arXiv subjects

C. P. Sun

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

At least 217 records · Page 12Linked to original sources

arXiv2006

Quantum Heat Engines Using Superconducting Quantum Circuits

We propose a quantum analog of the internal combustion engine used in most cars. Specifically, we study how to implement the Otto-type quantum heat engine (QHE) with the assistance of a Maxwell's demon. Three steps are required: thermalization, quantum measurement, and quantum feedback controlled by the Maxwell demon. We derive the positive-work condition of this composite QHE. Our QHE can be constructed using superconducting quantum circuits. We explicitly demonstrate the essential role of the demon in this macroscopic QHE.

quant-ph↗

arXiv2006

Electromagnetic manipulation for anti-Zeno effect in an engineered quantum tunneling process

We investigate the quantum Zeno and anti-Zeno effects for the irreversible quantum tunneling from a quantum dot to a ring array of quantum dots. By modeling the total system with the Anderson-Fano-Lee model, it is found that the transition from the quantum Zeno effect to quantum anti-Zeno effect can happen as the magnetic flux and the gate voltage were adjusted.

quant-ph↗

arXiv2006

Decoherence problem in quantum state transfer via an engineered spin chain

A perfect quantum state transfer(QST) has been shown in an engineered spin chain with "always-on interaction". Here, we consider a more realistic problem for such a protocol, the quantum decoherence induced by a spatially distributed environment, which is universally modeled as a bath of harmonic oscillators. By making use of the irreducible tensor method in angular momentum theory, we investigate the effect of decoherence on the efficiency of QST for both cases at zero and finite temperatures. We not only show the generic exponential decay of QST efficiency as the number of sites increase, but also find some counterintuitive effect, the QST can be enhanced as temperature increase.

quant-ph↗

arXiv2006

Possible coexistence of s-wave and unconventional pairing in Na_xCoO_2-yH2O:a new insight from impurity effects

To shed light on the symmetry of the superconducting order parameter in Na_xCoO_2-yH_2O, the Mn doping effects are studied. X-ray absorption spectroscopy verifies that the doped Mn impurities occupy the Co sites and are with a valance close to +4. Impurity scattering by Mn is in the unitary limit that, however, does not lead to strong Tc suppression. This absence of the strong impurity effects on T_c is not consistent with the simple picture of a sign-changing order parameter. Coexistence of the s-wave and unconventional order parameters is proposed to reconcile all existing experiments and has been directly observed by the specific heat experiments.

cond-mat.supr-con↗

arXiv2006

Probing Quantum Phase Transition in Macroscopic Qubit Array via circuit QED architecture

We demonstrate a universal physical mechanism to probe the macroscopic quantum phase transition based on circuit QED architecture. We found that, with certain parameters, the Josephson junction qubit array behaves as an antiferromagnetic Ising model in transverse field and the coupled transmission line resonator serves as a bosonic quantum probe. Our investigation shows that, at the critical point, the drastic broadening of the spectrum of the probe indicates the quantum phase transition.

quant-ph↗

arXiv2006

Dynamic generation of entangling wave packets in XY spin system with decaying long range couplings

We study the dynamic generation of spin entanglement between two distant sites in an XY model with $1/r^{2}$-decay long range couplings. Due to the linear dispersion relation $ε(k)\sim |k|$ of magnons in such model, we show that a well-located spin state can be dynamically split into two moving entangled local wave packets without changing their shapes. Interestingly, when such two wave packets meet at the diametrically opposite site after the fast period $τ=π$, the initial well-located state can be recurrent completely. Numerical calculation is performed to confirm the analytical result even the ring system of sizes $N$ up to several thousands are considered. The truncation approximation for the coupling strengths is also studied. Numerical simulation shows that the above conclusions still hold even the range of the coupling strength is truncated at a relative shorter scale comparing to the size of the spin system.

quant-ph↗

arXiv2006

Peierls distorted chain as a quantum data bus for quantum state transfer

We systematically study the transfer of quantum state of electron spin as the flying qubit along a half-filled Peierls distorted tight-binding chain described by the Su-Schrieffer-Heeger (SSH) model, which behaves as a quantum data bus. This enables a novel physical mechanism for quantum communication with always-on interaction: the effective hopping of the spin carrier between sites $A$ and $B$ connected to two sites in this SSH chain can be induced by the quasi-excitations of the SSH model. As we prove, it is the Peierls energy gap of the SSH quasi-excitations that plays a crucial role to protect the robustness of the quantum state transfer process. Moreover, our observation also indicates that such a scheme can also be employed to explore the intrinsic property of the quantum system.

cond-mat.mes-hall↗

arXiv2006

Correlated Photons from Collective Excitations of Three-Level Atomic Ensemble

We systematically study the interaction between two quantized optical fields and a cyclic atomic ensemble driven by a classic optical field. This so-called atomic cyclic ensemble consists of three-level atoms with Delta-type transitions due to the symmetry breaking, which can also be implemented in the superconducting quantum circuit by Yu-xi Liu et al. [Phys. Rev. Lett. 95, 087001 (2005)]. We explore the dynamic mechanisms to creating the quantum entanglements among photon states, and between photons and atomic collective excitations by the coherent manipulation of the atom-photon system. It is shown that the quantum information can be completely transferred from one quantized optical mode to another, and the quantum information carried by the two quantized optical fields can be stored in the collective modes of this atomic ensemble by adiabatically controlling the classic field Rabi frequencies.

quant-ph↗

arXiv2006

Decay of Loschmidt Echo Enhanced by Quantum Criticality

We study the transition of a quantum system $S $ from a pure state to a mixed one, which is induced by the quantum criticality of the surrounding system $E$ coupled to it. To characterize this transition quantitatively, we carefully examine the behavior of the Loschmidt echo (LE) of $E$ modelled as an Ising model in a transverse field, which behaves as a measuring apparatus in quantum measurement. It is found that the quantum critical behavior of $E$ strongly affects its capability of enhancing the decay of LE: near the critical value of the transverse field entailing the happening of quantum phase transition, the off-diagonal elements of the reduced density matrix describing $S$ vanish sharply.

quant-ph↗

arXiv2006

Spin wave based quantum storage of electronic spin with a ring array of nuclear spins

We propose a solid state based protocol to implement the universal quantum storage for electronic spin qubit. The quantum memory in this scheme is the spin wave excitation in the ring array of nuclei in a quantum dot. We show that the quantum information carried by an arbitrary state of the electronic spin can be coherently mapped onto the spin wave excitations or the magnon states. With an appropriate external control, the stored quantum state in quantum memory can be read out reversibly. We also explore in detail the quantum decoherence mechanism due to the inhomogeneous couplings between the electronic spin and the nuclear spins.

quant-ph↗

arXiv2006

Quantum-Classical Transition of Photon-Carnot Engine Induced by Quantum Decoherence

We study the physical implementation of the Photon Carnot engine (PCE) based on the cavity QED system [M. Scully et al, Science, \textbf{299}, 862 (2003)]. Here, we analyze two decoherence mechanisms for the more practical systems of PCE, the dissipation of photon field and the pure dephasing of the input atoms. As a result we find that (I) the PCE can work well to some extent even in the existence of the cavity loss (photon dissipation); and (II) the short-time atomic dephasing, which can destroy the PCE, is a fatal problem to be overcome.

quant-ph↗

arXiv2006

Spin entanglement induced by spin-orbit interactions in coupled quantum dots

We theoretically explore the possibility of creating spin quantum entanglement in a system of two electrons confined respectively in two vertically coupled quantum dots in the presence of Rashba type spin-orbit coupling. We find that the system can be described by a generalized Jaynes - Cummings model of two modes bosons interacting with two spins. The lower excitation states of this model are calculated to reveal the underlying physics of the far infrared absorption spectra. The analytic perturbation approach shows that an effective transverse coupling of spins can be obtained by eliminating the orbital degrees of freedom in the large detuning limit. Here, the orbital degrees of freedom of the two electrons, which are described by two modes of bosons, serve as a quantized data bus to exchange the quantum information between two electrons. Then a nontrivial two-qubit logic gate is realized and spin entanglement between the two electrons is created by virtue of spin-orbit coupling.

cond-mat.mes-hall↗

arXiv2006

Quantum control limited by quantum decoherence

We describe quantum controllability under the influences of the quantum decoherence induced by the quantum control itself. It is shown that, when the controller is considered as a quantum system, it will entangle with its controlled system and then cause quantum decoherence in the controlled system. In competition with this induced decoherence, the controllability will be limited by some uncertainty relation in a well-armed quantum control process. In association with the phase uncertainty and the standard quantum limit, a general model is studied to demonstrate the possibility of realizing a decoherence-free quantum control with a finite energy within a finite time. It is also shown that if the operations of quantum control are to be determined by the initial state of the controller, then due to the decoherence which results from the quantum control itself, there exists a low bound for quantum controllability.

quant-ph↗

arXiv2006

Analogue of Cavity QED for Coupling between Spin and Nanomechanical Resonator

We describe a cavity QED analogue for the coupling system of a spin and a nanomechanical resonator with a magnetic tip. For the quantized nanomechanical resonator, a spin-boson model for this coupling system can refer to a Jaynes-Cummings(JC) or an anti-JC model. These observations predict some quantum optical phenomena, such as squeezing and "collapse-revival" in the single oscillation mode of the nanomechanical resonator when it is initially prepared in the quasi-classical state. By modulating the phase of RF magnetic field one can switch the system between the JC and anti-JC model, which provides a potential protocol for the detection of the single spin. A damping mechanism is also analyzed.

quant-ph↗

arXiv2006

Quantum dynamics of magnetically controlled network for Bloch electrons

We study quantum dynamics of wave packet motion of Bloch electrons in quantum networks with the tight-binding approach for different types of nearest-neighbor interactions. For various geometrical configurations, these networks can function as some optical devices, such as beam splitters and interferometers. When the Bloch electrons with the Gaussian wave packets input these devices, various quantum coherence phenomena can be observed, e.g., the perfect quantum state transfer without reflection in a Y-shaped beam, the multi- mode entanglers of electron wave by star shaped network and Bloch electron interferometer with the lattice Aharonov-Bohm effects. Behind these conceptual quantum devices are the physical mechanism that, for hopping parameters with some specific values, a connected quantum networks can be reduced into a virtual network, which is a direct sum of some irreducible subnetworks. Thus, the perfect quantum state transfer in each subnetwork in this virtual network can be regarded as a coherent beam splitting process. Analytical and numerical investigations show the controllability of wave packet motion in these quantum networks by the magnetic flux through some loops of these networks, or by adjusting the couplings on nodes. We find the essential differences in these quantum coherence effects when the different wave packets enter these quantum networks initially. With these quantum coherent features, they are expected to be used as quantum information processors for the fermion system based on the possible engineered solid state systems, such as the array of quantum dots that can be implemented experimentally.

quant-ph↗

arXiv2006

Beam Splitter for Spin Waves in Quantum Spin Network

We theoretically design and analytically study a controllable beam splitter for the spin wave propagating in a star-shaped (e.g., a $Y$-shaped beam) spin network. Such a solid state beam splitter can display quantum interference and quantum entanglement by the well-aimed controls of interaction on nodes. It will enable an elementary interferometric device for scalable quantum information processing based on the solid system.

quant-ph↗

arXiv2006

Macroscopic Quantum Criticality in a Circuit QED

Cavity quantum electrodynamic (QED) is studied for two strongly-coupled charge qubits interacting with a single-mode quantized field, which is provided by a on-chip transmission line resonator. We analyze the dressed state structure of this superconducting circuit QED system and the selection rules of electromagnetic-induced transitions between any two of these dressed states. Its macroscopic quantum criticality, in the form of ground state level crossing, is also analyzed, resulting from competition between the Ising-type inter-qubit coupling and the controllable on-site potentials.

quant-ph↗

arXiv2005

Anomalous Magnetic and Thermal Behavior in Some RMn2O5 Oxides

The RMn2O5 (R=Pr, Nd, Sm, and Eu) oxides showing magnetoelectric (ME) behavior have been prepared in polycrystalline form by a standard citrate route. The lattice parameters, obtained from the powder XRD analysis, follow the rare-earth contraction indicating the trivalent character of the R ions. Cusp-like anomalies in the magnetic susceptibility curve and sharp peaks in the specific heat were reported at the corresponding temperatures in RMn2O5 (R=Pr, Nd, Sm, and Eu) indicating the magnetic or electric ordering transitions.

cond-mat.mtrl-sci↗