We study transfer of a quantum state through XX spin chains with static imperfections. We combine the two standard approaches for state transfer based on (i) modulated couplings between neighboring spins throughout the spin chain and (ii) weak coupling of the outermost spins to an unmodulated spin c...

[Phys. Rev. A 85, 022312] Published Thu Feb 09, 2012

We propose an optimal nonlocal entanglement concentration protocol (ECP) for multiphoton systems in a partially entangled pure state, resorting to the projection measurement on an additional photon. One party in quantum communication first performs a parity-check measurement on her photon in an N-ph...

[Phys. Rev. A 85, 022311] Published Wed Feb 08, 2012

We consider the coherent control of a quantum bit by the use of short pulses with finite duration τ_p. By shaping the pulse, we perturbatively decouple the dynamics of the bath from the dynamics of the quantum bit during the pulse. Such shaped pulses provide single quantum bit gates robust against de...

[Phys. Rev. A 85, 022310] Published Tue Feb 07, 2012

Deciding whether a mixed quantum state is separable or entangled is a difficult problem in general. Separable states are positive under partial transposition [they are positive-partial-transpose (PPT) states], but this simple test does not exclude all entangled states. In order to understand the ent...

[Phys. Rev. A 85, 022309] Published Mon Feb 06, 2012

We propose a fast and nondestructive spectroscopic method for single molecular ions that implements quantum logic schemes between an atomic ion and the molecular ion of interest. Our proposal relies on a hybrid coherent manipulation of the two-ion system, using optical or magnetic forces depending o...

[Phys. Rev. A 85, 022308] Published Mon Feb 06, 2012

We consider two independent quantum walks on separate lines augmented by partial or full swapping of coins after each step. For classical random walks, swapping or not swapping coins makes little difference to the random-walk characteristics, but we show that quantum walks with partial swapping of c...

[Phys. Rev. A 85, 022307] Published Mon Feb 06, 2012

We numerically demonstrate the control of motional degrees of freedom of an ensemble of neutral atoms in an optical lattice with a shallow trapping potential. Taking into account the range of quasimomenta across different Brillouin zones results in an ensemble whose members effectively have inhomoge...

[Phys. Rev. A 85, 022306] Published Mon Feb 06, 2012

We study the backaction of a driven nonlinear resonator on a multilevel superconducting qubit. Using unitary transformations on the multilevel Jaynes-Cummings Hamiltonian and quantum optics master equation, we derive an analytical model that goes beyond linear response theory. Within the limits of v...

[Phys. Rev. A 85, 022305] Published Fri Feb 03, 2012

An obstacle affecting any proposal for a topological quantum computer based on Ising anyons is that quasiparticle braiding can only implement a finite (nonuniversal) set of quantum operations. The computational power of this restricted set of operations (often called stabilizer operations) has been ...

[Phys. Rev. A 85, 022304] Published Fri Feb 03, 2012

Transferring a quantum state of a light field to a memory is of particular importance. However, this transfer is usually hampered because the memory system is subjected to some noise and this can limit the performance of the state transfer greatly. In this paper, we consider the transfer of a Gaussi...

[Phys. Rev. A 85, 022303] Published Fri Feb 03, 2012

We study the ability to control d-dimensional quantum systems (qudits) encoded in the hyperfine spin of alkali-metal atoms through the application of radio- and microwave-frequency magnetic fields in the presence of inhomogeneities in amplitude and detuning. Such a capability is essential to the des...

[Phys. Rev. A 85, 022302] Published Fri Feb 03, 2012

We show that a positive homogeneous function that is invariant under determinant-1 stochastic local operations and classical communication (SLOCC) transformations defines an N-qubit entanglement monotone if and only if the homogeneous degree is not larger than 4. We then describe a common basis and ...

[Phys. Rev. A 85, 022301] Published Fri Feb 03, 2012

We establish the optimal quantum teleportation protocol for the realistic scenario where both the input state and quantum channel are afflicted by noise. By taking these effects into account, higher fidelities are achieved. The optimality of the proposed protocol prevails even when restricted to a r...

[Phys. Rev. A 85, 020301] Published Wed Feb 01, 2012

We study a general quantum key distribution protocol in higher dimension. In this protocol, quantum states in arbitrary g+1 (1≤g≤d) out of all d+1 mutually unbiased bases in a d-dimensional system can be used for the key encoding. This provides a natural generalization of the quantum key distributio...

[Phys. Rev. A 85, 012334] Published Tue Jan 31, 2012

Entanglement of formation is an important measure of quantum entanglement. We present an experimental way to measure the entanglement of formation for arbitrary dimensional pure states. The measurement only evolves local quantum mechanical observables.

[Phys. Rev. A 85, 014304] Published Mon Jan 30, 2012

We present an experimental feasible scheme to generate two-mode entangled states of two spatially separated atomic spin ensembles, using superconducting quantum circuits consisting of two coplanar waveguide resonators and a charge qubit. We show that, with the charge qubit as a tunable coupler and t...

[Phys. Rev. A 85, 014303] Published Mon Jan 30, 2012

We report the storage and retrieval of a small microwave field from a superconducting resonator into collective excitations of a spin ensemble. The spins are nitrogen-vacancy centers in a diamond crystal. The storage time of the order of 30 ns is limited by inhomogeneous broadening of the spin ensem...

[Phys. Rev. A 85, 012333] Published Mon Jan 30, 2012

We exploit the notion of SWM- (simultaneous-weak-measurements)-induced quantum state reduction, enabling the use of measurement backaction to produce the desired quantum states. The probabilistic generation of maximally entangled Bell states and the |GHZ〉 (Greenberger-Horne-Zeilinger) state are then...

[Phys. Rev. A 85, 012332] Published Mon Jan 30, 2012

The problems of the design, control, and interaction of the single-electron double-dot charge qubits coherently coupled to the optical cavity mode are studied theoretically. A way to overcome the challenges concerned with the use of classical laser pulses for a qubit state engineering is described, ...

[Phys. Rev. A 85, 012331] Published Mon Jan 30, 2012

While it is usually known that the mean value of a single observable is enough to detect entanglement or its distillability, the counterpart of such an approach in the case of quantum privacy has been missing. Here we develop the concept of a privacy witness, i.e., a single observable that may detec...

[Phys. Rev. A 85, 012330] Published Fri Jan 27, 2012

We observe that changing a phase at a single point in a discrete quantum walk results in a rather surprising localization effect. For certain values of this phase change the possibility of localization strongly depends on the internal coin state of the walker.

[Phys. Rev. A 85, 012329] Published Fri Jan 27, 2012

We propose to apply the notion of the Schmidt number in order to show that a quantum memory or gate process is capable of maintaining a genuine multilevel quantum coherence. We present a simple criterion in terms of an average gate fidelity with respect to the input states of two mutually unbiased b...

[Phys. Rev. A 85, 010305] Published Thu Jan 26, 2012

In this Brief Report, we propose a theoretical scheme to transfer quantum states between superconducting charge qubits and semiconductor spin qubits in a circuit QED device. Under dispersive conditions, resonator-assisted state transfer between qubits can be performed controllably only by addressing...

[Phys. Rev. A 85, 014302] Published Wed Jan 25, 2012

We present a scalable method for generation of a cluster state for measurement-based quantum computing using van der Waals or dipole-dipole interactions between neutral atoms or polar molecules in an optical lattice. Nearest neighbor entanglement is accomplished by performing a phase gate using inte...

[Phys. Rev. A 85, 012328] Published Wed Jan 25, 2012

The effect of a number of mechanisms designed to suppress decoherence in open quantum systems are studied with respect to their effectiveness at slowing down the loss of entanglement. The effect of photonic band-gap materials and frequency modulation of the system-bath coupling are along expected li...

[Phys. Rev. A 85, 012327] Published Wed Jan 25, 2012

Two classes of quantum channels, which we call more capable and less noisy, are introduced. The more capable class consists of channels such that the quantum capacities of the complementary channels to the environments are zero. The less noisy class consists of channels such that the private capacit...

[Phys. Rev. A 85, 012326] Published Tue Jan 24, 2012

We study multiple-spin coherence transfers in linear Ising spin chains with nearest-neighbor couplings. These constitute a model for efficient information transfers in future quantum computing devices and for many multidimensional experiments for the assignment of complex spectra in nuclear magnetic...

[Phys. Rev. A 85, 012325] Published Tue Jan 24, 2012

In recent years it has been found that quantum systems can posses entanglement in equilibrium thermal states provided temperature is low enough. In the present work we explore a possibility of having entanglement in nonequilibrium stationary states. We show analytically that, in a simple one-dimensi...

[Phys. Rev. A 85, 012324] Published Mon Jan 23, 2012

A method to systematically construct the XX quantum spin chains with nearest-neighbor interactions that allow perfect state transfer (PST) is shown. Sets of orthogonal polynomials (OPs) are in correspondence with such systems. The key observation is that for any admissible one-excitation energy spec...

[Phys. Rev. A 85, 012323] Published Mon Jan 23, 2012

The communication capacity of Gaussian bosonic channels with memory has recently attracted much interest. Here, we investigate a method to prepare the multimode entangled input symbol states for encoding classical information into these channels. In particular, we study the usefulness of a Gaussian ...

[Phys. Rev. A 85, 012322] Published Mon Jan 23, 2012