RU2812341C2 - Method for detecting phases of low-photon coherent light fields at side frequencies in quantum key distribution system - Google Patents

Abstract

FIELD: quantum methods.

SUBSTANCE: optical communication method used to transmit information over a distance, in relation to photonic quantum communication systems. A method for detecting the phases of low-photon coherent light fields at side frequencies in a quantum key distribution system consists in detecting the result of mixing the side frequencies of a low-photon coherent pulse of light modes of signal radiation, which has encoded phases with the modes of reference radiation. The input radiation is encoded into one of the phases: ϕ=0,π/2, π, 3π/2, so that the low-frequency and high-frequency modes acquire phases (-ϕ) and (+ϕ), and the radiation, which has an unbiased central frequency, enters the phase modulator of the receiver, where it is encoded by one of the randomly selected phases: 0, π/2, π, 3π/2. When choosing the same bases in the transmitter and receiver, if the phases of the signal and reference frequency modes coincide, then the output low-frequency and high-frequency modes are sent to one detector, whereas if the phases of the signal and reference frequency modes are shifted by 180 degrees, then the output modes are sent to another detector.

EFFECT: increasing the speed of key generation due to the simultaneous measurement of two-phase states of side frequencies that differ by 180 degrees.

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Description

The invention relates to optical communication technology and can be used to transmit information over a distance, in relation to photonic quantum communication systems.

There are known methods for transmitting information based on the use of sending quantum particles, one of which are photons: US 2010/0046754 A 1 publ. 02/25/2010; JP 2001/028584 A publ. 01/30/2001; US 2007/0070353 A 1 publ. 03/29/2007.

A device for quantum distribution of symmetric bit sequences is also known [US Patent No. 6272224 B1, priority date 04/07/2001. MKI: H04L 9/08; H04K 1/00], containing, connected via a fiber-optic communication line, a transmitting device, including a monochromatic radiation source, an electro-optical phase modulator and an attenuator located in series along the propagation of radiation, as well as a phase shift device, the output of which is connected to the control input of the electro-optical phase modulator, and the input of the phase shift device is connected to the output of the radio frequency signal generator, and a receiving device including an electro-optical phase modulator, the output of which is optically coupled to a spectral filter, which is optically coupled to a classical radiation receiver and a single photon detector, the control input of the electro-optical phase modulator is connected to the output of the phase shift device, to the input of which the output of the radio frequency signal generator is connected, the fiber-optic communication line is optically coupled with the attenuator of the transmitting device and with the input of the electro-optical phase modulator of the receiving device, the device contains a synchronization unit, the first and second outputs of which are connected to the inputs of the radio frequency signal generator receiving and transmitting devices, respectively, as well as a phase shift control unit, the first and second outputs of which are connected to the synchronizing inputs of the phase shift device of the receiving and transmitting devices, respectively.

The disadvantage of this device is the low rate of transmission of quantum states, due to the loss of part of the signal, which is caused by the polarization dependence of the phase modulator in the electro-optical modulation unit of the receiver. An electro-optical phase modulator effectively modulates only that part of the radiation whose polarization coincides with the optical axis of the modulator crystal. Linearly polarized radiation emerging from the transmitter undergoes arbitrary polarization distortions when passing through a quantum channel, and therefore the receiver can register only part of the radiation reaching it, depending on the nature of the distortions and, thus, the rate of transmission of quantum states can change unpredictably over a wide range , which is unacceptable for practical use.

The closest analogue is the device described in patent RU 2 454 810 C1, containing a transmitting device connected via a fiber-optic communication line, including a monochromatic radiation source located in series along the radiation path, an electro-optical phase modulator and an attenuator, as well as a phase shift device, the output of which connected to the control input of the electro-optical phase modulator, and the input of the phase shifter is connected to the output of the radio frequency signal generator, and a receiving device including an electro-optical phase modulator, a classical radiation receiver optically coupled with a spectral filter, and a single photon detector, the electro-optical phase modulator is connected to the device phase shift, to the input of which the output of the radio frequency signal generator is connected, the fiber-optic communication line is optically coupled with the attenuator of the transmitting device, the device contains a synchronization unit, the first and second outputs of which are connected to the inputs of the radio frequency signal generator of the receiving and transmitting devices, respectively, characterized in that that the electro-optical phase modulator in the receiving device is made of two electro-optical phase modulators located along the radiation path, the control inputs of which are connected to the first and second outputs of the phase shift device, respectively, and the output of the first electro-optical phase modulator is optically coupled with the output of the second electro-optical phase modulator, behind the modulators by During the course of radiation, a Faraday mirror is installed, optically coupled to the input of the second electro-optical phase modulator, an optical circulator is introduced into the receiving device, the first port of which is optically coupled to the fiber-optic communication line, the second port is optically coupled to the input of the first electro-optical phase modulator, the third port is optically coupled to spectral filter, and the fourth port is optically coupled to the input of the single photon detector, the synchronization device has third and fourth outputs, which are connected to the synchronization inputs of the phase shift devices of the receiving and transmitting devices, respectively.

The disadvantage of this device is high optical losses, which reduce the transfer rate of quantum states.

The problem of polarization sensitivity of the phase modulator of the receiving device is solved in this device, but this solution leads to a significant increase in optical losses in the receiver and, as a result, to a low transmission rate of quantum states. The present invention proposes a new detection method that reduces losses in the receiving device, thereby increasing the transmission rate of quantum states.

The technical problem of the proposed detection method is solved by using the ability to distinguish the phases of coherent states of light at side frequencies, attenuated to a single-photon level.

The technical result of the claimed method is to increase the speed of key generation due to the simultaneous measurement of two phase states of side frequencies differing by 180 degrees.

To do this, we propose a new method for detecting the phases of low-photon coherent light fields at side frequencies in a quantum key distribution system, which includes:

transmitter - a transmitting unit containing

random number generator,

laser,

phase modulator,

Control block;

receiver - a receiving block containing

random number generator,

circulators,

phase modulator,

spectral filters, interferometer,

a photoreceiving unit having two avalanche detectors for recording quantum information states,

processing unit,

Control block;

an optical communication line made in the form of a single-mode optical fiber connecting the transmitting and receiving units;

The method consists in detecting the result of mixing the side frequencies of a low-photon coherent pulse of light modes of the signal radiation having encoded phases with the modes of the reference radiation, which makes it possible to achieve detection of both phases of the signal radiation when the bases coincide.

To obtain phase-coded signal coherent modes at side frequencies, phase modulation of single-mode coherent radiation by a radio frequency signal is used on the modulator 2 of the transmitter. The input radiation is encoded into one of 4 phases: ϕ=0, π/2, π, 3π/2, so that the “red” (low-frequency) and “blue” (high-frequency) modes acquire phases (-ϕ) and (+ϕ ). The modulated radiation arrives at the receiver at port 1 of circulator 3a. From port 2 of the circulator 3a, this radiation enters the spectral filter 4a, where the side frequencies of the signal radiation are sent to port 1 of the semi-transparent mirror 5, and the radiation at the central frequency is reflected into port 2 of the circulator 3a. From port 3 of circulator 3a, radiation with an unbiased central frequency enters phase modulator 2 of the receiver, where it is encoded by one of 4 randomly selected phases: 0, π/2, π, 3π/2, after which it is sent to port 1 of circulator 3b . From port 2 of circulator 3b, radiation at the central frequency and the generated phase-modulated radiation at side frequencies (reference radiation) are sent to spectral filter 4b, where the side frequencies pass to port 2 of the semi-transparent mirror 5, and the field at the central frequency is reflected into port 2 of circulator 3b. From port 3 of circulator 3b, radiation at the central frequency enters synchronization detector 6. Phase-coded “red” and “blue” frequency modes of the signal pulse of radiation coming from filter 4a, phase-coded (-ϕ, +ϕ), are mixed on a translucent mirror 5 with “red” and “blue” frequency modes of the reference radiation pulse coming from filter 4b, having phases (-ϕ, +ϕ), where ϕ=0, or ϕ=π/2. The detection results obtained only when choosing identical bases in the transmitter and receiver are taken into account. In these cases, after the semi-transparent mirror 5, as a result of the interference of frequency modes of the signal and reference radiation pulses, output frequency modes are formed, which are sent to two single-photon detectors 7a and 7b. If the phases of the signal and reference frequency modes coincide, then the output “red” and “blue” modes are sent to detector 7a, whereas if the phases of the signal and reference frequency modes are shifted π, then the output modes are sent to detector 7b. Triggering of detectors 7a or 7b makes it possible to detect both phases of the signal low-photon radiation pulse (see Scheme 1).

The technical result is achieved due to the method of detecting both phases of a separate pulse of signal radiation attenuated to a single-photon level when the bases used in phase coding on the transmitter and receiver coincide, which differs from the phase detection methods used in known quantum key distribution systems.

Claims (1)

A method for detecting the phases of low-photon coherent light fields at side frequencies in a quantum key distribution system, which consists in detecting the result of mixing the side frequencies of a low-photon coherent pulse of light modes of signal radiation, which has encoded phases with modes of reference radiation, while the input radiation is encoded with one of the phases: ϕ = 0, π/2, π, 3π/2, so that the low-frequency and high-frequency modes acquire phases (-ϕ) and (+ϕ), and the radiation, which has an unshifted central frequency, enters the phase modulator of the receiver, where it is encoded by one of the random selected phases: 0, π/2, π, 3π/2, when choosing the same bases in the transmitter and receiver, if the phases of the signal and reference frequency modes coincide, then the output low-frequency and high-frequency modes are sent to one detector, whereas if the phases of the signal and the reference frequency modes are shifted by 180 degrees, then the output modes are sent to another detector.