Distributed quantum computing in decoherence-free subspace via adiabatic passage

A scheme is proposed to implement distributed quantum computation in decoherence-free subspaces (DFSs) via adiabatic passage. The logical single-qubit is encoded in two atoms trapped in a single-mode cavity and the cavities are connected by an optical fiber. Our scheme is immune from the decoherence due to dephasing in virtue of encoding scheme and the decoherence due to spontaneous emission from excited states as the system in our scheme evolves along a dark state. Furthermore, the decoherence due to photon decay is greatly suppressed since the fiber mode remains in a vacuum state and the populations of the cavities’ modes being excited can be negligible under certain condition. It is shown that the minimum fidelity of the resultant gate operation for an arbitrary input state could be over 0.97.     

Generation of NOON Sates for Two Atomic Samples Trapped in Two Distant Cavities

A scheme is proposed for generating NOON states for two atomic samples trapped in two distant cavities connected by a third cavity and optical fibers. In the scheme, all the atoms are always populated in the two degenerate ground states, so the atoms’ spontaneous emission can be omitted approximately. During the operation neither the cavity modes nor fiber modes are excited, which is important in view of decoherence. The scheme does not include projective measurement and the NOON state is generated deterministically.     

Entangler and analyzer for multiphoton Greenberger-Horne-Zeilinger states using weak nonlinearities

In the regime of weak nonlinearity we present two general, feasible schemes for manipulating photon states. One is an entangler for generating any one of the n-photon Greenberger-Horne-Zeilinger (GHZ) states. Interactions of the incoming photons with cross-Kerr media followed by a phase shift gate and a measurement on a probe beam plus appropriate local operations using classical feed-forward of the measurement results allow one to obtain the desired states in a nearly deterministic manner. The second scheme discussed is an analyzer for multiphoton maximally entangled states, which is derived from the above entangler. In this scheme, all of the 2 ⁿ n-photon GHZ states can, nearly deterministically, be discriminated.     

Scalable preparation of three-atom and four-atom W states via atom-cavity-laser interaction

We propose the optical generation of W states for three atomic and four atomic qubits, with each qubit trapped in a separate cavity and coupled to the cavity laser. A single-photon source and two classical fields are employed in the present scheme. By encoding the quantum information of each qubit on the degenerate ground states of the atom, we obtain the atomic entanglement that is relatively stable against spontaneous emission. It is demonstrated that the three- and four-atomic W states can be produced deterministically via a proper manipulation of the atom-cavity interaction sequence and time. Generalization of the present scheme to prepare multi-atomic W states is also discussed.     

Q-phonon scheme in the collective nuclear model

The Q-phonon scheme developed on the basis of the algebraic collective nuclear model is discussed. It is shown that, within this scheme, low-lying collective states of even-even nuclei can be described to a precision higher than 90% of the norm by using one or, at maximum, two components of the Q-phonon basis constructed by applying a fixed number of the quadrupole operators Q to the exact ground state of the system. Various applications of this approximate scheme are discussed. It is shown that, by using this scheme, the relations between several E2-transition probabilities or between the energies of the collective states can be derived. It is also shown that the Q-phonon scheme can be used to extract information about the equilibrium shapes of nuclei and their fluctuations from data on the E2-transition probabilities.     

Probabilistic Joint Remote Preparation of Four-Particle Cluster-Type States with Quaternate Partially Entangled Channels

We put forward a new and feasible scheme to realize joint remote preparation of four-particle cluster-type states based on two quaternate partially entangled states as quantum channels. During the preparation, each of the states’ senders is just required to perform a bipartite projective measurement in a 2×2-dimensional Hilbert space, and the receiver needs to implement some appropriate unitary operations including a local triplet collective transformation. It is proved that our scheme can be accomplished in a probabilistic manner, and the success probability of preparation (SPP) is dependent on the entangled states set up in prior. Moreover, it is explored that SPP can be greatly enhanced to be quadruple of that in general case, when the prepared states belong to some special ensembles. And the scheme feasibility is evaluated finally.     

Robust Scheme for Entangling Two Distant Atoms

A robust scheme is presented for realizing entangled states for two atoms trapped in separate cavities connected by an optical fiber. The first atom is initially in a superposition of the excited state and an auxiliary ground state not coupled to the first cavity, while the second one is initially in the ground state coupled to the second cavity. The scheme involves two atom-cavity-fiber interactions accompanied by the monitoring of the cavity decay and atomic spontaneous emission. The two atoms evolve to an entangled state through exchanging an excitation after the first interaction. The states with the excitation failing to be transferred are eliminated when a photon is detected during the second interaction. Therefore, the scheme is insensitive to the decoherence effect and detection inefficiency.     

Acousto-optic generation of orbital angular momentum states of light in a tapered optical fiber

We demonstrate an acousto-optic mode converter based on a tapered optical fiber to efficiently generate orbital angular momentum states of light. In our scheme an acoustic wave is deployed to the waist of tapered optical fiber where two degenerate HE₂₁ modes leading to +1 and −1 orbital angular momentum eigen-modes are resonantly excited. The excitation of TM₀₁ and TE₀₁ modes is suppressed by enlarging the intermodal index difference between near-degenerate spatial modes. Numerical calculation for optimization of the taper diameter is provided. The experimental characterization of generated states is performed by analyzing the output far-field pattern and the spatial interference fringes with a uniform reference beam.     

Implementing W States and an Ancilla-Free 1→M Phase-Covariant Cloning via Adiabatic Evolution

A scheme is proposed for generating maximally multi-atom W states and realizing an ancilla-free 1→M phase-covariant quantum cloning. Compared to the previous scheme, our scheme is based on performing an adiabatic evolution through three separated cavities which remain in their respective vacuum states during the whole operation, the process is much more simple, and operations are reduced relatively. During the process of cloning, our scheme is not necessary to perform a measurement on atoms. And our scheme is robust for atomic spontaneous emission, cavity decay and photon leakage out of the fiber. All these convenient places make our scheme more feasible.     

Generation of free-travelling four-mode cluster-type entangled coherent states

A new scheme is proposed to generate free-travelling four-mode cluster-type entangled coherent states. Compared with other previously proposed schemes ours is very simple in execution using only one π-cross-Kerr medium, two 50:50 beam-splitters and two π/2-phase-shifters. Despite the setup simplicity, our scheme, unlike the others which are all probabilistic, is efficient with 100% success probability since no measurements are involved at all.     

Calculation of gain at X-ray wavelengths resulting from optical pumping of helium-like ions

An equivalent two level scheme is described for calculating level populations of helium-like ions in dense optically thick plasmas. The scheme is used to show how population inversions due to optical pumping of 3Por 4P states are affected by line trapping. The importance of various line broadening mechanisms for determining gain is briefly discussed.     

Self-calibration in a fiber optic sensing system using the walk-off compensation

We firstly present a concept of a self-calibration of the classical and quantum parameters’ measurement using a fiber optic system. The measurement of the change in phase of the optimum entangled states’ visibility is performed in terms of a walk-off length, i.e. birefringence. The applied physical parameters on the sensing fiber can be simultaneously measured and the self-calibration respecting the birefringence performed. The scheme of the entangled photons generation in fiber optic is reviewed and the walk-off on the polarization entangled states presented. The potential of self-calibration and simultaneous measurement using an interferometric sensing technique and fiber grating sensor are proposed and discussed. The walk-off on the entangled states in the thermal-controlled environment is presented and discussed.     

Linear optical scheme for producing polarization-entangled NOON states

We propose a linear optical scheme that can conditionally generate high NOON states using polarization modes. This scheme provides advantages over the previous proposals on path-entangled NOON states in view of success probability or required resources of optical elements. We also investigate two experimental schemes feasible within existing technology that can produce the NOON-like or the NOON state for N = 4.     

Controllable preparation of entangled coherent states with superconducting system

Utilizing a current-biased Josephson junction (CBJJ) as a tunable coupler for superconducting transmission line resonators (TLRs), we propose a potentially practical scheme to create entangled coherent states of the two TLR modes. Then, the influence of TLRs decay on the prepared entangled states is analyzed. And an interesting phenomenon that even entangled coherent states are robustness against decay with small α is found. At last, the experimental feasibility and the challenge of our schemes have been discussed.     

Design and analysis of all-optical switches based on fiber parametric devices

We propose a novel 2 × 2 wavelength-convertible optical switch based on dual-pump fiber parametric devices: one is driven by linearly parallel pumps, and the other one by perpendicular pumps. Theoretical analysis is made on the polarization effects on the switching performance of the two devices. The result predicts that two incident signals which are positioned symmetrically with respect to one pump can be switched independently, with judicious combinations of the relative pump-signal polarization states. Simulations performed in OptiSystem show that the scheme can achieve crosstalk-free packet switching with acceptable extinction ratios for both signals. Besides, future applications of this novel scheme in high-speed switching nodes are discussed.     

Quantum key distribution via fiber optic, fidelity and error corrections

We propose an interesting scheme on photon states generation using a fiber optic Mach Zehnder interferometer incorporating a fiber optic ring resonator without any optical pumping parts including in the system, which is available for long-distance link. In principle, the state of a quantum bit, it is known, unknown, or entangled to other systems. The desired quantum states are generated and transmitted in the link via a fiber optic. The transmission quality in terms of quantum fidelity is analyzed, where a high fidelity to the noiseless quantum channel is achieved by adding an ancillary photon after the signal photon within the correlation time of the fiber noise and by performing the quantum parity checking method. The error correction is also analyzed. For simplicity, feature and robustness against path-length mismatches among the nodes make our scheme suitable for multi-user quantum communication networks.     

Polarization entanglement generation at 1.5 μm based on walk-off effect due to fiber birefringence

In this Letter, a linear scheme to generate polarization entanglement at 1.5 μm based on commercial polarization maintained dispersion shifted fiber (PM-DSF) is proposed. The birefringent walk-off effect of the pulsed pump light in the PM-DSF provides an effective way to suppress the vector scattering processes of spontaneous four-wave mixing. A 90 deg offset of fiber polarization axes is introduced at the midpoint of the fiber to realize the quantum superposition of the two correlated photon states generated by the two scalar processes on different fiber polarization axes, leading to polarization entanglement generation. Experiments of the indistinguishable property on single-side and two-photon interference in two nonorthogonal polarization bases are demonstrated. A two-photon interference fringe visibility of 89±3% is achieved without subtracting the background counts, demonstrating its great potential in developing highly a efficient and stable fiber based polarization-entangled quantum light source at the optical communication band.     

Generation of Quantum States for Atomic Ensemble via Cavity QED

A scheme is proposed for generating quantum states of atomic ensemble. In this scheme, a beam of three-level atoms in the Λ configuration is trapped in a cavity, then squeezed vacuum state and squeezed coherent state of the atomic ensemble are prepared by choosing different initial states of the system. The scheme is based on the off-resonant interaction between the atom and cavities, so the high-level of the atom is eliminated adiabatically.     

Engineering steady-state entanglement for two atoms held in separate cavities through laser cooling

We propose a scheme to prepare the steady-state entanglement for two atoms, which are held in separate cavities that are coupled through a short optical fiber or optical resonator. The entangled steady-state with a high fidelity can be achieved even with a low cooperativity parameter, by making use of the driving laser fields. Such a cooling mechanism is based on a resonant laser pump of the unwanted ground states to the excited states, which finally decay to the desired steady-state.     

Efficient scheme for the preparation of symmetric Dicke states via cross-Kerr nonlinearity

A scheme is proposed for generating maximally entangled Dicke states among four modes. The scheme only uses Kerr medium and homodyne measurements on coherent light fields, which can be efficiently made in quantum optical laboratories. The scheme can be generalized to produce maximally entangled 2k-qubit states.