基于自发参量下转换源二阶激发过程产生四光子超纠缠态

目前,多光子纠缠态的制备大多通过线性光学器件演化自发参量下转换一阶激发过程产生的纠缠光子对得到.本文考虑由自发参量下转换源二阶激发产生四个不可区分的纠缠光子制备四光子超纠缠态的情况.通过几组分束器、半波片和偏振分束器等线性光学器件设计量子线路演化四光子系统,结合四模符合探测,可得到同时具有偏振纠缠和空间纠缠的四光子超纠缠态.     

Polarization control of multi-photon absorption under intermediate femtosecond laser field

It has been shown that the femtosecond laser polarization modulation is a very simple and well-established method to control the multi-photon absorption process by the light–matter interaction. Previous studies mainly focused on the multiphoton absorption control in the weak field. In this paper, we further explore the polarization control behavior of multiphoton absorption process in the intermediate femtosecond laser field. In the weak femtosecond laser field, the secondorder perturbation theory can well describe the non-resonant two-photon absorption process. However, the higher order nonlinear effect(e.g., four-photon absorption) can occur in the intermediate femtosecond laser field, and thus it is necessary to establish new theoretical model to describe the multi-photon absorption process, which includes the two-photon and four-photon transitions. Here, we construct a fourth-order perturbation theory to study the polarization control behavior of this multi-photon absorption under the intermediate femtosecond laser field excitation, and our theoretical results show that the two-photon and four-photon excitation pathways can induce a coherent interference, while the coherent interference is constructive or destructive that depends on the femtosecond laser center frequency. Moreover, the two-photon and fourphoton transitions have the different polarization control efficiency, and the four-photon absorption can obtain the higher polarization control efficiency. Thus, the polarization control efficiency of the whole excitation process can be increased or decreased by properly designing the femtosecond laser field intensity and laser center frequency. These studies can provide a clear physical picture for understanding and controlling the multi-photon absorption process in the intermediate femtosecond laser field, and also can provide a theoretical guidance for the future experimental realization.     

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.     

Phase Sensitivity of a Multi-mode Nonlinear Interferometer with Single Photons

We propose a multi-mode nonlinear interferometer and study its phase sensitivity for two, three and four modes with single photons as inputs. We find that the phase sensitivity of the nonlinear interferometers can be improved by increasing the nonlinear index k. And when the total number of the input photons is fixed, increasing the number of modes is detrimental to the phase sensitivity.     

Entanglement Concentration for Arbitrary Four-Photon Cluster State Assisted with Single Photons

We present an entanglement concentration protocol (ECP) to concentrate arbitrary four-photon less-entangled cluster state into maximally entangled cluster state. Different from other ECPs for cluster state, we only exploit the single photon as auxiliary, which makes this protocol feasible and economic. In our ECP, the concentrated maximally entangled state can be retained for further application and the discarded state can be reused for a higher success probability. This ECP works with the help of cross-Kerr nonlinearity and conventional photon detectors. This ECP may be useful in future one-way quantum computation.     

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 squeezing of multi-photon field interacting with a single Cooper-pair box

We investigate the entropy squeezing and entanglement of the Cooper-pair box interacting with multi-photon cavity field. The field is prepared initially in the coherent state, while the Cooper-pair box is assumed to start from a mixed state. We find that the number of photons and the detuning parameters play an important role in the entropy squeezing and entanglement. We observe that the entropy squeezing can be used as a good indicator of the entanglement. This study opens promising perspectives for creating remote quantum information processing networks.     

多光子相长干涉解释光子的广义聚束效应

光的聚束效应的发现是量子光学的重要里程碑.对其进行合理解释的努力,促进了量子光学理论与技术的大发展,使量子光学迅速发展和完善起来.现在一般认为,光场二阶相干所体现的聚束效应的物理本质是双光子干涉.本文发现,在高阶相干中有类似的广义聚束效应,这种广义聚束效应也可以用光子相长干涉解释.而且在一些典型的光路中,Ⅳ个光子参与的干涉,可以使这种广义聚束效应的增强因子达到N!.     

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.     

Heralded path-entangled NOON states generation from a reconfigurable photonic chip

Maximal multi-photon entangled states, known as NOON states, play an essential role in quantum metrology. With the number of photons growing, NOON states are becoming increasingly powerful and advantageous for obtaining supersensitive and super-resolved measurements. In this paper, we propose a universal scheme for generating three- and four-photon path-entangled NOON states on a reconfigurable photonic chip via photons subtracted from pairs and detected by heralding counters. Our method is postselection free, enabling phase supersensitive measurements and sensing at the Heisenberg limit. Our NOON-state generator allows for integration of quantum light sources as well as practical and portable precision phase-related measurements.     

Preparing Multi-photon Entangled State with Beam Splitter

We propose a scheme for preparing multi-photon entangled state which is useful for sub-shot-noise sensitivity with beam splitter, where the entangled degree is measured by the reciprocal of the mean quantum Fisher information per particle (RMQFIP). The multi-photon entangled state can be prepared by adjusting the transmissivity of the beam splitter and the better multi-photon entangled state can be prepared by increasing the total number of photons.     

Four-photon interference with asymmetric beam splitters

Two experiments of four-photon interference are performed with two pairs of photons from parametric downconversion with the help of asymmetric beam splitters. The first experiment is a generalization of the Hong-Ou-Mandel interference effect to two pairs of photons while the second one utilizes this effect to demonstrate a four-photon de Broglie wavelength of lambda/4 by projection measurement.     

Demonstration of temporal distinguishability in a four-photon state and a six-photon state

An experiment is performed to demonstrate the temporal distinguishability of a four-photon state and a six-photon state, both from parametric down-conversion. The experiment is based on a multiphoton interference scheme in a recently discovered projection measurement of a maximally entangled N-photon state. By measuring the visibility of the interference dip, we can distinguish the various scenarios in the temporal distribution of the pairs and, thus, quantitatively determine the degree of temporal distinguishability of a multiphoton state.     

Generating a four-photon polarization-entangled cluster state with homodyne measurement via cross-Kerr nonlinearity

We propose a protocol to generate a four-photon polarization-entangled cluster state with cross-Kerr nonlinearity by using the interference of polarized photons. The protocol is based on optical elements, cross-Kerr nonlinearity, and homodyne measurement, therefore it is feasible with current experimental technology. The success probability of our protocol is optimal, this property makes our protocol more efficient than others in the applications of quantum communication.     

Entanglement Dynamics of Electrons and Photons

Entanglement is a fundamental feature of quantum theory as well as a key resource for quantum computing and quantum communication, but the entanglement mechanism has not been found at present. We think when the two subsystems exist interaction directly or indirectly, they can be in entanglement state. such as, in the Jaynes-Cummings model, the entanglement between the atom and the light field comes from their interaction. In this paper, we have studied the entanglement mechanism of electron-electron and photon-photon, which are from the spin-spin interaction. We found their total entanglement states are relevant both space state and spin state. When two electrons or two photons are far away, their entanglement states should be disappeared even if their spin state is entangled.     

Efficient generation of large number-path entanglement using only linear optics and feed-forward

We show how an idealized measurement procedure can condense photons from two modes into one and how, by feeding forward the results of the measurement, it is possible to generate efficiently superposition states commonly called N00N states. For the basic procedure sources of number states leak onto a beam splitter, and the output ports are monitored by photodetectors. We find that detecting a fixed fraction of the input at one output port suffices to direct the remainder to the same port, with high probability, however large the initial state. When instead photons are detected at both ports, macroscopic quantum superposition states are produced. We describe a linear-optical circuit for making the components of such a state orthogonal, and another to convert the output to a N00N state. Our approach scales exponentially better than existing proposals. Important applications include quantum imaging and metrology.     

Generation of Gaussian-Shape Single Photons for High Efficiency Quantum Storage

We report the generation of heralded single photons with Gaussian-shape temporal waveforms through the spatial light modulation technique in an atomic ensemble. Both the full width at half maximum and the peak position of the Gaussian waveform can be controlled while the single photon nature holds well. We also analyze the bandwidth of the generated single photons in frequency domain and show how the sidebands of the frequency spectrum are modified by the shape of the temporal waveform. The generated single photons are especially suited for the realization of high efficiency quantum storage based on electromagnetically induced transparency.     

An Efficient Scheme for Entanglement Concentration of Arbitrary Four-Photon States

The maximally entanglement will become less-entangled state because of the effects of decoherence. However, maximally entangled states are usually important for most of the quantum information processing protocols. So we propose a practical entanglement concentration protocol of an arbitrary four-photon less-entangled state assisted with single photons, which greatly reduces the difficulty of the experimental realization. In our scheme, we only employ three steps, a four-photon cluster state can be acquired with a certain probability by three affiliated photons. The protocol adopts the linear optical elements and weak cross-Kerr media, which make the scheme more simple and feasible in the experiment.     

Heralding two-photon and four-photon path entanglement on a chip

Generating quantum entanglement is not only an important scientific endeavor, but will be essential to realizing quantum-enhanced technologies, in particular, quantum-enhanced measurements with precision beyond classical limits. We investigate the heralded generation of multiphoton entanglement for quantum metrology using a reconfigurable integrated waveguide device in which projective measurement of auxiliary photons heralds the generation of path-entangled states. We use four and six-photon inputs, to analyze the heralding process of two- and four-photon NOON states-a superposition of N photons in two paths, capable of enabling phase supersensitive measurements at the Heisenberg limit. Realistic devices will include imperfections; as part of the heralded state preparation, we demonstrate phase superresolution within our chip with a state that is more robust to photon loss.     

Controlled-NOT gate for manipulating photonic polarization states in a two-mode optical cavity

We investigate the coherent interaction between two cavity modes with left or right circular polarizations and a traveling atom in the four-level Y configuration. With large single-photon detunings for both cavity modes and in the presence of a strong classical field, all one-photon and two-photon transitions may be well eliminated so that a close-loop four-photon transition starting from the only populated ground state becomes absolutely dominant. This physical scenario is then exploited to devise an efficient controlled-NOT gate for manipulating polarization states of two intracavity photons. In the Lamb-Dicke limit and in the strong coupling regime, the expected controlled-NOT operation is found to have a rather high gate fidelity and a moderate photon loss.