Bose-Einstein Condensation of Photons and Photon Pairs

We investigate the Bose-Einstein condensation of photons and photon pairs in a two-dimension optical microcavity. We find that in the paraxial approximation, the mixed gas of photons and photon pairs is formally equivalent to a two dimension system of massive bosons with non-vanishing chemical potential, which implies the existence of two possible condensate phase. We also discuss the quantum phase transition of the system and obtain the critical point analytically. Moreover, we find that the quantum phase transition of the system can be interpreted as second harmonic generation.     

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.     

Nonlinear quantum optics in a waveguide: distinct single photons strongly interacting at the single atom level

We propose a waveguide-QED system where two single photons of distinct frequency or polarization interact strongly. The system consists of a single ladder-type three level atom coupled to a waveguide. When both optical transitions are coupled strongly to the waveguide's mode, we show that a control photon tuned to the upper transition induces a π phase shift and tunneling of a probe photon tuned to the otherwise reflective lower transition. Furthermore, the system exhibits single photon scattering by a classical control beam. Waveguide-QED schemes could be an alternative to high quality cavities or dense atomic ensembles in quantum information processing.     

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.     

Geometric Phase in a Two Energy Level <Emphasis Type="Italic">k</Emphasis>-Photon Jaynes-Cummings Model with Imaginary Photon Process

By using the Lewis-Riesenfeld invariant theory, we have studied the dynamical phase and the geometric phase in a two energy level k-photon Jaynes-Cummings model with imaginary photon process. We find that the geometric phase in a cycle case is independent of the frequency of the photon field, the coupling coefficient between photons and atoms, and the atom transition frequency. We predict the physical effect of the geometric phase in the imaginary photon process may be measured.     

Dicke quantum spin glass of atoms and photons

Recent studies of strongly interacting atoms and photons in optical cavities have rekindled interest in the Dicke model of atomic qubits coupled to discrete photon cavity modes. We study the multimode Dicke model with variable atom-photon couplings. We argue that a quantum spin-glass phase can appear, with a random linear combination of the cavity modes superradiant. We compute atomic and photon spectral response functions across this quantum phase transition, both of which should be accessible in experiments.     

Superfluid state of coherent light in nonlinear waveguides

We establish the photonic superfluid theory in waveguides made of self-defocussing polar crystals. In quantum theory it is shown that photons can sense an attractive effective interaction by exchange of virtual optical phonons. Such an interaction leads to the photonic superfluid state, in which a propagating photon pair consists of a combination of two photons with opposite transverse wave vector and spins. The most important property of the photonic superfluid state is that the system of photon pairs evolves without scattering attenuations. The traveling-wave superfluid state has the squeezing property and the soliton effect.     

Model of a proposed superconducting phase slip oscillator: a method for obtaining few-photon nonlinearities

We theoretically investigate a driven oscillator with the superconducting inductance subject to quantum phase slips (QPS). We find uncommon nonlinearities in the proposed device: they oscillate as a function of the number of photons N with a local period of the order of √N. We prove that such nonlinearities result in multiple metastable states encompassing few photons and study oscillatory dependence of various responses of the oscillator. Such nonlinearities enable new possibilities for quantum manipulation of photon states and very sensitive measurements to confirm the coherence of phase slips.     

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.     

Dynamical Properties of Two Coupled Dissipative QED Cavities Driven by Coherent Fields

When two identical QED cavities driven by the coherent fields are located in a uniform environment, in addition to dissipation, there appears an indirect coupling between the two cavities induced by the background fields. We investigate the effects of the coherent fields, the dissipation as well as the incoherent coupling on the following dynamical properties of the system： photon transfer, reversible decoherence, and quantum state transfer, etc. We find that the photons in the cavities do not leak completely into the environment due to the collective coupling between the cavities and the enviroment, and the photons are transferred irreversibly from the cavity with more photons to the cavity with less ones due to the incoherent coupling so that they are equally distributed among the two cavities. The coherent field pumping on the two cavities increases the mean photons, complements the revived magnitude of the reversible decoherence, but hinders the quantum state transfer between the two cavities. The above phenomena may find applications in quantum communication and other basic fields.     

Dynamical Properties of Two Coupled Dissipative QED Cavities Driven by Coherent Fields

When two identical QED cavities driven by the coherent fields are located in a uniform environment, in addition to dissipation, there appears an indirect coupling between the two cavities induced by the background fields. We investigate the effects of the coherent fields, the dissipation as well as the incoherent coupling on the following dynamical properties of the system: photon transfer, reversible decoherence, and quantum state transfer, etc. We find that the photons in the cavities do not leak completely into the environment due to the collective coupling between the cavities and the enviroment, and the photons are transferred irreversibly from the cavity with more photons to the cavity with less ones due to the incoherent coupling so that they are equally distributed among the two cavities. The coherent field pumping on the two cavities increases the mean photons, complements the revived magnitude of the reversible decoherence, but hinders the quantum state transfer between the two cavities. The above phenomena may find applications in quantum communication and other basic fields.     

Quasiprobability distribution in a Kerr-nonlinear blackbody

In a Kerr-nonlinear blackbody, bare photons with opposite wave vectors and helicities are bound into pairs and unpaired photons are transformed into a new kind of quasiparticle, the nonpolariton. The nonpolariton system constitutes free thermal radiation in the blackbody. In this paper, the quasiprobability (Q function) distribution of thermal radiation is investigated. Non-classical effects of quadrature squeezing have been observed. The structure of nonpolaritons is unsteady and governed by the temperature. The phase space of the photon system is considered and found that in the transition from the normal to the squeezed thermal radiation state, the phase symmetry of the photon system is spontaneously broken.     

Entanglement generation by Fock-state filtration

We demonstrate a Fock-state filter which is capable of preferentially blocking single photons over photon pairs. The large conditional nonlinearities are based on higher-order quantum interference, using linear optics, an ancilla photon, and measurement. We demonstrate that the filter acts coherently by using it to convert unentangled photon pairs to a path-entangled state. We quantify the degree of entanglement by transforming the path information to polarization information; applying quantum state tomography we measure a tangle of T=(20+/-9)%.     

High-Capacity Three-Party Quantum Secret Sharing With Hyperentanglement

We presents a novel scheme for high-capacity three-party quantum secret sharing (QSS) with the hyperentanglement in both the polarization and the spatial-mode degrees of freedom of photon pairs. The boss Alice need only prepare a sequence of photon pairs and some decoy photons. Her two agents measure their photons received from the boss Alice with two bases by choosing two unsymmetrical probabilities. The present QSS scheme has a high capacity as each pair can carry 2 bits of information, several times as other QSS schemes. Moreover, our setups with linear optical elements show that our QSS scheme does not increase the difficulty of its implementation in experiment and it is feasible with current techniques.     

Heralded generation of ultrafast single photons in pure quantum States

We present an experimental demonstration of heralded single photons prepared in pure quantum states from a parametric down-conversion source. It is shown that, through controlling the modal structure of the photon pair emission, one can generate pairs in factorable states and thence eliminate the need for spectral filters in multiple-source interference schemes. Indistinguishable heralded photons were generated in two independent spectrally engineered sources and Hong-Ou-Mandel interference observed between them without spectral filters. The measured visibility of 94.4% sets a minimum bound on the mean photon purity.     

High-Capacity Quantum Secret Sharing with Hyperdense Coding Assisted by Hyperentangled Photon Pairs

We presents a high-capacity three-party quantum secret sharing (QSS) protocol with a sequence of photon pairs in hyperentangled Bell states in both the polarization and the spatial-mode degrees of freedom. In our scheme, the boss Alice prepares a sequence of photon pairs in hyperentangled Bell states and divides them into two photon sequences which are sent the two agents, respectively. Alice exploits four subsets of decoy photons to assure the security of the photon transmission between her and her agents. The present QSS scheme has the advantage of having a high channel capacity as each photon pair can carry 4 bits of secret message in principle, two times of that by Deng et al. (Phys. Lett. A 372: 1957, 2008). We give out the setups for the preparation of the photon pairs in hyperentangled Bell states with a beta barium borate crystal and the manipulation of the photons with linear optical elements. It will be shown that our QSS protocol is feasible with current experimental technology.     

Off-equilibrium photon production during the chiral phase transition

In the early stage of ultrarelativistic heavy-ion collisions chiral symmetry is restored temporarily. During this so-called chiral phase transition, the quark masses change from their constituent to their bare values. This mass shift leads to the spontaneous non-perturbative creation of quark–antiquark pairs, which effectively contributes to the formation of the quark–gluon plasma. We investigate the photon production induced by this creation process. We provide an approach that eliminates possible unphysical contributions from the vacuum polarization and renders the resulting photon spectra integrable in the ultraviolet domain. The off-equilibrium photon numbers are of quadratic order in the perturbative coupling constants while a thermal production is only of quartic order. Quantitatively, we find, however, that for the most physical mass-shift scenarios and for photon momenta larger than 1 GeV the off-equilibrium processes contribute less photons than the thermal processes.     

费米超流气体的非线性Landau-Zener 隧穿

在平均场近似下,通过对相平面和不动点的分析, 研究了非线性两能级系统中费米超流气体的Landau-Zener 隧穿现象. 研究发现,费米子间的相互作用能够显著地影响量子隧穿. 当相互作用参数c小于临界值c^*时,在绝热极限下隧穿仍然满足量子绝热定理, 而大于这一临界值时,量子绝热定理不再满足. 最后通过和线性情况比较,得到了c*时隧穿率与扫描速率间满足的指数关系.     

一种基于纠缠态的量子中继通信系统

提出了一种基于纠缠态的量子中继通信系统,该系统应用纠缠为基本资源.纠缠为量子隐形传态和绝对安全的量子通信提供了保证.量子中继器用来延长高纠缠度的纠缠光子对的纠缠距离,利用纠缠交换和纠缠纯化在系统的发信者与受信者之间建立光子对的纠缠.应用量子隐形传态的原理传输量子信息.系统分析表明,量子通信系统的吞吐率随着通信双方成功建立纠缠的概率增大而显著增加,量子信号的传输距离取决于量子中继节点的级数.