arXiv subjects

Guang-Can Guo

Publications and source records attributed to Guang-Can Guo.

At least 901 records · Page 50Linked to original sources

arXiv2008

Active Quantum Memory Using Oscillating Dark States

An active method for long time storage of quantum superposition state in atomic system using the Oscillating Dark States (ODS) is presented. Quantum state of a three-level $Λ$ configuration atomic system oscillates periodically between two ground levels, when two pairs of classical detuning laser fields driving the system into the ODS under evolving adiabatic conditions. When considering another uploading/unloading adiabatic conditions and applying the oscillation of the ODS to quantum state storage, surprisingly, we can obtain the greatly suppressed decoherence of the system and high fidelity of the retrieved state, even if decay factor of coherence term of the system density matrix $γ_{21}$$\cdot$ $t$$ \gg$1. The storage time is not limited by coherence decay time of the atomic system any longer, and can be thousands of times longer than that in those passive schemes without additional laser fields.

quant-ph↗

arXiv2008

Local Distinguishability of Orthogonal Quantum States and Generators of SU(N)

We investigate the possibility of distinguishing a set of mutually orthogonal multipartite quantum states by local operations and classical communication (LOCC). We connect this problem with generators of SU(N) and present a new condition that is necessary for a set of orthogonal states to be locally distinguishable. We show that even in multipartite cases there exists a systematic way to check whether the presented condition is satisfied for a given set of orthogonal states. Based on the proposed checking method, we find that LOCC cannot distinguish three mutually orthogonal states in which two of them are GHZ-like states.

quant-ph↗

arXiv2008

A scheme for tunable quantum phase gate and effective preparation of graph-state entanglement

A scheme is presented for realizing a quantum phase gate with three-level atoms, solid-state qubits--often called artificial atoms, or ions that share a quantum data bus such as a single mode field in cavity QED system or a collective vibrational state of trapped ions. In this scheme, the conditional phase shift is tunable and controllable via the total effective interaction time. Furthermore, we show that the method can be used for effective preparation of graph-state entanglement, which are important resources for quantum computation, quantum error correction, studies of multiparticle entanglement, fundamental tests of non-locality and decoherence.

quant-ph↗

arXiv2008

Frequency-up conversion and quantum swap gate in an optical cavity with atomic cloud

A scheme is presented for realizing frequency-up conversion and a two-qubit quantum swap gate for intracavity fields. In the scheme, a V-type atomic ensemble prepared in their ground states collectively mediates the interaction between the two cavity modes. Under certain conditions, the cavity-field degree of freedom is decoupled from the atomic degrees of freedom, and the effective coupling strength between the two cavity modes scales up with $\sqrt{N}$ ($N$ is the number of atoms). The numerical simulation shows that the quantum swap gate still has a high fidelity under the influence of the atomic spontaneous emission and the decay of the cavity modes.

quant-ph↗

arXiv2008

Two kinds of quantum adiabatic approximation

A simple proof of quantum adiabatic theorem is provided. Quantum adiabatic approximation is divided into two kinds. For Hamiltonian H(t/T), a relation between the size of the error caused by quantum adiabatic approximation and the parameter T is given.

quant-ph↗

arXiv2007

First-principles study of the lattice and electronic structures of TbMn$_2$O$_5$

The structural, electronic and lattice dielectric properties of multiferroic TbMn$_2$O$_5$ are investigated using density functional theory within the generalized gradient approximation (GGA). We use collinear spin approximations and ignore the spin-orbit coupling. The calculated structural parameters are in excellent agreement with the experiments. We confirm that the ground state structure of TbMn$_2$O$_5$ is of space group $Pb2_1m$, allowing polarizations along the b-axis. The spontaneous electric polarization is calculated to be 1187 $nC\cdot$cm$^{-2}$. The calculated zone-center optical phonons frequencies and the oscillator strengths of IR phonons agree very well with the experimental values. We then derive an effective Hamiltonian to explain the magnetically-induced ferroelectricity in this compound. Our results strongly suggest that the ferroelectricity in TbMn$_2$O$_5$ is driven by the magnetic ordering that breaks the the inversion symmetry, without invoking the spin-orbit coupling.

cond-mat.mtrl-sci↗

arXiv2007

Coupled quantum electrodynamics in photonic crystal nanocavities

We show that a scalable photonic crystal nanocavity array, in which single embedded quantum dots are coherently interacting, can perform as an universal single-operation quantum gate. In a passive system, the optical analogue of electromagnetically-induced-transparency is observed. The presence of a single two-level system in the array dramatically controls the spectral lineshapes. When each cavity couples with a two-level system, our scheme achieves two-qubit gate operations with high fidelity and low photon loss, even in the bad cavity limit and with non-ideal detuning and decoherence.

quant-ph↗

arXiv2007

Ferroelectricity driven by the non-centrosymmetric magnetic ordering in multiferroic TbMn_2O_5: a first-principles study

The ground state structural, electronic and magnetic properties of multiferroic TbMn$_2$O$_5$ are investigated via first-principles calculations. We show that the ferroelectricity in TbMn$_2$O$_5$ is driven by the non-centrosymmetric magnetic ordering, without invoking the spin-orbit coupling and non-collinear spins. The {\it intrinsic} electric polarization in this compound is calculated to be 1187 $nC\cdot$ cm$^{-2}$, an order of magnitude larger than previously thought.

cond-mat.mtrl-sci↗

arXiv2007

Nonclassicality of quantum excitation of classical coherent fields in thermal environments

The nonclassicality of photon-added coherent fields in the thermal channel is investigated by exploring the volume of the negative part of the Wigner function which reduces with the dissipative time. The Wigner functions become positive when the decay time exceeds a threshold value. For the case of the single photon-added coherent state, we derive the exact threshold values of decay time in the thermal channel. For arbitrary partial negative Wigner distribution function, a generic analytical relation between the mean photon number of heat bath and the threshold value of decay time is presented. Finally, the possible application of SPACSs in quantum computation has been briefly discussed. OCIS codes: 270.0270, 270.2500, 000.5490

quant-ph↗

arXiv2007

Quantum Decoherence Modulated by Special Relativity

By investigating the evolution of a moving spin-1/2 Dirac electron coupled with a background magnetic noise, we demonstrate that the effects of special relativity will significantly modify the decoherence properties of the spin state. The dephasing could be much suppressed, and for a sufficiently long time the decoherence even seems to halt. This interesting phenomenon stems from the dressed environment induced by special relativity.

quant-ph↗

arXiv2007

Quantum Computation and Bell-state Measurement with Double-Dot Molecules

We propose a quantum computation architecture of double-dot molecules, where the qubit is encoded in the molecule two-electron spin states. By arranging the two dots inside each molecule perpendicular to the qubit scaling line, the interactions between neighboring qubits are largely simplified and the scaling to multi-qubit system becomes straightforward. As an Ising-model effective interaction can be expediently switched on and off between any two neighboring molecules by adjusting the potential offset between the two dots, universal two-qubit gates can be implemented without requiring time-dependent control of the tunnel coupling between the dots. A Bell-state measurement scheme for qubit encoded in double-dot singlet and triplet states is also proposed for quantum molecules arranged in this way.

quant-ph↗

arXiv2007

Direct observation of excitonic polaron in InAs/GaAs quantum dots

Excitonic polaron is directly demonstrated for the first time in InAs/GaAs quantum dots with photoluminescence method. A new peak ($s'$) below the ground state of exciton ($s$) comes out as the temperature varies from 4.2 K to 285 K, and a huge anticrossing energy of 31 meV between $s'$ and $s$ is observed at 225 K, which can only be explained by the formation of excitonic polaron. The results also provide a strong evidence for the invalidity of Huang-Rhys formulism in dealing with carrier-longitudinal optical phonon interaction in quantum dot. Instead, we propose a simple two-band model, and it fits the experimental data quite well. The reason for the finding of the anticrossing is also discussed.

cond-mat.mtrl-sci↗

arXiv2007

Nanocrystals in silicon photonic crystal standing-wave cavities as spin-photon phase gates for quantum information processing

By virtue of a silicon high-Q photonic crystal nanocavity, we propose and examine theoretically interactions between a stationary electron spin qubit of a semiconductor nanocrystal and a flying photon qubit. Firstly, we introduce, derive and demonstrate for the first time the explicit conditions towards realization of a spin-photon two-qubit phase gate, and propose these interactions as a generalized quantum interface for quantum information processing. Secondly, we examine novel single-spin-induced reflections as direct evidence of intrinsic bare and dressed modes in our coupled nanocrystal-cavity system. The excellent physical integration of this silicon system provides tremendous potential for large-scale quantum information processing.

quant-ph↗

arXiv2007

Optimal entanglement witnesses based on local orthogonal observables

We show that the entanglement witnesses based on local orthogonal observables which are introduced in [S. Yu and N.-L. Liu, Phys. Rev. Lett. 95, 150504 (2005)] and [O. Gühne, M. Mechler, G. Tóth and P. Adam, Phys. Rev. A 74, 010301 (2006)] in linear and nonlinear forms can be optimized, respectively. As applications, we calculate the optimal nonlinear witnesses of pure bipartite states and show a lower bound on the I-concurrence of bipartite higher dimensional systems with our method.

quant-ph↗

arXiv2007

Analogue to multiple electromagnetically induced transparency in all-optical drop-filter systems

We theoretically study a parallel optical configuration which includes N periodically coupled whispering-gallery-mode resonators. The model shows an obvious effect which has a direct analogy with the phenomenon of multiple electromagnetically induced transparency in quantum systems. The numerical simulations illuminate that the frequency transparency windows are sharp and highly transparent. We also briefly discuss the experimental feasibility of the current scheme in two practical systems, microrings and microdisks.

quant-ph↗

arXiv2007

Experimental Decoy Quantum Key Distribution Up To 130KM Fiber

Decoy State Quantum Key Distribution (QKD), being capable of beating PNS attack and uncon- ditionally secure, have become an attractive one recently. But, in many QKD systems, disturbances of transmission channel make quantum bit error rate (QBER) increase which limits both security distance and key bit rate of real-life decoy state QKD systems. We demonstrate the two-intensity decoy QKD with one-way Faraday-Michelson phase modulation system, which is free of channel dis- turbance and keeps interference fringe visibility (99%) long period, near 130KM single mode optical fiber in telecom (1550 nm) wavelength. This is longest distance fiber decoy state QKD system based on two intensity protocol.

quant-ph↗

arXiv2007

Compatibility conditions from multipartite entanglement measures

We consider an arbitrary d_{1}\otimes d_{2}\otimes ... \otimes d_{N} composite quantum system and find necessary conditions for general m-party subsystem states to be the reduced states of a common N-party state. These conditions will lead to various monogamy inequalities for bipartite quantum entanglement and partial disorder in multipartite states. Our results are tightly connected with the measures of multipartite entanglement.

quant-ph↗

arXiv2007

Unambiguous discrimination of mixed states: A description based on system-ancilla coupling

We propose a general description on the unambiguous discrimination of mixed states according to the system-environment coupling, and present a procedure to reduce this to a standard semidefinite programming problem. In the two states case, we introduce the canonical vectors and partly simplify the problem to the case of discrimination between pairs of canonical vectors. By considering the positivity of the two by two matrices, we obtain a series of new upper bounds of the total success probability which depends on both the prior probabilities and specific state structures.

quant-ph↗