CN111585747A - Transmitting end, encoding method and quantum communication system for realizing six polarization state encoding - Google Patents
Transmitting end, encoding method and quantum communication system for realizing six polarization state encoding Download PDFInfo
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Abstract
The invention relates to a transmitting end, an encoding method and a quantum communication system for realizing six polarization state codes, which can realize four polarization state codes commonly used in the preparation of BB84 protocols, such as H, V, and the like, with the least optical elements and in the most stable way,
In addition, two other polarization state encodings can be prepared, e.g.
Description
Technical Field
The invention relates to the field of quantum secure communication, in particular to a transmitting end, an encoding method and a quantum communication system for realizing six polarization state encoding.
Background
Quantum key distribution techniques have received much attention as they enable the generation of perfectly consistent unconditionally secure keys between two communicating parties. Since the proposal of BB84 in 1984, various theoretical schemes are perfected day by day, and the technology is gradually mature and goes to practical application. In quantum communication systems, encoding qubits using polarization information of quantum states is one of the most popular schemes. Polarization encoded quantum communication generally requires emission of 4 quantum states of different polarization, and there are two main implementations. One is a multi-laser scheme, i.e. 4 lasers are used to emit polarization states such as 0-degree linear polarization state, 45-degree linear polarization state, 90-degree linear polarization state, 135-degree linear polarization state, etc. respectively, and then are synthesized into one beam for output, because inevitable differences (such as central wavelength, spectral distribution, pulse shape, etc.) among different lasers bring some security holes, and the laser is gradually eliminated in quantum communication. The other approach is a single laser approach, where the light is modulated into different polarization states by a polarization modulator, which avoids the vulnerability of parameter inconsistency of multiple lasers, but the use of polarization modulators is costly, has poor performance, and especially has too slow a rate.
In fiber optic systems, where the phase modulator is very high in speed, there have been proposals to use phase modulators in interferometric rings to produce polarization states. The main principle is that two paths of light with different polarization states are firstly modulated by a phase controller, and then the phase difference between the two paths of light is formed into new polarization state output through beam combination interference. The study on the phase-polarization encoded quantum secure communication system published in physical science and newspaper, 54(06),2005: 2534-; the article "high-speed polarization control scheme in all-fiber quantum communication system" published by Li Shen et al in physical science, 62,2013:084214 proposes a Sagnac interferometer system formed by using a polarization beam splitter PBS to prepare four polarization states, namely a 45-degree linear polarization state, a 135-degree linear polarization state, a right-handed circular polarization state, a left-handed circular polarization state and the like; mohseng et al proposed an unequal arm interferometer system using a polarizing beam splitter PBS in patent document No. CN101150371B, and prepared four polarization states, i.e., a 45 ° linear polarization state, a 135 ° linear polarization state, a right-handed circular polarization state, and a left-handed circular polarization state; patent application publication No. CN104579564A by royal kingdong et al discloses a Sagnac interferometer system constructed using a polarizing beamsplitter PBS to produce four polarization states, 45 ° linear, 135 ° linear, right-hand and left-hand circular.
It is obvious from the prior art that no matter what interferometer structure is used, the prior scheme can only prepare four polarization states, but cannot realize quantum communication protocols with more than 4 polarization states, such as 6-state protocols using 6 polarization states, i.e. 0-degree linear polarization state, 45-degree linear polarization state, 90-degree linear polarization state, 135-degree linear polarization state, right-handed circular polarization state, left-handed circular polarization state, and the like, and reference system unrelated quantum communication protocols using the 6 polarization states.
Therefore, the combination of two four-state quantum encoders is often used in the prior art to prepare six polarization states. For example, chinese patent application No. ZL03139638.0 discloses a quantum cryptography communication method using a six-state quantum encoder and based on BB84 protocol, as shown in fig. 1, the six-state quantum encoder is formed by connecting two four-state quantum encoders through a synchronous flip-flop, wherein the two four-state quantum encoders are rotated by 45 °, and the polarization directions of polarization beam splitters in the two four-state quantum encoders are set to be 45 °, so that the 45 ° and 135 ° linearly polarized lights output by the first four-state quantum encoder become 0 ° and 90 ° linearly polarized lights for the second four-state quantum encoder, and the left-handed and right-handed circularly polarized lights output by the first four-state quantum encoder remain left-handed and right-handed circularly polarized lights for the second four-state quantum encoder. Thus, it is possible to provideFor the second four-state quantum encoder, there are four polarization states of input photons, i.e. linear polarization of 0 ° and 90 ° and left-and right-hand circular polarization, when the input voltage of the phase modulator of the second four-state quantum encoder is randomly generated by the true random generator to be 0, V0/2、V0、3V0When the four output voltages are provided, the polarization states of the output photons are respectively one of the six non-orthogonal polarization states of linear polarization of 0 degrees, 45 degrees, 90 degrees and 135 degrees and left-handed and right-handed circular polarization. Under the structure of the quantum encoder, a sender randomly prepares six non-orthogonal polarization state photons by using a six-state quantum encoder, the photons are transmitted to a receiver through an optical fiber, the receiver randomly generates six non-orthogonal polarization state measurement bases by using a six-state quantum decoder, the six non-orthogonal polarization state photons transmitted by the sender are detected, the used measurement bases are transmitted to the sender through a public channel under the condition that the photons are detected, the sender tells the receiver that the measurement bases are selected, then the corresponding bits are reserved by the sender and the receiver when the bases are consistent, other data are abandoned, the receiver randomly publishes some bits for the sender to confirm whether errors exist, and finally, after the sender confirms that no errors exist and can confirm that no people eavesdrop, the remaining bit sequence is reserved as a cipher book.
However, two 4-state encoders are required in the six-state quantum encoder, and a plurality of modulation devices are required, the optical path is complicated, and the overall cost is greatly increased due to the complicated devices. On the other hand, in such an encoder, a single-mode fiber is used to connect two interferometers, so a polarization controller (code 10 in fig. 1) needs to be introduced to accurately control the polarization state, which greatly reduces the stability of the system and is easily affected by environmental temperature changes or vibration. In addition, because a plurality of devices are adopted in the encoder, the system volume is difficult to reduce, and the integration level is poor.
The patent application CN 201811496007.1 discloses a quantum secure communication optical path, but it is used for realizing the preparation of six quantum states in time phase state, and the realization optical path thereof also has the problems of complex structure, multiple modulation devices, difficult reduction of system volume, poor integration level, etc., and cannot be used for preparing 6 polarization quantum states.
Disclosure of Invention
In view of the above problems in the prior art, the present invention provides a transmitting end and an encoding method for quantum communication system, which can realize four polarization state codes, such as H, V, and the like, commonly used in the preparation of BB84 protocol with a minimum of optical elements (such as phase modulator and other elements),
In addition, two other polarization state encodings can be prepared, e.g.
One aspect of the present invention relates to a transmitting end for realizing six polarization state codes, which includes a light source 1, a phase modulation module 2 and an optical path folding module 3. The light source 1 is used for providing signal light to be coded; the phase modulation module 2 includes a first phase modulation unit 211 and a polarization beam splitting unit 212, and is configured to: the signal light to be encoded is divided into a first signal light component and a second signal light component, and a first phase difference θ is formed between the first and second signal light components by the first phase modulation unit 2111And causing the first and second signal light components to interfere to output a first interfering light signal; the optical path folding module 3 is configured to receive the first interference optical signal and fold the first interference optical signal, and includes a second phase adjustment unit for performing phase adjustment on the first interference optical signal; and the phase modulation module 2 receives the folded first interference optical signal, and is further configured to: enabling the folded first interference optical signal to generate interference action to form a second interference optical signal, and enabling the second interference optical signal to be output through the polarization beam splitting unit 212; alternatively, the folded first interference optical signal is divided into first and second folded optical signal components, and a second phase difference θ is formed between the first and second folded optical signal components by the first phase modulation unit 2112And anThe first and second folded optical signal components are coupled out through the polarization beam splitting unit 212.
Further, the phase modulation module 2 further comprises a beam splitting unit 213, and is further configured to: at the first phase difference theta1Outputting two first interference optical signals respectively via two output ports of the beam splitting unit 213 when the first preset value is reached, and outputting the first interference optical signals at the first phase difference theta1Outputting the first interference optical signal via only one of two output ports of the beam splitting unit 213 when the first interference optical signal is a second preset value; the optical path folding module 3 is further configured to: at the first phase difference theta1When the first preset value is reached, the phase difference between the two first interference light signals is changed by theta3(ii) a And the phase modulation module 2 is further arranged to: make the turn back and have the phase difference change theta3The two first interference optical signals interfere at the beam splitting unit 213 to form the second interference optical signal; and at the first phase difference theta1When the first preset value is the second preset value, the folded first interference optical signal is split into the first and second folded optical signal components at the beam splitting unit 213.
Further, the optical path folding module 3 further includes an optical loop, and the second phase adjustment unit includes a phase modulator that is provided on the optical loop and has different optical paths with respect to two output ports of the phase modulation module 2 for the first interference optical signal; alternatively, the optical path folding module 3 further includes two reflection units respectively connected to the two output ports of the phase modulation module 2 for the first interference optical signal, and the second phase adjustment unit includes a phase shifter or a phase modulator provided in at least one of the two reflection units.
Further, the polarization beam splitting unit 212 and the beam splitting unit 213 are connected by a first optical path and a second optical path having the same optical path to form an equal-arm MZ interferometer, and the first phase modulation unit 211 is disposed on the first optical path and/or the second optical path; and/or, the polarization beam splitting unit 212 includes a polarization beam splitter; and/or, the beam splitting unit 213 comprises a beam splitter; and/or, the first phase modulation unit 211 includes a phase modulator.
Further, the transmitting end of the invention further comprises a phase detection feedback module, wherein the phase detection feedback module comprises a light splitting unit, a phase detection unit, a phase feedback algorithm unit, a phase shifter driving unit and a phase shifter.
Preferably, the first preset value is pi/2 or 3 pi/2, and the phase difference change theta3Is pi/2; and/or the second preset value is 0 or pi, and the second phase difference theta2Is 0, pi/2 or 3 pi/2.
Preferably, the six polarization state encodings include H, V,
Another aspect of the present invention relates to a quantum communication system including the above-described transmitting end.
Yet another aspect of the invention relates to an encoding method for implementing six polarization state encoding, comprising, for a first time, an interference step: the signal light to be encoded is divided into a first signal light component and a second signal light component, and a first phase difference θ is formed between the first and second signal light components by a first phase modulation unit 2111And causing the first and second signal light components to interfere to output a first interfering light signal; a light path folding step: enabling the first interference optical signals to be folded back, and carrying out phase adjustment on the first interference optical signals when the number of the first interference optical signals is two; a polarization state generation step: enabling the folded first interference optical signal to generate interference action to form a second interference optical signal, and enabling the second interference optical signal to be output through a polarization beam splitting unit 212; or, the folded first interference optical signal is divided into first and second folded optical signal components,forming a second phase difference θ between the first and second folded optical signal components using the first phase modulation unit 2112And having the second phase difference θ2Is coupled out via the polarization beam splitting unit 212.
Further, the first interference step further comprises the steps of: splitting the signal light to be encoded into the first and second signal light components via the polarization beam splitting unit 212; at the first phase difference theta1When the first preset value is the first preset value, two first interference optical signals are output through two output ports of the beam splitting unit 213 respectively; and at the first phase difference theta1When the first preset value is the second preset value, outputting the first interference optical signal through only one of the two output ports of the beam splitting unit 213; and/or the optical path folding step further comprises the steps of: at the first phase difference theta1When the first preset value is reached, the phase difference between the two first interference optical signals is subjected to phase difference change theta3(ii) a And/or the polarization state generating step further comprises the steps of: make the folded part have a phase difference change theta3The two first interference optical signals interfere at the beam splitting unit 213 to form the second interference optical signal; and at the first phase difference theta1When the first preset value is the second preset value, the folded first interference optical signal is split into the first and second folded optical signal components at the beam splitting unit 213.
Preferably, the first preset value is pi/2 or 3 pi/2, and the phase difference change theta3Is pi/2; and/or the second preset value is 0 or pi, and the second phase difference theta2Is 0, pi/2 or 3 pi/2.
Preferably, the encoding method of the present invention further comprises a phase detection feedback step.
Preferably, the six polarization state encodings include H, V,
Preferably, the encoding method of the present invention is implemented by using the transmitting end described above.
Drawings
FIG. 1 illustrates a prior art six-state quantum encoder;
FIG. 2 illustrates another prior art six-state quantum encoder;
FIG. 3 illustrates an exemplary embodiment of a transmitting end for implementing six polarization state encoding according to the present invention; and
fig. 4a and 4b show two exemplary embodiments of the optical path folding module of the present invention, respectively.
Detailed Description
Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings. The following examples are provided by way of illustration in order to fully convey the spirit of the invention to those skilled in the art to which the invention pertains. Accordingly, the present invention is not limited to the embodiments disclosed herein.
In the encoding method for realizing six polarization state encoding of the present invention, it basically comprises the following steps. By way of example, the six polarization states may include H, V,
A first interference step: splitting the signal light to be encoded into a first signal light component and a second signal light component by a polarization splitting unit, for example, splitting the two components of the signal light to be encoded in such a manner that polarization directions thereof are perpendicular to each other; performing a first phase modulation on at least one of the first and second signal light components using a first phase modulation unit to form a first phase difference θ therebetween1(ii) a And has a first phase difference theta1To generate a first interference optical signal.
As a preferred example, the first interference action step may be realized by means of an equal-arm MZ interferometer constituted by a polarization beam splitting unit and a beam splitting unit, and a first phase modulation unit provided in the interferometer. As an example, the polarization beam splitting unit may include a polarization beam splitter, the beam splitting unit may include a beam splitter, and the first phase modulation unit may include a phase modulator. As is readily understood by those skilled in the art, with such an equal-arm MZ interferometer structure, the first signal light component and the second signal light component will appear to have the same polarization direction at the beam splitter, so that interference occurs.
As a preferred example, the first signal light component and the second signal light component may have the same light intensity.
As a preferred example, the first phase difference θ1May be 0, pi/2 or 3 pi/2.
A light path folding step: the first interference light signal is folded back, wherein, when the first phase difference theta1When the first preset value is used for enabling the first interference step to generate two first interference optical signals, the second phase adjusting unit is used for enabling the phase difference between the two first interference optical signals to generate phase difference change theta3。
As a preferred example, the folding back of the light may be achieved by connecting two outputs for the first interfering light signal with an optical loop, wherein the second phase adjustment unit may comprise a phase modulator.
As another preferred example, reflection units may be connected to the two output ends for the first interference light signal, respectively, to achieve the folding of the light, wherein the second phase adjustment unit may be implemented by including a phase modulator or a phase shifter, preferably a phase shifter. Although the phase modulator is more expensive than the phase shifter, the phase modulator is a high-speed phase adjusting unit, the phase shifter is a slow-speed phase adjusting unit, and in the example of using the optical path loop, since the phase adjustment needs to be completed within a time interval of two first interference optical signals simultaneously existing in the loop, the second phase adjusting unit can be realized only by using the phase modulator with a faster speed; in the example of respectively connecting the reflection units to realize the optical path folding module, the phase adjustment time is not limited as described above, so that the second phase adjustment unit can be realized by using a phase shifter with a slower speed, thereby effectively reducing the system cost.
As a preferred example, the first preset value may be pi/2 or 3 pi/2, and the phase difference change θ3May be pi/2.
A polarization state generation step: when the first phase difference theta1When the phase difference is a first preset value, the phase difference is changed by theta3The two first interference optical signals generate a second interference action to generate a second interference optical signal; and the second interference light signal is output after the action of the polarization beam splitting unit, so that the coded signal light is obtained.
As an example, when the first preset value is pi/2 or 3 pi/2, the phase difference varies by theta3At π/2, the encoded signal light may have an H or V polarization state.
When the first phase difference theta1When the first preset value is a first preset value, dividing the folded first interference optical signal into a first folded optical signal component and a second folded optical signal component; performing a second phase modulation on at least one of the first and second folded optical signal components with a first phase modulation unit to form a second phase difference θ therebetween2(ii) a And has a second phase difference theta2For example, the two components interfere at the polarization beam splitting unit, and the output interference light signal is the encoded signal light.
As a preferred example, the second preset value may be 0 or pi, and the second phase difference θ2May be 0, pi/2 or 3 pi/2, in which case the encoded signal light may have
And the four polarization states.
It can be seen that in the encoding method of the present invention, the pass is θ1、θ2And theta3By selecting appropriate values, the encoded signal light can have any one of six polarization states, so that the six polarization states can be encoded. E.g. at theta1When the value is 0 or pi, the value corresponds to theta2The value of 0, pi/2 or 3 pi/2 can make the encoded signal light have polarization state
Or
At theta1Take a value of pi/2 or 3 pi/2, and theta3When the value is pi/2, the coded signal light can have the polarization state H or V. Furthermore, by means of the optical path folding step, the first interference step and the polarization state generation step can be realized by means of the same optical structure, except that in the two steps, the propagation directions of light are opposite.
To further illustrate the encoding principle of the present invention, fig. 3 shows an exemplary embodiment of the transmitting end for six polarization state encoding of the present invention.
As shown in the figure, the transmitting end of the present invention may include a light source 1, a phase modulation module 2, and an optical path folding module 3.
In the present invention, the light source 1 is used to output signal light to be encoded, which may be in the form of a laser, for example, to output a laser pulse signal.
The phase modulation module 2 is used to implement the first interference step and the polarization state generation step in the encoding method of the present invention. In this embodiment, as shown in fig. 3, the phase modulation module 2 may include a first phase modulation unit 211, a polarization beam splitting unit 212, and a beam splitting unit 213.
The polarization beam splitting unit 212 receives the signal light to be encoded and equally divides it into first and second signal light components according to two components of the signal light to be encoded whose polarization directions are perpendicular to each other. As an example, the polarization beam splitting unit 212 may include a Polarization Beam Splitter (PBS), and as shown in fig. 3, the signal light enters through the 3 rd port of the polarization beam splitting unit 212, and the split first and second signal light components are output through the 1 st port and the 2 nd port thereof, respectively.
The first and second signal light components output from polarization beam splitting unit 212 propagate toward the two input ends of beam splitting unit 213 along a first optical path and a second optical path, respectively. The first optical path and the second optical path are set to have the same optical path length so that the first and second signal light components can reach the beam splitting unit 213 at the same time. Therefore, the polarization beam splitting unit 212, the beam splitting unit 213, and the first and second optical paths therebetween substantially form an equal-arm MZ interferometer structure. Here, the beam splitting unit 213 may include a Beam Splitter (BS). As will be readily understood by those skilled in the art, the first and second signal light components will exhibit the same polarization direction for the beam splitting unit 213.
The first phase modulation unit 211 is disposed on one of the first optical path and the second optical path, and performs phase modulation on the passed signal light component to form a preset phase difference between the two components. For example, as shown in fig. 3, the first phase modulation unit 211 is disposed on the first optical path for performing a process of making the magnitude of the first signal light component, which is transmitted along the first optical path, be θ1So as to form a first phase difference θ between the first and second signal light components1. As an example, the first phase modulation unit 211 may include a phase modulator.
With a phase difference theta1Reaches the beam splitting unit 213 at the same time, where the first interference occurs to generate the first interference optical signal. Those skilled in the art will readily appreciate that the result of the first interference is out of phase with the first phase difference θ1Relating, for example, to the first phase difference θ1The output of the first interference is determined to be one or two interference optical signals, and the interference optical signals are output from the beam splitting unit 213. As an example, when θ1When the value is 0, only the interference optical signal is output at the port 7 of the beam splitting unit 213 as a result of the first interference, that is, only one first interference optical signal enters the optical path returning module 3; when theta is1When pi, the result of the first interference is only to output the interference optical signal at port 8 of the beam splitting unit 213That is, only one first interference optical signal enters the optical path returning module 3, but the output port is changed; when theta is1When the interference is pi/2 or 3 pi/2, the first interference light signal is output at the ports 7 and 8 of the beam splitting unit 213 at the same time, that is, two first interference light signals enter the optical path returning module 3 through different output ports respectively.
Two output ports 7 and 8 of the beam splitting unit 213 are connected to the optical path folding module 3, which is used for folding the first interference optical signal and generating a first phase difference θ1When the first preset value is used for enabling the first interference step to generate two first interference optical signals, the second phase adjusting unit is used for enabling the phase difference between the two first interference optical signals to generate phase difference change theta3. As an example, fig. 4a and 4b show two exemplary embodiments of the optical path folding module 3, respectively.
As shown in fig. 4a, the optical path folding module 3 may include an optical loop connecting the two output ports 7 and 8 of the beam splitting unit 213, and a second phase adjustment unit provided in the loop. In this embodiment, the second phase adjustment unit is arranged to have a different optical path length from the two output ports 7 and 8, so that the first interference light signals respectively output by the two output ports 7 and 8 can pass through the second phase adjustment unit at different times, thereby enabling phase modulation of at least one of the two first interference light signals to cause a phase difference change θ in the phase difference therebetween3. As an example, the second phase adjustment unit here may comprise a phase modulator.
As another embodiment, as shown in fig. 4b, the optical path folding module 3 may include a first reflection unit 31 and a second reflection unit 32, which are respectively connected to the two output ports 7 and 8 of the beam splitting unit 213, and are used for folding the input first interference optical signal. Wherein, at least one of the first reflection unit and the second reflection unit is provided with a second phase adjustment unit for performing phase modulation on at least one of two first interference optical signals simultaneously propagating in the first reflection unit and the second reflection unit respectively so as to change the phase difference between the two signals by theta3. AsThe second phase adjustment unit here may comprise, for example, a phase modulator or a phase shifter, preferably a phase shifter.
The first interference optical signal is reflected back to the phase modulation module 2, i.e. the beam splitting unit 213, after being acted by the optical path reflection module 3.
In the phase modulation module 2, when the first phase difference θ1When the phase difference is a first preset value, the phase difference is changed by theta3The two first interference optical signals generate a second interference action at the beam splitting unit 213 to generate a second interference optical signal, and the second interference optical signal is acted by the polarization beam splitting unit 212 to output encoded signal light. As an example, when the first preset value is pi/2 or 3 pi/2, the phase difference varies by theta3At π/2, the encoded signal light may have an H or V polarization state.
When the first phase difference theta1When the first preset value is the second preset value, the folded first interference optical signal is divided into a first folded optical signal component and a second folded optical signal component, and at least one of the first folded optical signal component and the second folded optical signal component is subjected to second phase modulation by using the first phase modulation unit so as to form a second phase difference theta between the first phase modulation unit and the second phase modulation2And has a second phase difference theta2The two components are coupled by the polarization beam splitting unit and then output encoded signal light, for example, the two components interfere with each other to generate encoded signal light.
As a preferred example, the second preset value may be 0 or pi, and the second phase difference θ2May be 0, pi/2 or 3 pi/2, in which case the encoded signal light may be
Taking fig. 4a as an example, the following table illustrates six polarization state codes that can be implemented by using the transmitting end and the encoding method of the present invention.
Phase modulation theta1、θ2、θ3Correspondence table with codes
In the transmitting end structure of the present invention, an equiarm MZ interferometer with a phase modulation function is formed by a polarization beam splitting unit 212 (e.g., a polarization beam splitter), a beam splitting unit 213 (e.g., a beam splitter) and a phase modulation unit 211, and 6 polarization state codes based on the BB84 protocol are realized in a very simple and efficient structure by multiplexing the equiarm MZ interferometer through a foldback module 3, and since the multiplexing interferometer provides two phase modulation and interference effects, a phase drift caused by the interferometer in the prior art can be eliminated, and the coding stability of the transmitting end is improved.
Further, the present inventors have noticed that, due to the use of the equiarm MZ interferometer structure in the transmitting-end structure of the present invention, phase drift may occur in the signal light while propagating therein. Therefore, the phase detection feedback module is also preferably in the transmitting end in the present invention. As an example of the phase detection feedback module, it may include a light splitting unit, a phase detection unit, a phase feedback algorithm unit, a phase shifter driving unit, and a phase shifter. A light splitting unit is provided in the optical path folding module 3 and/or the phase modulation module 2 for splitting the optical signal to the phase detection unit to detect the phase difference signal of the two arms of the interferometer and/or the phase change provided by the second phase adjustment unit, for example, in fig. 4b, a light splitting module may be provided in both the optical path folding module 3 and the phase modulation module 2 to respectively correspond to θ1And theta3Detection is performed to provide feedback. The phase detection unit is used for detecting the phase drift amount; the phase feedback algorithm unit is used for calculating the phase quantity needing feedback compensation according to the detected phase drift quantity; the phase shifter is arranged in the equal-arm MZ interferometer and/or the optical path folding module 3, and performs corresponding phase compensation on the signal light under the driving signal provided by the phase shifter driving unit(e.g. for θ)1And/or theta3) Therefore, closed loop phase locking is realized, and phase stability is ensured. Since those skilled in the art can understand and implement the corresponding phase detection, calculation and feedback processes under the condition that the functions of the units of the phase detection feedback module have been described, details thereof are not repeated herein.
Illustratively, the first phase difference θ will be briefly described below in conjunction with fig. 4a1When the polarization state is 0, pi/2 and 3 pi/2 respectively, the coding method and the coding process under the structure of the transmitting terminal are based on the six polarization state coding methods.
θ1When the ratio is 0: the signal light to be encoded, which is generated by the light source 1, is input from the port 3 of the polarization beam splitter 212, and is split into first and second signal light components, which enter the first optical path and the second optical path via the ports 1 and 2 of the polarization beam splitter 212, respectively. The phase modulator 211 on the first optical path does not modulate the phase of the first component, i.e. the first phase difference θ provided by the first phase modulator 211 between the first and second components10. The two signal light components with zero phase difference arrive at ports 5 and 6, respectively, of the beam splitter 213 at the same time and there occurs a first interference, where only the first interfering light signal will appear at the output port 7 of the beam splitter. The first interference optical signal returns to port 8 of the beam splitter 213 after being acted by the optical path returning module. The folded optical signal is split into two folded optical signal components by the beam splitter 213 to propagate along the first optical path and the second optical path, respectively, wherein the first phase modulator 211 performs the second phase modulation on the folded optical signal component on the first optical path again to form a second phase difference θ between the two2. Having a second phase difference theta2The two folded optical signal components arrive at the polarization beam splitter 212 at the same time and are coupled out through the polarization beam splitter 212, for example, an interference occurs at the polarization beam splitter 212, and at this time, the polarization state of the signal light output through the coupling (for example, through the interference) can be expressed as
Thus, by selecting the appropriate θ2Can doNow any polarization state other than H, V. For example, select θ20, pi/2, 3 pi/2, four different polarization states can be obtained on the signal light finally output by the polarization beam splitter of the interferometer,
θ1when pi is equal to: the signal light to be encoded, which is generated by the light source 1, is input from the port 3 of the polarization beam splitter 212, and is split into first and second signal light components, which enter the first optical path and the second optical path via the ports 1 and 2 of the polarization beam splitter 212, respectively. The phase modulator 211 on the first optical path applies a modulation amount of pi to the phase of the first component, i.e., a first phase difference θ provided by the first phase modulator 211 between the first and second components1Pi. The two optical signal components with a phase difference of pi arrive at ports 5 and 6 of the beam splitter 213 at the same time and interfere there for the first time, only the first interfering optical signal will be present at the output port 8 of the beam splitter. The first interference optical signal returns to port 7 of the beam splitter 213 after being acted by the optical path returning module. The folded optical signal is split into two folded optical signal components by the beam splitter 213 to propagate along the first optical path and the second optical path, respectively, wherein the first phase modulator 211 performs the second phase modulation on the folded optical signal component on the first optical path again to form a second phase difference θ between the two2. Having a second phase difference theta2The two folded optical signal components arrive at the polarization beam splitter 212 at the same time and are coupled out through the polarization beam splitter 212, for example, at the polarization beam splitter 212, an interference occurs, and at this time, the polarization state of the signal light output through the coupling (for example, through the interference) can be expressed as
Thus, by selecting the appropriate θ2Any polarization state other than H, V may be achieved. For example, select θ20, pi/2, 3 pi/2, can be inFour different polarization states are finally obtained on the signal light output by the polarization beam splitter of the interferometer,
θ1when pi/2: the signal light to be encoded, which is generated by the light source 1, is input from the port 3 of the polarization beam splitter 212, and is split into first and second signal light components, which enter the first optical path and the second optical path via the ports 1 and 2 of the polarization beam splitter 212, respectively. The phase modulator 211 on the first optical path applies a modulation amount of pi/2 to the phase of the first component, i.e., a first phase difference θ provided by the phase modulator 211 between the first and second components1Pi/2. The two signal light components with a phase difference of pi/2 arrive at ports 5 and 6, respectively, of the beam splitter 213 at the same time and interfere for the first time there, when the first interfering light signal appears at both output ports 7 and 8 of the beam splitter. The two first interference optical signals respectively return to the ports 8 and 7 of the beam splitter 213 after being acted by the optical path returning module, wherein a second phase adjusting unit arranged in the optical path returning module enables the phase difference between the first interference optical signal and the second interference optical signal to generate pi/2 change. The two optical signals that are folded back and have a pi/2 change in phase difference interfere with each other again at the beam splitter 213, and only the port 5 of the beam splitter 213 outputs an interference optical signal, which is finally output from the port 4 of the polarization beam splitter 212 after being acted on by the polarization beam splitter, and the polarization state of the output signal light is H.
θ1When the ratio is 3 pi/2: the signal light to be encoded, which is generated by the light source 1, is input from the port 3 of the polarization beam splitter 212, and is split into first and second signal light components, which enter the first optical path and the second optical path via the ports 1 and 2 of the polarization beam splitter 212, respectively. The first phase modulator 211 on the first optical path applies a modulation amount of 3 pi/2 to the phase of the first component, i.e., a first phase difference θ provided by the first phase modulator 211 between the first and second components 13 pi/2. The two optical signal components with a phase difference of pi/2 arrive at ports 5 and 6, respectively, of beam splitter 213 simultaneously and interfere for the first time there, at output ports 7 and 8 of the beam splitterA first interfering light signal is present. The two first interference optical signals respectively return to the ports 8 and 7 of the beam splitter 213 after being acted by the optical path returning module, wherein a second phase adjusting unit arranged in the optical path returning module enables the phase difference between the first interference optical signal and the second interference optical signal to generate pi/2 change. The two optical pulses that are folded back and have a pi/2 change in phase difference interfere with each other again at the beam splitter 213, and only the port 6 of the beam splitter 213 outputs an interference optical signal, which is finally output from the port 4 after being acted on by the polarization beam splitter 212, and the polarization state of the output signal light is V.
In addition, the invention also discloses a quantum communication system comprising the transmitting end for realizing the six polarization state codes and a quantum communication method comprising the transmitting end for realizing the six polarization state codes.
The above description is not intended to limit the present invention, and the present invention is not limited to the above examples, and the above alternatives may be used in combination with each other without contradiction. Those skilled in the art will also appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.
Claims (14)
1. A transmitting end for realizing six polarization state coding comprises a light source (1), a phase modulation module (2) and an optical path reentry module (3), wherein,
the light source (1) is used for providing signal light to be coded;
the phase modulation module (2) comprises a first phase modulation unit (211) and a polarization beam splitting unit (212), and is arranged to: the signal light to be coded is divided into a first signal light component and a second signal light component, a first phase difference theta is formed between the first and second signal light components by the first phase modulation unit (211)1And causing the first and second signal light components to interfere to output a first interfering light signal;
the optical path folding module (3) is arranged for receiving the first interference optical signal and folding the first interference optical signal, and comprises a second phase adjusting unit for adjusting the phase of the first interference optical signal; and is
The phase modulation module (2) receives the folded first interference optical signal and is further configured to: enabling the folded first interference optical signal to generate interference action to form a second interference optical signal, and enabling the second interference optical signal to be output through the polarization beam splitting unit (212); or, the first interference optical signal which is folded back is divided into a first and a second folded optical signal component, and a second phase difference theta is formed between the first and the second folded optical signal component by the first phase modulation unit (211)2And coupling out the first and second folded optical signal components via the polarization beam splitting unit (212).
2. The transmission end according to claim 1, wherein the phase modulation module (2) further comprises a beam splitting unit (213) and is further arranged to: at the first phase difference theta1Outputting two first interference light signals respectively via two output ports of the beam splitting unit (213) at a first preset value, and outputting the first phase difference theta1Outputting the first interference optical signal only through one of two output ports of the beam splitting unit (213) when the first interference optical signal is a second preset value;
the light path folding module (3) is further configured to: at the first phase difference theta1When the first preset value is reached, the phase difference between the two first interference light signals is changed by theta3(ii) a And
the phase modulation module (2) is further arranged to: make the turn back and have the phase difference change theta3Is interfered at the beam splitting unit (213) to form the second interference light signal; and at the first phase difference theta1When the first preset value is the second preset value, the first interference optical signal which is folded back is divided into the first folded back optical signal component and the second folded back optical signal component at the beam splitting unit (213).
3. The transmitting end according to claim 1, wherein the optical path folding module (3) further comprises an optical loop, and the second phase adjustment unit comprises a phase modulator provided on the optical loop and having different optical paths with respect to two output ports of the phase modulation module (2) for the first interference optical signal; alternatively, the first and second electrodes may be,
the optical path folding module (3) further comprises two reflection units respectively connected to two output ports of the phase modulation module (2) for the first interference optical signal, and the second phase adjustment unit comprises a phase shifter or a phase modulator provided in at least one of the two reflection units.
4. The transmitting end according to claim 2, wherein the polarization beam splitting unit (212) and the beam splitting unit (213) are connected by a first optical path and a second optical path having the same optical path length to form an equal-arm MZ interferometer, and the first phase modulation unit (211) is disposed on the first optical path and/or the second optical path; and/or the polarizing beam splitting unit (212) comprises a polarizing beam splitter; and/or the beam splitting unit (213) comprises a beam splitter; and/or the first phase modulation unit (211) comprises a phase modulator.
5. The transmitting end according to claim 1, further comprising a phase detection feedback module, wherein the phase detection feedback module comprises a light splitting unit, a phase detection unit, a phase feedback algorithm unit, a phase shifter driving unit, and a phase shifter.
6. The transmitting end of claim 2, wherein the first preset value is pi/2 or 3 pi/2, and the phase difference change θ is3Is pi/2; and/or the like and/or,
the second preset value is 0 or pi, and the second phase difference theta2Is 0, pi/2 or 3 pi/2.
7. The transmitting end according to claim 1, whereinThe six polarization state codes comprise H, V,
8. A quantum communication system comprising the transmitting end of any one of claims 1-7.
9. An encoding method for realizing six polarization state encoding comprises the following steps,
a first interference step: the signal light to be coded is divided into a first signal light component and a second signal light component, and a first phase difference theta is formed between the first and second signal light components by means of a first phase modulation unit (211)1And causing the first and second signal light components to interfere to output a first interfering light signal;
a light path folding step: enabling the first interference optical signals to be folded back, and carrying out phase adjustment on the first interference optical signals when the number of the first interference optical signals is two;
a polarization state generation step: enabling the folded first interference optical signal to generate interference action to form a second interference optical signal, and enabling the second interference optical signal to be output through a polarization beam splitting unit (212); or, the first interference optical signal which is folded back is divided into a first and a second folded optical signal component, and a second phase difference theta is formed between the first and the second folded optical signal component by the first phase modulation unit (211)2And having the second phase difference θ2Is coupled out via said polarization beam splitting unit (212).
10. The encoding method of claim 9, wherein the first interfering step further comprises the steps of: -splitting the signal light to be encoded into the first and second signal light components via the polarization beam splitting unit (212); at the first phase difference theta1At a first preset value, respectively passing through two outputs of the beam splitting unit (213)Outputting two first interference optical signals from an output port; and at the first phase difference theta1When the first preset value is a second preset value, the first interference optical signal is output through only one of two output ports of the beam splitting unit (213); and/or the like and/or,
the light path folding step further includes the steps of: at the first phase difference theta1When the first preset value is reached, the phase difference between the two first interference optical signals is subjected to phase difference change theta3(ii) a And/or the like and/or,
the polarization state generating step further comprises the steps of: make the folded part have a phase difference change theta3Is interfered at a beam splitting unit (213) to form the second interference light signal; and at the first phase difference theta1When the first preset value is a second preset value, the first interference optical signal which is folded back is divided into the first folded back optical signal component and the second folded back optical signal component at the beam splitting unit (213).
11. The encoding method of claim 10, wherein the first preset value is pi/2 or 3 pi/2, and the phase difference change θ is3Is pi/2; and/or the like and/or,
the second preset value is 0 or pi, and the second phase difference theta2Is 0, pi/2 or 3 pi/2.
12. The encoding method of claim 9, further comprising a phase detection feedback step.
13. The encoding method of claim 9, wherein the six polarization state encodings comprise H, V,
14. The encoding method according to claim 9, implemented with the transmitting end according to any of claims 1-7.
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