CN114584224A - Quantum key distribution phase encoding device - Google Patents
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Abstract
A quantum key distribution phase coding device comprises a laser, an orthogonal polarization time window generating element, a first path selecting element, a second path selecting element, a third path selecting element, a half-wave plate and a quarter-wave plate, wherein the orthogonal polarization time window generating element is used for changing a single light pulse into two time windows with time interval t and sub-pulses with orthogonal polarization; the laser and the input port of the orthogonal polarization time window generating element are connected after being welded at 45 degrees through the polarization maintaining optical fiber. Compared with the prior art, the invention can prepare 4 phase coding states without a phase modulator, and the polarization of the two time window pulses is mutually vertical, so that the non-interference component can be eliminated to improve the energy utilization rate, and further the safe code rate of the system can be improved. The optical switch is adopted and only needs to be driven by a digital circuit, so that stable coding can be realized, the stability and the practicability of the system are improved, and the cost and the complexity of the coding device are reduced.
Description
Technical Field
The invention relates to the technical field of quantum phase encoding, in particular to a quantum key distribution phase encoding device.
Background
The phase encoding is a common encoding mode of a BB84 quantum key distribution protocol, an unequal arm Mach-Zehnder interferometer or a Faraday Michelson interferometer is adopted in a conventional phase encoding scheme, and a phase modulator is added in one arm of the unequal arm Mach-Zehnder interferometer or the Faraday Michelson interferometer to randomly modulate the phase difference of a long arm and a short arm, so that 4 phase encoding states are prepared. However, the phase modulator has high insertion loss and cost, so that the loss of two time windows before and after the coding state is inconsistent, and the system safety is influenced. And 4 voltages need to be loaded on the phase adjusting circuit for adjusting 4 phases, at least 3 driving voltages are needed, the highest voltage is more than 1.5 times of half-wave voltage, the requirements on an analog-to-digital converter and an amplifier of the phase adjusting driving circuit are high, and the cost and the complexity of the system are increased. In addition, because the two time windows in the coding state have the same polarization, a non-interference peak is generated when a receiving end decodes, the energy utilization rate of photons is reduced, and the safe code rate of the system is limited finally.
Disclosure of Invention
Aiming at the defects in the prior art, the invention provides a quantum key distribution phase coding device which is used for solving the technical defects that the insertion loss of a phase modulator of a quantum key distribution system is large, so that the loss of two time windows is inconsistent, the requirement on a phase modulator driving circuit is high, the cost and the complexity are high and the like in the prior art, and polarization multiplexing is adopted to improve the energy utilization rate of an interferometer so as to improve the safe code rate of the system.
The invention provides a quantum key distribution phase encoding device, which comprises the following components:
the technical scheme of the invention is realized as follows:
a quantum key distribution phase encoding apparatus comprising a laser LD, an orthogonal polarization time window generating element OPTG for converting a single optical pulse into orthogonal polarized sub-pulses located in two time windows of time interval t, respectively, a first path selecting element PS1, a second path selecting element PS2, a third path selecting element PS3, a half-wave plate HWP, and a quarter-wave plate QWP; the laser LD is connected with the input port of the orthogonal polarization time window generating element OPTG after being welded for 45 degrees through a polarization maintaining optical fiber, and the output port of the orthogonal polarization time window generating element OPTG is connected with the first port of the first path selection element PS 1; the second port and the third port of the first path selection element PS1 are respectively connected with the second port and the first port of the second path selection element PS2 through polarization maintaining fibers to form a first equal-arm mach-zehnder interferometer MZ1, and the half-wave plate HWP is located on one arm of the first equal-arm mach-zehnder interferometer MZ 1; the third port and the fourth port of the second path selection element PS2 are respectively connected with the first port and the second port of the third path selection element PS3 through polarization maintaining fibers to form a second-arm mach-zehnder interferometer MZ2, and the quarter-wave plate QWP is located on one arm of the second-arm mach-zehnder interferometer MZ 2; the third port of the third path selection element PS3 is used to output 4 phase encoded states.
Preferably, the orthogonal polarization time window generating element OPTG is a four-port polarization beam splitter PBS, a first port and a third port of the polarization beam splitter PBS are respectively used as an input port and an output port of the orthogonal polarization time window generating element OPTG, and a second port and a fourth port of the polarization beam splitter are directly connected through a polarization maintaining fiber with a length of L1, the length of L1= t ns c, where ns is a refractive index of a slow axis of the polarization maintaining fiber, and c is an optical speed in vacuum.
Preferably, the orthogonal polarization time window generating element OPTG is a high birefringence polarization maintaining fiber PMF with a length L2, said length L2= t (ns-nf) c, wherein ns and nf are the refractive indices of the slow axis and the fast axis of the polarization maintaining fiber, respectively, and c is the speed of light in vacuum.
Preferably, the half-wave plate HWP is a fiber half-wave plate wound from a single-mode fiber, and the plane of the fiber ring of the fiber half-wave plate is perpendicular to the polarization-maintaining fiber slow axis of one arm of the first equal-arm mach-zehnder interferometer MZ 1.
Preferably, the quarter-wave plate QWP is an optical fiber quarter-wave plate wound by a single-mode optical fiber, and a plane of an optical fiber ring of the optical fiber quarter-wave plate is perpendicular to a slow axis of the polarization-maintaining optical fiber of one arm of the second-arm mach-zehnder interferometer MZ 2.
Preferably, the first path selection element PS1 is a first beam splitter BS1, the second path selection element PS2 is a first optical switch OS1, the third path selection element PS3 is a second optical switch OS2, the first beam splitter BS1 is a 1X2 beam splitter, the first optical switch OS1 is a 2X2 optical switch, and the second optical switch OS2 is a 2X1 optical switch.
Preferably, the first path selection element PS1 is a third optical switch OS3, the second path selection element PS2 is a second beam splitter BS2, the third path selection element PS3 is a fourth optical switch OS4, the third optical switch OS3 is a 1X2 optical switch, the fourth optical switch OS4 is a 2X1 optical switch, and the second beam splitter BS2 is a 2X2 beam splitter.
Preferably, the first path selection element PS1 is a fifth optical switch OS5, the second path selection element PS2 is a sixth optical switch OS6, the third path selection element PS3 is a third beam splitter BS3, the fifth optical switch OS5 is a 1X2 optical switch, the sixth optical switch OS6 is a 2X2 optical switch, and the third beam splitter BS3 is a 2X1 beam splitter.
Compared with the prior art, the invention has the following beneficial effects:
the invention provides a quantum key distribution phase coding device, which can prepare 4 phase coding states without a phase modulator, and the polarization of two time window pulses is mutually vertical, so that the non-interference component can be eliminated to improve the energy utilization rate, and further the safe code rate of a system can be improved. The optical switch is adopted and only needs to be driven by a digital circuit, so that stable coding can be realized, the stability and the practicability of the system are improved, and the cost and the complexity of the coding device are reduced.
Drawings
FIG. 1 is a schematic block diagram of a quantum key distribution phase encoding apparatus of the present invention;
FIG. 2 is a schematic block diagram of a first embodiment of a quantum key distribution phase encoding apparatus according to the present invention;
FIG. 3 is a schematic block diagram of a second embodiment of a quantum key distribution phase encoding apparatus of the present invention;
fig. 4 is a schematic block diagram of a third embodiment of a quantum key distribution phase encoding apparatus according to the present invention.
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, a quantum key distribution phase encoding apparatus includes a laser LD, an orthogonal polarization time window generating element OPTG for converting a single optical pulse into sub-pulses of orthogonal polarizations in two time windows at time intervals t, a first path selecting element PS1, a second path selecting element PS2, a third path selecting element PS3, a half-wave plate HWP, and a quarter-wave plate QWP; the laser LD is connected with the input port of the orthogonal polarization time window generating element OPTG after being welded for 45 degrees through a polarization maintaining optical fiber, and the output port of the orthogonal polarization time window generating element OPTG is connected with the first port of the first path selection element PS 1; the second port and the third port of the first path selection element PS1 are respectively connected with the second port and the first port of the second path selection element PS2 through polarization-maintaining optical fibers a1 and a2 to form a first equal-arm mach-zehnder interferometer MZ1, and the half-wave plate HWP is located on the polarization-maintaining optical fiber a 1; the third port and the fourth port of the second path selection element PS2 are respectively connected with the first port and the second port of the third path selection element PS3 through polarization-maintaining optical fibers A3 and a4 to form a second equal-arm mach-zehnder interferometer MZ2, and the quarter-wave plate QWP is positioned on the polarization-maintaining optical fiber A3; the third port of the third path selection element PS3 is used to output 4 phase encoded states.
The specific phase encoding process is as follows:
the laser LD generates a horizontally polarized light pulse P0, which is first subjected to a 45 ° polarization rotation to become 45 ° linearly polarized, and then enters the orthogonal polarization time window generating element OPTG to become two sub-pulses P1 and P2 with a time interval t, where P1 is the horizontal polarization H in the previous time window and P2 is the vertical polarization V in the next time window. The P1 and P2 then arrive at the first port of the first path selection element PS1, and then enter the first arm Mach-Zehnder interferometer MZ 1. The P1 and P2 may then exit the second or third port of the first path selection element PS1, propagating along polarization maintaining fiber a1 or a2, respectively. After reaching the second routing element PS2, the P1 and P2 may exit from the third port or the fourth port of the second routing element PS2, propagate along the polarization maintaining optical fiber A3 or a4, respectively, and finally reach the third routing element PS4, exiting from the third port thereof. Thus, by controlling the first PS1, the second PS2, and the third PS3, the P1 and the P2 can have 4 propagation paths, i.e., the paths along the polarization maintaining fiber "a 1+ A3" or "a 1+ a 4" or "a 2+ A3" or "a 2+ a 4", and ensure that only pulses propagating along one of the paths exit the third port of the third PS3 at a time.
Because the included angles of the slow axes of the half-wave plate HWP and the quarter-wave plate QWP and the slow axis of the polarization maintaining fiber are both 0 degree, the Jones matrixes of the half-wave plate HWP and the quarter-wave plate QWP can be respectively written as
The horizontal H and vertical V polarizations are converted into half-wave HWP or quarter-wave QWP respectively
That is, the phase of the horizontal polarization H is unchanged after passing through the half-wave plate HWP and the quarter-wave plate QWP; the vertical polarization V increases in phase by π after passing through the half-wave plate HWP and by π/2 after passing through the quarter-wave plate QWP.
When the P1 and the P2 propagate along the path of the polarization-maintaining fiber "A1 + A3", the two pass through the half-wave plate HWP and the quarter-wave plate QWP in sequence, the phase of the P1 with horizontal polarization is unchanged, and the phase of the P2 with vertical polarization is increased by pi + pi/2 =3 pi/2, so that the phase difference of the P1 and the P2 is 3 pi/2.
When the P1 and the P2 propagate along the path of the polarization-maintaining fiber "a 1+ a 4", the two pass through the half-wave plate HWP in sequence, the phase of the P1 with horizontal polarization is unchanged, and the phase of the P2 with vertical polarization is increased by pi, so that the phase difference between the P1 and the P2 is pi.
When the P1 and the P2 propagate along the path of the polarization-maintaining optical fiber "A2 + A3", the two pass through the quarter-wave plate QWP one after the other, the phase of the P1 with horizontal polarization is unchanged, and the phase of the P2 with vertical polarization is increased by pi/2, so that the phase difference between the P1 and the P2 is pi/2.
When the P1 and P2 propagate along the path of the polarization maintaining fiber "a 2+ a 4", the phase difference between the two is not changed and is still 0.
From the above analysis, it can be seen that the 4 phase encoded states required by the BB84 protocol can be prepared without a phase modulator by merely controlling the path selection elements to optically select the optical pulses.
As shown in fig. 2, a first embodiment of the quantum key distribution phase encoding apparatus of the present invention:
the structure of the coding device is as follows: the orthogonal polarization time window generation element OPTG is a four-port polarization beam splitter PBS, a first port and a third port of the polarization beam splitter PBS are respectively used as an input port and an output port of the orthogonal polarization time window generation element OPTG, a second port and a fourth port of the polarization beam splitter are directly connected through a polarization-maintaining optical fiber with the length of L1, and the length of L1= t ns c (ns is the refractive index of a slow axis of the polarization-maintaining optical fiber, and c is the light speed in vacuum). The first path selection element PS1 is a first beam splitter BS1, the second path selection element PS2 is a first optical switch OS1, the third path selection element PS3 is a second optical switch OS2, the first beam splitter BS1 is a 1X2 beam splitter, the first optical switch OS1 is a 2X2 optical switch, and the second optical switch OS2 is a 2X1 optical switch.
An embodiment of a phase encoding process comprises:
the laser LD generates a horizontally polarized light pulse P0, enters a first light path selection module, first undergoes 45-degree polarization rotation to become 45-degree linear polarization, and then enters a first port of a polarization beam splitter PBS, wherein a horizontal component is directly transmitted to a third port of the polarization beam splitter PBS, which is marked as a pulse P1 and still is horizontally polarized H; the vertically polarized component is reflected to the second port of the polarization beam splitter PBS, propagates along the polarization maintaining fiber with length L1 to the fourth port of the polarization beam splitter PBS, and is reflected to the third port, denoted as pulse P2, and still has vertical polarization V. P1 and P2 exit the third port of the polarizing beam splitter PBS for a time interval t, where P1 is in the previous time window and P2 is in the next time window. Then P1 and P2 arrive at the first port of the first beam splitter BS1 in sequence, are divided into sub-pulses P11, P12, P21 and P22 with equal amplitude and same polarization, wherein P11 and P21 exit from the second port of the first beam splitter BS1, propagate along the polarization-maintaining optical fiber A1, reach the second port of the first optical switch OS1 after passing through the half-wave plate HWP, and the phase difference of the two becomes pi; p12 and P22 exit from the third port of the first splitter BS1, propagate along the polarization maintaining fiber a2 and reach the first port of the first optical switch, and the phase difference between the two is still 0.
When the first optical switch OS1 is controlled to be in state 0, the P11 and the P21 exit from the third port of the first optical switch OS1, propagate along the polarization maintaining fiber A3, and reach the first port of the second optical switch OS2 after passing through the quarter-wave plate QWP, and the phase difference is increased by pi/2, that is, the phase difference is 3 pi/2. The P12 and P22 exit the fourth port of the first optical switch OS1, propagate along the polarization maintaining fiber a4 and reach the second port of the second optical switch OS2, and the phase difference is still 0. At this time, if the second optical switch OS2 is controlled to be in the state 0, only P11 and P21 can be emitted from the third port of the second optical switch OS2, so that the phase difference prepared by the phase encoding device is 3 pi/2; if the second optical switch OS2 is controlled to be in the state 1, only P12 and P22 can be emitted from the third port of the second optical switch OS2, and thus the phase difference prepared by the phase encoding device is 0.
When the first optical switch OS1 is controlled to be in state 1, P11 and P21 exit the fourth port of the first optical switch OS1, propagate along the polarization maintaining fiber a4 and reach the second port of the second optical switch OS2, and the phase difference is still pi. The P12 and P22 exit the third port of the first optical switch OS1, propagate along the polarization maintaining fiber A3, and reach the first port of the second optical switch OS2 after passing through the quarter wave plate QWP, and the phase difference becomes pi/2. At this time, if the second optical switch OS2 is controlled to be in the state 0, only P12 and P22 can be emitted from the third port of the second optical switch OS2, and thus the phase difference prepared by the phase encoding device is pi/2; if the second optical switch OS2 is controlled to be in state 1, only P11 and P21 can exit from the third port of the second optical switch OS2, and thus the phase difference prepared by the phase encoding apparatus is pi.
From the above analysis, it can be seen that, without a phase modulator, the optical path selection is performed on the optical pulse only by switching the states of the first optical switch OS1 and the second optical switch OS2, and the 4 phase encoding states can be obtained, where the code of the first embodiment of the phase encoding apparatus is shown in table 1, where state 0 of the first optical switch OS1 indicates that the first port to the fourth port and the second port to the third port are paths; state 1 indicates that the first port to the third port and the second port to the fourth port are pass-through. State 0 of the second optical switch OS2 indicates that the first port to the third port is on and the second port to the third port is off; state 1 indicates that the second port is closed to the third port and the first port is open to the third port.
Table 1: code table of first embodiment of phase encoding device
As shown in fig. 3, a second embodiment of the quantum key distribution phase encoding apparatus of the present invention:
the structure of the coding device is as follows: the orthogonal polarization time window generation element OPTG is a four-port polarization beam splitter PBS, a first port and a third port of the polarization beam splitter PBS are respectively used as an input port and an output port of the orthogonal polarization time window generation element OPTG, a second port and a fourth port of the polarization beam splitter are directly connected through a polarization-maintaining optical fiber with the length of L1, and the length of L1= t ns c (ns is the refractive index of a slow axis of the polarization-maintaining optical fiber, and c is the light speed in vacuum). The first path selection element PS1 is a third optical switch OS3, the second path selection element PS2 is a second beam splitter BS2, the third path selection element PS3 is a fourth optical switch OS4, the third optical switch OS3 is a 1X2 optical switch, the fourth optical switch OS4 is a 2X1 optical switch, and the second beam splitter BS2 is a 2X2 beam splitter.
An embodiment two-phase encoding process is:
the laser LD generates a horizontally polarized light pulse P0, enters a first light path selection module, first undergoes 45-degree polarization rotation to become 45-degree linear polarization, and then enters a first port of a polarization beam splitter PBS, wherein a horizontal component is directly transmitted to a third port of the polarization beam splitter PBS, which is marked as a pulse P1 and still is horizontally polarized H; the vertically polarized component is reflected to the second port of the polarization beam splitter PBS, propagates along the polarization maintaining fiber of length L1 to the fourth port of the polarization beam splitter PBS, and is reflected to the third port, denoted as pulse P2, still with vertical polarization V. P1 and P2 exit the third port of the polarizing beamsplitter PBS for a time interval t, where P1 was in the previous time window and P2 was in the next time window.
Then the P1 and the P2 successively reach the first port of the third optical switch OS3, when the state of the third optical switch OS3 is controlled to be 0, the P1 and the P2 exit from the second port of the third optical switch OS3, propagate along the polarization maintaining optical fiber a1, and reach the second port of the second beam splitter BS2 after passing through the half-wave plate HWP, and the phase difference between the two becomes pi. The P1 and P2 are then split into sub-pulses P110, P120 and P210, P220 of equal amplitude and polarization, respectively, where P110, P210 exits the third port of the second beam splitter BS2, propagates along the polarization maintaining fiber A3, and reaches the first port of the fourth optical switch OS4 after passing through the quarter wave plate QWP, with a phase difference of 3 pi/2, which is increased by pi/2. The P120, P220 exit the fourth port of the second beam splitter BS2, propagate along the polarization maintaining fiber a4 and reach the second port of the fourth optical switch OS4, with a phase difference of pi. At this time, if the fourth optical switch OS4 is controlled to be in the state 0, only P110 and P210 can be emitted from the third port of the fourth optical switch OS4, and thus the phase difference prepared by the phase encoding device is 3 pi/2; if the fourth optical switch OS4 is controlled to be in the state 1, only P120 and P220 can be emitted from the third port of the fourth optical switch OS4, and thus the phase difference prepared by the phase encoder is pi.
When the state of the third optical switch OS3 is controlled to be 1, the P1 and the P2 exit from the third port of the third optical switch OS3, propagate along the polarization maintaining optical fiber a2 to reach the first port of the second beam splitter BS2, and the phase difference between the two is still 0. Then P1 and P2 are split into sub-pulses P111, P121 and P211, P221 of equal amplitude and polarization, respectively, where P111, P211 exits the third port of the second beam splitter BS2, propagates along the polarization maintaining fiber A3, and reaches the first port of the fourth optical switch OS4 after passing through the quarter wave plate QWP, the phase difference becomes pi/2. P121, P221 exit the fourth port of the second beam splitter BS2, propagate along the polarization maintaining fiber a4 and reach the second port of the fourth optical switch OS4, and the phase difference is still 0. At this time, if the fourth optical switch OS4 is controlled to be in the state 0, only P111 and P211 can be emitted from the third port of the fourth optical switch OS4, and thus the phase difference prepared by the phase encoder is pi/2; if the fourth optical switch OS4 is controlled to be in the state 1, only P121 and P221 can be emitted from the third port of the fourth optical switch OS4, and thus the phase difference prepared by the phase encoder is 0.
From the above analysis, it can be seen that, without a phase modulator, the optical path selection is performed on the optical pulse only by switching the states of the third optical switch OS3 and the fourth optical switch OS4, and the 4 phase encoding states can be obtained, where the code of the second embodiment of the phase encoding apparatus is shown in table 2, where state 0 of the third optical switch OS3 indicates that the first port is closed to the second port, and the first port is open to the third port; state 1 indicates that the first port is open to the third port and the first port is open to the second port. State 0 of the fourth optical switch OS4 indicates that the first port to the third port are on and the second port to the third port are off; state 1 indicates that the second port is open to the third port and the first port is open to the third port.
Table 2: code table of the second embodiment of phase encoding device
As shown in fig. 4, a third embodiment of the quantum key distribution phase encoding apparatus of the present invention:
the structure of the coding device is as follows: the orthogonal polarization time window generating element OPTG is a high birefringence polarization maintaining fiber PMF with a length L2, the length L2= t (ns-nf) c (ns and nf are refractive indices of the slow axis and the fast axis of the polarization maintaining fiber, respectively, and c is the speed of light in vacuum). The first path selection element PS1 is a fifth optical switch OS5, the second path selection element PS2 is a sixth optical switch OS6, the third path selection element PS3 is a third beam splitter BS3, the fifth optical switch OS5 is a 1X2 optical switch, the sixth optical switch OS6 is a 2X2 optical switch, and the third beam splitter BS3 is a 2X1 beam splitter.
The three-phase encoding process of the embodiment is as follows:
the laser LD generates a horizontally polarized light pulse P0 that enters the first optical path selection module, first undergoes 45 ° polarization rotation to become 45 ° linear polarization, and then enters a high birefringence polarization maintaining fiber PMF with a length L2. The horizontal component and the vertical component of the P0 propagate along the slow axis and the fast axis respectively in the high birefringence polarization maintaining fiber PMF, and because the refractive indexes of the two axes are different greatly and the propagation speeds are different, the two components will generate a time difference t when emerging from the PMF. The component of the pulse propagating along the slow axis is denoted as pulse P1 in the previous time window, which is horizontally polarized H; the component of the pulse propagating along the fast axis is denoted as pulse P2, in the latter time window, vertical polarization V.
Then, the P1 and the P2 reach the first port of the fifth optical switch OS5 in sequence, when the state of the fifth optical switch OS5 is controlled to be 0, the P1 and the P2 exit from the second port of the fifth optical switch OS5, propagate along the polarization maintaining optical fiber a1, pass through the half-wave plate HWP, and reach the second port of the sixth optical switch OS6, and the phase difference between the two becomes pi. When the sixth optical switch OS6 is controlled to be in a state 0, P11 and P21 are emitted from the third port of the sixth optical switch OS6, propagate along the polarization maintaining optical fiber A3, pass through the quarter-wave plate QWP and reach the first port of the third beam splitter BS3, the phase difference is increased by pi/2, namely the phase difference is 3 pi/2, so that the phase difference prepared by the phase encoding device is 3 pi/2; when the sixth optical switch OS6 is controlled to be in the state 1, the P11 and the P21 exit from the fourth port of the sixth optical switch OS6, propagate along the polarization maintaining fiber a4 and reach the second port of the third beam splitter BS3, and the phase difference is still pi, so the phase difference prepared by the phase encoding apparatus is pi.
When the state of the fifth optical switch OS5 is controlled to be 1, P1 and P2 exit the third port of the fifth optical switch OS5, propagate along the polarization maintaining fiber a2 to reach the first port of the sixth optical switch OS6, and the phase difference between them is still 0. When the sixth optical switch OS6 is controlled to be in the state 0, the phase difference between the P11 and the P21, which are emitted from the fourth port of the sixth optical switch OS6 and propagate along the polarization maintaining optical fiber a4, at the second port of the third beam splitter BS3 is still 0, so the phase difference prepared by the phase encoding apparatus is 0; when the sixth optical switch OS6 is controlled to be in the state 1, the P11 and the P21 exit from the third port of the sixth optical switch OS6, propagate along the polarization maintaining fiber A3, pass through the quarter-wave plate QWP, and reach the first port of the third beam splitter BS3, and the phase difference is increased by pi/2, so that the phase difference prepared by the phase encoding apparatus is pi/2.
As can be seen from the above analysis, without a phase modulator, the optical path selection is performed on the optical pulse only by switching the states of the fifth optical switch OS5 and the sixth optical switch OS6, so as to obtain 4 phase encoding states, where the code of the third embodiment of the phase encoding apparatus is shown in table 3, where state 0 of the fifth optical switch OS5 indicates that the first port is connected to the second port, and the first port is disconnected to the third port; state 1 indicates that the first port is open to the third port and the first port is open to the second port. State 0 of the sixth optical switch OS6 indicates that the first port to the fourth port and the second port to the third port are on; state 1 indicates that the first port to the third port and the second port to the fourth port are pass-through.
Table 3: code table of phase encoding device embodiment three
The present invention also provides a phase encoded quantum key distribution system which may comprise any of the above described phase encoding devices.
According to the embodiments of the invention, the invention provides a quantum key distribution phase coding device, 4 phase coding states can be prepared without a phase modulator, the polarization of two time window pulses is perpendicular to each other, non-interference components can be eliminated to improve the energy utilization rate, and further the safe code rate of a system can be improved. The optical switch is adopted and only needs to be driven by a digital circuit, so that stable coding can be realized, the stability and the practicability of the system are improved, and the cost and the complexity of the coding device are reduced.
Claims (8)
1. A quantum key distribution phase encoding apparatus comprising a laser LD, an orthogonal polarization time window generating element OPTG for converting a single optical pulse into orthogonal polarized sub-pulses positioned in two time windows at time intervals t, a first path selecting element PS1, a second path selecting element PS2, a third path selecting element PS3, a half-wave plate HWP, and a quarter-wave plate QWP; the laser LD is connected with the input port of the orthogonal polarization time window generating element OPTG after being welded for 45 degrees through a polarization maintaining optical fiber, and the output port of the orthogonal polarization time window generating element OPTG is connected with the first port of the first path selection element PS 1; the second port and the third port of the first path selection element PS1 are respectively connected with the second port and the first port of the second path selection element PS2 through polarization maintaining fibers to form a first equal-arm mach-zehnder interferometer MZ1, and the half-wave plate HWP is located on one arm of the first equal-arm mach-zehnder interferometer MZ 1; the third port and the fourth port of the second path selection element PS2 are respectively connected with the first port and the second port of the third path selection element PS3 through polarization maintaining fibers to form a second-arm mach-zehnder interferometer MZ2, and the quarter-wave plate QWP is located on one arm of the second-arm mach-zehnder interferometer MZ 2; the third port of the third path selection element PS3 is used to output 4 phase encoded states.
2. The quantum key distribution phase encoding apparatus according to claim 1, wherein the orthogonal polarization time window generating element OPTG is a four-port polarization beam splitter PBS, a first port and a third port of the polarization beam splitter PBS are respectively an input port and an output port of the orthogonal polarization time window generating element OPTG, a second port and a fourth port of the polarization beam splitter are directly connected by a polarization maintaining fiber with a length of L1, the length of L1= t ns c, where ns is a refractive index of a slow axis of the polarization maintaining fiber, and c is an optical speed in vacuum.
3. The quantum key distribution phase encoding apparatus of claim 1, wherein the orthogonal polarization time window generating element OPTG is a high birefringence polarization maintaining fiber PMF with a length L2, the length L2= t (ns-nf) c, wherein ns and nf are refractive indices of a slow axis and a fast axis of the polarization maintaining fiber respectively, and c is an optical speed in vacuum.
4. A quantum key distribution phase encoding apparatus according to claim 1, 2 or 3, wherein the half-wave plate HWP is a fiber half-wave plate wound from a single-mode fiber, the fiber loop of the fiber half-wave plate lying in a plane perpendicular to the polarization maintaining fiber slow axis of one arm of the first arm mach-zehnder interferometer MZ 1.
5. The quantum key distribution phase encoding apparatus of claim 1, 2 or 3, wherein the quarter-wave plate QWP is a fiber quarter-wave plate wound from single-mode fiber, and a plane of a fiber ring of the fiber quarter-wave plate is perpendicular to a polarization maintaining fiber slow axis of one arm of the second-arm Mach-Zehnder interferometer MZ 2.
6. A quantum key distribution phase encoding apparatus as claimed in claim 1 or 2 or 3 wherein the first path selection element PS1 is a first beam splitter BS1, the second path selection element PS2 is a first optical switch OS1, the third path selection element PS3 is a second optical switch OS2, the first beam splitter BS1 is a 1X2 beam splitter, the first optical switch OS1 is a 2X2 optical switch, and the second optical switch OS2 is a 2X1 optical switch.
7. A quantum key distribution phase encoding apparatus as claimed in claim 1, 2 or 3 wherein the first path selection element PS1 is a third optical switch OS3, the second path selection element PS2 is a second beam splitter BS2, the third path selection element PS3 is a fourth optical switch OS4, the third optical switch OS3 is a 1X2 optical switch, the fourth optical switch OS4 is a 2X1 optical switch, and the second beam splitter BS2 is a 2X2 beam splitter.
8. A quantum key distribution phase encoding apparatus as claimed in claim 1, 2 or 3 wherein the first path selection element PS1 is a fifth optical switch OS5, the second path selection element PS2 is a sixth optical switch OS6, the third path selection element PS3 is a third beam splitter BS3, the fifth optical switch OS5 is a 1X2 optical switch, the sixth optical switch OS6 is a 2X2 optical switch, and the third beam splitter BS3 is a 2X1 beam splitter.
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