CN114465725B - Quantum key distribution coding device - Google Patents
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Abstract
A quantum key distribution coding device comprises a polarization interferometer, a first phase modulator, a second phase modulator and a polarization beam splitting module, wherein a port a of the polarization beam splitting module is an input port of the coding device; the port B of the polarization beam splitting module and the port A of the polarization interferometer are respectively connected with the input port of the first phase modulator and the output port of the first phase modulator, and the port c of the polarization beam splitting module and the port B of the polarization interferometer are respectively connected with the input port of the second phase modulator and the output port of the second phase modulator, so that a Sagnac ring is formed. Compared with the prior art, the invention can realize polarization and phase combined coding, improve the information quantity carried by a single photon bit and further improve the safe code rate of the system. In addition, the preparation of X, Y, Z three groups of basic vectors can be realized, the free switching of BB84 protocol, six-state protocol and RFI protocol is supported, and the preparation of signal state and decoy state can be realized.
Description
Technical Field
The invention relates to the technical field of quantum phase encoding, in particular to a quantum key distribution encoding device.
Background
The quantum key distribution has information theory security, the security of the quantum key distribution is guaranteed by the basic principle of quantum mechanics, the threat from a quantum computer can be resisted, and the quantum key distribution has an important role in secret communication. The BB84 protocol was proposed as the first protocol in the Quantum Key Distribution (QKD) protocol, and with the development of theory and experiment, a plurality of protocols were proposed in succession, such as a six-state protocol, a reference frame independent protocol, etc., which have respective advantages in various application scenarios. The encoding states that can simultaneously implement the above-mentioned protocols are various superposition states of modes |0> and |1>, and generally include three sets of basis vectors, X basis, Y basis, and Z basis. The modes |0>, |1> may be the front and back time windows or H, V polarization states, respectively, corresponding to the time phase encoding or polarization encoding, respectively.
At present, for a specific protocol or a coding mode of the specific protocol, a corresponding optical path needs to be designed to implement the protocol or the coding mode, and most of the optical paths are not compatible with other protocols or coding modes. Therefore, the encoding apparatus lacks versatility, and cannot switch the protocol or encoding method according to the requirements of practical applications, so as to satisfy the optimization of system performance. According to the prior art, in order to realize the switching of protocols or coding modes, a plurality of special coding devices of the protocols or the coding modes are required to be cascaded, so that the complexity and the cost of a system are greatly increased.
Disclosure of Invention
Aiming at the defects in the prior art, the invention provides a quantum key distribution coding device.
The technical scheme of the invention is realized as follows:
a quantum key distribution coding device comprises a polarization interferometer, a first phase modulator, a second phase modulator and a polarization beam splitting module, wherein a port a of the polarization beam splitting module is an input port of the coding device; the port B of the polarization beam splitting module and the port A of the polarization interferometer are respectively connected with the input port of the first phase modulator and the output port of the first phase modulator, and the port c of the polarization beam splitting module and the port B of the polarization interferometer are respectively connected with the input port of the second phase modulator and the output port of the second phase modulator to form a Sagnac ring; the port d of the polarization beam splitting module is used for outputting a coding state; the polarization beam splitting module is used for splitting a 45-degree linearly polarized light pulse input from a port a of the polarization beam splitting module into two pulse components with the same amplitude, respectively emitting from a port b and a port c of the polarization beam splitting module, and respectively transmitting the two pulse components in the Sagnac ring along the counterclockwise direction and the clockwise direction; the polarization interferometer is used for generating front and rear sub-pulses with mutually vertical polarization and time difference t; the first phase modulator and the second phase modulator are respectively used for modulating the phase of an optical pulse component which propagates in the Sagnac ring along the anticlockwise direction and the clockwise direction; the optical fibers in the coding device are all polarization maintaining optical fibers; the TE mode of the first phase modulator is aligned with the slow axis of the polarization-maintaining optical fiber of the input port of the first phase modulator, and the included angle of the TE mode of the first phase modulator and the slow axis of the polarization-maintaining optical fiber of the output port of the first phase modulator is 45 degrees; the TE mode of the second phase modulator is in a direction aligned with the fast axis of the polarization-maintaining optical fiber of the input port of the second phase modulator, and the included angle of the TE mode and the slow axis of the polarization-maintaining optical fiber of the output port of the second phase modulator is 45 degrees.
Preferably, the polarization beam splitting module comprises a circulator and a first polarization beam splitter, and the port a of the circulator is used as the port a of the polarization beam splitting module; the port b of the circulator is connected with the port a of the first polarization beam splitter; the port b and the port c of the first polarization beam splitter are respectively used as the port b and the port c of a polarization beam splitting module; and the output polarization-maintaining optical fiber of the port c of the circulator is welded at 45 degrees to be used as a port d of the polarization beam splitting module.
Preferably, the polarization beam splitting module is a fourth polarization beam splitter, and ports a-c of the fourth polarization beam splitter are respectively used as ports a-c of the polarization beam splitting module; and the output polarization-maintaining optical fiber of the port d of the fourth polarization beam splitter is welded at 45 degrees to be used as the port d of the polarization beam splitting module.
Preferably, the polarization beam splitting module comprises a first beam splitter, a first polarizer, a second polarizer and a third polarizer, and the port a of the first beam splitter is used as the port a of the polarization beam splitting module; the ports b-d of the first beam splitter are respectively connected with the input ports of the second polarizer, the third polarizer and the first polarizer, and the polarization maintaining optical fiber between the port d of the first beam splitter and the first polarizer is welded at 45 degrees; the output ports of the second polarizer, the third polarizer and the first polarizer are respectively used as ports b-d of the polarization beam splitting module; the polarization directions of the first polarizer and the second polarizer are aligned with the horizontal polarization direction, and the polarization direction of the third polarizer is aligned with the vertical polarization direction.
Preferably, the polarization beam splitting module includes a second beam splitter and a fourth polarizer, and a port d of the second beam splitter is connected to an input port of the fourth polarizer after being fused at 45 ° by a polarization maintaining fiber; the ports a-c of the second beam splitter are respectively used as the ports a-c of the polarization beam splitting module, and the output port of the fourth polarizer is used as the port d of the polarization beam splitting module; the first phase modulator and the second phase modulator both operate with a single polarization.
Preferably, the polarization interferometer comprises a second polarization beam splitter and a third polarization beam splitter, each of which comprises one input port and two output ports; the input ports of the second polarization beam splitter and the third polarization beam splitter are respectively used as a port A and a port B of the polarization interferometer; and the two output ports of the second polarization beam splitter and the third polarization beam splitter are respectively connected through a long-arm optical fiber and a short-arm optical fiber to form the unequal-arm Mach-Zehnder interferometer.
Preferably, the polarization interferometer includes a fifth polarization beam splitter, a first faraday mirror and a second faraday mirror, the fifth polarization beam splitter includes two input ports and two output ports, the two input ports of the fifth polarization beam splitter are respectively used as a port a and a port B of the polarization interferometer, and the two output ports of the fifth polarization beam splitter are respectively connected with the first faraday mirror and the second faraday mirror through two optical fibers with different lengths.
Preferably, the polarization interferometer is a sixth polarization beam splitter, the sixth polarization beam splitter includes two input ports and two output ports, one input port and one output port of the sixth polarization beam splitter are respectively used as the port a and the port B of the polarization interferometer, and the other input port and the other output port of the sixth polarization beam splitter are directly connected through a polarization-maintaining fiber.
Preferably, the polarization interferometer is a high birefringence polarization-maintaining fiber with a length L
Compared with the prior art, the invention has the following beneficial effects:
the invention provides a quantum key distribution coding device, which adopts a structure of combining a Sagnac ring and a polarization interferometer 100, can realize polarization and phase combined coding by respectively carrying out polarization modulation on two time window modes, improves the information quantity carried by a single photon bit, and further improves the safe code rate of a system. In addition, a polarizer is added at an output port of the encoding device, so that preparation of X, Y, Z three groups of basis vectors can be realized, free switching of a BB84 protocol, a six-state protocol and an RFI protocol is supported, and preparation of a signal state and a decoy state can be realized. Not only can realize stable coding, but also can finish the stable preparation of signal state and decoy state simultaneously, need not extra intensity modulator and phase compensation, and two time mode losses are unanimous, have promoted the stability and the practicality of system.
Drawings
Fig. 1 is a schematic block diagram of the structure of a quantum key distribution encoding apparatus according to the present invention;
FIG. 2 is a schematic block diagram of a first embodiment of a quantum key distribution encoding apparatus of the present invention;
FIG. 3 is a schematic block diagram of a second embodiment of a quantum key distribution encoding apparatus of the present invention;
FIG. 4 is a schematic block diagram of a third embodiment of a quantum key distribution encoding apparatus of the present invention;
fig. 5 is a schematic block diagram of a fourth embodiment of the quantum key distribution encoding apparatus of the present invention.
In the figure: the polarization beam splitter comprises a polarization interferometer-100, a second polarization beam splitter-110, a third polarization beam splitter-120, a fifth polarization beam splitter-130, a first Faraday mirror-140, a second Faraday mirror-150, a sixth polarization beam splitter-160, a first phase modulator-200, a second phase modulator-300, a polarization beam splitting module-400, a first polarization beam splitter-410, a circulator-420, a fourth polarization beam splitter-430, a first polarizer-440, a second polarizer-450, a first beam splitter-460, a third polarizer-470, a fourth polarizer-480 and a second beam splitter-490.
Detailed Description
The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which embodiments of the invention are shown.
As shown in fig. 1, an encoding apparatus (hereinafter referred to as encoding apparatus) for quantum key distribution includes a polarization interferometer 100, a first phase modulator 200, a second phase modulator 300, and a polarization beam splitting module 400, where a port a of the polarization beam splitting module 400 is an input port of the encoding apparatus; the port B of the polarization beam splitting module 400 and the port a of the polarization interferometer 100 are respectively connected with the input port and the output port of the first phase modulator 200, and the port c of the polarization beam splitting module 400 and the port B of the polarization interferometer 100 are respectively connected with the input port and the output port of the second phase modulator 300 to form a sagnac loop; the port d of the polarization beam splitting module 400 is used for outputting a coding state; the polarization beam splitting module 400 is configured to split a 45 ° linearly polarized light pulse input from a port a thereof into two pulse components with the same amplitude, exit from a port b and a port c thereof, and propagate in the sagnac loop along counterclockwise and clockwise directions, respectively; the polarization interferometer 100 is used for generating front and back sub-pulses with mutually perpendicular polarization and time difference t; the first phase modulator 200 and the second phase modulator 300 are respectively used for modulating the phase of an optical pulse component propagating in the sagnac loop along the anticlockwise direction and the clockwise direction; the optical fibers in the coding device are all polarization maintaining optical fibers; the TE mode of the first phase modulator 200 is aligned with the slow axis of the polarization maintaining optical fiber of the input port, and the included angle of the TE mode and the slow axis of the polarization maintaining optical fiber of the output port is 45 degrees; the TE mode of second phase modulator 300 is oriented to align with the fast axis of the polarization maintaining fiber at the input port and has an included angle of 45 ° with the slow axis of the polarization maintaining fiber at the output port.
The specific encoding process is as follows:
the 45 ° linearly polarized light pulse P0 enters from the input port of the encoding device, first enters the port a of the polarization splitting module 400, and is split into a vertical polarization component P1 and a horizontal polarization component P2. The P1 component exits from port B of the polarization beam splitting module 400, propagates along the sagnac loop in a clockwise direction, does not phase-modulate when passing through the first phase modulator 200, changes to 45-degree polarization when exiting from the first phase modulator 200, enters from port a of the polarization interferometer 100, outputs from port B, forms two front and rear sub-pulses P11 and P12 with a time difference of t, and respectively modulates the phase through the second phase modulator 300 in sequence
And
and then enters the port c of the polarization splitting module 400. The P2 component exits from port c of the polarization beam splitting module 400, propagates along the Sagnac loop in a counterclockwise direction, is not phase-modulated when passing through the second phase modulator 300, is changed into 45-degree polarization when exiting from the second phase modulator 300, enters from port B of the polarization interferometer 100, is output from port A to form front and rear sub-pulses P21 and P22, and respectively modulates the phase through the first phase modulator 200 in sequence
And
also with a time difference T, finally in sequenceEnters the port b of the polarization splitting module 400. P11 and P21 have the same propagation time, and exit from port d of the polarization beam splitting module 400, the polarizations of the two are perpendicular to each other, and the two are combined into a first polarization pulse through the polarization beam splitting module 400 and located in the previous time window |0>(ii) a P12 and P22 also have the same propagation time, and exit from port d of the polarization beam splitting module 400, the polarizations of the two are perpendicular to each other, and the two are combined into a second polarization pulse through the polarization beam splitting module 400 and located in the subsequent time window |1>. And the time difference between the first polarized pulse and the second polarized pulse is T, and the time difference is output from an output port of the encoding device.
Wherein P11 and P21 respectively propagate in the Sagnac ring along the counterclockwise direction and the clockwise direction, the optical paths of the P11 and the P21 are the same, the phase difference is only related to the phase modulated by the phase modulator, namely
The polarization state of the synthesized first polarized pulse is
Finally, after 45 DEG rotation, the polarization state of the first polarized pulse is changed into
P12 and P22 propagate in the Sagnac loop in the counter-clockwise and clockwise directions, respectively, with a phase difference of
The polarization state of the synthesized second polarized pulse is
After a 45 ° rotation before exiting from port d of the polarization beam splitting module 400, the polarization state changes to
By controlling the phase difference
And
when the polarization states are respectively 0, pi/2 and 3 pi/2, 4 different polarization states | V can be obtained>、|H>、|R>、|L>. Meanwhile, the phase difference between the first polarization pulse and the second polarization pulse is controlled to be 0, pi/2, pi and 3 pi/2 respectively, and polarization and phase composite encoding can be realized when the polarization states of the first polarization pulse and the second polarization pulse are modulated to be the same. The phase and code states modulated by P11, P12, P21, and P22 as they pass through first phase modulator 200 and second phase modulator 300, respectively, are shown in Table 1. Wherein |0>Represents the previous temporal pattern, |1>Indicating the latter time mode, the subscripts are polarization states.
TABLE 1 polarization and phase composite encoding table
If the port d of the polarization beam splitting module 400 passes through a polarizer before exiting, the polarizer works for the slow axis of the polarization maintaining fiber and is used for filtering the polarized light propagating along the fast axis of the polarization maintaining fiber through the polarized light propagating along the slow axis of the polarization maintaining fiber.
The Jones matrix of the polarizer is
Thus, the polarization state of the first polarized pulse after passing through the polarizer becomes horizontally polarized:
corresponding light intensity becomes
Similarly, the light intensity of the second polarized pulse after passing through the polarizer becomes
By modulating the first phase modulator 200 and the second phase modulator 300, the light intensities of the first polarized pulse and the second polarized pulse and the phase difference between the first polarized pulse and the second polarized pulse can be controlled respectively to prepare the coding states under the X base, the Y base and the Z base, and simultaneously prepare a signal state and a decoy state.
When signal states are to be prepared, the phase and code states modulated by P11, P12, P21, and P22 passing through first phase modulator 200 and second phase modulator 300, respectively, are shown in Table 2.
TABLE 2 coding states of X, Y and Z bases in signal state
When it is desired to prepare a decoy state with intensity u, the phase and code states modulated by P11, P12, P21 and P22 passing through first phase modulator 200 and second phase modulator 300, respectively, are shown in table 3.
TABLE 3 coded states of X-, Y-and Z-base in decoy state
Therefore, the encoding device can prepare encoding states under X base, Y base and Z base, and signal states and decoy states, can support free switching of BB84 protocol, six-state protocol and reference system independent protocol, and has stable encoding and no need of additional intensity modulator to prepare decoy states.
As shown in fig. 2, a first embodiment of the encoding apparatus of the present invention:
the structure of the coding device is as follows: the polarization beam splitting module 400 comprises a circulator 420 and a first polarization beam splitter 410, wherein a port a of the circulator 420 is used as a port a of the polarization beam splitting module 400; the port b of the circulator 420 is connected with the port a of the first polarization beam splitter 410; the port b and the port c of the first polarization beam splitter 410 are respectively used as the port b and the port c of the polarization beam splitting module 400; the output polarization maintaining fiber at port c of the circulator 420 is fused at 45 ° as port d of the polarization beam splitting module 400. The polarization interferometer 100 includes a second polarization beam splitter 110 and a third polarization beam splitter 120, and each of the second polarization beam splitter 110 and the third polarization beam splitter 120 includes an input port and two output ports; the input ports of the second polarization beam splitter 110 and the third polarization beam splitter 120 are respectively used as a port a and a port B of the polarization interferometer 100; the two output ports of the second polarization beam splitter 110 and the third polarization beam splitter 120 are connected through a long-arm optical fiber and a short-arm optical fiber respectively to form an unequal-arm mach-zehnder interferometer.
In one embodiment, the method can realize polarization phase composite encoding, and includes the following steps:
the 45 ° linearly polarized light pulse P0 enters from the input port of the encoding apparatus, first enters port a of the circulator 420, and enters port a of the first polarization beam splitter 410 after being output from port b, and is split into a vertical polarization component P1 and a horizontal polarization component P2. The P1 component exits from the port b of the first polarization beam splitter 410, propagates along the sagnac loop in a clockwise direction, is not phase-modulated when passing through the first phase modulator 200, is changed into 45-degree polarization when exiting from the first phase modulator 200, then enters the unequal arm mach-zehnder interferometer from the input port of the second polarization beam splitter 110, is output from the input port of the third polarization beam splitter 120 to form a front sub-pulse P11 and a rear sub-pulse P12 with a time difference of t, and respectively modulates the phase through the second phase modulator 300 in sequence
And
and then into port c of the first polarizing beam splitter 410. The P2 component exits from the port c of the first polarization beam splitter 410, propagates along the Sagnac loop in the counterclockwise direction, is not phase-modulated when passing through the second phase modulator 300, is changed into 45-degree polarization when exiting from the second phase modulator 300, then enters the unequal arm Mach-Zehnder interferometer from the input port of the third polarization beam splitter 120, is output from the input port of the second polarization beam splitter 110 to form a front sub-pulse P21 and a rear sub-pulse P22, and respectively modulates the phases through the first phase modulator 200 in sequence
And
and also with the time difference t, and finally enters the port b of the first polarization beam splitter 410 one after the other. P11 and P21 have the same propagation time, and exit from port a of the first polarization beam splitter 410, the polarizations of the two are perpendicular to each other, and the two are combined into a first polarization pulse through the first polarization beam splitter 410 and positioned in the previous time window |0>(ii) a P12 and P22 also have the same propagation time, and exit from the port a of the first polarization beam splitter 410, the polarizations of the two are perpendicular to each other, and the two are combined into a second polarization pulse through the first polarization beam splitter 410, and the second polarization pulse is located in the subsequent time window |1>. And the time difference between the first polarization pulse and the second polarization pulse is t, and the first polarization pulse and the second polarization pulse are output from an output port of the encoding device after sequentially passing through the circulator 420 and rotating the polarization by 45 degrees.
Wherein P11 and P21 respectively propagate in the Sagnac ring along the counterclockwise direction and the clockwise direction, the optical paths of the P11 and the P21 are the same, the phase difference is only related to the phase modulated by the phase modulator, namely
. Similarly, P12 and P22 propagate in the Sagnac loop counterclockwise and clockwise, respectively, with a phase difference of
. By controlling the phase difference
And
when the polarization states are respectively 0, pi/2 and 3 pi/2, 4 different polarization states | V can be obtained>、|H>、|R>、|L>. Meanwhile, the phase difference between the first polarization pulse and the second polarization pulse is controlled to be 0, pi/2, pi and 3 pi/2 respectively, and polarization and phase composite encoding can be realized when the polarization states of the first polarization pulse and the second polarization pulse are modulated to be the same. The phase and code states modulated by P11, P12, P21, and P22 as they pass through first phase modulator 200 and second phase modulator 300, respectively, are shown in Table 1.
As shown in fig. 3, the second embodiment of the encoding apparatus of the present invention:
the structure of the coding device is as follows: the polarization beam splitting module 400 is a fourth polarization beam splitter 430, and ports a-c of the fourth polarization beam splitter 430 are respectively used as ports a-c of the polarization beam splitting module 400; the output polarization maintaining fiber of the port d of the fourth polarization beam splitter 430 is welded at 45 ° as the port d of the polarization beam splitting module 400. The polarization interferometer 100 includes a fifth polarization beam splitter 130, a first faraday mirror 140 and a second faraday mirror 150, the fifth polarization beam splitter 130 includes two input ports and two output ports, the two input ports of the fifth polarization beam splitter 130 are respectively used as a port a and a port B of the polarization interferometer 100, and the two output ports of the fifth polarization beam splitter 130 are respectively connected with the first faraday mirror 140 and the second faraday mirror 150 through two optical fibers with different lengths.
The second encoding process of the embodiment comprises the following steps:
a 45 ° linearly polarized light pulse P0 enters from the input port of the encoding device, first enters port a of the fourth polarization beam splitter 430, and is split into a vertical polarization component P1 and a horizontal polarization component P2. Where the P1 component exits port b of fourth polarization splitter 430, propagates along the polarization maintaining fiber slow axis, passes through first phase modulator 200 without phase modulation, exits first phase modulator 200 with a 45 ° polarization, enters the input port of fifth polarization splitter 130, and is split into vertical componentsThe straight polarization component P11 and the horizontal polarization component P12 are respectively emitted from two output ports of the fifth polarization beam splitter 130, reflected by the first faraday mirror and the second faraday mirror and then output from the other input port of the fifth polarization beam splitter 130, the polarizations are mutually perpendicular, the time difference is t, and the phase modulation signals are respectively modulated by the second phase modulator 300 in sequence
And
and finally into port c of the fourth polarizing beam splitter 430. The P2 component exits from the port c of the fourth polarization beam splitter 430, propagates along the Sagnac loop in a counterclockwise direction, does not modulate phase when passing through the second phase modulator 300, changes into 45-degree polarization when exiting from the second phase modulator 300, then enters the other input port of the fifth polarization beam splitter 130, is decomposed into a vertical polarization component P21 and a horizontal polarization component P22, exits from the two output ports of the fifth polarization beam splitter 130, respectively, is reflected by the first Faraday mirror and the second Faraday mirror, then is output from the input port of the fifth polarization beam splitter 130, the polarizations are perpendicular to each other with time difference of t, and are modulated by the first phase modulator 200 in sequence to modulate the phase, respectively
And
and finally into port b of the fourth polarization beam splitter 430. P11 and P21 have the same propagation time, and exit from port d of the fourth polarization beam splitter 430, the polarizations of the two are perpendicular to each other, and the two are combined into a first polarization pulse through the fourth polarization beam splitter 430, and the first polarization pulse is in the previous time window |0>(ii) a P12 and P22 also have the same propagation time, and exit from port d of the fourth polarization beam splitter 430, the polarizations of the two are perpendicular to each other, and the two are combined into a second polarization pulse through the fourth polarization beam splitter 430, and the second polarization pulse is located in the subsequent time window |1>. The time difference between the first polarized pulse and the second polarized pulse is t, and the slave encoder rotates the polarization by 45 degreesAnd the output port of the code device outputs.
Wherein P11 and P21 respectively propagate in the Sagnac ring along the counterclockwise direction and the clockwise direction, the optical paths of the P11 and the P21 are the same, the phase difference is only related to the phase modulated by the phase modulator, namely
. Similarly, P12 and P22 propagate in the Sagnac loop counterclockwise and clockwise, respectively, with a phase difference of
. By controlling the phase difference
And
when the polarization states are respectively 0, pi/2 and 3 pi/2, 4 different polarization states | V can be obtained>、|H>、|R>、|L>. Meanwhile, the phase difference between the first polarization pulse and the second polarization pulse is controlled to be 0, pi/2, pi and 3 pi/2 respectively, and polarization and phase composite encoding can be realized when the polarization states of the first polarization pulse and the second polarization pulse are modulated to be the same. The phase and code states modulated by P11, P12, P21, and P22 as they pass through first phase modulator 200 and second phase modulator 300, respectively, are shown in Table 1.
As shown in fig. 4, a third embodiment of the encoding apparatus of the present invention:
the structure of the coding device is as follows: the polarization beam splitting module 400 comprises a first beam splitter 460, a first polarizer 440, a second polarizer 450 and a third polarizer 470, wherein the port a of the first beam splitter 460 is used as the port a of the polarization beam splitting module 400; the ports b-d of the first beam splitter 460 are respectively connected with the input ports of the second polarizer 450, the third polarizer 470 and the first polarizer 440, wherein the port d of the first beam splitter 460 is welded with the polarization maintaining fiber between the first polarizer 440 by 45 degrees; the output ports of the second polarizer 450, the third polarizer 470 and the first polarizer 440 are respectively used as ports b-d of the polarization beam splitting module 400; the polarization directions of the first polarizer 440 and the second polarizer 450 are aligned with the horizontal polarization direction, and the polarization direction of the third polarizer 470 is aligned with the vertical polarization direction. The polarization interferometer 100 is a sixth polarization beam splitter 160, the sixth polarization beam splitter 160 includes two input ports and two output ports, one input port and one output port of the sixth polarization beam splitter 160 are respectively used as the port a and the port B of the polarization interferometer 100, and the other input port and the other output port of the sixth polarization beam splitter 160 are directly connected through a polarization-maintaining fiber.
The third encoding process of the embodiment comprises the following steps:
a 45 ° linearly polarized light pulse P0 enters from the input port of the encoding device, first enters port a of the first beam splitter 460, and is split into two pulse components P1 and P2 of equal intensity, both in the 45 ° polarization state. Wherein the P1 component exits port b of the first beam splitter 460, passes through the second polarizer 450, transmits only the horizontal polarization component, propagates along the polarization maintaining fiber slow axis, passes through the first phase modulator 200 without phase modulation, exits the first phase modulator 200 with 45 ° polarization, and then enters the input port of the sixth polarization beam splitter 160 to be split into the vertical polarization component P11 and the horizontal polarization component P12. Where P11 exits directly from one output port of the sixth polarization beam splitter 160 and propagates along the slow axis of the polarization maintaining fiber. P12 exits from another output port of the sixth polarization beam splitter 160, enters another input port along the polarization maintaining fiber, exits from the output port, and propagates along the fast axis of the polarization maintaining fiber. The two polarizations are perpendicular to each other with time difference t, and are sequentially modulated in phase by a second phase modulator 300
And
and enters the port c of the first beam splitter 460 after passing through the third polarizer 470. The P2 component exits port c of the first beam splitter 460, passes through the third polarizer 470, transmits only the vertically polarized component, propagates along the polarization maintaining fiber slow axis, passes through the second phase modulator 300 without phase modulation, exits the second phase modulator 300 with 45 ° polarization, and then enters the sixth phase modulator 300The input port of the polarization beam splitter 160 is decomposed into a vertical polarization component P21 and a horizontal polarization component P22. Where P21 exits directly from one output port of the sixth polarization beam splitter 160 and propagates along the slow axis of the polarization maintaining fiber. P22 exits from another output port of the sixth polarization beam splitter 160, enters another input port along the polarization maintaining fiber, exits from the output port, and propagates along the fast axis of the polarization maintaining fiber. The two polarizations are perpendicular to each other with time difference t, and then pass through the second phase modulator 300 to modulate the phases respectively
And
and enters the port b of the first beam splitter 460 after passing through the second polarizer 450.
The P11 and the P21 have the same propagation time, and simultaneously exit from the port d of the first beam splitter 460, the polarizations of the two are perpendicular to each other, and the two are combined into a first polarization pulse through the first beam splitter 460, and the first polarization pulse is located in the previous time window |0 >; p12 and P22 also have the same propagation time, and exit from port d of the first beam splitter 460, with their polarizations perpendicular to each other, and are combined into a second polarization pulse via the first beam splitter 460, which is located in the subsequent time window |1 >. The time difference between the first polarized pulse and the second polarized pulse is t, and the first polarized pulse and the second polarized pulse are output from the output port of the encoding apparatus after being subjected to polarization rotation by 45 ° and the first polarizer 440.
By modulating the first phase modulator 200 and the second phase modulator 300, the light intensity of the first polarized pulse and the second polarized pulse and the phase difference between the two can be controlled respectively, the coding states under the X base, the Y base and the Z base can be prepared, and simultaneously, the signal state and the decoy state can be prepared. The phase and code states modulated by P11, P12, P21, and P22 when passing through first phase modulator 200 and second phase modulator 300, respectively, in the case of the preparation signal state and the decoy state are shown in tables 2 and 3, respectively.
As shown in fig. 5, the fourth embodiment of the encoding apparatus of the present invention:
the structure of the coding device is as follows: the polarization beam splitting module 400 comprises a second beam splitter 490 and a fourth polarizer 480, wherein a port d of the second beam splitter 490 is connected with an input port of the fourth polarizer 480 through a polarization maintaining fiber after being welded at 45 degrees; the ports a-c of the second beam splitter 490 are respectively used as the ports a-c of the polarization beam splitting module 400, and the output port of the fourth polarizer 480 is used as the port d of the polarization beam splitting module 400; both first phase modulator 200 and second phase modulator 300 operate with a single polarization. The polarization interferometer 100 is a high birefringence polarization maintaining fiber with a length L.
An embodiment four encoding process comprises:
a 45 ° linearly polarized light pulse P0 enters from the input port of the encoding device, first enters port a of the second beam splitter 490, splitting into two pulse components P1 and P2 of equal intensity, both in the 45 ° polarization state. Wherein the P1 component exits from port b of the second beam splitter 490, since the first phase modulator 200 operates with a single polarization, only light propagating along the slow axis of the polarization maintaining fiber can pass through the first phase modulator 200, without phase modulation during passing, and when exiting from the first phase modulator 200, the polarization is rotated 45 ° into the high birefringent polarization maintaining fiber with length L, and the polarization is decomposed into a vertical polarization component P11 and a horizontal polarization component P12, which propagate along the slow axis and the fast axis of the polarization maintaining fiber, respectively. Since the fast and slow axes have a refractive index difference Δ n, the time difference t = Δ n × L/c (c is the speed of light in vacuum) after P11 and P12 pass through the polarization maintaining fiber of length L. The two are respectively phase-modulated by a second phase modulator 300
And
and since second phase modulator 300 operates for a single polarization, only light propagating along the fast axis of the polarization maintaining fiber may pass through second phase modulator 300 and finally enter port c of second beam splitter 490. The P2 component exits from port c of second beam splitter 490, and since second phase modulator 300 operates for a single polarization, only light propagating along the fast axis of the polarization maintaining fiber can pass through second phase modulator 300 without phase modulation, and upon exiting second phase modulator 300, polarization is rotated 45 degrees into a high birefringence polarization maintaining fiber of length L, which is resolved into vertically polarized lightComponent P21 and horizontal polarization component P22, travel along the polarization maintaining fiber slow and fast axes, respectively. Due to the refractive index difference Δ n between the fast and slow axes, the time difference t = Δ n × L/c after P11 and P12 pass through the polarization maintaining fiber of length L. The two are respectively phase-modulated by a first phase modulator 200
And
and since first phase modulator 200 operates for a single polarization, only light propagating along the slow axis of the polarization maintaining fiber may pass through first phase modulator 200 and finally enter port b of second beam splitter 490.
The P11 and the P21 have the same propagation time, and simultaneously exit from the port d of the second beam splitter 490, the polarizations of the two are perpendicular to each other, and the two are combined into a first polarization pulse through the second beam splitter 490, and the first polarization pulse is located in the previous time window |0 >; the P12 and the P22 also have the same propagation time, and exit from the port d of the second beam splitter 490, the polarizations of the two are perpendicular to each other, and the two are combined into a second polarization pulse through the second beam splitter 490, and the second polarization pulse is located in the subsequent time window |1 >. The time difference between the first polarized pulse and the second polarized pulse is t, and the first polarized pulse and the second polarized pulse are output from the output port of the encoding apparatus after passing through the polarization rotation of 45 ° and the fourth polarizer 480.
By modulating the first phase modulator 200 and the second phase modulator 300, the light intensities of the first polarized pulse and the second polarized pulse and the phase difference between the first polarized pulse and the second polarized pulse can be controlled respectively, the coding states under the X base, the Y base and the Z base can be prepared, and meanwhile, the signal state and the decoy state can be prepared. The phase and code states modulated by P11, P12, P21, and P22 when passing through first phase modulator 200 and second phase modulator 300, respectively, in the case of the preparation signal state and the decoy state are shown in tables 2 and 3, respectively.
The invention also discloses a transmitting end of the quantum key distribution system, which comprises a laser, a coding device and an adjustable attenuator, wherein an input port and an output port of the coding device are respectively connected with the laser and the adjustable attenuator, the laser is used for generating optical pulses, the coding device is used for coding of various protocols and generating coded pulses, and the adjustable attenuator is used for attenuating the coded pulses to a single photon magnitude.
According to the embodiments of the present invention, the invention provides a quantum key distribution encoding device, which adopts a structure combining a sagnac loop and a polarization interferometer 100, and performs polarization modulation on two time window modes respectively, so as to realize polarization and phase joint encoding, improve the information amount carried by a single photon bit, and further improve the safe code rate of a system. In addition, a polarizer is added at an output port of the encoding device, so that preparation of X, Y, Z three groups of basis vectors can be realized, free switching of a BB84 protocol, a six-state protocol and an RFI protocol is supported, and preparation of a signal state and a decoy state can be realized. Not only can realize stable coding, but also can finish the stable preparation of signal state and decoy state simultaneously, need not extra intensity modulator and phase compensation, and two time mode losses are unanimous, have promoted the stability and the practicality of system.
Claims (9)
1. The quantum key distribution coding device is characterized by comprising a polarization interferometer (100), a first phase modulator (200), a second phase modulator (300) and a polarization beam splitting module (400), wherein a port a of the polarization beam splitting module (400) is an input port of the coding device; a port B of the polarization beam splitting module (400) and a port A of the polarization interferometer (100) are respectively connected with an input port of the first phase modulator (200) and an output port of the first phase modulator (200), and a port c of the polarization beam splitting module (400) and a port B of the polarization interferometer (100) are respectively connected with an input port of the second phase modulator (300) and an output port of the second phase modulator (300) to form a Sagnac ring; the port d of the polarization beam splitting module (400) is used for outputting a coding state; the polarization beam splitting module (400) is used for splitting a 45-degree linearly polarized light pulse input from a port a of the polarization beam splitting module into two pulse components with the same amplitude, respectively emitting the two pulse components from a port b and a port c of the polarization beam splitting module, and respectively transmitting the two pulse components in the Sagnac ring along the counterclockwise direction and the clockwise direction; the polarization beam splitting module (400) is also used for combining the pulses incident to the port b and the port c thereof, and carrying out polarization combination and polarization rotation; the polarization interferometer (100) is used for generating front and back sub-pulses with mutually perpendicular polarization and time difference t; the first phase modulator (200) and the second phase modulator (300) are respectively used for modulating the phase of an optical pulse component propagating in the Sagnac ring along the anticlockwise direction and the clockwise direction; the optical fibers in the coding device are all polarization maintaining optical fibers; the TE mode of the first phase modulator (200) is aligned with the slow axis of the polarization-maintaining optical fiber of the input port of the first phase modulator, and the included angle of the TE mode of the first phase modulator and the slow axis of the polarization-maintaining optical fiber of the output port of the first phase modulator is 45 degrees; the TE mode of the second phase modulator (300) is aligned with the fast axis of the polarization-maintaining optical fiber of the input port of the second phase modulator, the included angle of the TE mode with the slow axis of the polarization-maintaining optical fiber of the output port of the second phase modulator is 45 degrees, and the encoding process of the quantum key distribution encoding device is as follows:
s1: the 45-degree linearly polarized light pulse enters a port a of the polarization beam splitting module (400), is split into a vertical polarization component P1 and a horizontal polarization component P2, and is emitted from a port b and a port c of the polarization beam splitting module (400) respectively;
s2: the vertical polarization component P1 propagates along the Sagnac loop in the clockwise direction, forms a front sub-pulse P11 and a rear sub-pulse P12 after passing through a first phase modulator (200) and a polarization interferometer (100), and respectively modulates the phase through a second phase modulator 300
And phase
Sequentially enters the ports c of the polarization beam splitting module (400);
s3: the horizontal polarization component P2 propagates along the Sagnac loop in the counterclockwise direction, forms a front sub-pulse P21 and a rear sub-pulse P22 after passing through a second phase modulator (200) and a polarization interferometer (100), and respectively modulates the phase through a first phase modulator (300)
And phase
Sequentially enters the ports b of the polarization beam splitting module (400);
s4: the sub-pulse P11 and the sub-pulse P21 simultaneously exit from the port d of the polarization beam splitting module (400), the polarization directions of the sub-pulse P11 and the sub-pulse P21 are perpendicular to each other, and then the sub-pulse P11 and the sub-pulse P21 are combined into a first polarization pulse by the polarization beam splitting module (400) and perform 45-degree polarization rotation; the sub-pulse P12 and the sub-pulse P22 simultaneously exit from the port d of the polarization beam splitting module (400), the polarization directions of the sub-pulse P12 and the sub-pulse P22 are perpendicular to each other, and then the sub-pulse P12 and the sub-pulse P22 are combined into a second polarization pulse by the polarization beam splitting module (400) and subjected to 45-degree polarization rotation;
s5: by modulating different phases
Phase of
Phase of
And phase
And realizing the polarization state modulation of the first polarization pulse and the second polarization pulse, and finally outputting the corresponding coding state from the port d of the polarization beam splitting module (400).
2. The quantum key distribution encoding apparatus according to claim 1, wherein the polarization beam splitting module (400) comprises a circulator (420) and a first polarization beam splitter (410), and a port a of the circulator (420) is used as a port a of the polarization beam splitting module (400); the port b of the circulator (420) is connected with the port a of the first polarization beam splitter (410); the port b and the port c of the first polarization beam splitter (410) are respectively used as the port b and the port c of a polarization beam splitting module (400); and the output polarization-maintaining optical fiber of the port c of the circulator (420) is welded at 45 degrees to be used as the port d of the polarization beam splitting module (400).
3. The quantum key distribution encoding apparatus according to claim 1, wherein the polarization beam splitting module (400) is a fourth polarization beam splitter (430), and ports a-c of the fourth polarization beam splitter (430) are respectively the ports a-c of the polarization beam splitting module (400); and the output polarization-maintaining optical fiber of the port d of the fourth polarization beam splitter (430) is welded at 45 degrees to be used as the port d of the polarization beam splitting module (400).
4. The quantum key distribution encoding apparatus of claim 1, wherein the polarization beam splitting module (400) comprises a first beam splitter (460), a first polarizer (440), a second polarizer (450), and a third polarizer (470), the first beam splitter (460) having port a as port a of the polarization beam splitting module (400); the ports b-d of the first beam splitter (460) are respectively connected with the input ports of the second polarizer (450), the third polarizer (470) and the first polarizer (440), wherein the port d of the first beam splitter (460) is welded with the polarization-maintaining optical fiber between the first polarizer (440) at an angle of 45 degrees; the output ports of the second polarizer (450), the third polarizer (470) and the first polarizer (440) are respectively used as ports b-d of the polarization beam splitting module (400); the polarization directions of the first polarizer (440) and the second polarizer (450) are aligned with the horizontal polarization direction, and the polarization direction of the third polarizer (470) is aligned with the vertical polarization direction.
5. The quantum key distribution encoding device according to claim 1, wherein the polarization beam splitting module (400) comprises a second beam splitter (490) and a fourth polarizer (480), and a port d of the second beam splitter (490) is connected with an input port of the fourth polarizer (480) after being fused by 45 degrees through a polarization-maintaining fiber; the ports a-c of the second beam splitter (490) are respectively used as the ports a-c of the polarization beam splitting module (400), and the output port of the fourth polarizer (480) is used as the port d of the polarization beam splitting module (400); the first phase modulator (200) and the second phase modulator (300) both operate with a single polarization.
6. The quantum key distribution encoding apparatus of claim 1 or 2 or 3 or 4 or 5, wherein the polarization interferometer (100) comprises a second polarization beam splitter (110) and a third polarization beam splitter (120), the second polarization beam splitter (110) and the third polarization beam splitter (120) each comprising one input port and two output ports; the input ports of the second polarization beam splitter (110) and the third polarization beam splitter (120) are respectively used as a port A and a port B of the polarization interferometer (100); two output ports of the second polarization beam splitter (110) and the third polarization beam splitter (120) are respectively connected through a long-arm optical fiber and a short-arm optical fiber to form an unequal-arm Mach-Zehnder interferometer.
7. The quantum key distribution encoding apparatus according to claim 1, 2, 3, 4 or 5, wherein the polarization interferometer (100) comprises a fifth polarization beam splitter (130), a first Faraday mirror (140) and a second Faraday mirror (150), the fifth polarization beam splitter (130) comprises two input ports and two output ports, the two input ports of the fifth polarization beam splitter (130) are respectively used as the port A and the port B of the polarization interferometer (100), and the two output ports of the fifth polarization beam splitter (130) are respectively connected with the first Faraday mirror (140) and the second Faraday mirror (150) through two optical fibers with different lengths.
8. The quantum key distribution encoding apparatus according to claim 1, 2, 3, 4 or 5, wherein the polarization interferometer (100) is a sixth polarization beam splitter (160), the sixth polarization beam splitter (160) includes two input ports and two output ports, one input port and one output port of the sixth polarization beam splitter (160) are respectively used as the port A and the port B of the polarization interferometer (100), and the other input port and the other output port of the sixth polarization beam splitter (160) are directly connected through a polarization-maintaining fiber.
9. The quantum key distribution encoding device of claim 1, 2, 3, 4 or 5, wherein the polarization interferometer (100) is a high birefringence polarization maintaining fiber with a length L.
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