We propose a single-photon router using a single atom with an inversion center coupled to quantum multichannels made of coupled-resonator waveguides. We show that the spontaneous emission of the atom can direct single photons from one quantum channel into another. The on-demand classical field perfectly switches-off the single-photon routing due to the quantum interference in the atomic amplitudes of optical transitions. Total reflections in the incident channel are due to the photonic bound state in the continuum. Two virtual channels, named as scatter-free and controllable channels, are found, which are coherent superpositions of quantum channels. Any incident photon in the scatter-free channel is totally transmitted. The propagating states of the controllable channel are orthogonal to those of the scatter-free channel. Single photons in the controllable channel can be perfectly reflected or transmitted by the atom.

We propose two alternative entanglement concentration protocols (ECPs) using the Faraday rotation of photonic polarization. Through the single-photon input-output process in cavity QED, it is shown that the maximally entangled atomic (photonic) state can be extracted from two partially entangled states. The distinct feature of our protocols is that we can concentrate both atomic and photonic entangled states via photonic Faraday rotation, and thus they may be universal and useful for entanglement concentration in the experiment. Furthermore, as photonic Faraday rotation works in low-Q cavities and only involves virtual excitation of atoms, our ECPs are insensitive to both cavity decay and atomic spontaneous emission.PACS numbers: 03.67.-a, 03.67. Bg, 42.50.Dv Entanglement is the key resource in quantum information processing (QIP), such as quantum teleportation [1], quantum key distribution [2] and quantum dense coding [3]. In order to complete such QIP protocols perfectly, the maximally entangled states are usually required. However, the entanglement will inevitably degrade in the process of distribution and storage due to the interaction between system and its external environment. To overcome the dissipation and decoherence, Bennett et al. proposed the protocols of entanglement purification [4] and entanglement concentration [5]. By use of entanglement purification protocols (EPPs), one can distill a set of mixed entangled states into a subset of highly entangled states with local operation and classical communication [4]. However, EPPs can only improve the quality of the mixed state and can not get the maximally entangled state. On the other hand, entanglement concentration protocols (ECPs) [5] can be used to convert the partially entangled pairs to the maximally entangled ones. In the early days, many efforts have been devoted to photonic ECPs with linear [6,7] or nonlinear [8] optical elements. Recently, ECPs of solid state qubits (such as atomic [9][10][11] or electric qubits [12]) have also been investigated frequently.Cavity quantum electrodynamics (QED) system [13] is an excellent platform for understanding the fundamental principle of quantum mechanics and investigating QIP. In most of QIP protocols based on cavity QED, they usually require that atoms strongly interact with high-Q cavity field, which guarantees not only entanglement preparation but also further implementation of QIP tasks. However, as the high-Q cavity is well isolated from the environment, it seems unsuitable for efficiently accomplishing the input-output process of photons, which is the key step to implement long-distance QIP in a scalable fashion. Recently, An et al. [14] proposed a novel scheme to

We propose an approach to enhance the mechanical effects of single photons in a two-mode optomechanical system. This is achieved by introducing a resonance-frequency modulation to the cavity fields. When the modulation frequency and amplitude satisfy certain conditions, the mechanical displacement induced by single photons could be larger than the quantum zero-point fluctuation of the oscillating resonator. This method can be used to create distinct mechanical superposition states.

Intrinsic defects in optomechanical devices are generally viewed to be detrimental for achieving coherent amplification of phonons, and great care has thus been exercised in fabricating devices and materials with no (or a minimal number of) defects. Contrary to this view, here we show that, by surpassing an exceptional point (EP), both the mechanical gain and the phonon number can be enhanced despite increasing defect losses. This counterintuitive effect, well described by an effective non-Hermitian phonon-defect model, provides a mechanical analog of the loss-induced purely optical lasing. This opens the way to operating random-defect phonon devices at EPs.Comment: 9 pages, 6 figure

We propose and study an approach to realize quantum switch for single-photon transport in a coupled superconducting transmission-line-resonator ͑TLR͒ array with one controllable hopping interaction. We find that the single photon with arbitrary wave vector can transport in a controllable way in this system. We also study how to realize controllable hopping interaction between two TLRs via a Cooper-pair box ͑CPB͒. When the frequency of the CPB is largely detuned from those of the two TLRs, the variables of the CPB can be adiabatically eliminated and thus a controllable interaction between two TLRs can be obtained.Coupled cavity arrays ͑CCAs͒ ͓1͔ have recently attracted considerable attentions of both theorists and experimentalists. The CCAs have been proposed to implement quantum simulators for many-body physics, such as discovering new matter phases of photons ͓2-4͔ and providing a new platform to study spin systems ͓5,6͔. The CCAs are also suggested to manipulate photons for optical quantum information processing ͓7-9͔. Moreover, photon transport in the CCAs has been investigated ͓10-14͔. There are several possible ways to construct the CCAs, for example: ͑i͒ coupled defect cavities in photonic crystals ͓15͔; ͑ii͒ coupled toroidal microresonators ͓16͔; and ͑iii͒ coupled superconducting transmission-line resonators ͑TLRs͒ ͓11,12͔.In CCAs, there have been many proposals to realize quantum switch ͓17,18͔, which is used to control single-photon transport ͓11,19-21͔. For example, the reflection and transmission of photons in a coupled resonator waveguide can be controlled by a tunable two-level quantum system ͓11,18͔, acting as a controller.Here, we study another approach to control the singlephoton transport in a CCA, which consists of a chain of TLRs ͓22,23͔. In our proposal, the controllable transport is realized by a tunable coupling. As we know, how to control coupling between two solid devices is a major challenge in scalable quantum computing circuits ͓24-30͔. To obtain a tunable coupling, we propose that a Cooper-pair box ͑CPB͒ acts as a coupler. When the frequency of the coupler is largely detuned from those of the two resonators, the variables of the coupler can be adiabatically eliminated and thus a controllable interaction can be induced. Compared with previous approach ͓11͔, this approach has following advantage: dynamical variables of the coupler are adiabatically eliminated, therefore the coupler is a passive controlling element, which makes robust to prevent from the environment of the coupler.As shown in Fig. 1, one-dimensional CCA is a chain of N cavities, each is only coupled to its nearest-neighbor ones, Figs. 1͑a͒ and 1͑b͒ are the site lattice model and the schematic of coupled TLR array, respectively. The TLRs are assumed to have the same frequency. We also assume that the coupling strength between two nearest-neighbor TLRs is the same, except one between the lth and ͑l +1͒th TLRs. The Hamiltonian of the system is H = ͚ n a n † a n − t ͚ n ͑a n † a n+1 + a n+1 † a n ͒ − t͑a l † a l+1 + a l+1 † a...

We study analytically the dynamic behaviors of quantum correlation measured by quantum discord between two uncoupled qubits, which are immersed in a common Ohmic environment. We show that the quantum discord of the two noninteracting qubits can be greatly amplified or protected for certain initially prepared X-type states in the time evolution. Especially, it is found that there does exist the stable amplification of the quantum discord for the case of two identical qubits, and the quantum discord can be protected for the case of two different qubits with a large detuning. It is also indicated that in general there does exist a sudden change of the quantum discord in the time evolution at a critic time point t c , and the discord amplification and protection may occur only in the time interval 0 < t ≤ t c for certain X-type states. This sheds new light on the creation and protection of quantum correlation.

We present a new theoretical treatment of macroscopic quantum self-trapping (MQST) and quantum coherent atomic tunneling in a zero-temperature two-species Bose-Einstein condensate system in the presence of the nonlinear self-interaction of each species, the interspecies nonlinear interaction, and the Josephson-like tunneling interaction. It is shown that the nonlinear interactions can dramatically affect the MQST and the atomic tunneling, and lead to the collapses and revivals (CR) of population imbalance between the two condensates. The competing effects between the self-interaction of each species and the interspecies interaction can lead to the quenching of the MQST and the suppression of the CR and the Shapiro-like steps of the atomic tunneling current. It is revealed that the interatomic nonlinear interactions can induce the coherent atomic tunneling between two condensates even though there does not exist the interspecies Josephson-like tunneling coupling.

In this paper, we present a method to generate continuous-variable-type entangled states between photons and atoms in atomic Bose-Einstein condensate (BEC). The proposed method involves an atomic BEC with three internal states, a weak quantized probe laser and a strong classical coupling laser, which form a three-level Λ-shaped BEC system. We consider a situation where the BEC is in electromagnetically induced transparency (EIT) with the coupling laser being much stronger than the probe laser. In this case, the upper and intermediate levels are unpopulated, so that their adiabatic elimination enables an effective two-mode model involving only the atomic field at the lowest internal level and the quantized probe laser field. Atom-photon quantum entanglement is created through laser-atom and inter-atomic interactions, and two-photon detuning. We show how to generate atom-photon entangled coherent states and entangled states between photon (atom) coherent states and atom-(photon-) macroscopic quantum superposition (MQS) states, and between photon-MQS and atom-MQS states.