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EP-4742560-A1 - DEVICE, SYSTEM AND METHOD FOR AUTOMATIC PHOTON POLARIZATION CORRECTION IN QUANTUM KEY DISTRIBUTION SYSTEMS

EP4742560A1EP 4742560 A1EP4742560 A1EP 4742560A1EP-4742560-A1

Abstract

The present invention discloses a device (1) for receiving an initial sequence (2) of polarized photons, modifying the polarization of each of said photons and performing measurements upon each of said photons according to an embodiment of the present invention. The device (1) comprises a beam splitter (3); two half-wave plates (4, 4'), two quarter-wave plates (5, 5'); two polarizer beam splitters (6, 6'), four single-photon detectors (7, 7', 7", 7‴), tagging means (8) connected to the single-photon detectors (7, 7', 7", 7‴) and rotating means (9) connected to the half-wave (4, 4') and quarter-wave plates (5, 5'). These elements are arranged and configured in such a way that the polarization of each photon comprised in the incoming light beam or initial sequence (2) of photons can be corrected before decoding. A system for transmitting a cryptographic key according to a Quantum Key Distribution, protocol with polarization correction and a method for correcting the polarization of each photon comprised in the sequence of photons used in the transmission of a cryptographic key according to a QKD protocol are also disclosed.

Inventors

  • LLANOS VELASCO, Adrián
  • ARTEAGA DÍAZ, Pablo
  • FERNÁNDEZ MÁRMOL, Verónica

Assignees

  • Consejo Superior De Investigaciones Científicas - CSIC

Dates

Publication Date
20260513
Application Date
20241111

Claims (10)

  1. Device (1) for receiving an initial sequence (2) of photons, modifying the polarization of said photons individually and performing measurements upon said photons, comprising: - a beam splitter (3); - a first half-wave plate (4); - first (6) and second (6') polarizer beam splitters; - four single-photon detectors (7, 7', 7", 7‴); and - tagging means (8) connected to the single-photon detectors (7, 7', 7", 7‴) adapted to register a timestamp of arrival of the photons to each of the four single-photon detectors (7, 7', 7", 7‴) and a detection rate of each single-photon detector (7, 7', 7", 7‴); wherein: - the beam splitter (3) is adapted to split in a substantially even way the initial sequence (2) of photons into a first (11) and a second (11') subsequences of photons, the first subsequence of photons (11) propagating along a first optical path (12) and the second subsequence of photons (11') propagating along a second optical path (12'); - the first polarizer beam splitter (6) is arranged along the first optical path (12) and is adapted to split the first subsequence of photons (11) into a third (13) and fourth (13') subsequences of photons of substantially orthogonal polarizations, the third subsequence of photons (13) propagating along a third optical path (14) and the fourth subsequence of photons (13') propagating along a fourth optical path (14'); - the second polarizer beam splitter (6') is arranged along the second optical path (12') and is adapted to split the second subsequence of photons (11') into a fifth (15) and sixth (15') subsequences of photons of substantially orthogonal polarizations, the fifth subsequence of photons (15) propagating along a fifth optical path (16) and the sixth subsequence of photons (15') propagating along a sixth optical (16') path; - the first half-wave plate (4) is arranged between the beam splitter (3) and the first polarizer beam splitter (6) along the first optical path (12); and - the four single-photon detectors (7, 7', 7", 7‴) are arranged after the polarizer beam splitters (6, 6'), in a configuration such that there is one single-photon detector (7, 7', 7", 7‴) arranged along each of the third (14), fourth (14'), fifth (16) and sixth (16') optical paths; and characterized in that the device further comprises: a second half-wave plate (4') arranged between the beam splitter (3) and the second polarizer beam splitter (6') along the second optical path (12'); a first quarter-wave plate (5) arranged between the beam splitter (3) and the first polarizer beam splitter (6) along the first optical path (12); a second quarter-wave plate (5') arranged between the beam splitter (3) and the second polarizer beam splitter (6') along the second optical path (12'); and rotating means (9) connected to the first and second half-wave plates (4, 4') and to the first and second quarter-wave plates (5, 5') and adapted to rotate each plate (4, 4', 5, 5') individually.
  2. A device (1) according to claim 1, wherein: - the first optical path (12) and the second optical path (12') are substantially perpendicular; - the third optical path (14) and the fourth optical path (14') are substantially perpendicular; and/or - the fifth optical path (16) and the sixth optical path (16') are substantially perpendicular.
  3. A device (1) according to any one of claims 1 or 2, wherein the rotating means (9) comprise four electronically controlled rotary mounts, each one connected to one of the first and second half-wave plates (4, 4') or the first and second quarter-wave plates (5, 5').
  4. A device (1) according to any one of claims 1 or 3, further comprising a microcontroller (17) connected to the rotating means (9) and adapted to send an instruction to said rotating means (9) to rotate each plate (4, 4', 5, 5') individually.
  5. A device (1) according to claim 4, wherein the microcontroller (17) is further connected to the tagging means (8) and wherein the instruction sent by the microcontroller (17) to the rotating means (9) is based on the detection rate of each single-photon detector (7, 7', 7", 7‴) as measured by the tagging means (8).
  6. A device (1) according to claim 4, wherein the microcontroller (17) is further connected to a conditioning circuit at the electronic output of each of the single-photon detectors (7, 7', 7", 7‴) and wherein the instruction sent by the microcontroller (17) to the rotating means (9) is based on a number of pulses per second counted by the microcontroller (17).
  7. A system for transmitting a cryptographic key according to a Quantum Key Distribution, QKD, protocol with polarization correction comprising: - a transmitter adapted to prepare and send a sequence of photons in which the polarization of each photon has been previously selected from a group comprising two pairs of orthogonal linear polarization states, being said pairs of orthogonal linear polarization states rotated at 45° between one another; - an atmospheric or free space transmission channel, and - a device (1) according to any one of claims 1 to 6.
  8. A system according to claim 7, wherein the QKD protocol comprises a BB84 protocol.
  9. A system according to any one of claims 7 or 8, wherein the transmitter is arranged substantially on the Earth's surface and wherein the device (1) for receiving an initial sequence (2) of polarized photons, modifying the polarization of said photons individualy and performing measurements upon said photons is arranged on a satellite placed in orbit.
  10. A method for correcting the polarization of photons comprised in an initial sequence of polarized photons (2) used in the transmission of a cryptographic key according to a QKD protocol, the method comprising operating a system according to any one of claims 7 to 9 and performing the following steps: a) producing and sending, with the transmitter, a sequence of identically polarized photons; b) configuring the first and second half-wave plates (4, 4') and the first and second quarter-wave plates (5, 5') in an initial orientation with respect to the direction of incidence (10) of the sequence of photons; c) receiving the sequence of photons sent in step a) in the device (1); d) measuring the photon detection rate in the four single-photon detectors (7, 7', 7", 7‴); e) identifying, as first single-photon detector, the single-photon detector (7, 7', 7", 7‴) with the highest rate, identifying, as second single-photon detector, the single-photon detector (7, 7', 7", 7‴) arranged after the same polarizer beam splitter (6 or 6') as the first single-photon detector and identifying the optical path (12 or 12') along which said polarizer beam splitter (6 or 6') is arranged; f) rotating, by using the rotating means (9), the half-wave (4 or 4') plate arranged at the optical path identified in step e) (12 or 12') and measuring the photon detection rate in the first and second single-photon detectors; g) repeating step f) until the ratio between the detection rates in the first single-photon detector and in the second single-photon detector is maximized for that configuration; h) rotating, by using the rotating means (9), the quarter-wave plate (5 or 5') arranged on the optical path identified in step e) (12 or 12') and measuring the photon detection rate in the first and second single-photon detectors; i) repeating step h) until the ratio between the detection rates in the first single-photon detector and in the second single-photon detector is maximized for that configuration; j) repeating steps f)-i) until the ratio between the detection rates in the first single-photon detector and in the second single-photon detector is maximized. k) producing and sending, with the transmitter, a sequence of identically polarized photons, the polarization of said photons being rotated 45° with respect to that of the photons produced and sent in step a); l) receiving the sequence of photons sent in step h) in the device (1); m) measuring the photon detection rate in the four single-photon detectors; n) identifying, as third single-photon detector, the single-photon detector (7, 7', 7", 7‴) with the highest rate, identifying, as fourth single-photon detector, the single-photon detector (7, 7', 7", 7‴) arranged after the same polarizer beam splitter (6 or 6') as the third single-photon detector and identifying the optical path (12 or 12') along which said polarizer beam splitter (6 or 6') is arranged; o) rotating, by using the rotating means (9), the half-wave plate (4 or 4') arranged on the optical path identified in step j) (12 or 12') and measuring the photon detection rate in the third and fourth single-photon detectors; p) repeating step n) until the ratio between the detection rates in the third single-photon detector and in the fourth single-photon detector is maximized for that configuration; q) rotating, by using the rotating means (9), the quarter-wave plate (5 or 5') arranged on the optical path identified in step j) (12 or 12') and measuring the photon detection rate in the third and fourth single-photon detectors; r) repeating step p) until the ratio between the detection rates in the third single-photon detector and in the fourth single-photon detector is maximized for that configuration; and s) repeating steps n)-q) until the ratio between the detection rates in the third single-photon detector and in the fourth single-photon detector is maximized, obtaining a device (1) for receiving an initial sequence (2) of photons and performing measurements upon said photons wherein the polarization of said photons for that specific transmitter has been corrected.

Description

FIELD OF THE INVENTION The present invention belongs to the technical field of quantum communications. More specifically, the invention relates to a system adapted to correct the polarization of photons used to transmit a quantum key within the BB84 protocol. The system of the invention works in free space, i.e., in an atmospheric transmission channel, as opposed to optical fiber, avoiding signal loss associated with the latter. BACKGROUND OF THE INVENTION Among other effects, globalization and the development and deployment of many kinds of activities over the internet has brought, in the last decades, a considerable increase in communications all around the world and even beyond. These communications, such as those related to commercial or security operations, often include the exchange of sensible information which must remain confidential from the well-being of the processes and persons involved. For these reasons, the transmission of information is usually done by encrypting the transferred data so that only the appropriate recipient/s can access the information. Encryption is the process of transforming or encoding information in a way that, ideally, only authorized parties can decode. This process converts the original representation of the information into an alternative form, which is often done according to a key that serves for encoding and decoding the encrypted information. Encryption does not itself prevent that the information does not reach undesired recipients but denies the intelligible content to a would-be interceptor. This is so since the encrypted information may be public as long as the key is transferred securely between the authorized parties involved in the communication. Nevertheless, the development of new technologies has led to the creation of new ways of hacking or eavesdropping the usual encryption and key-transmission systems, so new, more secure communication frameworks, such as quantum cryptography, have become necessary. Quantum cryptography is based on the exchange of specific quantum states between a transmitter and a receiver and benefits from the characteristics of quantum systems in order to provide a secure way of transmitting information. The best-known example of quantum cryptography is quantum key distribution (QKD), which offers a secure solution to the key exchange problem. An important and unique property of QKD is the ability of the two communicating users to detect the presence of any third party trying to gain knowledge of the key. This results from a fundamental aspect of quantum mechanics: the process of measuring a quantum system disturbs the system. Therefore, a third party trying to eavesdrop on the key must in some way measure it, thus introducing detectable anomalies. By using quantum superposition or quantum entanglement and transmitting information in quantum states, a communication system that detects eavesdropping can be implemented. If the level of eavesdropping is below a certain threshold, a key can be produced that is guaranteed to be secure. Otherwise, no secure key is possible, and communication is aborted. The BB84 Quantum Key Distribution protocol is one of the most common ones in practical QKD implementations. This protocol uses four quantum states to encode the key information, which typically consist of polarization states of low-intensity optical pulses. These polarization states are grouped into two pairs of orthogonal linear polarization states, and at 45° between the pairs, each of these pairs representing an encoding basis. At the receiver, two measurement bases are used to project the states and decode their information by measuring their polarization. Each of these measurement bases projects the states onto two orthogonal polarization axes, and each projection is measured with a single-photon detector. In this way, the measurement of each detector corresponds to a different decoding value. The BB84 protocol operates with maximum efficiency when the transmitter and receiver use the same bases to encode and decode the information. However, in practice, the transmission of states through the optical elements of the transmitter and receiver systems, and through the transmission channel, can introduce variations in the polarization states. These polarization variations cause the photons to be projected onto the opposite axis with a certain probability, resulting in measurements in the wrong single-photon detector. This produces measurement errors even when the transmitter and receiver use the same bases to encode and decode the information, which degrades or disables key exchange. In systems with an atmospheric transmission channel, the incidence angles of the beams on different optical components change each time the systems are optically aligned, depending on thermal movements of the buildings in which they are located, mechanical tolerances, thermoelasticity of optical components, etc., which implies that the polarization of the s