RU2806780C1 - Device for quantum distribution of symmetrical bit sequence based on continuous variables using polarization combiner - Google Patents

Abstract

FIELD: photonic quantum communication systems.

SUBSTANCE: device comprises a sender unit comprising a coherent radiation source, a fibre optical isolator, a fibre amplitude modulator, a fibre beam splitter, fibre amplitude and phase modulators, a delay line, a fibre polarization combiner and a control and synchronization unit, and a receiver unit comprising a polarization controller, a polarization beam splitter, a delay line, a fibre beam splitter with two inputs and two outputs, a balanced detector, a control and synchronization unit, wherein the sender unit is connected to the receiver unit via a quantum channel and a synchronization channel.

EFFECT: providing the possibility of quantum distribution of a symmetric bit sequence based on continuous variables using a polarization combiner.

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Description

The present invention relates to optical technology, namely to photonic quantum communication systems.

A device for quantum distribution of a symmetric bit sequence based on continuous variables is known [Patent US 7929700 B2, priority date: 12/15/2005. MKI: H04L 9/0852], which is closest to what is described. The device consists of two blocks, a sender and a receiver. In the sender block, an optical signal is generated, the quadratures of which are distributed according to the normal law. The receiver block uses a homodyne method for detecting quadratures of the optical signal.

In the circuit of the described device, a Faraday mirror and a delay line are used to provide time multiplexing. When using a Faraday mirror, the complexity of the work arises, since there is a need to monitor its operation in real time. The present invention uses a delay line and a polarization combiner to provide a configuration of polarizations of the combined radiation such that the radiation remains linearly polarized and the polarizations are perpendicular to each other, thereby providing polarization multiplexing.

In terms of a simplified theoretical model, the operation of the system is described as follows:

- the sender, through phase and amplitude modulation, generates a quantum state, the quadratures of which are statistically distributed according to the normal law, as well as a classical state, which is used as a local oscillator;

- the sender sends states through a quantum channel;

- the receiver, through a polarizing fiber beam splitter, separates the quantum and classical states;

- performing detection using a homodyne detection system, the recipient extracts information about the quadrature of the quantum state;

- based on the obtained quadrature values, a symmetrical bit sequence will be generated.

The technical result is the use of a polarization combiner to provide polarization multiplexing. The polarization combiner replaces the Faraday mirror, which makes the circuit easier to implement and operate.

The claimed device is divided into sender and receiver blocks. The sender block contains a source of coherent radiation, an optical fiber isolator, an amplitude fiber modulator to provide a pulsed laser mode, a fiber beam splitter to separate the laser radiation into a signal arm and a local oscillator arm. The signal arm contains fiber amplitude and phase modulators, as well as a delay line to provide time multiplexing. The local oscillator arm and the signal arm are connected by a polarization combiner. The programmable logic integrated circuit (FPGA) contains interfaces for controlling modulators and a laser module. The SFP standard module is also connected to and controlled by the FPGA.

The receiver block contains, in addition to an FPGA similar to that installed in the sender block, a polarization controller, a polarization fiber beam splitter, and a delay line is installed in the arm of the local oscillator. To detect the signal, a homodyne detection system is installed in the receiver block.

Control user modules contain SFP modules. A low-pass filter is installed for the synchronization signal.

The method of work will be specified further and supported by mathematical calculations. The advantages of the invention will be realized and achieved by the technical implementation of the invention set forth in the written description and claims, as well as in the accompanying drawing.

Descriptions of the claimed device are presented in explanatory form and are aimed at explaining the basic principles of the invention and clarifying its differences from previously declared similar inventions.

Fig. 1 illustrates a functional diagram of the claimed QKD device.

The invention implements the generation, transmission and reception of quantum information contained in the signal state quadrature state via a fiber quantum channel. The device is designed in such a way that it can be assembled from standard telecommunications equipment. Below we consider the mathematical model of the system in a simplified classical form and the optical design.

The QKD device works as follows. The coherent radiation source generates a light beam passing through an optical fiber isolator that protects from reflected laser light. Then the radiation is modulated using an amplitude modulator, which sets the pulsed operating mode of the laser. The radiation is then split at a polarization beam splitter. Radiation in one arm, not subject to modulation, is used as a local oscillator (LO). Laser radiation in the second arm is modulated by fiber amplitude and phase modulators, which set random quadrature values distributed according to the normal law. Control signals to the fiber modulators are supplied through appropriate interfaces with the FPGA. Also, the radiation in this arm is delayed relative to the LO using a delay line, thus providing time multiplexing. Next, the radiation of the two arms is connected to a polarization combiner, which provides polarization multiplexing.

In the receiver block, the radiation passes through the polarization controller, it changes the polarization state for precise division on the polarization beam splitter so that the radiation of the local oscillator passes completely into one arm of the polarization fiber beam splitter, and the radiation of the quantum signal into the other. The polarization controller is controlled by an FPGA. A delay line is installed in the arm of the local oscillator. Next, the radiation of the local oscillator and the signal are confused at a beam splitter with two inputs and two outputs, at which interference of the signal and the local oscillator occurs. The result of interference is recorded by a homodyne detector. The detection result is further processed on the FPGA.

Mutual information “sender-receiver” in this case is evaluated as:

where μ is the homo-/heterodyning parameter;

SNR - signal-to-noise ratio;

T=T _ch T _det is the channel transmittance, T _ch is the transmittance of the channel itself, T _det =T _rec η _det is the product of the transmittance associated with optical losses in the receiver block and the efficiency of the balanced detector;

V _A - dispersion of the sender signal;

ξ - excess noise.

To synchronize users, the sender's FPGA sends a synchronization signal through its SFP module, which the recipient receives on its SFP module. The resulting synchronization signal passes through a low-pass filter and then arrives at the computer.

Claims (1)

A device for quantum distribution of a symmetric bit sequence based on continuous variables using a polarization combiner, containing a sender unit including a coherent radiation source connected via a fiber-optic path, an optical fiber isolator connected to the coherent radiation source, a fiber beam splitter connected to the fiber insulator, amplitude and phase modulators connected to the first of the fiber beam splitter arms, a delay line connected to the phase modulator, a fiber combiner connected to the second output of the fiber beam splitter and delay line, a control and synchronization unit connected to the fiber amplitude and phase modulators, and a block receiver containing a polarization controller, a polarization beam splitter connected to the polarization controller, a delay line connected to the first output of the polarization beam splitter, a two-input, two-output fiber beam splitter connected to the second output of the polarization beam splitter and a delay line, a balanced detector connected to the fiber beam splitter with two inputs and two outputs, a control and synchronization unit connected to a balanced detector and to a polarization controller.