WO2018196478A1 - 一种利用量子干涉控制量子关联光子对频谱特性的方法 - Google Patents

一种利用量子干涉控制量子关联光子对频谱特性的方法 Download PDF

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WO2018196478A1
WO2018196478A1 PCT/CN2018/077374 CN2018077374W WO2018196478A1 WO 2018196478 A1 WO2018196478 A1 WO 2018196478A1 CN 2018077374 W CN2018077374 W CN 2018077374W WO 2018196478 A1 WO2018196478 A1 WO 2018196478A1
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medium
cascade structure
dispersion
quantum
stage cascade
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李小英
崔亮
苏杰
李嘉敏
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Tianjin University
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    • G—PHYSICS
    • G02—OPTICS
    • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
    • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
    • G02F1/35—Non-linear optics
    • G02F1/3501—Constructional details or arrangements of non-linear optical devices, e.g. shape of non-linear crystals
    • G—PHYSICS
    • G02—OPTICS
    • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
    • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
    • G02F1/35—Non-linear optics
    • G—PHYSICS
    • G02—OPTICS
    • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
    • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
    • G02F1/35—Non-linear optics
    • G02F1/353—Frequency conversion, i.e. wherein a light beam is generated with frequency components different from those of the incident light beams
    • G02F1/3536—Four-wave interaction
    • G—PHYSICS
    • G02—OPTICS
    • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
    • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
    • G02F1/35—Non-linear optics
    • G02F1/365—Non-linear optics in an optical waveguide structure
    • G—PHYSICS
    • G02—OPTICS
    • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
    • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
    • G02F1/35—Non-linear optics
    • G02F1/39—Non-linear optics for parametric generation or amplification of light, infrared or ultraviolet waves
    • G02F1/395—Non-linear optics for parametric generation or amplification of light, infrared or ultraviolet waves in optical waveguides

Definitions

  • the invention belongs to the field of quantum information science and technology, relates to the preparation of quantum-related photon pairs, and the preparation of entangled photons and single photons based on quantum-related photon pairs, in particular a method for controlling the spectral characteristics of quantum-related photons.
  • the implementation of many quantum information processing techniques relies on quantum interference between photons of the transformation limit.
  • the self-luminous parametric process in nonlinear media is an effective method for preparing quantum light field and predictive single photon quantum light field.
  • Commonly used self-luminous parametric processes include spontaneous parametric down-conversion in x (2) second-order nonlinear media and spontaneous four-wave mixing in x (3) third-order nonlinear media.
  • one (or two) photons from a strong pumping light field are quenched by the x (2) (x( 3) ) nonlinear effect of the medium and simultaneously produce a pair of frequencies ⁇ s
  • Quantum-related photon pairs of ⁇ i usually the pair of photons are called signal photons and idle photons, respectively.
  • the self-luminous parametric process can be either single-frequency laser pumping or pulsed laser pumping. Since pulse laser pumping can define the generation time of quantum state light field, it can satisfy the synchronization conditions required for interference between independent quantum light sources, and realize quantum information processing tasks such as quantum calculation and quantum entanglement exchange.
  • the present invention is directed to a self-luminous parametric process for pulsed laser pumping.
  • the time-mode characteristics and collection efficiency of quantum-related photon pairs are two important factors in measuring their characteristics and quality.
  • Investigating the temporal mode characteristics of associated photon pairs can be derived from the joint spectral function F( ⁇ s , ⁇ i ) describing the spectral characteristics of the photons.
  • F( ⁇ s , ⁇ i ) is proportional to the probability amplitude of a pair of signals having a frequency of ⁇ s and ⁇ i and an idle photon pair, respectively.
  • K the closer K is to 1, the closer the associated photon pair is to the single mode, ie the closer the signal and the idle photon are to the transform limit.
  • the temporal mode characteristics of the associated photon pairs can also be characterized by the second order correlation function g (2) of the signal or idle frequency field. Taking the signal light field as an example, g (2) has the following relationship with the associated photon pair spectrum:
  • f s ( ⁇ s ) represents the filter transmission spectrum of the signal photon channel.
  • g (2) ⁇ 2 the closer it is to a single mode, the higher the mode purity.
  • the collection efficiency of associated photon pairs refers to the probability that each pair of signals and idle photons generated in a nonlinear medium are collected and output after passing through respective filters. If one or even two of a pair of photons are lost during filtering and collection, the output count rate of the associated photon pair will be met (ie, the detectors of the signal photon channel and the idler photon channel simultaneously detect the count rate of photons). The drop affects the brightness and background noise of the associated photon pair.
  • the collection efficiency of associated photon pairs can be characterized by the conditional collection efficiency of the signal or idler photons.
  • conditional collection efficiency ⁇ s(i) of the signal (idle) photon indicates the probability that the corresponding signal (idle) photon is collected when an idle (signal) photon is successfully collected, and the associated photon pair.
  • the spectrum has the following relationship:
  • f s(i) ( ⁇ s(i) ) represents the filter transmission spectrum of the signal (idle) photonic channel. If the collection efficiency ⁇ s ⁇ i on both sides is closer to 100%, it indicates that the collection efficiency of the associated photon pair is higher.
  • the associated photon pair In many quantum information applications, not only is the associated photon pair required to have a single model, but also a collection efficiency close to the ideal value. However, for most common bulk or waveguide nonlinear media, the joint spectral function of the associated photon pairs produced is usually not directly decomposed into a direct product form, ie F( ⁇ s , ⁇ i ) ⁇ ⁇ ( ⁇ s ) ⁇ ( ⁇ i ). In this case, according to the Schmidt decomposition formula, the average mode number of the associated photon pair is greater than 1, and is in a multi-modal state, and the corresponding signal and the idle photon are not photons of the transformation limit.
  • the following two methods are often used: one is to filter the signal photons and the idle photons with a narrowband filter, so that the coherent time of the filtered signal and the idle photons are both More than the pulse width of the pump pulse, generally the narrower the filter bandwidth, the closer the photon is to the single mode; the second is to control the dispersion of the nonlinear medium, so that the joint spectrum function can be directly decomposed.
  • the photon pair generated by the former method the addition of narrow-band filtering will result in a decrease in collection efficiency, and the narrower the filter bandwidth, the lower the collection efficiency; although the conversion-limited photon pair produced by the latter method can obtain higher brightness
  • the phase matching conditions of the parametric process and the requirements of the photon-pair spectral characteristics on the nonlinear medium dispersion affect each other and contain each other. There is a lack of photon-to-wavelength that cannot be flexibly changed. In addition, it is limited by the processing technology and theoretically achievable by design. Dispersion characteristics are often difficult to implement in engineering.
  • the object of the present invention is to overcome the deficiencies in the prior art and to provide a method for controlling the spectral characteristics of quantum correlation photons using quantum interference.
  • the method is based on the quantum interference of the multi-stage parametric process to regulate the nonlinear interaction; by using the cascade structure of the nonlinear medium and the dispersive medium, the phase matching of the parametric process and the control process of the photon pair spectral characteristics are separated, so Photons of different wavelengths flexibly regulate the spectrum.
  • the separation control process of the method is carried out by controlling the wavelength range of the photon pair by controlling the dispersion of the nonlinear medium; controlling the spectral characteristics of the photon pair by controlling the dispersion characteristics, the length, and the order of the interference of the dispersion medium between the nonlinear mediums. .
  • a plurality of quantum states can be further prepared, including a predicted pure single photon, a dual mode polarization entangled photon pair, and a multichannel pure single photon.
  • the present invention Compared with the method for controlling a nonlinear media to obtain a spectrally decomposable photon pair, the present invention has the advantages of simple process, flexible change of photon pair wavelength, and simultaneous realization of multi-wavelength output; and method for obtaining a transformed limit photon pair by using narrowband filtering
  • the photon pair produced by the present invention has the advantage of having high brightness.
  • a method for controlling spectral characteristics of a quantum correlation photon pair by using quantum interference includes the following steps: using a multi-stage cascade structure consisting of a non-linear medium, a piece of dispersion medium, ..., a piece of nonlinear medium, or a segment
  • a two-stage cascade structure consisting of a nonlinear medium, a length of dispersion controllable medium and a section of nonlinear medium; the nonlinear medium and the dispersion medium in the multi-stage cascade structure are alternately arranged in turn, and each non-linear medium is regarded as one Level, the number of stages of the multi-stage cascade structure is greater than or equal to two; light waves may be incident from one end of the multi-stage cascade structure or the two-stage cascade structure, and exit from the other end to form a light wave path;
  • the interference constructive length and the destructive corresponding wavelength can be tuned by changing the number of stages of the multi-stage cascade structure, the length of the dispersive medium, or replacing the dispersive medium having different dispersion characteristics;
  • the cascade structure, the wavelength of the interference constructive phase and the cancellation corresponding wavelength can be tuned by changing the dispersion characteristics and length of the dispersion controllable medium;
  • a quantum correlation photon pair at the wavelength corresponding to the interference construct is collected and output using a filtering and collecting device.
  • the nonlinear medium in the multi-stage cascade structure or the two-stage cascade structure is a monolithic medium satisfying the phase matching condition of the nonlinear parametric process or a waveguide structure medium satisfying the phase matching condition of the nonlinear parametric process; each segment of the nonlinear medium Having the same or different dispersion characteristics, when each segment of the nonlinear medium has different dispersion characteristics, it is necessary to ensure that the wavelength regions of the quantum-related photon pairs generated by the parametric process in each segment of the nonlinear medium overlap.
  • the dispersive medium in the multi-stage cascade structure is a monolithic medium having dispersion or a waveguide structure medium having dispersion;
  • the dispersion controllable medium in the two-stage cascade structure is a monolith with dispersion controllability a medium, a waveguide structure medium having dispersion controllability, or a device having dispersion controllability;
  • the device having dispersion controllability consists of a medium and a dispersion device.
  • the present invention is based on a multi-stage cascade structure in which a nonlinear medium and a dispersive medium are sequentially intersected (the number of stages is greater than or equal to two), or a two-stage cascade structure composed of a nonlinear medium and a dispersion controllable medium;
  • the joint parameter process produces correlated photon pairs.
  • the function of the cascade structure after the first segment of the nonlinear medium can be regarded as: actively filtering the associated photon pairs generated by the spontaneous parametric process in the first segment of the nonlinear medium.
  • the method can improve the collection efficiency while improving the purity of the photon pair mode.
  • a dispersion-controlled nonlinear medium such as a periodically-polarized nonlinear crystal or a photonic crystal fiber
  • the wavelength can be flexibly changed, and the preparation process is simple.
  • the spectral characteristics can be controlled by changing the number of stages of the cascade structure, the length of the dispersion medium, or replacing the dispersion medium having different dispersion characteristics. .
  • the correlated photon pair outputted by the two-stage cascade structure including the dispersion controllable medium can be controlled by changing the dispersion characteristics of the dispersion controllable medium.
  • Figure 1 is a schematic diagram of the generation of correlated photon pairs using a spontaneous parametric process of pulsed light pumping in a multi-stage cascade.
  • Figure 2 is a schematic diagram of the generation of correlated photon pairs using a multi-stage cascade structure based on a dispersion shifted fiber and a single mode fiber.
  • 3(a) to 3(d) are graphs showing the results of spectrum simulation of correlated photon pairs generated by a multi-stage cascade structure.
  • Fig. 4 is a schematic diagram of quantum interference between two independent cascaded single-photon sources based on cascade structure.
  • Figure 5 is a schematic diagram of the preparation of a polarization entangled photon pair based on a multi-stage cascade structure.
  • Fig. 6 is a schematic diagram of a pure-state single-photon prediction using a two-stage cascade structure based on a waveform shaper.
  • Fig. 7(a) and Fig. 7(b) are schematic diagrams and simulation results of a near-instant state single photon with a two-stage cascade structure based on a waveform shaper.
  • Figure 8 is a block diagram of a correlated photon pair source based on cascaded spontaneous parametric down-conversion in a bulk crystal.
  • the invention utilizes a pulse laser pumping multi-stage cascade structure (the number of stages is greater than or equal to two) or a two-stage cascade structure containing a dispersion controllable medium, and obtains a quantum correlation photon pair through a cascade optical parameter process, thereby preparing a plurality of quantum State light field.
  • a pulse laser pumping multi-stage cascade structure the number of stages is greater than or equal to two
  • a two-stage cascade structure containing a dispersion controllable medium By changing the parameters of the cascade structure, combined with the collection and filtering means, the control of the spectral characteristics of the associated photons can be achieved.
  • the following is an example of a spontaneous four-wave mixing parameter process in a Gaussian pulse-pumped single-mode optical waveguide, illustrating a multi-stage cascade structure (levels greater than or equal to two) and a two-stage cascade containing dispersion controllable media. The spectral characteristics of the photon pairs produced in the structure.
  • the phase mismatch of the first-stage nonlinear medium is ⁇ k 1 and the length is L 1 .
  • the phase mismatch of the N- th nonlinear medium is ⁇ k N and the length is L N .
  • ⁇ d ⁇ s + ⁇ i - 2 ⁇ p is the phase difference between the signal introduced by the dispersion controllable medium, the idle photon and the pump photon.
  • Embodiment 1 Generating a spectrally controllable correlated photon using a multi-stage cascade structure based on a dispersion-shifted fiber and a single-mode fiber.
  • the pulsed pump light used in the pump cascade structure is a standard Gaussian spectrum with a center wavelength of 1550 nm, a full width at half maximum of 0.9 nm, and a ⁇ P p of 1 km -1 .
  • the signal in the cascade structure and the idle photons are filtered out using a multi-channel filter.
  • Figure 3 (a) is a multi-level cascade structure, that is, the associated photon pair spectrum of a 100m dispersion-shifted fiber output
  • each graph is a two-dimensional contour plot of the associated photon pair spectrum
  • the idle photon frequency ⁇ s(i) is converted into a signal (idle) photon wavelength ⁇ s(i) , and the lower half is obtained by projecting
  • F NU ⁇ s , ⁇ i
  • the signal (idle) photon center wavelengths corresponding to the three island structures are 1556.0 nm (1544.0 nm), 1558.5 nm (1541.6 nm), and 1560.4 nm (1539.7 nm), respectively.
  • the multi-level cascade structure can change the spectral characteristics of the associated photon pairs, and cooperate with the corresponding filters to obtain a single mode, higher mode purity and near-ideal collection efficiency.
  • the tuning of the spectral characteristics of the associated photons can be achieved by changing the number of cascaded structures, the length of the dispersion medium, and the like.
  • Embodiment 2 Two independent quantum interferences between predictive pure single photon sources based on multi-stage cascade structure
  • Quantum interference between independent single photon sources is the basis of a variety of quantum information technologies.
  • the two independent predictive single photon sources in this embodiment are all based on a three-stage cascade structure composed of a dispersion-shifted fiber and a single-mode fiber, and the structural parameters are the same.
  • the length and dispersion of each dispersion-shifted fiber are the same, the length is 100m, the zero-dispersion wavelength is 1549nm, and the group velocity dispersion slope is 0.075ps/km/(nm) 2 ;
  • the length and dispersion of the segment single-mode fiber are the same, the length is 11m, and the group velocity dispersion coefficient at 1550nm is 20ps/km/nm.
  • Different sections of the fiber are welded in sequence.
  • the pulsed pump light used in the pump cascade structure is a standard Gaussian spectrum with a center wavelength of 1550 nm, a full width at half maximum of 0.9 nm, and a ⁇ P p of 1 km -1 .
  • the center wavelength of the signal photon channel of the rectangular multi-channel filter is 1558.5 nm, the center wavelength of the idle photon channel is 1541.6 nm, and the bandwidth of both channels is 2 nm.
  • Both predictive single photon sources use the detection signal of idle photons to predict the existence of signal photons, thereby projecting signal photons onto a single photon state.
  • the idle photons output by the two single photon sources are directly detected by two single photon detectors respectively, and the output signal photons pass through two input ports of the beam splitter, and two of the beam splitters.
  • the outputs are detected by two other single photon detectors, and one of the signal photons introduces a delay t through the delay.
  • a quantum interference pattern can be obtained.
  • Example 3 Preparation of polarization entangled photon pairs based on multi-stage cascade structure
  • this embodiment generates a polarization entangled photon pair using a bidirectionally pumped three-stage cascade structure.
  • the cascade structure parameters, pump parameters, and multi-channel filters in FIG. 5 are the same as the corresponding parameters in Embodiment 2 shown in FIG.
  • the pulsed pump light is incident on the polarizing beam splitter in a 45 degree linear polarization direction and is divided into two beams having a polarization direction parallel to the paper surface and perpendicular to the paper surface.
  • a beam with a polarization direction parallel to the paper surface is input clockwise to a three-stage cascade structure, and a signal having the same polarization direction as the pump light and an idle frequency associated photon pair are generated by a four-wave mixing process (with
  • the remaining pump light and associated photon pairs output in the clockwise and counterclockwise directions are output from the same output port after the polarization beam splitter meets.
  • the signal and the idler photons are then separated from the remaining pump light by a filter. Due to the coherent superposition, the signal output from the filter and the idler photons are in the polarization entangled state. on.
  • the phase difference of the associated photon pairs produced by controlling the two processes And the polarization state the four Bell states of polarization entanglement can also be obtained.
  • Embodiment 4 Two-channel predictive near-simple single photon source based on multi-stage cascade structure
  • the present embodiment is shown in FIG. 6.
  • the cascade structure parameters and pump parameters in FIG. 6 are the same as the corresponding parameters in Embodiment 1.
  • a multi-channel filter 1 is added after the third-stage dispersion-shifted fiber, and the center wavelengths of the two rectangular filter channels of the filter are 1558.5 nm and 1541.6 nm, respectively, and the bandwidth is 2 nm, respectively, correspondingly
  • the signal photons and the idle photons are filtered out to form a first output channel, and the remaining other wavelengths are output from the other output port to the third stage single mode fiber.
  • a multi-channel filter 2 is added after the fourth-stage dispersion-shifted fiber.
  • the center wavelengths of the two rectangular filter channels of the filter are 1556.0 nm and 1544.0 nm, respectively, and the bandwidth is 2 nm, respectively, and the corresponding signal photons and idle photons can be respectively Filter out to form a second output channel.
  • the multi-channel filter does not introduce dispersion, according to the calculation results in Embodiment 1, it can be seen that the photon pairs output by the two output channels have higher mode purity and declared efficiency. If the detection signal of the idle photon is used to predict the existence of the signal photon, a two-channel near-simple single photon source can be prepared.
  • Example 5 Preparation of a two-channel predictive nearly pure single photon using a two-stage cascade structure based on a waveform shaper
  • the device of this embodiment is shown in Fig. 7(a).
  • the two-stage cascade structure is composed of two-stage dispersion-shifting fiber and a waveform shaper.
  • the two-stage dispersion-shifted fiber has the same length and dispersion, and the length is 100m.
  • the zero-dispersion wavelength is 1549nm
  • the group velocity dispersion slope is 0.075ps/km/(nm) 2
  • the pulse pumping light used for the pump cascade structure is the standard Gaussian spectrum
  • the center wavelength is 1550nm
  • the half-maximum full width is 0.7 nm
  • ⁇ P p takes 1 km -1 .
  • phase shift is introduced by controlling the waveform plasticizer:
  • the bandwidth of each filter channel is 2 nm.
  • Embodiment 6 Associated photon pair generating device based on cascaded spontaneous parameter down conversion process in bulk crystal
  • a pulsed optical pumping nonlinear medium-dispersive medium-multi-stage cascade structure of nonlinear medium (the number of stages is equal to two) is generated, and the associated photon pair is generated by the cascaded spontaneous parameter down-conversion process.
  • the nonlinear medium is a bulk lithium niobate crystal having a thickness of 0.5 mm, and the crystal cutting satisfies the first type of phase matching of the parametric down conversion process
  • the dispersive medium is a carbon dioxide gas having a certain pressure.
  • the resulting signal and the idle photons and the remaining pump light exit the free space from the cascade at different angles and are filtered out through the filters and apertures. Since the spatial modes of the generated photons are not uniform, there are constructive and destructive modulation phenomena in the temporal and spatial modes, so the corresponding filters and apertures need to select the appropriate wavelength and position.

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Abstract

一种利用量子干涉控制量子关联光子对频谱特性的方法，包括以下步骤：采用由一段非线性介质、一段色散介质、……、一段非线性介质依次构成的多级级联结构，或采用由一段非线性介质、一段色散可控介质和一段非线性介质依次构成的两级级联结构；所述多级级联结构中的非线性介质与色散介质依次交替排列，每段非线性介质视为一级，所述多级级联结构的级数大于等于二；光波可从所述多级级联结构或两级级联结构的一端入射，从另一端出射，形成光波通路；利用脉冲激光对所述多级级联结构或两级级联结构进行泵浦，通过级联结构中的级联参量过程产生量子关联光子对；由于级联参量过程中的量子干涉效应，所述量子关联光子对的强度在不同波长处发生周期性的干涉相长和相消；对于所述多级级联结构，干涉相长和相消对应的波长可通过改变多级级联结构的级数、色散介质的长度或替换具有不同色散特性的色散介质进行调谐；对于所述两级级联结构，干涉相长和相消对应的波长可通过改变色散可控介质的色散特性和长度进行调谐；使用滤波和收集装置，将干涉相长所对应波长处的量子关联光子对进行收集和输出。

Description

一种利用量子干涉控制量子关联光子对频谱特性的方法 技术领域
本发明属于量子信息科学与技术领域，涉及量子关联光子对的制备，以及以量子关联光子对为基础的纠缠光子和单光子的制备，具体为一种控制量子关联光子对频谱特性的方法。
背景技术
很多量子信息处理技术的实现，依赖于变换极限的光子之间的量子干涉。非线性介质中的自发光学参量过程是一种制备量子关联光子对、预报式单光子等量子光场的有效方法。常用的自发光学参量过程包括x (2)二阶非线性介质中的自发参量下转换过程以及x (3)三阶非线性介质中的自发四波混频过程。在此类过程中，来源于强泵浦光场中的一个(或两个)光子通过介质的x (2)(x( 3))非线性效应湮灭、并同时产生一对频率分别为ω s和ω i的量子关联光子对，通常这对光子被分别称为信号光子和闲置光子。
自发光学参量过程可以由单频激光泵浦，也可以由脉冲激光泵浦。由于采用脉冲激光泵浦可以定义量子态光场的产生时间，因而可以满足独立量子光源之间实现干涉所要求的同步条件，实现量子计算和量子纠缠交换等量子信息处理任务。本发明即针对脉冲激光泵浦的自发光学参量过程。
脉冲激光泵浦情况下，量子关联光子对的时间模式特性以及收集效率是衡量其特性和质量的两个重要因素。考察关联光子对的时间模式特性，可以从描述光子对频谱特性的联合频谱函数F(ω s，ω i)出发。F(ω s，ω i)正比于产生一对频率分别为ω s和ω i的信号和闲置光子对的几率振幅。将联合频谱函数进行施密特分解：F(ω s，ω i)＝∑r kφ k(ω s)ψ k(ω i)(∑r k 2＝1)，即可得到光子对的平均模式数K＝1/∑r k 4。K越接近于1，表明关联光子对越接近单模，即信号和闲置光子越接近变换极限。另外，在实践中，关联光子对的时间模式特性也可以采用信号或闲频光场的二阶相关函数g (2)来表征。以信号光场为例，g (2)与关联光子对频谱有如下关系：
Figure PCTCN2018077374-appb-000001
其中，f s(ω s)表示信号光子通道的滤波器透射谱。根据Schwartz不等式，有g (2)≤2。一般地，光场的g (2)越接近于2，表明其越接近于单模，模式纯度越高。
关联光子对的收集效率是指，对于非线性介质中产生的每一对信号和闲置光子，经过各 自滤波器后，两者均被收集并输出的概率。若一对光子中一个甚至两个在滤波和收集过程中被散失，则会造成输出关联光子对的符合计数率(即信号光子通道和闲频光子通道的探测器同时探测到光子的计数率)的下降，影响关联光子对的亮度和背景噪声。关联光子对的收集效率可由信号或闲频光子的条件收集效率来表征。以信号(闲置)光子的条件收集效率η s(i)为例，其表示当一个闲置(信号)光子被成功收集后，其所对应的信号(闲置)光子被收集的概率，与关联光子对频谱有如下关系：
Figure PCTCN2018077374-appb-000002
其中f s(i)(ω s(i))表示信号(闲置)光子通道的滤波器透射谱。若两侧收集效率乘积η sη i越接近100％，则表明关联光子对的收集效率越高。
在很多量子信息应用中，不仅要求关联光子对具有单模特性，而且具有接近理想值的收集效率。然而，对于大多数普通的块状或波导非线性介质，其所产生关联光子对的联合频谱函数通常不可直接分解为直积形式，即F(ω s，ω i)≠φ(ω s)ψ(ω i)。这种情况下，根据施密特分解公式可知，关联光子对的平均模式数大于1，处于多模态，相应的信号和闲置光子均不是变换极限的光子。
为了使通过自发参量过程产生的信号光子和闲置光子均满足变换极限，常采用以下两种方法：一是采用窄带滤波器对信号光子和闲置光子滤波，使滤波后信号和闲置光子的相干时间均大于泵浦脉冲的脉宽，一般来说滤波器带宽越窄，光子越接近单模；二是控制非线性介质的色散，使得联合频谱函数可直接分解。然而，前一种方法产生的光子对，窄带滤波的加入会造成收集效率的降低，且滤波带宽越窄，收集效率越低；后一种方法产生的变换极限光子对虽然可获得较高亮度，但是参量过程位相匹配条件和光子对频谱特性对非线性介质色散的要求互相影响、互相牵制，存在不能灵活改变的光子对波长的不足；此外，受限于加工工艺，理论上通过设计可达到的色散特性在工程上实现起来通常都很困难。
发明内容
本发明的目的是为了克服现有技术中的不足，提供一种利用量子干涉控制量子关联光子对频谱特性的方法。该方法基于多级参量过程的量子干涉来调控非线性相互作用；通过采用非线性介质和色散介质的级联结构，从而将参量过程位相匹配和光子对频谱特性的控制过程相分离，因此可以对不同波长的光子对频谱进行灵活调控。该方法的分离控制过程体现在：通过控制非线性介质的色散控制光子对的波长范围；通过控制非线性介质之间色散介质的色散特性、长度、以及干涉的级次来控制光子对的频谱特性。利用本发明产生的频谱可控光子 对，可进一步制备出多种量子态，包括预报式纯态单光子，双模偏振纠缠光子对，以及多通道纯态单光子等。与控制非线性介质获得频谱可分解光子对的方法相比，本发明具有工艺简单、可灵活改变光子对波长、可同时实现多波长输出的优点；与利用窄带滤波获得变换极限光子对的方法相比，采用本发明产生的光子对具有高亮度的优点。
本发明的目的是通过以下技术方案实现的：
一种利用量子干涉控制量子关联光子对频谱特性的方法，包括以下步骤：采用由一段非线性介质、一段色散介质、……、一段非线性介质依次构成的多级级联结构，或采用由一段非线性介质、一段色散可控介质和一段非线性介质依次构成的两级级联结构；所述多级级联结构中的非线性介质与色散介质依次交替排列，每段非线性介质视为一级，所述多级级联结构的级数大于等于二；光波可从所述多级级联结构或两级级联结构的一端入射，从另一端出射，形成光波通路；
利用脉冲激光对所述多级级联结构或两级级联结构进行泵浦，通过级联结构中的级联参量过程产生量子关联光子对；由于级联参量过程中的量子干涉效应，所述量子关联光子对的强度在不同波长处发生周期性的干涉相长和相消；
对于所述多级级联结构，干涉相长和相消对应的波长可通过改变多级级联结构的级数、色散介质的长度或替换具有不同色散特性的色散介质进行调谐；对于所述两级级联结构，干涉相长和相消对应的波长可通过改变色散可控介质的色散特性和长度进行调谐；
使用滤波和收集装置，将干涉相长所对应波长处的量子关联光子对进行收集和输出。
所述多级级联结构或两级级联结构中的非线性介质是满足非线性参量过程相位匹配条件的整块介质或满足非线性参量过程相位匹配条件的波导结构介质；各段非线性介质具有相同或不同的色散特性，当各段非线性介质具有不同的色散特性时，需确保各段非线性介质中通过参量过程产生的量子关联光子对的波长区域具有重叠。
所述多级级联结构中的色散介质是具有色散性的整块介质或具有色散性的波导结构介质；所述两级级联结构中的色散可控介质是具有色散可控性的整块介质、具有色散可控性的波导结构介质或具有色散可控性的器件；所述具有色散可控性的器件由介质和色散器件组成。
与现有技术相比，本发明的技术方案所带来的有益效果是：
(1)本发明基于由非线性介质和色散介质依次交叉构成的多级级联结构(级数大于等于二)，或者由非线性介质和色散可控介质构成的两级级联结构；通过级联参量过程产生关联光子对。在此级联参量过程中，第一段非线性介质后的级联结构的作用可以视为：对第一段非线性介质中通过自发参量过程所产生的关联光子对进行主动滤波。与单独采用一段普通块状或波导非线性介质的关联光子对、通过被动滤波提高纯度的方法相比，本方法相比可在提 高光子对模式纯度的同时保持收集效率。与采用色散可控非线性介质(如周期极化非线性晶体或光子晶体光纤等)的方法相比，具有波长可灵活改变、且制备工序简单的特点。
(2)本发明中，对于多级级联结构所输出的关联光子对，可以通过改变级联结构的级数、色散介质的长度、或替换具有不同色散特性的色散介质对其频谱特性进行控制。
(3)本发明中，对于包含色散可控介质的两级级联结构所输出的关联光子对，可以通过改变色散可控介质的色散特性对其频谱特性进行控制。
附图说明
图1利用多级级联结构中脉冲光泵浦的自发参量过程产生关联光子对的示意图。
图2利用基于色散位移光纤和单模光纤的多级级联结构产生关联光子对的示意图。
图3(a)至图3(d)为多级级联结构所产生关联光子对的频谱模拟结果图。
图4两个独立的基于级联结构的预报式纯态单光子源间量子干涉示意图。
图5基于多级级联结构的偏振纠缠光子对制备示意图。
图6利用基于波形塑形器的两级级联结构制备双输出通道预报式纯态单光子示意图。
图7(a)和图7(b)是利用基于波形塑形器的两级级联结构制备双输出通道预报式近纯态单光子示意图及模拟结果图。
图8是基于块状晶体中级联自发参量下转换的关联光子对源的结构图。
具体实施方式
下面结合附图对本发明作进一步的描述：
本发明利用脉冲激光泵浦多级级联结构(级数大于等于二)或包含色散可控介质的两级级联结构，通过级联光学参量过程获得量子关联光子对，进而可以制备多种量子态光场。通过改变所述级联结构的参数，并结合收集和滤波装置，可以实现对关联光子对频谱特性的控制。下面以高斯脉冲泵浦的单模光波导中的自发四波混频参量过程为例，说明本发明中多级级联结构(级数大于等于二)以及包含色散可控介质的两级级联结构中所产生光子对的频谱特性。
首先，对于一段具有均匀色散的非线性介质，自发四波混频过程所产生关联光子对的频谱函数为：
Figure PCTCN2018077374-appb-000003
即高斯脉冲泵浦的包络函数
Figure PCTCN2018077374-appb-000004
和相位匹配函数
Figure PCTCN2018077374-appb-000005
的乘积。其 中，ω p为泵浦光的中心频率，σ p为泵浦光的带宽，信号、闲频光子和泵浦光的波长与频率的关系为λ j＝2πc/ω j(c代表光速)；L为非线性介质长度，Δk＝k s+k i-2k p+2kp+2γP p为相位失配，k p、k s和k i分别代表泵浦、信号和闲频光子的传播常数，由非线性介质的色散决定，γ代表介质的非线性系数，P p代表泵浦光的峰值功率。
对于本发明中的多级级联结构(级数大于等于二)，若假设其级数为N，如图1所示，第一段非线性介质的相位失配为Δk 1、长度为L 1，第N段非线性介质的相位失配为Δk N、长度为L N，第一段色散介质的相位失配为Δk d1(这里色散介质的相位失配定义为Δk d＝k s+k i-2k p)、长度为L d1，第N-1段色散介质的相位失配为Δk d(N-1)、长度为L d(N-1)，则级联结构输出的关联光子对频谱可以表示为：
Figure PCTCN2018077374-appb-000006
Figure PCTCN2018077374-appb-000007
上式中其它参数所代表的意义与公式(3)相同。
若进一步假设多级级联结构中各段非线性介质的色散和长度均相同，即Δk 1＝Δk 2＝…＝Δk N＝ΔK U、L 1＝L 2＝…＝L N＝L U，各段色散介质的色散和长度也相同，即Δk d1＝…Δk d(N-1)＝Δk dU、L d1＝…＝L d(N-1)＝L dU。则关联光子对的频谱函数可以表示为：
Figure PCTCN2018077374-appb-000008
上式中其它参数所代表的意义与公式(3)相同，其中
Figure PCTCN2018077374-appb-000009
此处
Figure PCTCN2018077374-appb-000010
比较公式H(θ)与公式(3),可以看出，由于后一段非线性介质对前一段非线性介质输出的相敏放大,多级级联结构输出的联合频谱受到干涉因子H(θ)修正。可以看出，公式H(θ)中的干涉因子表达式与多光束干涉的干涉因子类似。
对于本发明中的基于色散可控介质的两级级联结构，其级数为N＝2，根据公式(4)可知，其输出的关联光子对频谱可以表示为
Figure PCTCN2018077374-appb-000011
Figure PCTCN2018077374-appb-000012
这里，Φ d＝φ s+φ i-2φ p即为色散可控介质引入的信号、闲置光子与泵浦光子间的相位差。利用色散可控介质，可以对各个相位匹配波长处的Φ d进行个别或整体的调谐，从而实现对关联光子对频谱的控制。
实施例1:利用基于色散位移光纤和单模光纤的多级级联结构产生频谱可控关联光子对本实施例如图2所示，多级级联结构(级数N≥3)中的非线性介质为色散位移光纤，且各段色散位移光纤的长度和色散相同，长度均为L U＝100m，零色散波长均为1549nm、群速度色散斜率均为0.075ps/km/(nm) 2；多级级联结构中的色散介质为通讯波段标准单模光纤，且各段单模光纤的长度和色散相同，长度均为L dU＝11m，1550nm处的群速度色散系数为20ps/km/nm。不同段光纤被依次焊接。用于泵浦级联结构的脉冲泵浦光为标准高斯频谱，中心波长为1550nm，半最大全宽为0.9nm；γP p取1km -1。最后，利用多通道滤波器将级联结构中的信号和闲置光子滤出。将上述参数带入到公式(5)和公式(6)中，可计算得到不同参数下多级级联结构输出的关联光子对频谱|F NU(ω s，ω i)| 2。
本实施例的主要计算结果如图3(a)至图3(d)所示，计算中已假设光纤损耗以及焊接损耗均为零，并已对结果进行归一化。图3(a)是不采用多级级联结构，即仅一段100m色散位移光纤输出的关联光子对频谱，图3(b)是N＝2时的两级级联结构输出的关联光子对频谱。图3(c)和3(d)分别是采用了本发明之后的，三级级联结构(N＝3)和四级级联结构(N＝4)输出的关联光子对频谱。对于图3(a)-(d)，各图的上半部分是关联光子对频谱|F NU(ω s，ω i)| 2的二维等高线图，图中坐标轴已由信号(闲置)光子频率ω s(i)转化为了信号(闲置)光子波长λ s(i)，下半部分是将|F NU(ω s，ω i)| 2投影到信号光子波长坐标轴上得到的一维信号光子频谱。
从图3(a)至图3(d)可以看出，采用级联结构后，由于周期性的量子干涉相长和相消作用，光子对频谱变成分离的岛式结构。三个岛式结构对应的信号(闲置)光子中心波长，分别为1556.0nm(1544.0nm)、1558.5nm(1541.6nm)和1560.4nm(1539.7nm)。从图3(c)和3(d)可以看出，采用本发明中的多级(N≥3)级联结构后，所输出频谱的干涉的可见度得到加强(比较一维信号光子频谱)，岛式结构的分离特征更加明显，也即多级级联结构作为主动滤波器对相邻通道的隔离度随级次而增加，这就意味着更高的收集效率。此外，在两个岛式结构之间，还存在N-2个较不明显的小型岛式结构，而且通过改变级数还可以控制各个岛式结构的宽度和形状，因此对频谱的控制更加灵活。
考察图3(c)中信号(闲置)光子中心波长为1558.5nm(1541.6nm)的岛式机构，以其中 心波长为准，分别在信号和闲置光子通道设置带宽为2nm的矩形多通道滤波器，将信号和闲置光子分别滤出。这时计算得到的信号光场g (2)＝1.92，收集效率η sη i＝0.95，接近单模光场(g (2)＝2)和理想的收集效率(η sη i＝1)。
考察图3(d)中信号(闲置)光子中心波长为1556.0nm(1544.0nm)的岛式机构，以其中心波长为准，分别在信号和闲置光子通道设置带宽为2nm的矩形多通道滤波器，将信号和闲置光子分别滤出。这时计算得到的信号光场g (2)＝1.95，收集效率η sη i＝0.93，接近单模光场和理想的收集效率。
综上所述，采用多级级联结构可以改变关联光子对的频谱特性，与相应的滤波器配合，可获得接近单模的、较高的模式纯度和接近理想的收集效率。而且，通过改变级联结构级数、色散介质长度等途径可以实现对关联光子对频谱特性的调谐。
实施例2:两个独立的基于多级级联结构的预报式纯态单光子源间量子干涉
独立单光子源间的量子干涉是多种量子信息技术的基础。如图4所示，本实施例中的两个独立的预报式单光子源的均基于由色散位移光纤和单模光纤组成的三级级联结构，结构参数均相同。两个预报式单光子源中，各段色散位移光纤的长度和色散相同，长度均为100m，零色散波长均为1549nm、群速度色散斜率均为0.075ps/km/(nm) 2；且各段单模光纤的长度和色散相同，长度均为11m，1550nm处的群速度色散系数为20ps/km/nm。不同段光纤被依次焊接。用于泵浦级联结构的脉冲泵浦光为标准高斯频谱，中心波长为1550nm，半最大全宽为0.9nm；γP p取1km -1。矩形多通道滤波器的信号光子通道中心波长为1558.5nm，闲置光子通道中心波长为1541.6nm，两个通道的带宽均为2nm。
两个预报式单光子源均利用闲置光子的探测信号预报信号光子的存在，从而将信号光子投影到单光子态上。在图4所示的量子干涉装置中，两个单光子源输出的闲置光子被两个单光子探测器分别直接探测，输出的信号光子通过分束器的两个输入口，分束器的两个输出被另外两个单光子探测器探测，且其中一路信号光子通过延时器引入延时t。测量四个单光子探测器的四重符合计数与延时t的关系，可以得到量子干涉图样。在上述参数设置下，根据实施例1中的计算结果(信号光场g (2)＝1.92，收集效率η sη i＝0.95)，对应的量子干涉可见度V＝92％。
实施例3:基于多级级联结构的偏振纠缠光子对制备
如图5所示，本实施例采用双向泵浦的三级级联结构产生偏振纠缠光子对。图5中的级联结构参数、泵浦参数、多通道滤波器与图4所示的实施例2中的相应参数相同。脉冲泵浦 光以45度线偏振方向入射进偏振分束器，被分成偏振方向平行于纸面和垂直于纸面的两束。其中偏振方向平行于纸面的一束顺时针输入至三级级联结构，通过四波混频过程产生偏振方向与泵浦光相同的信号和闲频关联光子对(以|H> s|H> i表示)；而偏振方向垂直于纸面的一束逆时针输入至上述三级级联结构后，同样通过四波混频过程产生偏振方向与泵浦光相同的信号和闲频关联光子对(以|V> s|V> i表示)。通过控制三级级联结构所引入的偏振改变，使得顺时针和逆时针方向输出的剩余泵浦光和关联光子对在偏振分束器会合后，从相同的输出口输出。然后通过滤波器将信号和闲频光子与剩余泵浦光分开。由于相干叠加作用，滤波器输出的信号和闲频光子处于偏振纠缠态
Figure PCTCN2018077374-appb-000013
上。在此基础上，通过在控制两个过程所产生关联光子对的相位差
Figure PCTCN2018077374-appb-000014
和偏振态，还可得到偏振纠缠的四个Bell态。
实施例4:基于多级级联结构的双通道预报式近纯态单光子源
本实施例如图6所示，图6中的级联结构参数、泵浦参数与实施例1中的相应参数相同。本实施例中，在第3段色散位移光纤之后加入了多通道滤波器1，该滤波器的两个矩形滤波通道的中心波长分别为1558.5nm和1541.6nm，带宽为2nm，分别可以将相应的信号光子和闲置光子滤出，构成第一输出通道，剩余的其它波长由另一输出口输出到第3段单模光纤中。在第4段色散位移光纤后加入了多通道滤波器2，该滤波器的两个矩形滤波通道的中心波长分别为1556.0nm和1544.0nm，带宽为2nm，分别可以将相应的信号光子和闲置光子滤出，构成第二输出通道。假设多通道滤波器不引入色散，根据实施例1中的计算结果，可知两个输出通道输出的光子对均具有较高的模式纯度和宣布效率。若使用闲置光子的探测信号预报信号光子的存在，可制备双通道近纯态单光子源。
实施例5:利用基于波形塑形器的两级级联结构制备双通道预报式近纯态单光子
本实施例的装置如图7(a)所示，两级级联结构由两段色散位移光纤和波形塑形器构成，其中的两段色散位移光纤的长度和色散相同，长度均为100m，零色散波长均为1549nm、群速度色散斜率均为0.075ps/km/(nm) 2；用于泵浦级联结构的脉冲泵浦光为标准高斯频谱，中心波长为1550nm，半最大全宽为0.7nm；γP p取1km -1。
通过控制波形塑性器引入如下相移：
Figure PCTCN2018077374-appb-000015
其中，λ s1＝1554nm和λ i1＝1546nm分别是通道1的信号和闲置光子波长；λ s2＝1557nm和λ i2＝1543.1nm分别是通道2的信号和闲置光子波长；系数a为1.7nm -1。通过将上述相移代入到公式(7)中，可以得到相应的关联光子对频谱函数图。计算得到的二维等高线图如图7(b)所示，从图中可以看到两个岛式结构，分别对应于两个输出通道。实施例中采用图7(a)所示的多通道矩形滤波器将两个通道的信号和闲置光子分别输出，每个滤波通道的带宽为2nm。对于两个输出通道，计算可得信号(闲置)光场g (2)＝1.96，收集效率η sη i＝0.98。因此两个输出通道输出的光子对均具有较高的模式纯度和宣布效率，若使用闲置光子的探测信号预报信号光子的存在，可制备双通道近纯态单光子源。此外，还可将公式(8)进行拓展、改写，从而可以制备多通道近纯态单光子源。
实施例6：基于块状晶体中级联自发参量下转换过程的关联光子对产生装置
本实施例通过脉冲光泵浦非线性介质——色散介质——非线性介质的多级级联结构(级数等于二)，通过级联自发参量下转换过程产生关联光子对。如图8的装置图所示，其中的非线性介质为0.5mm厚度的块状铌酸锂晶体，晶体切割满足参量下转换过程的第一类相位匹配，色散介质为具有一定压强的二氧化碳气体。所产生的信号和闲置光子以及剩余泵浦光以不同角度从级联结构出射进自由空间，通过滤光片和孔阑将其滤出。由于所产生光子的空间模式不单一，在时间和空间模式上具有相长和相消的调制现象，因此相应的滤光片和孔阑需要选择合适的波长和位置。
本发明并不限于上文描述的实施方式。以上对具体实施方式的描述旨在描述和说明本发明的技术方案，上述的具体实施方式仅仅是示意性的，并不是限制性的。在不脱离本发明宗旨和权利要求所保护的范围情况下，本领域的普通技术人员在本发明的启示下还可做出很多形式的具体变换，这些均属于本发明的保护范围之内。

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  1. 一种利用量子干涉控制量子关联光子对频谱特性的方法，其特征在于，包括以下步骤：
    采用由一段非线性介质、一段色散介质、……、一段非线性介质依次构成的多级级联结构，或采用由一段非线性介质、一段色散可控介质和一段非线性介质依次构成的两级级联结构；所述多级级联结构中的非线性介质与色散介质依次交替排列，每段非线性介质视为一级，所述多级级联结构的级数大于等于二；光波可从所述多级级联结构或两级级联结构的一端入射，从另一端出射，形成光波通路；
    利用脉冲激光对所述多级级联结构或两级级联结构进行泵浦，通过级联结构中的级联参量过程产生量子关联光子对；由于级联参量过程中的量子干涉效应，所述量子关联光子对的强度在不同波长处发生周期性的干涉相长和相消；
    对于所述多级级联结构，干涉相长和相消对应的波长可通过改变多级级联结构的级数、色散介质的长度或替换具有不同色散特性的色散介质进行调谐；对于所述两级级联结构，干涉相长和相消对应的波长可通过改变色散可控介质的色散特性和长度进行调谐；
    使用滤波和收集装置，将干涉相长所对应波长处的量子关联光子对进行收集和输出。
  2. 根据权利要求1所述一种利用量子干涉控制量子关联光子对频谱特性的方法，其特征在于，所述多级级联结构或两级级联结构中的非线性介质是满足非线性参量过程相位匹配条件的整块介质或满足非线性参量过程相位匹配条件的波导结构介质；各段非线性介质具有相同或不同的色散特性，当各段非线性介质具有不同的色散特性时，需确保各段非线性介质中通过参量过程产生的量子关联光子对的波长区域具有重叠。
  3. 根据权利要求1所述一种利用量子干涉控制量子关联光子对频谱特性的方法，其特征在于，所述多级级联结构中的色散介质是具有色散性的整块介质或具有色散性的波导结构介质；所述两级级联结构中的色散可控介质是具有色散可控性的整块介质、具有色散可控性的波导结构介质或具有色散可控性的器件；所述具有色散可控性的器件由介质和色散器件组成。
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