CN114900245A - Polarization-independent phase decoding integrated chip and quantum key distribution system - Google Patents
Polarization-independent phase decoding integrated chip and quantum key distribution system Download PDFInfo
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Abstract
The invention discloses a polarization-independent phase decoding integrated chip, which belongs to the technical field of quantum secure communication and comprises a first polarization beam splitting rotator, a second polarization beam splitting rotator, a first beam splitter, a second beam splitter, a first waveguide delay line and a first phase modulator which are integrated on the same substrate. Compared with the prior art, the invention realizes the polarization-independent stable phase decoding, does not need to adopt devices which are difficult to integrate, such as magneto-optical crystals and the like, can reduce the overall size of the integrated waveguide interferometer, and greatly improves the integration level. And the input and output ports of the integrated chip can be further separated, and the use of a circulator is avoided. The quantum key distribution system greatly reduces the volume of an optical system, can realize equipment miniaturization, has the characteristic of polarization disturbance of an immune channel, and can realize the long-term working stability of the system.
Description
Technical Field
The invention relates to the technical field of quantum secure communication, in particular to a polarization-independent phase decoding integrated chip and a quantum key distribution system.
Background
Quantum key distribution can provide unconditionally secure key distribution for both communication parties at a long distance, and the most mature protocol at present is the BB84 quantum key distribution protocol. The optical fiber quantum key distribution system generally adopts a single-mode optical fiber as a transmission channel, but because the optical fiber channel has an inherent birefringence effect, the polarization state of photons can change in the transmission process and can change along with the change of the external environment. However, when the traditional scheme based on the double unequal arm mach-zehnder interferometer performs decoding interference at a receiving end, the polarization state changes randomly due to the disturbance of the optical fiber channel, and the long and short arm polarization changes of the interferometer are different, so that the stability of the interference is affected, and thus the system is poor in stability and is easily subjected to environmental interference.
In the prior art, one solution to the polarization disturbance is to use a faraday-michelson interferometer, so that the birefringence effect of the optical fiber and the influence of environmental disturbance on the polarization state can be eliminated, and the polarization change of the long and short arms can be automatically compensated, and the system is very stable. Yet another solution is an interferometer as disclosed in patent CN210041849U, which uses a faraday rotator to automatically compensate for channel polarization perturbations and different polarization changes in the long and short arms, also based on the faraday effect. However, interferometers constructed by discrete optical elements in the schemes have the disadvantages of large volume, complex structure, poor stability, high cost and difficulty in mass production, and the interferometer has low manufacturing precision due to the arm length difference, so that the system stability is poor, and the requirements of integration and miniaturization of the system requirements cannot be met.
In order to improve the integration of the interferometer, the patent CN109391471B and the document Zhang G W, et al, Polarization-induced interferometer based on a hybrid integrated planar light-wave circuit [ J ], Photonics Research, 2021, 9(11): 2176-. Similarly, the solution of CN210041849U also faces the problem of increasing the difficulty of integration due to the inclusion of magneto-optical crystal. Patent CN1106020662A and document Xu H, et al, Photonic Integrated Phase Decoder Scheme for High-Speed, Efficient and Stable Quantum Key Distribution System [ J ]. arXiv preprinting arXiv:1910.08327, 2019, propose an Integrated waveguide sagnac loop structure based on a three-port polarization beam splitter, which can replace a faraday mirror, so that the interferometer is convenient for integration. However, this solution requires the use of 2 integrated waveguide sagnac loop structures, which reduce the loss uniformity of the long and short arms, thereby affecting the interference visibility, and is a round-trip structure, where the round-trip through the waveguides increases the insertion loss of the signal.
Disclosure of Invention
Aiming at the defects in the prior art, the invention provides a polarization-independent phase decoding integrated chip.
The technical scheme of the invention is realized as follows:
a polarization-independent phase decoding integrated chip comprises a first polarization beam splitting rotator, a second polarization beam splitting rotator, a first beam splitter, a second beam splitter, a first waveguide delay line and a first phase modulator which are integrated on the same substrate,
the first port of the first polarization beam splitting rotator is used as an input port In and a first output port Out1 of the phase decoding integrated chip, and the first port of the second polarization beam splitting rotator is used as a second output port Out2 of the phase decoding integrated chip; the second port of the first polarization beam splitting rotator is connected with the first port of the first beam splitter through a waveguide L1; the third port of the first polarization beam splitting rotator is connected with the first port of the second beam splitter through a waveguide L2; the second port of the second polarization beam splitting rotator is connected with the fourth port of the first beam splitter through a waveguide L3; the third port of the second polarization beam splitting rotator is connected with the fourth port of the second beam splitter through a waveguide L4; the third port of the first beam splitter is connected with the third port of the second beam splitter through a first waveguide delay line to form a path M1; the second port of the first beam splitter is connected to the second port of the second beam splitter through a first phase modulator to form a path M2;
the first polarization beam splitting rotator is used for carrying out polarization beam splitting on input signal light to generate a first signal light component and a second signal light component which have the same polarization;
the first beam splitter is configured to split the first signal light component to generate a third signal light component and a fourth signal light component; the second beam splitter is configured to split the second signal light component to generate a fifth signal light component and a sixth signal light component;
the first waveguide delay line is used for simultaneously delaying the third signal light component and the fifth signal light component; the first phase modulator is configured to phase-modulate the fourth signal light component and the sixth signal light component at the same time;
the second beam splitter is further configured to interfere the third signal light component and the fourth signal light component to generate first interference light and second interference light; the first beam splitter is further configured to interfere the fifth signal light component and the sixth signal light component to generate third interference light and fourth interference light;
the second polarization beam splitting rotator is used for carrying out polarization beam combination on the first interference light and the third interference light and outputting the first interference light and the third interference light;
the first polarization beam splitting rotator is also used for carrying out polarization beam combination on the second interference light and the fourth interference light and outputting the second interference light and the fourth interference light.
Preferably, the phase decoding integrated chip further comprises a second phase modulator and a third phase modulator, and the second phase modulator and the third phase modulator are respectively located at symmetrical positions on two sides of the first waveguide delay line; the second phase modulator and the third phase modulator are applied with the same voltage for adjusting the phase of the third signal light component and the phase of the fifth signal light component.
Preferably, the phase decoding integrated chip further includes a first polarization rotation structure, a second waveguide delay line, a second polarization rotation structure, and a fourth phase modulator, and the first polarization beam splitting rotator further includes a fourth port;
the polarization rotation angles of the first polarization rotation structure and the second polarization rotation structure are both 90 degrees, are respectively located in the middle positions of the path M1 and the path M2, and are respectively used for rotating the polarization directions of the third signal light component, the fifth signal light component, the fourth signal light component and the sixth signal light component;
the first waveguide delay line and the second waveguide delay line have the same length, are respectively positioned at symmetrical positions on two sides of the first polarization rotation structure, and are used for delaying the third signal light component and the fifth signal light component;
the first phase modulator and the fourth phase modulator are respectively positioned at symmetrical positions on two sides of the second polarization rotating structure and are used for adjusting the phase of the fourth signal light component and the phase of the sixth signal light component, and the loaded voltages of the first phase modulator and the fourth phase modulator are the same;
and the fourth port of the first polarization beam splitting rotator is used for outputting an optical signal after the second interference light and the fourth interference light are polarized and combined.
Preferably, the lengths of the waveguide L1, the waveguide L2, the waveguide L3 and the waveguide L4 are all equal.
Preferably, the first waveguide delay line and the first phase modulator are located at an intermediate position of the path M1 and the path M2, respectively.
Preferably, the first beam splitter and the second beam splitter are both multimode interference couplers or 3dB directional couplers.
The invention also provides a quantum key distribution system, which comprises a sending end, a receiving end and a channel for connecting the sending end and the receiving end, wherein the sending end comprises a laser, an intensity modulator, a sending end interferometer and an adjustable attenuator which are sequentially connected; the sending end interferometer is an unequal arm Mach-Zehnder interferometer integrated chip and comprises a third beam splitter, a third waveguide delay line, a fifth phase modulator and a fourth beam splitter; the receiving end interferometer is one of the polarization-independent phase decoding integrated chips.
Compared with the prior art, the invention has the following beneficial effects:
the invention provides a polarization-independent phase decoding integrated chip, which is characterized in that a polarization beam splitting rotator is used for splitting two orthogonal polarization components of input signal light, the two orthogonal polarization components are interfered by the same unequal arm Mach-Zehnder interferometer from opposite directions respectively, and the interference results of the two polarization components are output after being polarized and combined, so that polarization-independent stable phase decoding can be realized, devices which are difficult to integrate such as magneto-optical crystals and the like do not need to be adopted, the overall size of the integrated waveguide interferometer can be reduced, and the integration level is greatly improved. And through respectively adding 90-degree polarization rotation structures on two arms of the interferometer, the input port and the output port of the integrated chip can be further separated, and the use of a circulator is avoided. The quantum key distribution system based on the phase decoding integrated chip greatly reduces the volume of an optical system, can realize miniaturization of equipment, has the characteristic of polarization disturbance of an immune channel, and can realize long-term working stability of the system.
Drawings
FIG. 1 is a block diagram of a first embodiment of a polarization independent phase decoding IC according to the present invention;
FIG. 2 is a block diagram of a second embodiment of a polarization independent phase decoding IC according to the present invention;
FIG. 3 is a block diagram of a third embodiment of a polarization independent phase decoding IC according to the present invention;
fig. 4 is a block diagram of a quantum key distribution system according to the present invention.
In the figure: a first polarization beam splitting rotator 1, a second polarization beam splitting rotator 2, a first beam splitter 3, a second beam splitter 4, a first waveguide delay line 5, a first phase modulator 6, a second phase modulator 7, a third phase modulator 8, a first polarization rotating structure 9, a second waveguide delay line 10, a second polarization rotating structure 11, a fourth phase modulator 12, a transmitting end A, a laser A-1, an intensity modulator A-2, a transmitting end interferometer A-3, a third beam splitter A-3-1, a third waveguide delay line A-3-2, a fifth phase modulator A-3-3, a fourth beam splitter A-3-4, an adjustable attenuator A-4, a receiving end B, a receiving end interferometer B-1, a first single photon detector B-2, a second single photon detector B-3, channel C.
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 first embodiment of a polarization-independent phase-decoding integrated chip includes a first polarization beam splitting rotator 1, a second polarization beam splitting rotator 2, a first beam splitter 3, a second beam splitter 4, a first waveguide delay line 5, and a first phase modulator 6 integrated on the same substrate, where a first port of the first polarization beam splitting rotator 1 serves as an input port In and a first output port Out1 of the phase-decoding integrated chip, and a first port of the second polarization beam splitting rotator 2 serves as a second output port Out2 of the phase-decoding integrated chip; the second port of the first polarization beam splitting rotator 1 is connected with the first port of the first beam splitter 3 through a waveguide L1; the third port of the first polarization beam splitting rotator 1 is connected with the first port of the second beam splitter 4 through a waveguide L2; the second port of the second polarization beam splitter rotator 2 is connected with the fourth port of the first beam splitter 3 through a waveguide L3; the third port of the second polarization beam splitting rotator 2 is connected with the fourth port of the second beam splitter 4 through a waveguide L4; the third port of the first beam splitter 3 is connected with the third port of the second beam splitter 4 through a first waveguide delay line 5 to form a path M1; the second port of the first beam splitter 3 is connected to the second port of the second beam splitter 4 through a first phase modulator 6, so as to form a path M2;
the first polarization beam splitting rotator 1 is configured to perform polarization beam splitting on input signal light to generate a first signal light component and a second signal light component with the same polarization; the first beam splitter 3 is configured to split the first signal light component to generate a third signal light component and a fourth signal light component; the second beam splitter 4 is configured to split the second signal light component to generate a fifth signal light component and a sixth signal light component; the first waveguide delay line 5 is configured to delay the third signal light component and the fifth signal light component at the same time; the first phase modulator 6 is configured to phase-modulate the fourth signal light component and the sixth signal light component at the same time; the second beam splitter 4 is further configured to interfere the third signal light component and the fourth signal light component to generate first interference light and second interference light; the first beam splitter 3 is further configured to interfere the fifth signal light component and the sixth signal light component to generate third interference light and fourth interference light; the second polarization beam splitting rotator 2 is used for polarization beam combination and output of the first interference light and the third interference light; the first polarization beam splitting rotator 1 is further configured to perform polarization beam combination on the second interference light and the fourth interference light and output the polarization beam combination;
the first beam splitter 3 and the second beam splitter 4 are both multimode interference couplers or 3dB directional couplers. The lengths of the waveguide L1, the waveguide L2, the waveguide L3 and the waveguide L4 are all equal. The first waveguide delay line 5 and the first phase modulator 6 are located in the middle of the path M1 and the path M2, respectively.
The specific decoding process is as follows:
the phase difference between the front time mode and the rear time mode of the phase coding state sent by the sending end is
And the same polarization can be written as
The polarization states of temporal modes |0> and |1> are assumed to be both horizontally polarized. After passing through a single-mode optical fiber channel, due to the existence of birefringence effect and the disturbance of the environment where the channel is located, the phase encoding state changes into a random polarization state when reaching a receiving end, and therefore the phase encoding state entering the decoding device can be written as
Wherein the content of the first and second substances,
。
an optical pulse with any polarization enters an input port In of the polarization-independent phase decoding integrated wave chip, first enters a first port of the first polarization beam splitting rotator 1, and is divided into a first signal light component and a second signal light component with the same polarization. The two are respectively emitted from the second port and the third port of the first polarization beam splitting rotator 1, respectively propagate along the waveguide L1 and the waveguide L2 in TE polarization mode, and simultaneously reach the first port of the first beam splitter 3 and the first port of the second beam splitter 4, and the quantum states of the two are respectively
Wherein, the first signal light component enters into the unequal arm Mach-Zehnder interferometer composed of the first beam splitter 3, the second beam splitter 4, the first waveguide delay line 5 and the first phase modulator 6 from the first port of the first beam splitter 3 for phase decoding, and is first split into a third signal light component and a fourth signal light component with the same amplitude and polarization by the first beam splitter 3, and the third signal light component and the fourth signal light component respectively pass through the first waveguide delay line 5 and the first phase modulator 6 and then are changed into quantum states
Since the length difference between the path M1 and the path M2 is the arm length difference of the unequal arm mach-zehnder interferometer, the time pattern |0> of the third signal light component and the time pattern |1> of the fourth signal light component overlap in time, the remaining time patterns do not interfere with each other, and the quantum states of the first interference light and the second interference light resulting from the interference of the third signal light component and the fourth signal light component are obtained as the quantum states of the first interference light and the second interference light, respectively
Further, the light intensity of the first interference light and the second interference light can be obtained as
，
A phase difference between the phase modulated by the first phase modulator 6 and the phase modulated at the transmitting end.
Similarly, the second signal light component enters the unequal arm mach zehnder interferometer composed of the first beam splitter 3, the second beam splitter 4, the first waveguide delay line 5 and the first phase modulator 6 from the first port of the second beam splitter 4 for phase decoding, and is first split into a fifth signal light component and a sixth signal light component with the same amplitude and polarization by the second beam splitter 4, and the fifth signal light component and the sixth signal light component respectively pass through the first waveguide delay line 5 and the first phase modulator 6 and then are subjected to quantum state change into a quantum state
Since the difference between the lengths of the path M1 and the path M2 is the arm length difference of the unequal arm mach-zehnder interferometer, the time pattern |0> of the fifth signal light component and the time pattern |1> of the sixth signal light component overlap in time, the remaining time patterns do not interfere with each other, and the quantum states of the third interference light and the fourth interference light resulting from the interference of the fifth signal light component and the sixth signal light component are obtained as the quantum states of the third interference light and the fourth interference light, respectively
Further, the light intensity of the third interference light and the fourth interference light can be obtained as
。
The first interference light and the third interference light enter the second polarization beam splitting rotator 2 simultaneously for polarization beam combination, and the light intensity output from the first port is
The second interference light and the fourth interference light enter the first polarization beam splitting rotator 1 simultaneously for polarization beam combination, and the light intensity output from the first port is
It follows that the interference results from the two output ports of the phase decoding integrated chip are only related to the modulated phase difference, and not to the incident polarization state, and therefore can be immune to random perturbations of the channel. In addition, the maximum value of the interference light intensity is 1/2, because there is a non-interference peak, only half of the pulses participate in the interference, and therefore the energy utilization rate of the photons is 1/2. When the transmitting end modulates 4 phases, the receiving end can modulate 4 phases for decoding, and the normalized light intensities output by the two corresponding output ports are shown in table 1
Table 1: normalized light intensity meter output by two output ports of phase decoding integrated chip
As shown in fig. 2, a second embodiment of the integrated chip for polarization independent phase decoding according to the present invention:
the structure of the polarization-independent phase decoding integrated chip is as follows: the phase decoding integrated chip further comprises a second phase modulator 7 and a third phase modulator 8, wherein the second phase modulator 7 and the third phase modulator 8 are respectively positioned at symmetrical positions on two sides of the first waveguide delay line 5; the second phase modulator 7 and the third phase modulator 8 are applied with the same voltage for adjusting the phase of the third signal light component and the fifth signal light component.
The specific decoding process of the second embodiment is as follows:
the phase difference between the front time mode and the rear time mode of the phase coding state sent by the sending end is
And have the same polarization, can be written as
The polarization states of temporal modes |0> and |1> are assumed to be both horizontally polarized. After passing through a single-mode optical fiber channel, due to the existence of birefringence effect and the disturbance of the environment where the channel is located, the phase encoding state changes into a random polarization state when reaching a receiving end, and therefore the phase encoding state entering the decoding device can be written as
Wherein the content of the first and second substances,
。
an optical pulse with any polarization enters an input port In of the polarization-independent phase decoding integrated wave chip, first enters a first port of the first polarization beam splitting rotator 1, and is divided into a first signal light component and a second signal light component with the same polarization. The two are respectively emitted from the second port and the third port of the first polarization beam splitting rotator 1, respectively propagate along the waveguide L1 and the waveguide L2 in TE polarization mode, and simultaneously reach the first port of the first beam splitter 3 and the first port of the second beam splitter 4, and the quantum states of the two are respectively
Wherein, the first signal light component enters into the unequal arm Mach-Zehnder interferometer composed of the first beam splitter 3, the second beam splitter 4, the first waveguide delay line 5 and the first phase modulator 6 from the first port of the first beam splitter 3 for phase decoding, and is first split into a third signal light component and a fourth signal light component with the same amplitude and polarization by the first beam splitter 3, the third signal light component passes through the second phase modulator 7, the first waveguide delay line 5 and the third phase modulator 8, and the fourth signal light component passes through the first phase modulator 6 and then is changed into a quantum state
Wherein the content of the first and second substances,
the phases respectively modulated by the first phase modulator 6, the second phase modulator 7 and the third phase modulator 8 are all randomly modulated with the phase 0 or pi/2 and satisfy
。
Since the length difference between the path M1 and the path M2 is the arm length difference of the unequal arm mach-zehnder interferometer, the time pattern |0> of the third signal light component and the time pattern |1> of the fourth signal light component overlap in time, the remaining time patterns do not interfere with each other, and the quantum states of the first interference light and the second interference light resulting from the interference of the third signal light component and the fourth signal light component are obtained as the quantum states of the first interference light and the second interference light, respectively
Wherein, the first and the second end of the pipe are connected with each other,
the possible modulation results are shown in table 2.
TABLE 2 phase differences
Modulation result of (2)
Further, the light intensity of the first interference light and the second interference light can be obtained as
，
。
Similarly, the second signal light component enters the unequal arm mach zehnder interferometer composed of the first beam splitter 3, the second beam splitter 4, the first waveguide delay line 5 and the first phase modulator 6 from the first port of the second beam splitter 4 to be phase-decoded, and is first split by the second beam splitter 4 into a fifth signal light component and a sixth signal light component which have the same amplitude and polarization, the third signal light component passes through the second phase modulator 7, the first waveguide delay line 5 and the third phase modulator 8, and the fourth signal light component passes through the first phase modulator 6 to be changed into a quantum state
Since the difference between the lengths of the path M1 and the path M2 is the arm length difference of the unequal arm mach-zehnder interferometer, the time pattern |0> of the fifth signal light component and the time pattern |1> of the sixth signal light component overlap in time, the remaining time patterns do not interfere with each other, and the quantum states of the third interference light and the fourth interference light resulting from the interference of the fifth signal light component and the sixth signal light component are obtained as the quantum states of the third interference light and the fourth interference light, respectively
Further, the light intensity of the third interference light and the fourth interference light are obtained as
。
The first interference light and the third interference light enter the second polarization beam splitting rotator 2 simultaneously for polarization beam combination, and the light intensity output from the first port is
The second interference light and the fourth interference light enter the first polarization beam splitting rotator 1 simultaneously for polarization beam combination, and the light intensity output from the first port is
It follows that the interference results from the two output ports of the phase decoding integrated chip are only related to the modulated phase difference, and not to the incident polarization state, and therefore can be immune to random perturbations of the channel. The phase of first phase modulator 6, second phase modulator 7 and third phase modulator 8 is adjusted according to tables 1 and 2, i.e. a polarization independent phase decoding is possible.
As shown in fig. 3, a third embodiment of a polarization-independent phase decoding integrated chip according to the present invention:
the phase decoding integrated chip structure is as follows: the phase decoding integrated chip further comprises a first polarization rotation structure 9, a second waveguide delay line 10, a second polarization rotation structure 11 and a fourth phase modulator 12, and the first polarization beam splitting rotator 1 further comprises a fourth port; the polarization rotation angles of the first polarization rotating structure 9 and the second polarization rotating structure 11 are both 90 °, and are respectively located at the middle positions of the path M1 and the path M2, and are respectively used for rotating the polarization directions of the third signal light component, the fifth signal light component, the fourth signal light component and the sixth signal light component; the first waveguide delay line 5 and the second waveguide delay line 10 have the same length, are respectively located at symmetrical positions on two sides of the first polarization rotation structure 9, and are used for delaying the third signal light component and the fifth signal light component; the first phase modulator 6 and the fourth phase modulator 12 are respectively located at symmetrical positions on two sides of the second polarization rotating structure 11, and are used for adjusting the phases of the fourth signal light component and the sixth signal light component, and the loaded voltages of the fourth signal light component and the sixth signal light component are the same; and the fourth port of the first polarization beam splitting rotator 1 is used for outputting an optical signal after the second interference light and the fourth interference light are polarized and combined.
The third specific decoding process of the embodiment comprises the following steps:
the phase difference between the front time mode and the rear time mode of the phase coding state sent by the sending end is
And have the same polarization, can be written as
The polarization states of temporal modes |0> and |1> are assumed to be both horizontally polarized. After passing through a single-mode optical fiber channel, due to the existence of birefringence effect and the disturbance of the environment where the channel is located, the phase encoding state changes into a random polarization state when reaching a receiving end, and therefore the phase encoding state entering the decoding device can be written as
Wherein the content of the first and second substances,
。
an optical pulse with any polarization enters an input port In of the polarization-independent phase decoding integrated wave chip, firstly enters a first port of the first polarization beam splitting rotator 1, and is divided into a first signal light component and a second signal light component which are both TE polarization. The two are respectively emitted from the second port and the third port of the first polarization beam splitting rotator 1, respectively propagate along the waveguide L1 and the waveguide L2 in TE polarization mode, and simultaneously reach the first port of the first beam splitter 3 and the first port of the second beam splitter 4, and the quantum states of the two are respectively
Wherein, the first signal light component enters into the unequal arm Mach-Zehnder interferometer composed of the first beam splitter 3, the second beam splitter 4, the first waveguide delay line 5 and the first phase modulator 6 from the first port of the first beam splitter 3 for phase decoding, and is first split into a third signal light component and a fourth signal light component with the same amplitude and polarization by the first beam splitter 3, wherein the third signal light component rotates the polarization state by 90 degrees through the first polarization rotation structure 9 after passing through the first waveguide delay line 5 to become TM polarization, and then reaches the third port of the second beam splitter 4 through the second waveguide delay line 10, the fourth signal light component rotates the polarization state by 90 degrees through the second polarization rotation structure 11 after passing through the first phase modulator 6 to become TM polarization, and then reaches the second port of the second beam splitter 4 through the fourth phase modulator 12, and the polarization states are the same and are both TM polarization, quantum state change to
Wherein the content of the first and second substances,
modulated by a first phase modulator 6 and a fourth phase modulator 10, respectivelyPhase, and satisfy
。
Since the length difference between the path M1 and the path M2 is the arm length difference of the unequal arm mach-zehnder interferometer, the time pattern |0> of the third signal light component and the time pattern |1> of the fourth signal light component overlap in time, and the remaining time patterns do not interfere with each other, thereby obtaining first interference light and second interference light generated by the interference of the third signal light component and the fourth signal light component, both of which are TM polarized light and have quantum states of TM polarization
Wherein the content of the first and second substances,
. Further, the light intensity of the first interference light and the second interference light can be obtained as
，
A phase difference between the phase modulated by the first phase modulator 6 and the phase modulated at the transmitting end.
Similarly, the second signal light component enters the unequal arm mach zehnder interferometer composed of the first beam splitter 3, the second beam splitter 4, the first waveguide delay line 5 and the first phase modulator 6 from the first port of the second beam splitter 4 for phase decoding, and is first split by the second beam splitter 4 into a fifth signal light component and a sixth signal light component with the same amplitude and polarization, wherein the fifth signal light component passes through the second waveguide delay line 10, rotates the polarization state by 90 ° through the first polarization rotation structure 9, becomes TM polarization, passes through the first waveguide delay line 5 to reach the third port of the first beam splitter 3, the sixth signal light component passes through the fourth waveguide delay line 12, rotates the polarization state by 90 ° through the second polarization rotation structure 11, also becomes TM polarization, passes through the first phase modulator 6 to reach the second port of the first beam splitter 3, the polarization states of the sixth signal light component are the same and are both TM polarization, quantum state change to
Since the difference between the lengths of the path M1 and the path M2 is the arm length difference of the unequal arm mach-zehnder interferometer, the time pattern |0> of the fifth signal light component and the time pattern |1> of the sixth signal light component overlap in time, the remaining time patterns do not interfere with each other, and the quantum states of the third interference light and the fourth interference light resulting from the interference of the fifth signal light component and the sixth signal light component are obtained as the quantum states of the third interference light and the fourth interference light, respectively
Further, the light intensity of the third interference light and the fourth interference light can be obtained as
。
The first interference light and the third interference light enter the second polarization beam splitting rotator 2 simultaneously for polarization beam combination, and the light intensity output from the first port is
The second interference light and the fourth interference light enter the first polarization beam splitting rotator 1 simultaneously for polarization beam combination, and because the polarization of the two interference lights is changed into TM polarization, the combined interference light is output from the fourth port of the first polarization beam splitting rotator 1, and the light intensity is
It follows that the interference results from the two output ports of the phase decoding integrated chip are only related to the modulated phase difference, and not to the incident polarization state, and therefore can be immune to random perturbations of the channel. Moreover, due to the fact that the 90-degree polarization rotation structure is added, one path of output port can be separated from the input port, and interference light does not need to be led out by using a circulator.
As shown in fig. 4, the quantum key distribution system based on the phase decoding integrated chip of the present invention:
the quantum key distribution system comprises a sending end A, a receiving end B and a channel C for connecting the sending end A and the receiving end B, wherein the sending end A comprises a laser A-1, an intensity modulator A-2, a sending end interferometer A-3 and an adjustable attenuator A-4 which are sequentially connected, the receiving end B comprises a receiving end interferometer B-1, a first single-photon detector B-2 and a second single-photon detector B-3, and two output ends of the receiving end interferometer B-1 are respectively connected with the first single-photon detector B-2 and the second single-photon detector B-3; the sending end interferometer A-3 is an unequal arm Mach-Zehnder interferometer integrated chip and comprises a third beam splitter A-3-1, a third waveguide delay line A-3-2, a fifth phase modulator A-3-3 and a fourth beam splitter A-3-4; the receiving end interferometer B-1 is a three-phase decoding integrated chip of the embodiment.
The specific process of quantum key distribution comprises the following steps:
a laser A-1 of a sending end A generates an optical pulse signal with repetition frequency f, a signal state and a decoy state are generated after the light intensity is randomly modulated by an intensity modulator A-2, and then the signal state and the decoy state enter a sending end interferometer A-3 for phase encoding. The sending end interferometer A-3 is an unequal arm Mach-Zehnder interferometer integrated chip, and the arm length difference is the same as that of the receiving end interferometer B-1. The time difference of two pulse components emitted from the output port of the fourth beam splitter A-3-4 is delta t, and the phase difference is
The light intensity is adjusted to single photon magnitude by an adjustable attenuator A-4 to obtain quantum state
Wherein
The phases 0, pi/2, pi, 3 pi/2 are randomly modulated.
The quantum state prepared by the sending terminal A reaches the receiving terminal B through the channel C, and after decoding is carried out according to the decoding process of the third embodiment, the two paths of interference light respectively enter the first single-photon detector B-2 and the second single-photon detector B-3. Modulating the phase according to Table 1
Corresponding detection results can be obtained.
And comparing the detection result with the basis vector information corresponding to the modulation phase to obtain an initial key, and performing post-processing processes such as error code estimation, error correction, secret amplification and the like to generate a safe quantum key between the sending end A and the receiving end B.
It can be known from the synthesis of the embodiments of the present invention that the present invention provides a polarization independent phase decoding integrated chip, wherein a polarization beam splitting rotator is used to split two orthogonal polarization components of an input signal light, the two orthogonal polarization components are interfered by the same unequal arm mach-zehnder interferometer from opposite directions, and the interference results of the two polarization components are output after polarization beam combining, so that polarization independent stable phase decoding can be realized, magneto-optical crystals and other devices which are difficult to integrate are not required, the overall size of the integrated waveguide interferometer can be reduced, and the integration level is greatly improved. And through respectively adding 90-degree polarization rotation structures on two arms of the interferometer, the input port and the output port of the integrated chip can be further separated, and the use of a circulator is avoided. The quantum key distribution system based on the phase decoding integrated chip greatly reduces the volume of an optical system, can realize miniaturization of equipment, has the characteristic of polarization disturbance of an immune channel, and can realize long-term working stability of the system.
Claims (7)
1. A polarization-independent phase decoding integrated chip is characterized by comprising a first polarization beam splitting rotator, a second polarization beam splitting rotator, a first beam splitter, a second beam splitter, a first waveguide delay line and a first phase modulator which are integrated on the same substrate,
the first port of the first polarization beam splitting rotator is used as an input port In and a first output port Out1 of the phase decoding integrated chip, and the first port of the second polarization beam splitting rotator is used as a second output port Out2 of the phase decoding integrated chip; the second port of the first polarization beam splitting rotator is connected with the first port of the first beam splitter through a waveguide L1; the third port of the first polarization beam splitting rotator is connected with the first port of the second beam splitter through a waveguide L2; the second port of the second polarization beam splitting rotator is connected with the fourth port of the first beam splitter through a waveguide L3; the third port of the second polarization beam splitting rotator is connected with the fourth port of the second beam splitting through a waveguide L4; the third port of the first beam splitter is connected with the third port of the second beam splitter through a first waveguide delay line to form a path M1; the second port of the first beam splitter is connected to the second port of the second beam splitter through a first phase modulator to form a path M2;
the first polarization beam splitting rotator is used for carrying out polarization beam splitting on input signal light to generate a first signal light component and a second signal light component which have the same polarization;
the first beam splitter is configured to split the first signal light component to generate a third signal light component and a fourth signal light component; the second beam splitter is configured to split the second signal light component to generate a fifth signal light component and a sixth signal light component;
the first waveguide delay line is used for simultaneously delaying the third signal light component and the fifth signal light component; the first phase modulator is configured to phase-modulate the fourth signal light component and the sixth signal light component at the same time;
the second beam splitter is further configured to interfere the third signal light component and the fourth signal light component to generate first interference light and second interference light; the first beam splitter is further configured to interfere the fifth signal light component and the sixth signal light component to generate third interference light and fourth interference light;
the second polarization beam splitting rotator is used for carrying out polarization beam combination on the first interference light and the third interference light and outputting the first interference light and the third interference light;
the first polarization beam splitting rotator is also used for carrying out polarization beam combination on the second interference light and the fourth interference light and outputting the second interference light and the fourth interference light.
2. The integrated polarization-independent phase decoding chip of claim 1, further comprising a second phase modulator and a third phase modulator, wherein the second phase modulator and the third phase modulator are respectively positioned symmetrically on two sides of the first waveguide delay line; the second phase modulator and the third phase modulator are applied with the same voltage for adjusting the phase of the third signal light component and the phase of the fifth signal light component.
3. The polarization-independent phase-decoding integrated chip of claim 1, wherein the phase-decoding integrated chip further comprises a first polarization-rotating structure, a second waveguide delay line, a second polarization-rotating structure, a fourth phase modulator, the first polarization splitting rotator further comprising a fourth port;
the polarization rotation angles of the first polarization rotation structure and the second polarization rotation structure are both 90 degrees, are respectively located in the middle positions of the path M1 and the path M2, and are respectively used for rotating the polarization directions of the third signal light component, the fifth signal light component, the fourth signal light component and the sixth signal light component;
the first waveguide delay line and the second waveguide delay line have the same length, are respectively positioned at symmetrical positions on two sides of the first polarization rotation structure, and are used for delaying the third signal light component and the fifth signal light component;
the first phase modulator and the fourth phase modulator are respectively positioned at symmetrical positions on two sides of the second polarization rotating structure and are used for adjusting the phase of the fourth signal light component and the phase of the sixth signal light component, and the loaded voltages of the first phase modulator and the fourth phase modulator are the same;
and the fourth port of the first polarization beam splitting rotator is used for outputting an optical signal after the second interference light and the fourth interference light are polarized and combined.
4. The polarization independent phase decoding integrated chip of claim 1, 2 or 3, wherein the lengths of waveguide L1, waveguide L2, waveguide L3 and waveguide L4 are all equal.
5. The integrated polarization independent phase decoding chip of claim 1, 2 or 3, wherein the first waveguide delay line and the first phase modulator are located midway between path M1 and path M2, respectively.
6. The polarization-independent phase decoding integrated chip of claim 1, 2 or 3, wherein the first beam splitter and the second beam splitter are both multimode interference couplers or 3dB directional couplers.
7. A quantum key distribution system comprises a sending end, a receiving end and a channel for connecting the sending end and the receiving end, and is characterized in that the sending end comprises a laser, an intensity modulator, a sending end interferometer and an adjustable attenuator which are sequentially connected, the receiving end comprises a receiving end interferometer, a first single-photon detector and a second single-photon detector, and two output ends of the receiving end interferometer are respectively connected with the first single-photon detector and the second single-photon detector; the sending end interferometer is an unequal arm Mach-Zehnder interferometer integrated chip and comprises a third beam splitter, a third waveguide delay line, a fifth phase modulator and a fourth beam splitter; the receiving-end interferometer is a polarization-independent phase decoding integrated chip in any one of claims 1 to 6.
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