A polarization compensation method for a multi-wavelength free-space quantum communication system

Abstract

The present invention discloses a polarization compensation method for a multi-wavelength free-space quantum communication system. The device is based on a multi-wavelength time-sharing free-space quantum communication system composed of a polarization-encoded quantum coding module, a dichroic mirror, a back optical path, a wave plate, a transmitting end telescope, a receiving end telescope, a half-wave plate and a quantum decoding module. The polarization state of the system is compensated by rotating two wave plates at the transmitting terminal to compensate for the degenerated circular polarization, and by compensating the rotation phase-matched half-wave plate at the receiving terminal to compensate for the base vector angle deviation of the linear polarization. The rotation angle of the two wave plates will be obtained by measuring the transmission matrix of each device and system and the phase delay of the wave plate before loading, and calculated based on the Mueller-Stokes algorithm. The present invention has a simple structure, a wide applicable spectrum, and easy access to devices. It can be applied to the polarization state maintenance design of a free-space quantum communication system working in multi-wavelength time-sharing and the design of a polarization-sensitive space optical communication optical system.

Description

Technical Field

[0001] The present invention relates to a polarization compensation method for a multi-wavelength free-space quantum communication system, and in particular to compensation for polarization state degradation of quantum coded light in a multi-wavelength time-sharing free-space quantum communication system. The method can be applied to polarization state maintenance design of a free-space quantum communication optical system and polarization-sensitive space optical communication optical system design. Background Art

[0002] Quantum communication is an inherently unconditionally secure information encryption method based on the quantum indivisibility principle and quantum non-cloning principle in physics, including quantum key distribution, quantum entanglement distribution, and quantum teleportation. The polarization state of photons is currently the most important information carrier.

[0003] Optical fiber and free space links are usually used to transmit photons. However, due to the loss, optical fiber is difficult to transmit information over long distances. Free space links, such as the information transmission between the "Micius" satellite and the ground station, have achieved information transmission over thousands of kilometers. However, due to the background light, Micius can only work at night. The use of a wavelength of 1550nm during the day is currently the main means of suppressing background light. However, the 1550nm wavelength is limited by detector efficiency, telescope aperture design and other reasons, and its efficiency at night cannot be compared with the traditional visible light band. Multi-wavelength quantum communication that uses the visible light band to work at night and the 1550nm wavelength to work during the day is one of the main trends in free space quantum communication.

[0004] The maintenance of polarization state is the key to the design of free-space quantum communication system. The degradation of polarization state will lead to an increase in bit error rate. Optical devices in optical systems (such as optical fibers and reflectors) will inevitably introduce degradation. Polarization state compensation design through wave plates is the main way to maintain the polarization state of the system. Limited by the level of technology, wave plates cannot achieve perfect phase matching over a wide band. The method of this patent avoids this problem. This patent is based on the Mueller-Stokes algorithm. By installing two wave plates of the same model at the transmitting terminal and a phase-matched half-wave plate at the receiving terminal, and based on the parameters of each system measured before loading, the angle at which the wave plate should be rotated at each wavelength is calculated to achieve polarization state compensation. This method has a simple structure and can achieve polarization state compensation over a wide spectrum. Summary of the invention

[0005] Specifically, it relates to a polarization maintaining method for a multi-wavelength free-space quantum communication system, which can be applied to the optical system design of a polarization-coded free-space quantum communication system.

[0006] The purpose of the present invention is to provide a polarization compensation method for a multi-wavelength free-space quantum communication system, by selecting two wave plates of the same type at the transmitting end and a phase-matched half-wave plate at the receiving end, and calculating the angle of rotation of the wave plate based on the measurement of the parameters of each optical path system before loading to achieve polarization state compensation. The present invention is based on the attached Figure 1 The system shown in FIG. 1 includes a working wavelength of λ 1 Quantum coding module 1, working wavelength is λ 2 The quantum coding module 2, dichroic mirror 3, back light path 4, first wave plate 5, second wave plate 6, transmitting end telescope 7, free space link 8, receiving end telescope 9, half wave plate 10, quantum decoding module 11. The quantum coding module 1 and quantum coding module 2 are polarization coding; the first wave plate 5 and the second wave plate 6 are the same type of wave plates, and the phase delay of the two at each wavelength is approximately equal, with an error of 2%. The first wave plate 5 and the second wave plate 6 are at λ 1 and 2 The phase delay at is greater than 0.25π and less than 0.75π; the half-wave plate 10 is a replaceable half-wave plate, and the phase delay accuracy is better than 1 / 300λ. 1 When the half-wave plate 10 is operated with a central wavelength of λ 1 half-wave plate, when λ 2 When the half-wave plate 10 is operated with a central wavelength of λ 2 The quantum decoding module 11 is a quantum decoding module that matches the quantum encoding module 1 and the quantum encoding module 2.

[0007] It is characterized by comprising the following steps:

[0008] 1) The measured wavelength before loading is λ 1 The joint transmission matrix M of the quantum coding module 1, the dichroic mirror 3, and the back optical path 4 is 1 ;

[0009] 2) The measured wavelength before loading is λ 2 The joint transmission matrix m of the quantum coding module 2, the dichroic mirror 3, and the back light path 4 is 1 ;

[0010] 3) The measured wavelength before loading is λ 1 The transmission matrix M of the transmitting telescope 7 is 2 , the transmission matrix M of the receiving telescope 9 3 ;

[0011] 4) The measured wavelength before loading is λ 2 The transmission matrix m of the transmitting telescope 7 is 2 , the receiving end telescope 9 transmission matrix m 3 ;

[0012] 5) Before loading, measure the first wave plate 5 at wavelength λ 1 ,λ 2 The phase delay at 1 ,δ 2

[0013] 6) Before loading, measure the second wave plate 6 at wavelength λ 1 ,λ 2 The phase delay at 1 ,ε 2

[0014] 7) Let R = [1, 0, 0, 1] T is the Stokes parameter of right-handed circularly polarized light. When the transmission wavelength λ 1 When the quantum light is , the transmission matrix of the system should satisfy:

[0015]

[0016] θ 1 ,θ 2 are the rotation angles of the first wave plate 5 and the second wave plate 6, and are the transmission matrices of the first wave plate 5 and the second wave plate 6, respectively, which are expressed in the form of Mueller matrix:

[0017]

[0018] The transmission matrix of the system also satisfies:

[0019]

[0020] Let SR = M 1 M 3 M 2 R=[1,SR 1 ,SR 2 ,SR 3 ] T , the above formula is expressed as:

[0021]

[0022] So we have:

[0023]

[0024] The rotation angle of the first wave plate 5 and the second wave plate 6 is solved by the following formula:

[0025]

[0026] When the equations have a solution, the angle between the first wave plate 5 and the second wave plate 6 should be within the following range:

[0027]

[0028] If the above inequality does not hold, then the equation needs to satisfy:

[0029]

[0030] L=[1,0,0,-1] T is the Stokes parameter of left-handed circularly polarized light. At this time, the rotation angle of the first wave plate 5 and the second wave plate 6 satisfies:

[0031]

[0032] The equation system should satisfy the following inequality:

[0033]

[0034] For all SRs 3 , when one of the two inequalities holds, the phase delays of the two wave plates should be nearly equal and satisfy:

[0035]

[0036] 8) When the transmission wavelength λ 2 When the quantum light is Sr, let Sr = m 1 m 3 m 2 R=[1,Sr 1 ,Sr 2 ,Sr 3 ] T ,like:

[0037]

[0038] The rotation angle β of the first wave plate 5 1 , the rotation angle β of the second wave plate 6 2 Solve by the following formula:

[0039]

[0040] If the above formula is not satisfied, when the following formula is satisfied:

[0041]

[0042] The rotation angle β of the first wave plate 5 1 , the rotation angle β of the second wave plate 6 2 Solve by the following formula:

[0043]

[0044] For all Sr3 , when one of the two inequalities holds, the phase delay of the two wave plates should be nearly equal and satisfy:

[0045]

[0046] 9) When the transmission wavelength λ 1 When the quantum light is transmitted, the quantum decoding module 11 detects the deviation angle of the polarization direction between the horizontal polarized light and the ideal horizontal polarized light. Rotate the half-wave plate (10) to Implement basis vector correction.

[0047] 10) When the transmission wavelength λ 2 When the quantum light is transmitted, the quantum decoding module 11 detects the polarization direction deviation angle τ between the horizontal polarized light and the ideal horizontal polarized light. The angle of rotating the half-wave plate (10) to Implement basis vector correction.

[0048] The advantages of the patent of the present invention are: 1. Simple structure; 2. Wide applicable spectrum; 3. Easy to obtain the device. BRIEF DESCRIPTION OF THE DRAWINGS

[0049] Figure 1 : Polarization-maintaining system for multi-wavelength free-space quantum communication. DETAILED DESCRIPTION

[0050] The following is a detailed description of an implementation example of the method of the present invention with reference to the accompanying drawings.

[0051] The main components and systems used in the embodiments of the present invention are described as follows:

[0052] It consists of a quantum coding module, a dichroic mirror, a back optical path, a wave plate, a transmitting telescope, a free space link, a receiving telescope, a half-wave plate, and a quantum coding module.

[0053] 1) Quantum coding module 1: 1550nm quantum coding module based on BB84 protocol;

[0054] 2) Quantum coding module 2: 850nm quantum coding module based on BB84 protocol;

[0055] 3) Dichroic mirror 3: DMLP1000T dichroic mirror produced by Thorlabs, cut-off wavelength 1000nm, 1550nm transmittance greater than 95%, 850nm reflectivity greater than 95%;

[0056] 4) Wave plate 4: QWP10-850B quarter wave plate produced by lbtek, working wavelength 850nm, phase at 850nm is 0.5π, phase at 1550nm is 0.258π

[0057] 5) First wave plate 5: QWP10-850B quarter wave plate produced by lbtek, with an operating wavelength of 850nm, a phase of 0.5π at 850nm, and a phase of 0.258π at 1550nm.

[0058] 6) Half-wave 10: When the wavelength of the light source is 850nm, the half-wave plate uses the product of lbtek, model HWP10-850B, the working wavelength is 850nm, and the transmittance is greater than 98%. When the wavelength of the light source is 1550nm, the half-wave plate uses the product of lbtek, model HWP10-1550B, the working wavelength is 1550nm, and the transmittance is greater than 98%;

[0059] 7) Quantum decoding module 11: When the wavelength is 850nm, the 850nm quantum decoding module based on the BB84 protocol is used. When the wavelength is 1550nm, the 1550nm quantum decoding module based on the BB84 protocol is used.

[0060] The polarization state is compensated by calculating the wave plate rotation angle through the following steps:

[0061] 1) Before loading, measure the joint transmission matrix M of the quantum coding module 1, the dichroic mirror 3, and the rear optical path 4 at 850nm 1 :

[0062]

[0063] 2) Before loading, measure the joint transmission matrix m of the quantum coding module 2, the dichroic mirror 3, and the rear optical path 4 at 1550nm 1 :

[0064]

[0065] 3) Before loading, the transmission matrix M of the transmitting end telescope 7 is measured at a wavelength of 850 nm 2 , the receiving end telescope 9 transmission matrix M 3 :

[0066]

[0067]

[0068] 4) Before loading, the transmission matrix m of the transmitting end telescope 7 is measured at a wavelength of 1550nm 2 , the receiving end telescope 9 transmission matrix m 3 :

[0069]

[0070]

[0071] 5) The phase retardation of the first wave plate 5 at a wavelength of 850nm is 0.5π, and the phase retardation at 1550nm is 0.258π. Therefore, δ 1 =0.5π,ε 1 =0.258π.

[0072] 6) The phase retardation of the second wave plate 6 at a wavelength of 850nm is 0.5π, and the phase retardation at a wavelength of 1550nm is 0.258π. Therefore, δ 2 =0.5π,ε 2 =0.258π.

[0073] 7) Let R = [1, 0, 0, 1] T is the Stokes parameter of right-handed circularly polarized light. When the quantum coding module 1 is working, SR=M 1 M 3 M 2 R=[1,SR 1 ,SR 2 ,SR 3 ] T ,at this time

[0074] SR=[1,-0.510,0.528,-0.678] T

[0075] at this time

[0076]

[0077] According to the following formula

[0078]

[0079] The rotation angle of the first wave plate 5 can be obtained as θ 1 =66.9°, the rotation angle of the second wave plate 6 is θ 2 =90.6°.

[0080] The polarization direction deviation angle of the horizontal polarized light and the ideal horizontal polarized light is 19° detected by the quantum decoding module 11. The angle of the half-wave plate 10 is rotated to -8.5° to achieve basis vector correction.

[0081] 8) Let R = [1, 0, 0, 1] T is the Stokes parameter of right-handed circularly polarized light. When the quantum coding module 2 is working, Sr=m 1 m 3 m 2 R=[1,Sr 1 ,Sr 2 ,Sr 3 ]T

[0082] at this time

[0083] Sr=[1,0.100,-0.445,0.890] T

[0084] at this time

[0085]

[0086] According to the following formula

[0087]

[0088] The rotation angle of the first wave plate 5 can be obtained as β 1 =148.0°, the rotation angle of the second wave plate 6 is β 2 =39.1°.

[0089] The quantum decoding module 11 detects the polarization direction deviation angle of the horizontal polarized light from the ideal horizontal polarized light by 27°. The angle of the half-wave plate 10 is rotated to -13.5° to achieve basis vector correction.

Claims

1. A polarization compensation method for a multi-wavelength free-space quantum communication system, the multi-wavelength free-space quantum communication system comprising: A quantum coding module (1) with a working wavelength of λ1, a quantum coding module (2) with a working wavelength of λ2, a dichroic mirror (3), a rear optical path (4), a first wave plate (5), a second wave plate (6), a transmitting end telescope (7), a free space link (8), a receiving end telescope (9), a half-wave plate (10), and a quantum decoding module (11), characterized in that the polarization compensation method comprises the following steps: 1) When the measured wavelength before loading is λ1, the joint polarization transmission matrix M1 of the quantum coding module (1), the dichroic mirror (3), and the rear optical path (4); 2) When the measured wavelength before loading is λ2, the joint polarization transmission matrix m1 of the quantum coding module (2), the dichroic mirror (3), and the rear optical path (4); 3) When measuring wavelength λ1 before loading, the transmitting end telescope (7) transmits the matrix M2, and the receiving end telescope (9) transmits the matrix M3; 4) When measuring wavelength λ2 before loading, the transmitting end telescope (7) transmits matrix m2, and the receiving end telescope (9) transmits matrix m3; 5) before loading, measuring the phase delays of the first wave plate (5) at wavelengths λ1 and λ2 as δ1 and δ2 respectively; 6) before loading, measuring the phase delays of the second wave plate (6) at wavelengths λ1 and λ2 as ε1 and ε2 respectively; 7) Let R = [1, 0, 0, 1] T is the Stokes parameter of right-handed circularly polarized light. When transmitting quantum light of wavelength λ1, let SR=M1M3M2R=[1,SR1,SR2,SR3] T ,like: The rotation angle θ1 of the first wave plate (5) and the rotation angle θ2 of the second wave plate (6) are calculated by the following equations: If not satisfied, when the following formula is satisfied The rotation angle θ1 of the first wave plate (5) and the rotation angle θ2 of the second wave plate (6) are calculated by the following equations: 8) When transmitting quantum light of wavelength λ2, let Sr = m1m3m2R = [1, Sr1, Sr2, Sr3] T ,like: The rotation angle β1 of the first wave plate (5) and the rotation angle β2 of the second wave plate (6) are calculated by the following equations: If not satisfied, when the following formula is satisfied The rotation angle β1 of the first wave plate (5) and the rotation angle β2 of the second wave plate (6) are calculated by the following equations: 9) When transmitting quantum light with a wavelength of λ1, the deviation angle between the horizontal polarized light and the ideal horizontal polarized light is detected by the quantum decoding module (11) Rotate the half-wave plate (10) to 10) When transmitting quantum light of wavelength λ2, the deviation angle τ between the horizontal polarized light and the ideal horizontal polarized light is detected by the quantum decoding module (11), and the angle of the half-wave plate (10) is rotated to 2. The polarization compensation method for a multi-wavelength free-space quantum communication system according to claim 1, characterized in that: The quantum coding module (1) with a working wavelength of λ1 and the quantum coding module (2) with a working wavelength of λ2 are polarization coding.

3. The polarization compensation method for a multi-wavelength free-space quantum communication system according to claim 1, characterized in that: The first wave plate (5) and the second wave plate (6) are wave plates of the same model, and their phase delays at each wavelength are approximately equal, with an error of 2%, and the phase delays at λ1 and λ2 are greater than 0.25π and less than 0.75π.

4. The polarization compensation method for a multi-wavelength free-space quantum communication system according to claim 1, characterized in that: The half-wave plate (10) is a replaceable half-wave plate with a phase delay accuracy better than 1 / 300λ. When the wavelength λ1 is used, the half-wave plate (10) uses a half-wave plate with a central wavelength of λ1. When the wavelength λ2 is used, the half-wave plate (10) uses a half-wave plate with a central wavelength of λ2.

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