Quantum communication system using entangled photons

1 . A quantum communication system, comprising:

an emitter of entangled photons, comprising a source configured in order to generate at least one pair of entangled photons comprising a first photon emitted on a first propagation path and simultaneously a second photon emitted on a second propagation path different to the first propagation path;

a first receiver arranged on the first propagation path, comprising:

at least one first instrument arranged in order to absorb the first photon in one of two complementary polarizations, the polarization of the photon being indeterminate in an observable basis according to which the first instrument absorbs the photons,

an optical selector arranged upstream of said at least one first instrument and configured in order either to allow the first photon to pass through toward said at least one first instrument or to prevent it from being measured;

a second receiver arranged on the second propagation path so as to be reached by the second photon after the first photon has reached the optical selector and/or after it has been able to reach the instrument of the first receiver, said second receiver comprising an optical amplifier configured to multiply the second photon while preserving its polarization, and, arranged downstream of the amplifier, a measuring instrument configured to measure the average quantum state of the multiplied photons.

2 . The quantum communication system according to claim 1 , wherein the first receiver comprises at least one second instrument, the optical selector being arranged upstream of the first and second instruments and configured in order either to allow the first photon to pass through while directing it toward one or other of the instruments or to prevent it from being absorbed.

3 . The quantum communication system according to claim 1 , wherein the emitter is configured in order to generate a plurality of pairs of entangled photons successively.

4 . The quantum communication system according to claim 1 , wherein the optical selector comprises a reflector.

5 . The quantum communication system according to claim 1 , wherein the optical selector comprises a device whose refractive index and/or reflection direction are controlled.

6 . The quantum communication system according to claim 1 , further comprising a device for protecting the quantum state of at least one photon which is arranged in proximity to the first receiver so as to protect the quantum state of the first photon in the event that it is prevented from being measured by the optical selector, said quantum state of the first photon being protected at least until the second photon has been multiplied at the second receiver.

7 . The quantum communication system according to claim 6 , wherein the device for protecting the quantum state comprises a transparent space.

8 . The quantum communication system according to claim 1 , wherein at least one instrument of the first receiver comprises at least one filter configured to send the first photon toward at least one photon detector.

9 . The quantum communication system according to claim 1 , wherein the measuring instrument of the second receiver comprises at least one photon detector arranged in order to measure the polarization of the light originating from the multiplication of the second photon.

10 . The quantum communication system according to claim 1 , wherein the measuring instrument of the second receiver comprises a succession of semireflective plates arranged downstream of the optical amplifier, said plates directing the flux of multiplied photons with an equal intensity toward polarizing filters, downstream of which detectors are arranged.

11 . The quantum communication system according to claim 1 , wherein the optical amplifier is a doped-fiber amplifier.

12 . The quantum communication system according to claim 1 , wherein the first receiver and/or the second receiver comprises one or more dichroic filters.

13 . The quantum communication system according to claim 1 , wherein the emitter and each of the receivers comprises a clock, the clocks of the emitter and of the receivers being synchronized with one another.

14 . The quantum communication system according to claim 1 , wherein the second receiver comprises a switch arranged in front of the optical amplifier and configured in order to absorb or reflect the photon or photons subsequent to a first photon reaching said second receiver in a predetermined time interval.

15 . The quantum communication system according to claim 1 , further comprising a second emitter capable of generating one or more pairs of entangled photons, the second emitter being located closer to the second receiver than to the first receiver.

16 . A quantum communication method using the quantum communication system according to claim 1 , comprising the steps of:

generating a pair of entangled photons from an emitter, the first photon of the pair being emitted toward a first receiver and the second photon of the pair being emitted simultaneously toward a second receiver, the first and second photons being entangled, the second receiver being located on the propagation path of the second photon further away from the emitter than the first receiver, so that it arrives there later,

according to information to be transmitted, selecting to absorb or not absorb the first photon in one of two pairs of complementary polarizations when it reaches the first receiver, by using an optical selector which does or does not direct said photon toward an instrument or toward one of a plurality of instruments,. in the event that the selection is made not to absorb the photon in a predefined polarization, trapping said first photon in a protection device configured to avoid its absorption at least so long as the second photon has not reached the second receiver,

at the second receiver, duplicating the second photon to form a flux of multiplied photons by using an amplification device, each multiplied photon having preserved the quantum state of the second photon,

measuring the average quantum state of multiplied photons and determining according to this measurement whether the first photon has been absorbed at the first receiver, and/or with which instrument, in order to deduce therefrom the information transmitted (“0”, “1”) by the first receiver.

17 . The quantum communication method according to claim 16 , wherein the polarization of the pair of entangled photons being is indeterminate in an observable basis or bases, in which the instruments of the first receiver absorb them.

18 . The quantum communication method according to claim 16 , wherein the pair of entangled photons reaches the receivers with a linear entangled polarization, one of the two complementary absorption polarizations being circular, the second receiver being arranged in order to distinguish whether the average polarization of the flux of multiplied photons is circular or linear, and to determine according to this distinction whether or not the first photon has been measured.

19 . The quantum communication method according to claim 16 , wherein the pair of entangled photons reaches the receivers with a circular entangled polarization, the quantum state measured at the first receiver being a linear polarization, the second receiver being arranged in order to distinguish whether the average polarization of the flux of multiplied photons is circular or linear, and to determine according to this distinction whether or not the first photon has been measured.

20 . The quantum communication method according to claim 16 , wherein the pair of entangled photons reaches the receivers with a vertical or horizontal linear polarization, the quantum state measured at the first receiver being a linear polarization at 45° or −45° to the vertical or the horizontal, the second receiver being arranged in order to distinguish whether the average linear polarization of the flux of multiplied photons is at 45° or −45°, or the vertical or the horizontal, and to determine according to this distinction whether or not the first photon has been measured.

21 . The quantum communication method according to claim 16 , wherein a plurality of pairs of entangled photons are generated successively by the emitter, each pair of photons are configured to transmit an item of information (“0”, “1”, “2”) from the first receiver to the second receiver.

22 . The quantum communication method according to claim 16 , wherein it is verified that an item of information transmitted by a flux of photons from the emitter to the receiver has not been eavesdropped by implementing the following steps:

the second receiver establishes a first list of reception dates of the photons that conveyed item of information, and relative polarization designating that of the two complementary polarizations in which each photon was received;

the second receiver generates a message containing the list collected in the preceding step, creates an electronic signature of this list and transmits said list and the signature to the receiver;

the first receiver receives the list and signature then verifies said signature;

the first receiver makes a second list including elements of the first list for which the relative polarizations of the photons received by the first and second receivers and are equal, or indeed for which the entangled photon never reached the first receiver, two entangled photons having different relative polarizations;

if the number of elements of the second list restricted to pairs of entangled photons each of the two photons of which reached their respective receivers is lower than the product of a predetermined ratio multiplied by the count of the elements of the first list restricted to pairs of entangled photons each of the two photons of which reached their respective receivers, then the item of information is declared to have been transmitted without having been eavesdropped.

23 . The quantum communication method according to claim 22 , wherein:

the item of information is declared to have been transmitted without having been eavesdropped, and then subsequently the second list is sent signed by the first receiver to the second receiver, then a third list is created by the first receiver, this list including the relative polarizations of the photons featuring in the first list but not featuring in the second list,

on reception of the second list by the second receiver and after verification of the signature, the third list is recreated in the second receiver using the first and second lists, then a signed message, confirming successful receipt of the second list, is sent by the second receiver to the first receiver; and

the second receiver uses the third list as shared key with the first receiver, and, on reception by the first receiver of the message transmitted by the second receiver in the preceding step, the first receiver uses the third list as shared key in exchanges with the second receiver.

24 . The quantum communication method according to claims 23 , wherein a polarizing filter is placed upstream of the second receiver.