Superconducting pi qubit with a ferromagnetic Josephson junction

Solid-state qubits have the potential for the large-scale integration and for the flexibility of layout for quantum computing. However, their short decoherence time due to the coupling to the environment remains an important problem to be overcome. We propose a new superconducting qubit which incorporates a spin-electronic device: the qubit consists of a superconducting ring with a ferromagnetic pi junction which has a metallic contact and a normal Josephson junction with an insulating barrier. Thus, a quantum coherent two-level state is formed without an external magnetic field. This feature and the simple structure of the qubit make it possible to reduce its size leading to a long decoherence time.     

Entanglement Dynamics of Quantum States Undergoing Decoherence from a Driven Critical Environment

In this paper,we have investigated the quantum entanglement of quantum states undergoing decoherence from a spin environment which drives a quantum phase transition.From our analysis,we find that the entanglement dynamics depends not only on the coupling strength but also on the external magnetic field and the number of the freedom degrees of the environment.Specially,our results imply that the decay of the entanglement can be enhanced by the quantum phase transition of the environment when the system is coupled to the environment weakly.Additionally,the discussion of the case of the multipartite states with high dimensions is made.     

Entanglement Dynamics of Quantum States Undergoing Decoherence from a Driven Critical Environment

In this paper, we have investigated the quantum entanglement of quantum states undergoing decoherence from a spin environment which drives a quantum phase transition. From our analysis, we find that the entanglement dynamics depends not only on the coupling strength but also on the external magnetic field and the number of the freedom degrees of the environment. Specially, our results imply that the decay of the entanglement can be enhanced by the quantum phase transition of the environment when the system is coupled to the environment weakly. Additionally, the discussion of the case of the multipartite states with high dimensions is made.     

The initial decoherence of the mesoscopic Josephson junction flux qubit

For a mesoscopic radio frequency superconducting quantum interference device (rfSQUID), at a degeneracy point, the system reduces to a quantum two-state system which can be used as a flux qubit. When the noise environment is equivalent to a harmonic oscillators bath, by virtue of an operator-norm measure for the short time decoherence, this paper investigates the initial decoherence of the flux qubit operating in the ohmic noise environment and illustrates its property by means of the numerical evaluation.     

Decoherence of quantum Brownian motion in noncommutative space

The decoherence of a harmonic oscillator under two-dimensional quantum Brownian motion on a noncommutative plane is investigated. The interaction with the environment is considered by two separate models so-called coupled and uncoupled. The two-dimensional master equation and its noncommutative counterpart are derived for both employed models. The rate of the linear entropy (predictability sieve) is chosen as a criterion to investigate the purity in the presence of the space noncommutativity. Besides, a two-dimensional charged harmonic oscillator on a plane which is imposed by a perpendicular magnetic field is introduced as a realization of our model. Therefore, our approach provides a formalism to investigate the influence of the magnetic field on the decoherence of the pure states. We show that in the high magnetic field limit the rate of the decoherence will be decreased.     

Dynamic Quantum Erasure Mediated by Electromagnetically Induced Transparency

We propose a scheme on control of the transition from quantum decoherence to coherence of a considered system S provided by the polarization degree of freedom of a probe field coupled to its motional degrees of freedom representing the environment B. A magneto-optically manipulated atomic ensemble with a tripod configuration is used to enhance the coupling between systems S and B. The spatial profile of the external fields induces a spatially varying potential for system B in the gas cell with identical and noninteracting atoms at different transverse points. It is found that the coherence of the system S can be maintained, lost or gained by properly choosing the incident positions of the probe field with respect to the center of the control laser field, and the two photon detuning for each components of the probe laser field.     

磁场引起的近藤单态退相干机制

磁场作为一个环境能够诱导近藤单态的退相干。我们采用格林函数方法,计算磁场下量子点耦合Aharonov-Bohm环系统的退相干特性,数值结果显示磁场引起的近藤单态的退相干是一个突然的过程。     

原子分子光子系统的耗散相互作用和退相干

原子分子系统与量子化的电磁场或光子模式耦合的系统是非相对论量子力学理论研究和实验研究的主要对象和模型. 现实系统必然与外界环境耦合,且即便原子隔绝较好、光学腔壁品质因子足够高,原子系统也不等价于少数几个能级构成的简单模型：它仍然有不为零的几率跃迁到不可控的能级空间、与原子相互作用的自由空间真空场的量子效应也必须考虑. 本文将结合开放量子系统理论的基本要素与原子光子的基本模型,对原子分子系统在电磁场中发生的耗散以及量子退相干过程做简单综述,并重点介绍描述量子系统退相干过程的主流理论工具——主方程.     

Quantum Correlation in Circuit QED Under Various Dissipative Modes

Dynamical evolutions of quantum correlations in circuit quantum electrodynamics (circuit-QED) are investigated under various dissipative modes. The influences of photon number, coupling strength, detuning and relative phase angle on quantum entanglement and quantum discord are compared as well. The results show that quantum discord may be less robust to decoherence than quantum entanglement since the death and revival also appears. Under certain dissipative mode, the decoherence subspace can be formed in circuit-QED due to the cooperative action of vacuum field. Whether a decoherence subspace can be formed not only depends on the form of quantum system but also relates closely to the dissipative mode of environment. One can manipulate decoherence through manipulating the correlation between environments, but the effect depends on the choice of initial quantum states and dissipative modes. Furthermore, we find that proper relative phase of initial quantum state provides one means of suppressing decoherence.     

Comparison between decoherence time for a two-state spin ½ system with its corresponding quantum retrieval period

The evolution of a two-state quantum system (a spin ½ particle) in both the framework of standard quantum mechanics and under the decoherence regime is considered. The former approach on this issue is the well-known quantum flipping process of a dichotomic system subjected to a time-dependent magnetic field. In the latter approach, the Spin-Boson model is utilized to describe the interaction of system with its environment and the Born-Markov master equation is derived to obtain the decoherence time. It is possible to show that under certain conditions, one may find a potential conflict between the predictions of decoherence theory and the result observed in a typical quantum flipping experiment.     

Breakdown of Landau Fermi liquid theory: Restrictions on the degrees of freedom of quantum electrons

One challenge in contemporary condensed matter physics is to understand unconventional electronic physics beyond the paradigm of Landau Fermi-liquid theory. Here, we present a perspective that posits that most such examples of unconventional electronic physics stem from restrictions on the degrees of freedom of quantum electrons in Landau Fermi liquids. Since the degrees of freedom are deeply connected to the system’s symmetries and topology, these restrictions can thus be realized by external constraints or by interaction-driven processes via the following mechanisms: (i) symmetry breaking, (ii) new emergent symmetries, and (iii) nontrivial topology. Various examples of unconventional electronic physics beyond the reach of traditional Landau Fermi liquid theory are extensively investigated from this point of view. Our perspective yields basic pathways to study the breakdown of Landau Fermi liquids and also provides a guiding principle in the search for novel electronic systems and devices.     

Cosmological perturbations and quantum fields in curved space

We identify the quantum theory of cosmological perturbations with the quantum field theory in curved spacetime with emphasis on its field concept. We materialize this idea by using a coherent state as a quantum analogue of a nontrivial classical field configuration. We present analytic results in a de Sitter universe for the massless and massive minimal free scalar fields. Some new features on the spectrum of perturbations are obtained for the massive case. We also show how such quantum field theories can be derived from quantum gravity using the semiclassical approximation. A physical degree of freedom is picked up from three scalar perturbations in the quantum gravity scalar system and its Schrödinger equation is derived. Peculiar features of quantum fields at imaginary time and its possible implications on boundary conditions for the wave function of the universe are also discussed.     

Information entropy and entanglement of a superconducting qubit coupled to a cavity field with its spontaneous decay

The quantum entanglement between superconducting qubit and cavity field is described quantitatively in the presence of spontaneous decay. Depending on how how a system is quantum correlated with its environment, the entanglement dynamics between the qubit and cavity is evaluated and investigated during the dissipative process. The motivation based on recent experiments wherein the Cooper box can be used to probe the decay of the resonator superposition state due to environmental decoherence, we theoretically investigate the dynamics of entanglement measured by the negativity. Wehrl entropy and Wehrl phase distribution of a superconducting qubit coupled to a cavity field induced by a superconducting qubit-damping reservoir governed by a master equation.     

Circuit design based manipulation of decoherence and disentanglement

In the process of quantum information transport, environment inevitably causes decoherence and disentanglement. It is effective to constitute a hybrid qubit system by taking advantages of different types of qubits to overcome the effects of decoherence and achieve quantum information transport. We find that energy relaxation exists in the process of information exchange bewteen the hybrid qubits. Combining this kind of energy relaxation with the decoherence effects from external environment, quantum information transport of non-disentangled effect can be achieved in phase damping channel if the exchange decay rate and decoherence time satisfy certain constraint relations. We discuss the scheme to achieve the constraint relations through combining specific quantum circuits.     

Quantum decoherence due to non-linear dynamics of environment

We discuss quantum decoherence in an open system which couples with a non-linear environment with finite degrees of freedom. Even if the degrees of freedom of the environment are finite, the strong non-linearity of the environment is expected to destroy quantum coherence of the open system like a heat bath with infinite degrees of freedom. In order to demonstrate this fact, we use two-dimensional kicked rotors as the environment and investigate a master equation for a reduced density matrix which is obtained by coarse-graining the environmental degrees of freedom. Our numerical simulation shows that when the non-linearity of the environment exceeds a critical strength, quantum coherence of the open system is irreversibly destroyed. This decoherence is due to the uncorrelated response of the environment to the open system and is related to the chaotic property of the non-linear environment.     

Steady Quantum Discord Behavior for Two Qubits Heisenberg XYZ Chain with Intrinsic Decoherence

By taking into account the intrinsic decoherence and the nonuniform magnetic field, quantum discord (QD) and steady quantum discord (SQD) behavior of a two-qubit anisotropic Heisenberg XYZ chain with different initial states are investigated. We find that properly tuning the external and self parameters not only can improve the quantum correlation and steady quantum correlation but also can weaken the effects of decoherence such as increasing anisotropic parameter Δ, decreasing B or b. When t is infinity, the SQD value and the physical about the SQD phenomenon are studied in detail, the SQD value is strongly dependent on the external and self parameters, which is increased evidently by increasing anisotropic parameter and decreasing nonuniform field. Through analyzing the physical about SQD phenomenon, the conditions about the existence of SQD phenomenon are analyzed with different initial states. These investigations can imply us more control parameters on quantum correlation and steady quantum correlation in solid state systems.     

Environment-assisted precision measurement

We describe a method to enhance the sensitivity of precision measurements that takes advantage of the environment of a quantum sensor to amplify the response of the sensor to weak external perturbations. An individual qubit is used to sense the dynamics of surrounding ancillary qubits, which are in turn affected by the external field to be measured. The resulting sensitivity enhancement is determined by the number of ancillas that are coupled strongly to the sensor qubit; it does not depend on the exact values of the coupling strengths and is resilient to many forms of decoherence. The method achieves nearly Heisenberg-limited precision measurement, using a novel class of entangled states. We discuss specific applications to improve clock sensitivity using trapped ions and magnetic sensing based on electronic spins in diamond.     

An intrinsic limit to quantum coherence due to spontaneous symmetry breaking

We investigate the influence of spontaneous symmetry breaking on the decoherence of a many-particle quantum system. This decoherence process is analyzed in an exactly solvable model system that is known to be representative of symmetry broken macroscopic systems in equilibrium. It is shown that spontaneous symmetry breaking imposes a fundamental limit to the time that a system can stay quantum coherent. This universal time scale is t(spon) approximately = 2piNH/(kBT), given in terms of the number of microscopic degrees of freedom N, temperature T, and the constants of Planck (h) and Boltzmann (kB).     

Boundary-induced localized structures in a nonlinear optical feedback experiment

Experimental and numerical evidence of symmetry-breaking bifurcations of a circular dissipative soliton with additional boundary conditions in the feedback of a liquid crystal light valve are reported. By tuning the strength of the nonlinearity or the size of the additional boundaries, the circular structure breaks up into polygonal symmetries and the system exhibits multistability. The experimental results are confirmed by numerical simulations with different configurations of the polarizers thus demonstrating the universality of the phenomenon.