Photon echo without a free induction decay in a double-Λ system

We have characterized a novel photon-echo pulse sequence for a double-Λ-type energy level system where the input and rephasing transitions are different from the applied π pulses. We show that, despite having imperfect π-pulses associated with large coherent emission due to free induction decay (FID), the noise added in the echo mode is only 0.2 ± 0.1 photons per shot, compared to 4 × 10? photons in the FID modes. Using this echo pulse sequence in the "rephased amplified spontaneous emission" (RASE) scheme [Phys. Rev. A 81, 012301 (2010)] will allow for generation of entangled photon pairs that are in different frequency, temporal, and potentially spatial modes to any bright driving fields. The coherence and efficiency properties of this sequence were characterized in a Pr(3+):Y?SiO? crystal.     

A Four-Dimensional Measurement Scheme for Single Photon Time-Polarization

We propose an experimental scheme to realize the four-dimensional projective measurements tor a single photon. The photon polarization and time-energy provide the four-dimensional Hilbert space. Based on this scheme, we suggest an experiment to test the violation of Bell inequalities of four-dimensional systems. In addition, by virtue of a maximally entangled biphoton state, we also show that it is possible to construct a quantum key distribution channel that can provide two-bit key with one pair of entangled photons.     

Triggered single photons and entangled photons from a quantum dot microcavity

Current quantum cryptography systems are limited by the attenuated coherent pulses they use as light sources: a security loophole is opened up by the possibility of multiple-photon pulses. By replacing the source with a single-photon emitter, transmission rates of secure information can be improved. We have investigated the use of single self-assembled InAs/GaAs quantum dots as such single-photon sources, and have seen a tenfold reduction in the multi-photon probability as compared to Poissonian pulses. An extension of our experiment should also allow for the generation of triggered, polarization-entangled photon pairs. The utility of these light sources is currently limited by the low efficiency with which photons are collected. However, by fabricating an optical microcavity containing a single quantum dot, the spontaneous emission rate into a single mode can be enhanced. Using this method, we have seen 78% coupling of single-dot radiation into a single cavity resonance. The enhanced spontaneous decay should also allow for higher photon pulse rates, up to about 3 GHz. Received 8 July 2001 and Received in final form 25 August 2001     

Triggered single photons from a quantum dot

We demonstrate a new method for generating triggered single photons. After a laser pulse generates excitons inside a single quantum dot, electrostatic interactions between them and the resulting spectral shifts allow a single emitted photon to be isolated. Correlation measurements show a reduction of the two-photon probability to 0.12 times the value for Poisson light. Strong antibunching persists when the emission is saturated. The emitted photons are also polarized.     

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     

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.     

An efficient and secure multiparty quantum secret sharing scheme based on single photons

A scheme of multiparty quantum secret sharing of classical messages (QSSCM) is proposed based on single photons and local unitary operations. In this scheme, eavesdropping checks are performed only twice, and one photon can generate one bit of classical secret message except those chosen for eavesdropping check; in addition, only the sender and one of the agents are required to store photons. Thus, this scheme is more practical and efficient.     

A second look at single-photon production in S + Au collisions at 200 A GeV and implications for quark–hadron phase transition

We reanalyze the production of single photons in S + Au collisions at CERN SPS to investigate: (i) the consequences of using a much richer equation of state for hadrons than the one used in an earlier study by us, and (ii) to see if the recent estimates of photon production in quark matter (at two-loop level) by Aurenche, et al. are consistent with the upper limit of the photon production measured by the WA80 group. We find that the data are consistent with a quark–hadron phase transition. The data are also consistent with a scenario where no phase transition takes place, but where the hadronic matter reaches a density of several hadrons per unit volume, which is rather unphysical. Received: 21 June 1999 / Revised version: 25 August 1999 / Published online: 16 November 1999     

Correlation evolution and monogamy of two geometric quantum discords in multipartite systems

We study the controlled photon emission of generic V-type three-level molecules (e.g. BaF) via a radio frequency field. The density matrix approach was employed to calculate the probabilities of emission of one or two X-polarised photons. The influence of the relative delay phase between the sequential pulse on the emission photon probabilities can be enhanced or eliminated by the polarisation of the radio frequency field. For θ ∈ [ 0.35π,0.75π ] + nπ, the influence of the relative delay phase is enhanced, whereas for θ ? [ 0.35π,0.75π ] + nπ the influence is suppressed. The probabilities of emitting one or two X-polarised photons oscillate with the modulation index ζ, and several zero points exist, which could be used as a trigger for photon emissions. The results also demonstrated the phenomenon of radio frequency field-assisted photon absorption.     

Efficient Three-Party Quantum Secret Sharing with Single Photons

A scheme for three-party quantum secret sharing of a private key is presented with single photons. The agent Bob first prepares a sequence of single photons with two biased bases and then sends them to the boss Alice who checks the security of the transmission with measurements and produces some decoy photons by rearranging the orders of some sample photons. Alice encodes her bits with two unitary operations on the photons and then sends them to the other agent. The security of this scheme is equivalent to that in the modified Bennett Brassard 1984 quantum key distribution protocol. Moreover, each photon can carry one bit of the private key and the intrinsic efficiency for qubits and the total efficiency both approach the maximal value 100% when the number of the bits in the key is very large.     

Efficient source of single photons: a single quantum dot in a micropost microcavity

We have demonstrated efficient production of triggered single photons by coupling a single semiconductor quantum dot to a three-dimensionally confined optical mode in a micropost microcavity. The efficiency of emitting single photons into a single-mode traveling wave is approximately 38%, which is nearly 2 orders of magnitude higher than for a quantum dot in bulk semiconductor material. At the same time, the probability of having more than one photon in a given pulse is reduced by a factor of 7 as compared to light with Poissonian photon statistics.     

Photon Correlations from the Mollow Triplet

Photon correlations between the photoluminescence peaks of the Mollow triplet have been known for a long time, and recently hailed as a resource for heralded single‐photon sources. Here, we provide the full picture of photon‐correlations at all orders (we deal explicitly with up to four photons) and with no restriction to the peculiar frequency windows that enclose the peaks. We show that a rich multi‐photon physics lies between the peaks, due to transitions involving virtual photons, and thereby much more strongly correlated than those transiting through the real states. Specifically, we show that such emissions occur in bundles of photons rather than as successive, albeit correlated, photons. We provide the recipe to frequency‐filter the emission of the Mollow triplet to turn it into a versatile and tunable photon source, allowing in principle all scenarios of photon emission, with advantages already at the one‐photon level, i.e., providing more strongly correlated heralded single‐photon sources than those already known.     

Efficient Generation of Frequency-Multiplexed Entangled Single Photons

We present two schemes to generate frequency-multiplexed entangled (FME) single photons by coherently mapping photonic entanglement into and out of a quantum memory based on Raman interactions. By splitting a single photon and performing subsequent state transfer, we separate the generation of entanglement and its frequency conversion, and find that the both progresses have the characteristic of inherent determinacy. Our theory can reproduce the prominent features of observed results including pulse shapes and the condition for deterministically generating the FME single photons. The schemes are suitable for the entangled photon pairs with a wider frequency range, and could be immune to the photon loss originating from cavity-mode damping, spontaneous emission, and the dephasing due to atomic thermal motion. The sources might have significant applications in wavelength-division-multiplexing quantum key distribution.     

Quantum storage of single photons with unknown arrival time and pulse shapes

We present a scheme for the quantum storage of single photons using electromagnetically induced transparency (EIT) in a low-finesse optical cavity, assisted by state-selected spontaneous atomic emission. Mediated by the dark mode of cavity EIT, the destructive quantum interference between the cavity input-output channel and state-selected atomic spontaneous emission leads to strong absorption of single photons with unknown arrival time and pulse shapes. We discuss the application of this phenomenon to photon counting using stored light.     

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.     

On the measurement of photon flux in parametric down-conversion

We report the measurement of the photons flux produced in parametric down-conversion, performed in photon counting regime with actively quenched silicon avalanche photodiodes as single photon detectors. Measurements are done with the detector in a well defined geometrical and spectral situation. By comparison of the experimental data with the theory, a value for the second order susceptibilities of the non linear crystal can be inferred. Received 2 July 1999 and Received in final form 2 August 1999     

Polarization Properties of Quantum-Dot-Based Single Photon Sources

Polarization properties of single photons emitted by optical pumping from a single quantum dot （ QD） are studied by using a four-level system model. The model is capable of explaining the polarization uncertainty observed in single photon emission experiments. It is found that the dependence of photon emission efficiency and polarization visibility on pump power are opposite in general cases. By employing QDs with small size and strong carrier confinement, the photon polarization visibility under high pump power can be improved. In addition, embedding a QD into a well designed microcavity is also found to be favourable, whereas the trade-off between high polarization visibility and multi-photon emission is noted.     

脉冲激发三能级体系半导体量子点的单光子发射效率

研究了脉冲激发下单个半导体量子点中单光子发射的统计特性.在旋转波近似条件下，由系统粒子数演化主方程并结合量子回归理论推导了二阶相关函数的运动方程，利用此方程讨论了二阶相关函数随输入脉冲面积的关系.在窄脉冲宽度的脉冲激发下，单光子的发射概率p和效率η都随着强度的增强而产生振荡.研究表明，采用窄脉冲宽度，当输入脉冲面积在π附近时可以得到较高的单光子发射效率. 关键词： 半导体量子点 单光子发射 三能级系统     

Simple cases of spontaneous emission from an atom-photon cluster localized in a microcavity

The spontaneous emission from an atomic ensemble localized in a microcavity with the participation of microcavity photons and an external broadband quantized electromagnetic field at the Raman resonance of photons with an optically forbidden (two-photon) atomic transition has been studied. The average spontaneous decay intensity has been calculated for simple cases. It is shown that the dynamics of spontaneous emission from this atomic ensemble differs generally from the conventional superradiance (spontaneous emission of an atomic ensemble at a one-photon optically allowed transition from excited to the ground state. When the atomic ensemble is strongly excited, the delay times and the emission pulse shape differ significantly. The parameter ranges where the spontaneous emission from the atomic ensemble under consideration at a two-photon Raman transition can be described as conventional superradiance with renormalized parameters are found. In the case of single excitation the photon emission probability depends on the number of photons and atoms in the microcavity.     

Extreme Ultraviolet Pulse Shaping with Aligned Molecules

We theoretically propose a simple scheme to shape the extreme ultraviolet pulse on the attosecond timescale with the aligned molecule. This may offer a new way to manipulate and control photon emission on the attosecond timescale.