远距离量子通信实验中的高维纠缠源

设计了适用于远距离量子通信实验的高维纠缠源.利用连续激光器抽运产生了极化-时间两体四维纠缠光子对,在抽运功率20 mW下测到每秒700对符合,保真度为89%±3%.相比已有的高维纠缠源,在本文中发展的源具有传输便利、相位稳定性好等优点,适用于未来远距离高维量子通信实验和量子力学基本问题实验检验,如远距离高维量子密码实验、两粒子Greenberger-Horne-Zeilinger定理检验、两粒子量子赝"心灵感应"(quantum pseudo telepathy)实验演示等. 关键词： 高维纠缠 极化纠缠 时间箱纠缠 量子通信     

Generation of photon pairs in dispersion shift fibers through spontaneous four wave mixing: Influence of self-phase modulation

Correlated signal and idler photon pairs with small detuning in the telecom band can be generated through spontaneous four-wave mixing in dispersion shift fibers. However, photons originated from other nonlinear processes in optical fibers, such as Raman scattering and self-phase modulation, may contaminate the photon pairs. It has been proved that photons produced by Raman scattering are the background noise of photon pairs. Here we show that photons induced by self-phase modulation of pump pulses are another origin of background noise. After studying the dependence of self-phase modulation induced photons in signal and idler bands, we demonstrate that the quantum correlation of photon pairs can be degraded by the self-phase modulation effect. The investigations are useful for characterizing and optimizing an all fiber source of photon pairs.     

Entanglement formation and violation of Bell's inequality with a semiconductor single photon source

We report the generation of polarization-entangled photons, using a quantum dot single photon source, linear optics, and photodetectors. Two photons created independently are observed to violate Bell's inequality. The density matrix describing the polarization state of the postselected photon pairs is reconstructed and agrees well with a simple model predicting the quality of entanglement from the known parameters of the single photon source. Our scheme provides a method to create no more than one entangled photon pair per cycle after postselection, a feature useful to enhance quantum cryptography protocols based on shared entanglement.     

一种新的预报单光子源诱骗态量子密钥分发方案

提出一种新的预报单光子源诱骗态量子密钥分发方案.在发端采用参量下变换产生纠缠光子对,其中之一用来进行预报探测,根据探测结果将另一路光脉冲分成两个集合,其中预报探测有响应的脉冲集合用作信号态,无响应的脉冲集合作为诱骗态.由于探测效率的问题,这两个集合都是有光子的,通过这两个集合的通过率和错误率估计出单光子的通过率和错误率.此方法不需要改变光强,简单可行.仿真结果表明：该方法可以达到完美单光子源的安全通信距离;与预报单光子源的量子密钥分发相比,密钥产生率有了很大的提高;和三强度预报单光子源诱骗态量子密钥分发的 关键词： 量子保密通信 量子密钥分发 诱骗态 预报单光子源     

Full-field quantum correlations of spatially entangled photons

Spatially entangled twin photons allow the study of high-dimensional entanglement, and the Laguerre-Gauss modes are the most commonly used basis to discretize the single-photon mode spaces. In this basis, to date only the azimuthal degree of freedom has been investigated experimentally due to its fundamental and experimental simplicity. We show that the full spatial entanglement is indeed accessible experimentally; i.e., we have found practicable radial detection modes with negligible cross correlations. This allows us to demonstrate hybrid azimuthal-radial quantum correlations in a Hilbert space with more than 100 dimensions per photon.     

Nonclassical interference and entanglement generation using a photonic crystal fiber pair photon source

We demonstrate two key components for optical quantum information processing: a bright source of heralded single photons; and a bright source of entangled photon pairs. A pair of pump photons produces a correlated pair of photons at widely spaced wavelengths (583 nm and 900 nm), via a chi((3)) four-wave mixing process. We demonstrate nonclassical interference between heralded photons from independent sources with a visibility of 95% (after correction for background), and an entangled photon pair source, with a fidelity of 89% with a Bell state.     

Photon pair source via two coupling single quantum emitters

We study the two coupling two-level single molecules driven by an external field as a photon pair source. The probability of emitting two photons, P2, is employed to describe the photon pair source quality in a short time, and the correlation coefficient RAB is employed to describe the photon pair source quality in a long time limit. The results demonstrate that the coupling single quantum emitters can be considered as a stable photon pair source.     

Single photon frequency up-conversion and its applications

Optical frequency up-conversion is a technique, based on sum frequency generation in a non-linear optical medium, in which signal light from one frequency (wavelength) is converted to another frequency. By using this technique, near infrared light can be converted to light in the visible or near visible range and therefore detected by commercially available visible detectors with high efficiency and low noise. The National Institute of Standards and Technology (NIST) has adapted the frequency up-conversion technique to develop highly efficient and sensitive single photon detectors and a spectrometer for use at telecommunication wavelengths. The NIST team used these single photon up-conversion detectors and spectrometer in a variety of pioneering research projects including the implementation of a quantum key distribution system; the demonstration of a detector with a temporal resolution beyond the jitter limitation of commercial single photon detectors; the characterization of an entangled photon pair source, including a direct spectrum measurement for photons generated in spontaneous parametric down-conversion; the characterization of single photons from quantum dots including the measurement of carrier lifetime with escalated high accuracy and the demonstration of the converted quantum dot photons preserving their non-classical features; the observation of 2nd, 3rd and 4th order temporal correlations of near infrared single photons from coherent and pseudo-thermal sources following frequency up-conversion; a study on the time-resolving measurement capability of the detectors using a short pulse pump and; evaluating the modulation of a single photon wave packet for better interfacing of independent sources. In this article, we will present an overview of the frequency up-conversion technique, introduce its applications in quantum information systems and discuss its unique features and prospects for the future.     

两个独立全光纤多通道光子纠缠源的Hong-Ou-Mandel干涉

独立光子源的干涉是实现复杂量子体系应用(比如多光子纠缠态产生和量子隐形传态等)的核心技术.利用100 GHz密集波分复用技术,实现了1.55μm全光纤多通道独立纠缠光子源的Hong-Ou-Mandel干涉,在不去除暗符合(随机符合计数)的情况下,可见度为53.2%±8.4%,去除暗符合可见度可达到82.9%±5.3%.给出了关于色散位移光纤中基于自发四波混频过程产生的单光子光谱纯度严格的理论描述,模拟了抽运脉冲宽度和滤波器带宽对单光子光谱纯度的影响,并给出了理论上的最佳条件(最佳的抽运脉冲宽度为8 ps,高斯滤波器带宽为40 GHz及以下).在测量Hong-Ou-Mandel干涉之前,先测量了液氮冷却状态下的色散位移光纤关联光子源的符合和随机符合比率,在抽运功率为23μW的情况下,最大比率可以达到131.Hong-Ou-Mandel干涉在高精度光学测量、测量装置无关的量子密钥分配等应用中扮演着极为重要的角色.     

Deutsch-jozsa algorithm using triggered single photons from a single quantum dot

We demonstrate a two-qubit Deutsch-Jozsa algorithm with single photons from a single InP quantum dot. The qubits are implemented via the spatial mode and the polarization of a single photon. Our photon source is operated both under continuous and pulsed excitation, the latter allowing deterministic quantum logic by generating photons on demand with a strong suppression of two-photon events. The computation reached a success probability of up to 79%. We also exploit the concept of decoherence-free subspaces that helps to make our experimental setup robust against sources of phase noise.     

Synchronization and Phase Shaping of Single Photons with High-Efficiency Quantum Memory

Time synchronization and phase shaping of single photons both play fundamental roles in quantum information applications that rely on multi-photon quantum interference.Phase shaping typically requires separate modulators with extra insertion losses.Here,we develop an all-optical built-in phase modulator for single photons using a quantum memory.The fast phase modulation of a single photon in both step and linear manner are verified by observing the efficient quantum-memory-assisted Hong-Ou-Mandel interference between two single photons,where the anti-coalescence effect of bosonic photon pairs is demonstrated.The developed phase modulator may push forward the practical quantum information applications.     

Measurement-device-independent quantum key distribution with hyper-encoding

Measurement device-independent quantum key distribution(MDI-QKD) protocols are immune to all possible attacks on the photon detectors during quantum communication, but their key generation rates are low compared with those of other QKD schemes.Increasing each individual photon's channel capacity is an efficient way to increase the key generation rate, and high-dimensional(HD) encoding is a powerful tool for increasing the channel capacity of photons. In this paper, we propose an HD MDI-QKD protocol with qudits hyper-encoded in spatial mode and polarization degrees of freedom(DOFs). In the proposed protocol, keys can be generated using the spatial mode and polarization DOFs simultaneously. The proposed protocol is unconditionally secure,even for weak coherent pulses with decoy states. The proposed MDI-QKD protocol may be useful for future quantum secure communication applications.     

Quantum diffraction of position-momentum entangled photons from a sharp edge

In this paper, an experiment of quantum diffraction of position-momentum entangled photons from a straight sharp edge is presented. Path of a single photon of an entangled pair is partially blocked by a sharp edge whereas the other photon is detected at a stationary location without revealing the which-path information of the other photon. Quantum diffraction pattern of the sharp edge is revealed only in the correlated conditional detection of spatially separated photons and no diffraction pattern is formed in local detections of individual photons. Theoretical analysis of the quantum diffraction of position-momentum entangled photons from a sharp edge is also presented in this paper. Experimental measurements of the quantum diffraction pattern are compared with theoretically calculated quantum diffraction pattern of position-momentum entangled photons.     

Aerial image formation of quantum lithography for diffraction limit

Since the diffraction limit of Rayleigh criterion hardly creates finer features, the development for the quantum lithography of entangled photons, one of technologies beyond the diffraction limit, is a key merit without the shorter wavelength source tool. In the arbitrary pattern formation for the commercialization of this lithography, however, this quantum lithography is required to implement mask patterns and the conventional optical lithography. In this paper, for the quantum lithography of entangled photons, collective behavior of N-photon entangled states is modeled and simulated to show the effect of photon entangled states for 3-dimensional arbitrary pattern formation by using the rigorous coupled wave analysis (RCWA) and Kirchhoff analysis. For the enhanced resolution from the point of view that the de Broglie wavelength of a quantum state comprised of two entangled photons is half the classical wavelength associated with either photon, simulation results of entangled photons are similar to those of short wavelengths. However, both of simulation results show that the resolution of entangled photons is better than those of the shorter wavelengths. Simulation results can predict realistic better performance of entangled photons.     

Deterministic and storable single-photon source based on a quantum memory

A single-photon source is realized with a cold atomic ensemble (87Rb atoms). A single excitation, written in an atomic quantum memory by Raman scattering of a laser pulse, is retrieved deterministically as a single photon at a predetermined time. It is shown that the production rate of single photons can be enhanced considerably by a feedback circuit while the single-photon quality is conserved. Such a single-photon source is well suited for future large-scale realization of quantum communication and linear optical quantum computation.     

Subnatural linewidth single photons from a quantum dot

The observation of quantum-dot resonance fluorescence enabled a new solid-state approach to generating single photons with a bandwidth approaching the natural linewidth of a quantum-dot transition. Here, we operate in the small Rabi frequency limit of resonance fluorescence--the Heitler regime--to generate subnatural linewidth and high-coherence quantum light from a single quantum dot. The measured single-photon coherence is 30 times longer than the lifetime of the quantum-dot transition, and the single photons exhibit a linewidth which is inherited from the excitation laser. In contrast, intensity-correlation measurements reveal that this photon source maintains a high degree of antibunching behavior on the order of the transition lifetime with vanishing two-photon scattering probability. Generating decoherence-free phase-locked single photons from multiple quantum systems will be feasible with our approach.     

Violation of Bell's inequality with photons from independent sources

We report a violation of Bell's inequality using one photon from a parametric down-conversion source and a second photon from an attenuated laser beam. The two photons were entangled at a beam splitter using the postselection technique of Shih and Alley [Phys. Rev. Lett. 61, 2921 (1988)]]. A quantum interference pattern with a visibility of 91% was obtained using the photons from these independent sources, as compared with a visibility of 99.4% using two photons from a central parametric down-conversion source.     

多光子纠缠实验新进展--五光子纠缠技术及终端开放的隐形传态实验

在实验上发展了多光子纠缠技术,利用已有的四光子纠缠技术结合新发展的单光子源技术,在世界上第一次实现了五光子纠缠．并在此基础上实现了新型的量子隐形传态——终端开放的隐形传态．实验结果在量子力学基础问题的检验、信息论、密码学和量子计算等重要应用方向上,都具有显著的意义和价值．实验方法将大大促进未来网络化量子通信、线性光学量子计算、量子力学基础检验等重要科学问题的研究。     

Entropy change and information gain in photon counting measurement

Entropy change in a quantum state of cavity photons is investigated in the continuous measurement of photon number that is characterized by the quantum Markov process. It is shown that the average value of the entropy change in the quantum state of cavity photons is equal to the Shannon mutual information obtained from the outcome exhibited by the photodetector. Furthermore it is found that the entropy change reflects the photon statistics of the quantum state of cavity photons.     

Quantum teleportation with a quantum dot single photon source

We report the experimental demonstration of a quantum teleportation protocol with a semiconductor single photon source. Two qubits, a target and an ancilla, each defined by a single photon occupying two optical modes (dual-rail qubit), were generated independently by the single photon source. Upon measurement of two modes from different qubits and postselection, the state of the two remaining modes was found to reproduce the state of the target qubit. In particular, the coherence between the target qubit modes was transferred to the output modes to a large extent. The observed fidelity is 80%, in agreement with the residual distinguishability between consecutive photons from the source. An improved version of this teleportation scheme using more ancillas is the building block of the recent Knill, Laflamme, and Milburn proposal for efficient linear optics quantum computation.