arXiv subjects

C. P. Sun

Publications and source records attributed to C. P. Sun.

At least 271 records · Page 15Linked to original sources

arXiv2004

Fast entanglement of two charge-phase qubits through nonadiabatic coupling to a large junction

We propose a theoretical protocol for quantum logic gates between two Josephson junction charge-phase qubits through the control of their coupling to a large junction. In the low excitation limit of the large junction when $E_{J}\gg E_{c}$, it behaves effectively as a quantum data-bus mode of a harmonic oscillator. Our protocol is efficient and fast. In addition, it does not require the data-bus to stay adiabatically in its ground state, as such it can be implemented over a wide parameter regime independent of the data-bus quantum state.

cond-mat.supr-con↗

arXiv2004

Nonadiabatic effects of atomic motion inside a high Q optical cavity

We revisit the topic of atomic center of mass motion of a three level atom Raman coupled strongly to an external laser field and the quantum field of a high Q optical cavity. We focus on the motion related nonadiabatic effects of the atomic internal dynamics and provide a quantitative answer to the validity regime for the application of the motional insensitive dark state as recently suggested in Ref. [Phys. Rev. A {\bf 67}, 032305 (2003)].

quant-ph↗

arXiv2003

Geometric Quantum Information Storage Based on Atomic Ensemble

Quantum information storage (QIS) is a physical process to write quantum states into a quantum memory (QM). We observe that in some general cases the quantum state can be retrieved up to a unitary transformation depicted by the non-Abelian Berry's geometric phase factor (holonomy). The QIS of photon with this geometric character can be implemented with the symmetric collective excitations of a $Λ$-atom ensemble, which is adiabatically controlled by a classical light with a small detuning $δ$. The cyclic change of the controlled Rabi frequency $Ω(t)$ with period $T=\frac{2nπ}δ$ accumulates additional geometric phases in different components of the final photon state. Then, in a purely geometric way the stored state can be decoded from the final state.

quant-ph↗

arXiv2003

Two photon interference and optical free induction decay

The two photon interference phenomenon is theoretically investigated for the general situations with an arbitrary input two photon state with and without photon polarization. For the case without polarization, the necessary-sufficient condition for the destructive interference of coincidence counting is given as the symmetric pairing of photons in the light pulses. For both case it is shown that the "dip" in coincidence curve can be understood in terms of the free induction decay mechanism. This observation predicts the destructive interference phenomenon to occur even for certain cases with separable input two photon state, but it can only be explained in terms of "the two photon (not two photons)interference ".

quant-ph↗

arXiv2003

Quasi-spin wave quantum memories with dynamic symmetry

For the two-mode exciton system formed by the quasi-spin wave collective excitation of many $Λ$ atoms fixed at the lattice sites of a crystal, we discover a dynamic symmetry depicted by the semi-direct product algebra $SU(2)\bar{\otimes} h_2 $ in the large $N$ limit with low excitations. With the help of the spectral generating algebra method, we obtain a larger class of exact zero-eigenvalue states adiabatically interpolating between the initial state of photon-type and the final state of quasi-spin wave exciton-type. The conditions for the adiabatic passage of dark states are shown to be valid, even with the presence of the level degeneracy. These theoretical results can lead to propose new protocol of implementing quantum memory robust against quantum decoherence.

quant-ph↗

arXiv2003

Quantum computation based on d-level cluster states

The concept of qudit (a d-level system) cluster state is proposed by generalizing the qubit cluster state (Phys. Rev. Lett. \textbf{86}, 910 (2001)) according to the finite dimensional representations of quantum plane algebra. We demonstrate their quantum correlations and prove a theorem which guarantees the availability of the qudit cluster states in quantum computation. We explicitly construct the network to show the universality of the one-way computer based on the defined qudit cluster states and single-qudit measurement. And the corresponding protocol of implementing one-way quantum computer can be suggested with the high dimensional "Ising" model which can be found in many magnetic systems.

quant-ph↗

arXiv2003

Adiabatic passage of collective excitations in atomic ensembles

We describe a theoretical scheme that allows for transfer of quantum states of atomic collective excitation between two macroscopic atomic ensembles localized in two spatially-separated domains. The conception is based on the occurrence of double-exciton dark states due to the collective destructive quantum interference of the emissions from the two atomic ensembles. With an adiabatically coherence manipulation for the atom-field couplings by stimulated Ramann scattering, the dark states will extrapolate from an exciton state of an ensemble to that of another. This realizes the transport of quantum information among atomic ensembles.

quant-ph↗

arXiv2003

Transient dynamics of light propagation in EIT medium and hidden symmetry of multi-bit quantum memory

We investigate the transient phenomenon or property of the propagation of an optical probe field in a medium consisting of many $Λ$-type three-level atoms coupled to this probe field and an classical driven field. We observe a hidden symmetry and obtain an exact solution for this light propagation problem by means of the spectral generating method. This solution enlightens us to propose a practical protocol implementing the quantum memory robust for quantum decoherence in a crystal. As an transient dynamic process this solution also manifests an exotic result that a wave-packet of light will split into three packets propagating at different group velocities. It is argued that "super-luminal group velocity" and "sub-luminal group velocity" can be observed simultaneously in the same system. This interesting phenomenon is expected to be demonstrated experimentally.

quant-ph↗

arXiv2003

Superradiance of low density Frenkel excitons in a crystal slab of three-level atoms: Quantum interference effect

We systematically study the fluorescence of low density Frenkel excitons in a crystal slab containing $N_T$ V-type three-level atoms. Based on symmetric quasi-spin realization of SU(3) in large $N$ limit, the two-mode exciton operators are invoked to depict various collective excitations of the collection of these V-type atoms starting from their ground state. By making use of the rotating wave approximation, the light intensity of radiation for the single lattice layer is investigated in detail. As a quantum coherence effect, the quantum beat phenomenon is discussed in detail for different initial excitonic states. We also test the above results analytically without the consideration of the rotating wave approximation and the self-interaction of radiance field is also included.

cond-mat↗

arXiv2003

Quantum teleportation using cluster states

A protocol of quantum communication is proposed in terms of the multi-qubit quantum teleportation through cluster states (Phys. Rev. Lett. \textbf{86}, 910 (2001)). Extending the cluster state based quantum teleportation on the basic unit of three qubits (or qudits), the corresponding multi-qubit network is constructed for both the qubits and qudits (multi-level) cases. The classical information costs to complete this communication task is also analyzed. It is also shown that this quantum communication protocol can be implemented in the spin-spin system on lattices.

quant-ph↗

arXiv2002

Engineering Progressive Decoherence with Quantum Jumps in Charge Qubit

For the Josephson junction charge qubits with macroscopically quantum natures, we propose a theoretical scheme to observe the loss of quantum coherence through coupling such qubit system to an engineered reservoir, the harmonic oscillator mode in the LC circuit formed by the inductor and the separated capacitors. Similar to the usual cavity QED system in form, this charge qubit system with engineered couplings shows the quantum jumps (C.P.Sun et al Fortschr. Phys. \textbf{43}, 585 (1995))in a progressive decoherence process. Corresponding to two components of superposition of two charge states, the inductor evolves simultaneously towards two distinct quasi-classical states entangling with two states of the charge qubit. Then it induces the quantum decoherence for the induced squeezing macroscopically in the LC mode.

quant-ph↗

arXiv2002

Generalized Quantum Games with Nash Equilibrium

We define generalized quantum games by introducing the coherent payoff operators and propose a simple scheme to illustrate it. The scheme is implemented with a single spin qubit system and two entangled qubit system. The Nash Equilibrium Theorem is proved for the models.

quant-ph↗

arXiv2002

Semiconductor-cavity QED in high-Q regimes: Detuning effect

The non-resonant interaction between the high-density excitons in a quantum well and a single mode cavity field is investigated. An analytical expression for the physical spectrum of the excitons is obtained. The spectral properties of the excitons, which are initially prepared in the number states or the superposed states of the two different number states by the resonant femtosecond pulse pumping experiment, are studied. Numerical study of the physical spectrum is carried out and a discussion of the detuning effect is presented.

quant-ph↗

arXiv2002

On the single mode approximation in spinor-1 atomic condensate

We investigate the validity conditions of the single mode approximation (SMA) in spinor-1 atomic condensate when effects due to residual magnetic fields are negligible. For atomic interactions of the ferromagnetic type, the SMA is shown to be exact, with a mode function different from what is commonly used. However, the quantitative deviation is small under current experimental conditions (for $^{87}$Rb atoms). For anti-ferromagnetic interactions, we find that the SMA becomes invalid in general. The differences among the mean field mode functions for the three spin components are shown to depend strongly on the system magnetization. Our results can be important for studies of beyond mean field quantum correlations, such as fragmentation, spin squeezing, and multi-partite entanglement.

cond-mat.supr-con↗

arXiv2002

An quantum approach of measurement based on the Zurek's triple model

In a close form without referring the time-dependent Hamiltonian to the total system, a consistent approach for quantum measurement is proposed based on Zurek's triple model of quantum decoherence [W.Zurek, Phys. Rev. D 24, 1516 (1981)]. An exactly-solvable model based on the intracavity system is dealt with in details to demonstrate the central idea in our approach: by peeling off one collective variable of the measuring apparatus from its many degrees of freedom, as the pointer of the apparatus, the collective variable de-couples with the internal environment formed by the effective internal variables, but still interacts with the measured system to form a triple entanglement among the measured system, the pointer and the internal environment. As another mechanism to cause decoherence, the uncertainty of relative phase and its many-particle amplification can be summed up to an ideal entanglement or an Shmidt decomposition with respect to the preferred basis.

quant-ph↗

arXiv2002

Decoherence of collective atomic spin states due to inhomogeneous coupling

We investigate the decoherence of a superposition of symmetric collective internal states of an atomic ensemble due to inhomogeneous coupling to external control fields. For asymptotically large system, we find the characteristic decoherence rate scales as $\sqrt{N}$ with $N$ being the total number of atoms. Our results shed new light on attempts for quantum information processing and storage with atomic ensembles.

quant-ph↗

arXiv2001

Multistability and Critical Fluctuation in a Split Bose-Einstein Condenstate

By using a two-mode description, we show that there exist the multistability, phase transition and associated critical fluctuations in the macroscopic tunneling process between the halves of a double-well trap containing a Bose-Einstein condenstate. The phase transition that two of the triple stable states and a unstable state merge into one stable state or a reverse process takes place whenever the ratio of the mean field energy per particle to the tunneling energy goes across a critical value of order one. The critical fluctuation phenomenon corresponds to squeezed states for the phase difference between the two wells accompanying with large fluctuations of atom numbers.

quant-ph↗

arXiv2001

Dynamics of optically driven exciton and quantum decoherence

By using the normal ordering method, we study the state evolution of an optically driven excitons in a quantum well immersed in a leaky cavity, which was introduced by Yu-xi Liu et.al. [Phys. Rev. A {\bf 63}, 033816 (2001)]. The influence of the external laser field on the quantum decoherence of a mesoscopically superposed states of the excitons is investigated. Our result shows that, thought the characteristic time of decoherence does not depend on the external field, the phase of the decoherence factor can be well controlled by adjusting the external parameters.

quant-ph↗