arXiv subjects

Li-Tuo Shen

Publications and source records attributed to Li-Tuo Shen.

31 records · Page 2Linked to original sources

arXiv2014

Ground state of the asymmetric Rabi model in the ultrastrong coupling regime

We study the ground states of the single- and two-qubit asymmetric Rabi models, in which the qubit-oscillator coupling strengths for the counterrotating-wave and corotating-wave interactions are unequal. We take the transformation method to obtain the approximately analytical ground states for both models and numerically verify its validity for a wide range of parameters under the near-resonance condition. We find that the ground-state energy in either the single- or two-qubit asymmetric Rabi model has an approximately quadratic dependence on the coupling strengths stemming from different contributions of the counterrotating-wave and corotating-wave interactions. For both models, we show that the ground-state energy is mainly contributed by the counterrotating-wave interaction. Interestingly, for the two-qubit asymmetric Rabi model, we find that, with the increase of the coupling strength in the counterrotating-wave or corotating-wave interaction, the two-qubit entanglement first reaches its maximum then drops to zero. Furthermore, the maximum of the two-qubit entanglement in the two-qubit asymmetric Rabi model can be much larger than that in the two-qubit symmetric Rabi model.

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arXiv2014

Quantum phase transition of polaritonic excitations in a multi-excitation coupled array

We analyze the quantum phase transition-like behavior in the lowest energy state of a two-site coupled atom-cavity system, where each cavity contains one atom but the total excitation number is not limited to two. Utilizing the variance of the total excitation number to distinguish the insulator and superfluid states, and the variance of the atomic excitation number to identify the polaritonic characteristics of these states, we find that the total excitation number plays a significant role in the lowest-energy-state phase transitions. In both the small hopping regime and the small atom-field interaction regime, we identify an interesting coexisting phase involving characteristics of both photonic superfluid and atomic insulator. For small hopping, we find that the signature of the photonic superfluid state becomes more pronounced with the increase in total excitation number, and that the boundaries of the various phases shift with respect to the case of $N=2$. In the limit of small atom-field interaction, the polaritonic superfluid region becomes broader as the total excitation number increases. We demonstrate that the variance of the total excitation number in a single site has a linear dependence on the total excitation number in the large-detuning limit.

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arXiv2014

Distributed manipulation of two-qubit entanglement with coupled continuous variables

We study the dynamics of two qubits separately sent through two coupled resonators, each initially containing a coherent state field. We present analytical arguments and numerical calculations for the qubit-field system under different two-qubit initial states, photon hopping strengths, and detunings. In far off-resonant regime, the maximal entanglement of two qubits can be generated with the initial qubit state in which one qubit is in the excited state and the other is in the ground state, and the initially maximal two-qubit entanglement can be frozen and fully revived even for large mean photon number. When the qubits are both initially in their excited states or ground states, the qubit-qubit entanglement birth and death apparently appear in the regime where the photon hopping strength is close to qubit-field detuning, and its peaks do not decrease monotonically as the interaction time increases. It is interesting to observe that when there is photon hopping strength between two fields, the field-field entanglement can be larger than one and increases as the initial amplitude of the coherent state grows. By postselecting the fields both in their coherent states, the entanglement of two initially unentangled qubits can be largely improved. Our present setup is fundamental for the distributed quantum information processing and applicable to different physical qubit-resonator systems.

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arXiv2014

Adiabatic approximation for three qubits ultrastrongly coupled to a harmonic oscillator

We study the system involving mutual interaction between three qubits and an oscillator within the ultrastrong coupling regime. We apply adiabatic approximation approach to explore two extreme regimes: (i) the oscillator's frequency is far larger than each qubit's frequency and (ii) the qubit's frequency is far larger than the oscillator's frequency, and analyze the energy-level spectrum and the ground-state property of the qubit-oscillator system under the conditions of various system parameters. For the energy-level spectrum, we concentrate on studying the degeneracy in low energy levels. For the ground state, we focus on its nonclassical properties that are necessary for preparing the nonclassical states. We show that the minimum qubit-oscillator coupling strength needed for generating the nonclassical states of the Schrödinger-cat type in the oscillator is just one half of that in the Rabi model. We find that the qubit-qubit entanglement in the ground state vanishes if the qubit-oscillator coupling strength is strong enough, for which the entropy of three qubits keeps larger than one. We also observe the phase-transition-like behavior in the regime where the qubit's frequency is far larger than the oscillator's frequency.

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arXiv2014

Shortcuts to adiabatic passage for population transfer and maximum entanglement creation between two atoms in a cavity

We use the approach of "transitionless quantum driving" proposed by Berry to construct shortcuts to the population transfer and the creation of maximal entanglement between two $Λ$-type atoms based on the cavity quantum electronic dynamics (CQED) system. An effective Hamiltonian is designed by resorting to an auxiliary excited level, a classical driving field and an extra cavity field mode to supplement or substitute the original reference Hamiltonian, and steer the system evolution along its instantaneous eigenstates in an arbitrarily short time, speeding up the rate of population transfer and creation of maximal entanglement between the two atoms inside a cavity. Numerical simulation demonstrates that our shortcuts' performance is robust against the decoherences caused by atomic spontaneous emission and cavity photon leakage.

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arXiv2013

Ground state of three qubits coupled to a harmonic oscillator with ultrastrong coupling

We study the Rabi model composed of three qubits coupled to a harmonic oscillator without involving the rotating-wave approximation. We show that the ground state of the three-qubit Rabi model can be analytically treated by using the transformation method, and the transformed ground state agrees well with the exactly numerical simulation under a wide range of qubit-oscillator coupling strengths for different detunings. We use the pairwise entanglement to characterize the ground-state entanglement between any two qubits and show that it has an approximately quadratic dependence on the qubit-oscillator coupling strength. Interestingly, we find that there is no qubit-qubit entanglement for the ground state if the qubit-oscillator coupling strength is large enough.

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arXiv2013

Quantum teleportation and computation with Rydberg atoms in an optical lattice

Neutral atoms excited to Rydberg states can interact with each other via dipole-dipole interaction, which results in a physical phenomenon named Rydberg blockade mechanism. The effect attracts much attention due to its potential applications in quantum computation and quantum simulation. Quantum teleportation has been the core protocol in quantum information science playing a key role in efficient long-distance quantum communication. Here, we first propose the implementation of teleportation scheme with neutral atoms via Rydberg blockade, in which the entangled states of qubits can readily be prepared and the Bell states measurements just require single qubit operations without precise control of Rydberg interaction. The rapid experimental progress of coherent control of Rydberg excitation, optical trapping techniques and state-selective atomic detection promise the application of the teleportation scheme for scalable quantum computation and many-body quantum simulation using the protocol proposed by D. Gottesman and I. L. Chuang [Nature (London) 402, 390 (1999)] with Rydberg atoms in optical lattice.

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arXiv2013

Using shortcut to adiabatic passage for the ultrafast quantum state transfer in cavity QED system

We propose an alternative scheme to implement the quantum state transfer between two three-level atoms based on the invariant-based inverse engineering in cavity quantum electronic dynamics (QED) system. The quantum information can be ultrafast transferred between the atoms by taking advantage of the cavity field as a medium for exchanging quantum information speedily. Through designing the time-dependent laser pulse and atom-cavity coupling, we send the atoms through the cavity with a short time interval experiencing the two processes of the invariant dynamics between each atom and the cavity field simultaneously. Numerical simulation shows that the target state can be ultrafast populated with a high fidelity even when considering the atomic spontaneous emission and the photon leakage out of the cavity field. We also redesign a reasonable Gaussian-type wave form in the atom-cavity coupling for the realistic experiment operation.

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arXiv2012

Control of two-atom entanglement with two thermal fields in coupled cavities

The dynamical evolution of a quantum system composed of two coupled cavities, each containing a two-level atom and a single-mode thermal field, is investigated under different conditions. The entanglement between the two atoms is controlled by the hopping strength and the detuning between the atomic transition and the cavities. We find that when the atomic transition is far off-resonant with both the eigenmodes of the coupled cavity system, the maximally entangled state for the two atoms can be generated with the initial state in which one atom is in the ground state and the other is in the excited state. When both the two atoms are initially in the excited state, the entanglement exhibits period sudden birth and death. By choosing appropriate parameter values, the initial maximal entanglement of the two atoms can be frozen. The relation between the concurrence and cooperative parameter is calculated.

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arXiv2012

Distributed phase-covariant cloning with atomic ensembles via quantum Zeno dynamics

We propose an interesting scheme for distributed orbital state quantum cloning with atomic ensembles based on the quantum Zeno dynamics. These atomic ensembles which consist of identical three-level atoms are trapped in distant cavities connected by a single-mode integrated optical star coupler. These qubits can be manipulated through appropriate modulation of the coupling constants between atomic ensemble and classical field, and the cavity decay can be largely suppressed as the number of atoms in the ensemble qubits increases. The fidelity of each cloned qubit can be obtained with analytic result. The present scheme provides a new way to construct the quantum communication network.

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arXiv2012

Distributed entanglement induced by dissipative bosonic media

We describe a scheme with analytic result that allows to generate steady-state entanglement for two atoms over a dissipative bosonic medium. The resonant coupling between the mediating bosonic mode and cavity modes produces three collective atomic decay channels. This dissipative dynamics, together with the unitary process induced by classical microwave fields, drives the two atoms to the symmetric or asymmetric entangled steady state conditional upon the choice of the phases of the microwave fields. The effects on the steady-state entanglement of off-resonance mediating bosonic modes are analyzed. The entanglement can be obtained with high fidelity regardless of the initial state and there is a linear relation in the scaling of the fidelity with the cooperativity parameter. The fidelity is insensitive to the fluctuation of the Rabi frequencies of the classical driving fields.

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arXiv2012

Engineering W-type steady states for three atoms via dissipation in an optical cavity

We propose a scheme for the dissipative preparation of W-type entangled steady-states of three atoms trapped in an optical cavity. The scheme is based on the competition between the decay processes into and out of the target state. By suitable choice of system parameters, we resolve the whole evolution process and employ the effective operator formalism to engineer four independent decay processes, so that the target state becomes the stationary state of the quantum system. The scheme requires neither the preparation of definite initial states nor the precise control of system parameters and preparation time.

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arXiv2011

Steady-State Entanglement for Distant Atoms by Dissipation in Coupled Cavities

We propose a scheme for the generation of entangled states for two atoms trapped in separate cavities coupled to each other. The scheme is based on the competition between the unitary dynamics induced by the classical fields and the collective decays induced by the dissipation of two delocalized field modes. Under certain conditions, the symmetric or asymmetric entangled state is produced in the steady state. The analytical result shows that the distributed steady entanglement can be achieved with high fidelity independent of the initial state, and is robust against parameter fluctuations. We also find out that the linear scaling of $F$ has a quadratic improvement compared to distributed entangled state preparation protocols based on unitary dynamics.

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