link between the sender and receiver. The quantum key can be used in encrypted communications. Similar apparatus and methods can be used for quantum imaging.

Claims

1. An apparatus for generating a shared quantum key between a sender and a receiver, the apparatus comprising a sending apparatus and a receiving apparatus,

the sending apparatus stochastically generating entangled photon pairs, the sending apparatus detecting a first photon of each entangled photon pair and transmitting a second photon of each entangled photon pair to the receiving apparatus,

the receiving apparatus detecting the second photon of each entangled pair,

the sending apparatus and the receiving apparatus being in communication through a communication link,

the shared quantum key being generated from stochastic temporal coincidences between sender photon detection data and receiver photon detection data shared over the communication link.

2. The apparatus of claim 1, wherein the stochastically generated entangled photon pairs that the sending apparatus generates are split into first and second entangled photon beams, the first entangled photon beam being retained by the sender and being comprised of photons having a first polarization, the second entangled photon beam being sent to the receiver and being comprised of photons having a second polarization, the sender photon detection data and receiver photon detection data including timing information correlated with detected photon polarization, and wherein the shared quantum key is derived from comparison of the sender and receiver data for instances when the first polarization is the conjugate of the second polarization at corresponding times.

3. The apparatus of claim 1, wherein the first photons have a first energy, the second photons have a second energy, the first energy being greater than the second energy, and further including filters which act as dichroic mirrors to pass photons with an energy interval encompassing said first energy and said second energy while excluding photon energies outside the energy interval, and the sender photon detection data and receiver photon detection data including information correlated with detected photon energy.

4. The apparatus of claim 2, wherein the first polarization and the second polarization are the conjugates of one another that have an angular difference of 90 degrees.

5. The apparatus of claim 1, wherein the the stochastically generated entangled photon pairs that the sending apparatus generates are split into first and second entangled photon beams, the first entangled photon beam being directed to the sender and being comprised of photons having a first polarization, the second entangled photon beam being sent to the receiver and being comprised of photons having a second polarization, the sender photon detection data and receiver photon detection data including timing information correlated with detected photon polarization, and wherein the shared quantum key is a sequence of bits derived from a comparison of the sender and receiver data for instances when the first polarization is the conjugate of the second polarization at a corresponding time;

and wherein for each instance when the first polarization is the conjugate of the second polarization at a corresponding time the first polarization may have only first or second characteristics; and

wherein

(a) if the first polarization has the first characteristic then a zero is selected for a predetermined bit position in the shared quantum key sequence of bits;

(b) if the first polarization has the second characteristic, then a one is selected for a predetermined position in the shared quantum key sequence of bits.

6. The apparatus of claim 5 wherein the first polarization characteristic is the angle of polarization.

7. The apparatus of claim 6, wherein the first polarization is 0 degrees, and the second polarization is 90 degrees, polarizations being angularly determined from a common plane.

8. The apparatus of claim 1, wherein the sending apparatus generates time-random photon pairs, to thereby eliminate the need for a modulation device.

9. The apparatus of claim 2, wherein the first and second entangled photon beams are provided using a laser, and a nonlinear crystal.

10. A sending apparatus in possession of a sender for enabling an encryption key to be shared between the sender and a receiver, the sending apparatus comprising:

a source of entangled photon pairs generated at stochastic times to form an entangled photon beam that is unmodulated and stochastic;

a first beamsplitter;

a first detector; and

a communications link, providing communication with the receiver,

the entangled photon beam being incident on the first beamsplitter, the first beamsplitter operational to split at least part of the entangled photon beam into a directed beam and a transmitted beam, the directed beam being incident on the detector and the transmitted beam being sent to the receiver,

the first detector providing detector data correlated with detection of photons in the directed beam, the detector data including time of detection data, the detector data being transmittable to the receiver over the communications link,

whereby the transmitted beam and detector data can be sent to the receiver, the receiver then generating the encryption key using stochastic temporal coincidences determined between detector data received by the receiver, and receiver detection data determined from the transmitted beam.

11. The apparatus of claim 10, wherein the source of entangled photon pairs provides entangled photon pairs with each of one of the pair being mutually interdependent on the other such that for each generated entangled photon pair, the plane of polarization of one of the pair depends on that of the other, the polarization of the first photon of the pair being detected by the detector and the polarization of the second photon of said photon pair adapted to be detected by the receiver.

12. The apparatus of claim 10, wherein the source of entangled photon pairs comprises a laser and a nonlinear crystal.

13. The apparatus of claim 10, wherein the detector is a photon counting detector.

14. The apparatus of claim 10, wherein the apparatus further comprises a receiver apparatus in possession of the receiver, the receiver apparatus comprising:

a receiver beamsplitter;

a first receiver detector; and

a second receiver detector,

the receiver beamsplitter splitting at least part of the transmitted beam received from the sender into a first receiver beam and a second receiver beam, the first receiver beam being incident on the first receiver detector and the second receiver beam being incident on the second receiver detector,

the receiver determining time-dependent coincidences between the receiver data obtained from the first receiver detector and the second receiver detector and the detector data received from the sender.

15. The apparatus of claim 10, further comprising a second beamsplitter and a second detector, and wherein the

source of entangled photons comprises a first source of entangled photons providing a first entangled photon beam;

and

a second source of entangled photons providing a second entangled photon beam;

the first entangled photon beam being incident on the first beamsplitter, the first beamsplitter operational to split at least part of the first entangled photon beam into a first directed beam and a first transmitted beam, the directed beam being incident on the detector, the second entangled photon beam being incident on the second beamsplitter, the second beamsplitter operational to split at least part of the second entangled photon beam into a second directed beam and a second transmitted beam, the second directed beam being incident on the second detector,

the first detector providing first detector data correlated with detection of photons in the first directed beam,

the second detector providing second detector data correlated with detection of photons in the second directed beam,

the first detector data and second detector data being combined in the detector data being transmittable to the receiver over the communications link,

the first transmitted beam and the second transmitted beam being combined into the transmitted beam being sent to the receiver.

16. The apparatus of claim 10, wherein the the directed beam is comprised of photons having a first polarization, the transmitted beam is comprised of photons having a second polarization, the detector and the receiver detection data including timing information correlated with detected photon polarization, and wherein the shared quantum key is a sequence of bits derived from a comparison of the detector and receiver detection data for instances when the first polarization is the conjugate of the second polarization at a corresponding time;

and wherein for each instance when the first polarization is the conjugate of the second polarization at a corresponding time the first polarization has either first or second characteristics; and

wherein

(c) if the first polarization has the first characteristic then a zero is selected for a predetermined bit position in the shared quantum key sequence of bits;

(d) if the first polarization has the second characteristic, then a one is selected for a predetermined position in the shared quantum key sequence of bits.

17. The apparatus of claim 10, the sending apparatus further including an image generator upon which at least a portion of the directed beam illuminates, the apparatus being further operable to transmit encrypted image data.

18. The apparatus of claim 17, wherein the source of entangled photons includes a laser and a type II nonlinear crystal,

the directed beam being incident on the image generator, the directed beam subsequently being split by a polarizing beamsplitter into a first split beam and a second split beam, the first split beam being incident on a first detector, the second split beam being incident on a second detector,

the detector data being transmittable to the receiver over the communications link including first detector data from the first detector and second detector data from the second detector.

19. A method of generating a quantum key for encrypted communication between a sender at a sender location and a receiver at a receiver location, the method comprising:

splitting a stochastic, unmodulated entangled photon beam into a directed beam and a transmitted beam at the sender location;

determining sender data relating to the directed beam at the sender location, the sender data including sender time-dependent photon detection data;

transmitting the transmitted beam to the receiver;

determining receiver data relating to the transmitted beam at the receiver location, the receiver data including receiver time-dependent photon detection data; and

generating the encryption key through an exchange of sender data and receiver data through a communication link between the sender location and the receiver location.

20. The method of claim 19, wherein the entangled photon beam comprises entangled photon pairs with each of one of the pair being mutually interdependent on the other such that for each generated entangled proton pair the plane of polarization of one of the pair depends on that of the other, the first photon of the pair being part of the directed beam and the second photon of said photon pair being part of the transmitted beam.

21. The method of claim 19, wherein the directed beam is comprised of photons having a first polarization, the transmitted beam is comprised of photons having a second polarization, the sender data and receiver data including timing information correlated with detected photon polarization, and wherein the shared quantum key is a sequence of bits derived from a comparison of the sender and receiver data for instances when the first polarization is the conjugate of the second polarization at a corresponding time;

and wherein for each instance when the first polarization is the conjugate of the second polarization at a corresponding time the first polarization may have only first or second characteristics; and

wherein

(a) if the first polarization has the first characteristic then a zero is selected for a predetermined bit position in the shared quantum key sequence of bits;

(b) if the first polarization has the second characteristic, then a one is selected.

22. The method of claim 19, wherein the transmitted beam is split into a first receiver beam and a second receiver beam at the receiver location, the receiver data relating to the first receiver beam and the second receiver beam.

23. The method of claim 19, wherein a second entangled photon beam is split into a second directed beam and a second transmitted beam at the sender location, the sender data relating to the directed beam and the second directed beam,

the second transmitted beam being combined with the transmitted beam, both being sent to the receiver.

24. An apparatus for facilitating the generation of a quantum key to be shared between a sender and a receiver, the apparatus comprising:

a sending apparatus, including

a first source of entangled photons, providing a first entangled beam absent modulation,

a second source of entangled photons, providing a second entangled beam absent modulation,

a first beamsplitter,

a first detector,

a second beamsplitter, and

a second detector,

the first entangled beam being incident on the first beamsplitter, the first beamsplitter producing a first directed beam incident on the first detector and a first transmitted beam transmitted to the receiver,

the second entangled beam being incident on the second beamsplitter, the second beamsplitter producing a second directed beam incident on the second detector and a second transmitted beam transmitted to the receiver;

a receiving apparatus, including

a first receiving beamsplitter,

a first receiver detector, and

a second receiver detector;

sender detector electronics, operable to provide sender data, the sender data including sender single photon detection data from the first detector and the second detector during predetermined time intervals;

receiver detector electronics, operable to provide receiver data, the receiver data including receiver single photon detection data from the first receiver detector and the second receiver detector during the predetermined time intervals; and

a communications link between the sender and the receiver, through which sender data and receiver data can be exchanged.

25. The apparatus of claim 24, wherein:

the first source of entangled photons includes a first laser and a first nonlinear crystal of Type I nonlinearity,

the second source of entangled photons includes a second laser and a second nonlinear crystal of Type I nonlinearity,

the first entangled beam having a first polarization,

the second entangled beam having a second polarization.

26. The apparatus of claim 25, wherein the first polarization is at an angle of zero degrees, the second polarization is at an angle of 45 degrees, the first receiver detector is responsive to photons polarized at −

45 degrees, and the second receiver detector is responsive to photons polarized at 90 degrees, polarizations being angularly determined from a common plane.

27. The apparatus of claim 24, wherein the communications link between the sender and the receiver is a public communications link.

28. The apparatus of claim 24, further comprising coincidence detection electronics at the receiver, the coincidence detection electronics receiving sender data and receiver data.

29. A sending apparatus for transmitting encoded image data from a sender to a receiver, the sending apparatus comprising:

a source of entangled photons providing an entangled beam, and

a first beamsplitter,

an image generator,

a second beamsplitter,

a first detector, and

a second detector,

the entangled beam being incident on the first beamsplitter, the beamsplitter providing a directed beam and a transmitted beam,

the transmitted beam being sent to the receiver,

the directed beam being incident on the image generator such that portions of the directed beam are reflected by the image generator and portions of the directed light are transmitted to the second beamsplitter, the second beamsplitter providing two orthogonally polarized beams incident on the first and second detectors respectively,

a communications link with the remote apparatus, through which detection time data can be transmitted,

whereby the receiver can receive the transmitted beam and the detection time data related to the directed beam, the receiver being able to decode the transmitted encoded image data using coincidence measurements between receiver detection data generated by the receiver and detection data received from the sending apparatus.

30. An apparatus for image transmission from a sender to a receiver, the apparatus comprising:

a sending apparatus, including;

a source of entangled photons providing an entangled beam,

a beamsplitter,

an image generator, and

a beam recombiner,

the entangled beam being incident on the beamsplitter, the beamsplitter providing a first beam which illuminates the image generator, and a second beam which does not interact with the image generator,

the beam recombiner combining the first beam and the second beam into a transmitted beam sent to the receiving apparatus; and

a receiving apparatus, including;

a receiver beamsplitter,

a first receiver detector, providing first receiver data,

a second receiver detector, providing second receiver data, and

a coincidence circuit,

the transmitted beam being incident on the receiver beamsplitter, the receiver beamsplitter providing a first receiver beam incident on the first receiver detector, and a second receiver beam incident on the second receiver detector,

the coincidence circuit receiving the first receiver data and the second receiver data, and reconstructing the image from determined coincidences between the first receiver data and the second receiver data.

31. The apparatus of claim 30, wherein the first receiver data and the second receiver data including single photon counting data obtained during synchronized time intervals.

32. The apparatus of claim 30, wherein the image is transmitted utilizing encoded image data using coincidence measurements between receiver detection data generated by the receiver and detection data received from the sending apparatus.

33. The apparatus of claim 31, wherein the image is transmitted utilizing encoded image data using coincidence measurements between receiver detection data generated by the receiver and detection data received from the sending apparatus.