Quantum non-demolition measurement of photon number using weak nonlinearities

We propose an alternative method for the quantum non-demolition measurement of photon numbers wherein weak cross-Kerr nonlinearities are to be used. The usual approach to quantum non-demolition measurements of quantum number involves encoding the photon number, through a cross-Kerr interaction, into a phase shift of a probe coherent state which is then detected through balanced homodyning. Weak nonlinearities produce small phase shifts which are difficult to detect and distinguish. In the method we propose, unbalanced homodyning acts as a displacement operator on the probe beam coherent state such that the cross-Kerr interaction encodes the photon number into the amplitude of a new coherent state. The value of the photon number can be determined by inefficient photon counting on the new coherent state. Our proposed method requires fewer resources than does the usual approach.     

Estimation of photon counting statistics with imperfect detectors

The study on photon counting statistics is of fundamental importance in quantum optics. We theoretically analyzed the imperfect detection of an arbitrary quantum state. We derived photon counting formulae for six typical quantum states(i.e.,Fock, coherent, squeeze-vacuum, thermal, odd and even coherent states) with finite quantum efficiencies and dark counts based on multiple on/off detector arrays. We applied the formulae to the simulation of multiphoton number detections and obtained both the simulated and ideal photon number distributions of each state. A comparison between the results by using the fidelity and relative entropy was carried out to evaluate the detection scheme and help select detectors for different quantum states.     

A new quantum mechanical photon counting distribution formula

By virtue of the density operator's P-representation in the coherent state representation, we derive a new quantum mechanical photon counting distribution formula. As its application, we calculate photon counting distributions for some given light fields. It is found that the pure squeezed state's photon counting distribution is related to the Legendre function, which is a new result.     

Quantum efficiency and signal bandwidth of thallium atomic line filters

We derive a new, simplified method for the computation of trapping effects in coupled three-level atomic systems. The linearity of the Holstein radiation trapping equation allows the reduction of the general rate equation to the solution of the basic two-level Holstein equation and to an algebraic eigenvalue problem. The method is applied to the numerical simulation of the quantum efficiency and the signal bandwidth of thallium atomic line filters. With a pumping scheme that achieves population inversion between the ground state and the metastable state, 90% quantum efficiency and 10 MHz signal bandwidth can be achieved, while in noninverting pumping schemes, even quite high pump intensities result in less than 60% quantum efficiency and 8 MHz signal bandwidth. A tradeoff between quantum efficiency, signal bandwidth and pump power is possible.     

Degenerate Bose-Fermi mixture of metastable atoms

We report the observation of simultaneous quantum degeneracy in a dilute gaseous Bose-Fermi mixture of metastable atoms. Sympathetic cooling of helium-3 (fermion) by helium-4 (boson), both in the lowest triplet state, allows us to produce ensembles containing more than 10(6) atoms of each isotope at temperatures below 1 microK, and achieve a fermionic degeneracy parameter of T/TF = 0.45. Because of their high internal energy, the detection of individual metastable atoms with subnanosecond time resolution is possible, permitting the study of bosonic and fermionic quantum gases with unprecedented precision. This may lead to metastable helium becoming the mainstay of quantum atom optics.     

Quantum entanglement of Fock states with perfectly efficient ultraslow single-probe photon four-wave mixing

We propose a method to achieve quantum entanglement of two Fock states with perfectly efficient, ultraslow propagation enhanced four-wave mixing. A cold atomic medium is illuminated with a two-mode cw control laser to produce coherent mixtures of excited states. An ultraslowly propagating, single-photon quantum probe field completes the four-wave mixing with 100% photon flux conversion efficiency, creating a depth dependent entanglement of two Fock states. We show that at a suitable propagation distance, a maximum entangled state is created with a single-photon wave-packet state that has 50% probability of being in each of two product-type Fock states.     

Quantum theory of continuous measurements and its applications in quantum optics

We present an overview of the quantum theory of continuous measurements and discuss some of its important applications in quantum optics. Quantum theory of continuous measurements is the appropriate generalization of the conventional formulation of quantum theory, which is adequate to deal with counting experiments where a detector monitors a system continuously over an interval of time and records the times of occurrence of a given type of event, such as the emission or arrival of a particle. We first discuss the classical theory of counting processes and indicate how one arrives at the celebrated photon counting formula of Mandel for classical optical fields. We then discuss the inadequacies of the so called quantum Mandel formula. We explain how the unphysical results that arise from the quantum Mandel formula are due to the fact that the formula is obtained on the basis of an erroneous identification of the coincidence probability densities associated with a continuous measurement situation. We then summarize the basic framework of the quantum theory of continuous measurements as developed by Davies. We explain how a complete characterization of the counting process can be achieved by specifying merely the measurement transformation associated with the change in the state of the system when a single event is observed in an infinitesimal interval of time. In order to illustrate the applications of the quantum theory of continuoius measurements in quantum optics, we first derive the photon counting probabilities of a single-mode free field and also of a single-mode field in interaction with an external source. We then discuss the general quantum counting formula of Chmara for a multi-mode electromagnetic field coupled to an external source. We explain how the Chmara counting formula is indeed the appropriate quantum generalization of the classical Mandel formula. To illustrate the fact that the quantum theory of continuous measurements has other diverse applications in quantum optics, besides the theory of photodetection, we summarize the theory of ‘quantum jumps’ developed by Zoller, Marte and Walls and Barchielli, where the continuous measurements framework is employed to evaluate the statistics of photon emission events in the resonance fluorescence of an atomic system.     

Hyperentanglement-assisted hyperdistillation for hyper-encoding photon system

In quantum information processing, the quality of photon system is decreased by the inevitable interaction with environment, which will greatly reduce the efficiency and security of quantum information processing. In this paper, we propose hyperentanglement-assisted hyperdistillation schemes to guarantee the quality of hyper-encoding photon system based on the method of quantum hyper-teleportation, which can increase the success probability of hyperdistillation and reduce the resource consumption. First, we propose a hyperentanglement-assisted single-photon hyperdistillation (HASPHD) scheme for polarization and spatial qubits to get rid of the vacuum state component caused by transmission loss, whose success probability can achieve the optimal one by increasing the efficiency of quantum hyper-teleportation. Subsequently, we present two hyperentanglement-assisted hyperentanglement distillation (HAHED) schemes for photon system to protect hyperentanglement from both transmission loss and quantum channel noise, which can recover the less-entangled mixed state to maximally hyperentangled state for known-parameter and unknown-parameter cases with high success probability and low resource consumption. In these hyperdistillation schemes, the influence of imperfect effects of optical elements can be largely decreased by the quantum hyper-teleportation method. These characters make the hyperentanglement-assisted hyperdistillation schemes have potential application prospects in practical quantum information processing.     

Homodyne tomography characterization and nonlocality of a dual-mode optical qubit

A single photon, delocalized over two optical modes, is characterized by means of quantum homodyne tomography. The reconstructed four-dimensional density matrix extends over the entire Hilbert space and thus reveals, for the first time, complete information about the dual-rail optical quantum bit as a state of the electromagnetic field. The experimental data violate the Bell inequality albeit with a loophole similar to the detection loophole in photon counting experiments.     

Precision Zeeman-Stark spectroscopy of the metastable a1[3Sigma+] state of PbO

The metastable a(1)[(3)Sigma(+)] state of PbO has been suggested as a suitable system in which to search for the electric dipole moment of the electron. We report here the development of experimental techniques allowing high-sensitivity measurements of Zeeman and Stark effects in this system, similar to those required for such a search. We observe Zeeman quantum beats in fluorescence from a vapor cell of PbO, with shot-noise limited extraction of the quantum beat frequencies, high counting rates, and long coherence times. We argue that improvement in sensitivity to the electron electric dipole moment by at least 2 orders of magnitude appears possible using these techniques.     

How good must single photon sources and detectors be for efficient linear optical quantum computation?

We present a scheme for linear optical quantum computation that is highly robust to imperfect single photon sources and inefficient detectors. In particular we show that if the product of the detector efficiency with the source efficiency is greater than 2/3, then efficient linear optical quantum computation is possible. This high threshold is achieved within the cluster state paradigm for quantum computation.     

光纤耦合对量子密钥分配系统光子探测的影响

借助量子密码术和卫星可以实现全球性的保密通信网络。但使用现有的单光子探测模块搭建星地量子密钥分配(QKD)系统,接收端就面临着空间光-多模光纤耦合的技术挑战。空间光-多模光纤耦合条件对星地量子密钥分配系统的跟瞄精度提出了严格要求。理论分析和定量计算表明,跟瞄精度ε与光束发散角θdiv的比值ε/dθiv≤0.5时,星地量子密钥分配系统的光子探测概率较高,系统可以正常工作;ε/dθiv≤0.1时,系统处于量子密钥产生速率为几kb/s的更理想状况。采用短波长更有利于满足空间光-多模光纤耦合条件,同时有利于系统获得更高的密钥产生速率。     

光子两自由度超并行量子计算与超纠缠态操控

光子系统在量子信息处理和传输过程中有非常重要的应用. 譬如, 利用光子与原子(或人工原子)之间的相互作用, 可以完成信息的安全传输、存储和快速的并行计算处理等任务. 光子系统具有多个自由度, 如极化、空间模式、轨道角动量、时间-能量、频率等自由度. 光子系统的多个自由度可以同时应用于量子信息处理过程. 超并行量子计算利用光子系统多个自由度的光量子态同时进行量子并行计算, 使量子计算具有更强的并行性, 且需要的量子资源少, 更能抵抗光子数损耗等噪声的影响. 多个自由度同时存在纠缠的光子系统量子态称为超纠缠态, 它能够提高量子通信的容量与安全性, 辅助完成一些重要的量子通信任务. 在本综述中, 我们简要介绍了光子系统两自由度量子态在量子信息中的一些新应用, 包括超并行量子计算、超纠缠态分析、超纠缠浓缩和纯化三个部分.     

Derivation of generalized quantum jump operators and comparison of the microscopic single photon detector models

The recent experiment of Parigi et al. [Science 317, 1890 (2007)] shows, in agreement with theory, that subtraction of one photon can increase the expectation value of the number of photons in the thermal state. This observation agrees with the standard photon counting model in which the quantum jump superoperator (QJS) gives a count rate proportional to the number of photons. An alternate model for indirect photon counting has been introduced by Dodonov et al. [Phys. Rev. A 72, 023816 (2005)]. In their model the count rate is proportional to the probability that there are photons in the cavity, and the cavity field is bidirectionally coupled with a two state quantum system which is unidirectionally coupled to a counting device. We give a consistent first principle derivation of the QJSs for the indirect photon counting scheme and establish the complete relations between the physical measurement setup and the QJSs. It is shown that the time-dependent probability for photoelectron emission event must include normalization of the conditional probability. This normalization was neglected in the previous derivation of the QJSs. We include the normalization and obtain the correct photoelectron emission rates and the correct QJSs and show in which coupling parameter regimes these QJSs are applicable. Our analytical results are compared with the exact numerical solution of the Lindblad equation of the system. The derived QJSs enable analysis of experimental photon count rates in a case where a one-to-one correspondence does not exist between the decay of photons and the detection events.     

Robust entanglement through macroscopic quantum jumps

We propose an entanglement generation scheme that requires neither the coherent evolution of a quantum system nor the detection of single photons. Instead, the desired state is heralded by a macroscopic quantum jump. Macroscopic quantum jumps manifest themselves as a random telegraph signal with long intervals of intense fluorescence (light periods) interrupted by the complete absence of photons (dark periods). Here we show that a system of two atoms trapped inside an optical cavity can be designed such that a dark period prepares the atoms in a maximally entangled ground state. Achieving fidelities above 0.9 is possible even when the single-atom cooperativity parameter is as low as 10 and when using a photon detector with an efficiency as low as eta=0.2.     

Photon counts modulation in optical time domain reflectometry

The quantum fluctuation of photon counting limits the field application of optical time domain reflection. A method of photon counts modulation optics time domain reflection with single photon detection at 1.55 μm is presented. The influence of quantum fluctuation can be effectively controlled by demodulation technology since quantum fluctuation shows a uniform distribution in the frequency domain. Combined with the changing of the integration time of the lock-in amplifier, the signal to noise ratio is significantly enhanced. Accordingly the signal to noise improvement ratio reaches 31.7 dB compared with the direct photon counting measurement.     

利用多像素光子计数器的光学串话效应提高光子探测效率

通过分析多像素光子计数器(MPPC)的工作原理和其光学串话(OC)效应的特点,提出在使用MPPC输出雪崩信号的幅度或电荷量作为光子计数的参量时,利用MPPC的OC效应能提高MPPC的光子探测效率的观点,并从理论上分析了OC效应对光子探测效率的影响。理论分析结果显示,在这两种光子计数模式下,利用OC效应能明显提高MPPC的光子探测效率。利用本文模型计算得出当MPPC的雪崩单元数M为1 600个,忽略OC效应时的光子探测效率等于30%,光学串话概率等于50%,以及单脉冲入射光子数均值为10时,包含OC效应影响的等效光子探测效率可提高50%,达45%左右。该结果对MPPC在天体物理、粒子物理、荧光光谱探测等弱光探测场合的应用有一定指导意义。     

Two narrow bandwidth photons interfering in an electromagnetically induced transparency （EIT） system

In this paper, we have analysed in detail the quantum interference of the degenerate narrowband two-photon state by using a Mach-Zehnder interferometer, in which an electromagnetically induced transparency （EIT） medium is placed in one of two interfering beams. Our results clearly show that it is possible to coherently keep the quantum state at a single photon level in the EIT process, especially when the transparent window of the EIT medium is much larger than the bandwidth of the single photon. This shows that the EIT medium is possibly a kind of memory or repeater for the narrowband photons in the areas of quantum communication and quantum computer. This kind of experiment is feasible within the current technology.     

Single-photon detector for long-distance fiber-optic quantum key distribution

We have investigated the performance of an InGaAs/InP avalanche photodiode for photon counting at 1550 nm in the temperature range of thermoelectric cooling. A quantum efficiency of 13.7% was obtained at -55 degrees C , while the dark-count probability per gate (?1 ns) was kept as small as 2.4x10(-5) . The developed single-photon detector would yield a 104.4-km fiber-optic quantum key distribution in the ideal condition.     

基于相关光子多模式相关性的InSb模拟探测器定标方法

利用基于参量下转换产生的相关光子可以实现“无溯源”的绝对定标. 将该方法推广应用于模拟探测器定标的过程中, 获取两路模拟光电流信号的有效相关信息是主要难点. 在相关光子的多模式相关性理论模型的基础上, 提出了一种新的光电流处理方案. 通过将某一时刻采集到的光电流所包含的电荷量转换为等效光子计数, 采用双光路平衡探测和双通道数据波动校正的技术思路, 开展了红外模拟探测器量子效率定标验证实验研究. 利用532 nm单波长激光器为抽运源、PPLN晶体为非线性晶体, 在25 ℃工作温度下获取了631和3390 nm的相关光子对, 定标了InSb红外模拟探测器在3390 nm处的绝对功率响应度. 结果表明, 对InSb模拟探测器的合成不确定度为7.785%. 根据量子效率与绝对功率响应度之间的函数关系, 定标结果与国内计量单位的校准结果的相对偏差为3.6%. 利用多模式相关性在模拟信号下实现红外模拟探测器的绝对功率响应度定标在国际上暂无此方面的报道, 该方法验证了应用多模式相关性理论开展模拟探测器定标方法的可行性, 对于探索基于相关光子的定标技术和拓宽辐射定标应用领域具有重要意义.