Towards a dephasing diode: asymmetric and geometric dephasing

We study the effect of a noisy environment on spin and charge transport in ballistic quantum wires with spin-orbit coupling (Rashba coupling). We find that the wire then acts as a dephasing diode, inducing very different dephasing of the spins of right and left movers. We also show how Berry phase (geometric phase) in a curved wire can induce such asymmetric dephasing, in addition to purely geometric dephasing. We propose ways to measure these effects through spin detectors, spin-echo techniques, and Aharanov-Bohm interferometry.     

Effect of Different Environments on Multipartite Global Discord

We investigate the influences of non-Markovian dissipation and global dephasing process on the dynamical behaviors of global discord among four qubits.We find that for the non-Markovian dissipation model W state is the most robust to decoherence compared to Dicke and GHZ states;however,for the global dephasing model Dicke state is the most robust to decoherence among them.Also the dynamical behaviors of global quantum discord are quite different from that of the multipartite entanglement for the non-Markovian dissipation model,while they are very similar to each other for the global dephasing model.     

Geometric nature of the environment-induced Berry phase and geometric dephasing

We investigate the geometric phase or Berry phase acquired by a spin half which is both subject to a slowly varying magnetic field and weakly coupled to a dissipative environment (either quantum or classical). We study how this phase is modified by the environment and find that the modification is of a geometric nature. While the original Berry phase (for an isolated system) is the flux of a monopole field through the loop traversed by the magnetic field, the environment-induced modification of the phase is the flux of a quadrupolelike field. We find that the environment-induced phase is complex, and its imaginary part is a geometric contribution to dephasing. Its sign depends on the direction of the loop. Unlike the Berry phase, this geometric dephasing is gauge invariant for open paths of the magnetic field.     

Quantum Monty Hall Problem under Decoherence

We study the effect of decoherence on quantum Monty Hall problem under theinfluence of amplitude damping, depolarizing, and dephasing channels. It isshown that under the effect of decoherence, there is a Nash equilibrium ofthe game in case of depolarizing channel for Alice's quantum strategy.Whereas in case of dephasing noise, the game is not influenced by thequantum channel. For amplitude damping channel, Bob's payoffs are foundsymmetrical about a decoherence of 50% and the maximum occurs at this value of decoherence for his classical strategy. However, it is worth-mentioning that in case of depolarizing channel, Bob's classical strategy remains always dominant against any choice of Alice's strategy.     

Correlation dynamics of qubit-qutrit systems in a classical dephasing environment

We study the time evolution of classical and quantum correlations for hybrid qubit-qutrit systems in independent and common dephasing environments. Our discussion involves a comparative analysis of the Markovian dynamics of negativity, quantum discord, geometric measure of quantum discord and classical correlation. For the case of independent environments, we have demonstrated the phenomenon of sudden transition between classical and quantum decoherence for qubit-qutrit states. In the common environment case, we have shown that dynamics of quantum and geometric discords might be completely independent of each other for a certain time interval, although they tend to be eventually in accord.     

Optimal dynamical decoherence control of a qubit

We present a theory of dynamical control by modulation for optimal decoherence reduction. The theory is based on the non-Markovian Euler-Lagrange equation for the energy-constrained field that minimizes the average dephasing rate of a qubit for any given dephasing spectrum.     

Disentanglement of 3-qubit states with stochastic dephasing

The effect of stochastic dephasing on the entanglement of 3-qubit states is analyzed. We find that the extent to which the entanglement vanishes depends not only on the strength of the stochastic dephasing, but also on the structure of states of concern under decoherence induced by the stochastic dephasing. The linear entropy used to measure coherence loss is evaluated.     

Geometric phase of an open double-quantum-dot system detected by a quantum point contact

We study theoretically the geometric phase of a double-quantum-dot(DQD) system measured by a quantum point contact(QPC) in the pure dephasing and dissipative environments, respectively. The results show that in these two environments, the coupling strength between the quantum dots has an enhanced impact on the geometric phase during a quasiperiod. This is due to the fact that the expansion of the width of the tunneling channel connecting the two quantum dots accelerates the oscillations of the electron between the quantum dots and makes the length of the evolution path longer.In addition, there is a notable near-zero region in the geometric phase because the stronger coupling between the system and the QPC freezes the electron in one quantum dot and the solid angle enclosed by the evolution path is approximately zero,which is associated with the quantum Zeno effect. For the pure dephasing environment, the geometric phase is suppressed as the dephasing rate increases which is caused only by the phase damping of the system. In the dissipative environment,the geometric phase is reduced with the increase of the relaxation rate which results from both the energy dissipation and phase damping of the system. Our results are helpful for using the geometric phase to construct the fault-tolerant quantum devices based on quantum dot systems in quantum information.     

Geometric phase and nonadiabatic effects in an electronic harmonic oscillator

Steering a quantum harmonic oscillator state along cyclic trajectories leads to a path-dependent geometric phase. Here we describe its experimental observation in an electronic harmonic oscillator. We use a superconducting qubit as a nonlinear probe of the phase, which is otherwise unobservable due to the linearity of the oscillator. We show that the geometric phase is, for a variety of cyclic paths, proportional to the area enclosed in the quadrature plane. At the transition to the nonadiabatic regime, we study corrections to the phase and dephasing of the qubit caused by qubit-resonator entanglement. In particular, we identify parameters for which this dephasing mechanism is negligible even in the nonadiabatic regime. The demonstrated controllability makes our system a versatile tool to study geometric phases in open quantum systems and to investigate their potential for quantum information processing.     

Decoherence and 1/f noise in Josephson qubits

We propose and study a model of dephasing due to an environment of bistable fluctuators. We apply our analysis to the decoherence of Josephson qubits, induced by background charges present in the substrate, which are also responsible for the 1/f noise. The discrete nature of the environment leads to a number of new features which are mostly pronounced for slowly moving charges. Far away from the degeneracy this model for the dephasing is solved exactly.     

Quantum Probing Topological Phase Transitions by Non‐Markovianity

Understanding the physical significance and probing the global invariants characterizing quantum topological phases in extended systems is a main challenge in modern physics with major impact in different areas of science. Here, a quantum‐information‐inspired probing method is proposed where topological phase transitions are revealed by a non‐Markovianity quantifier. The idea is illustrated by considering the decoherence dynamics of an external read‐out qubit that probes a Su–Schrieffer–Heeger (SSH) chain with either pure dephasing or dissipative coupling. Qubit decoherence features and non‐Markovianity measure clearly signal the topological phase transition of the SSH chain.     

Effect of dephasing on the current statistics of mesoscopic devices

We investigate the effects of dephasing on the current statistics of mesoscopic conductors with a recently developed statistical model, focusing, in particular, on mesoscopic cavities and Aharonov-Bohm rings. For such devices, we analyze the influence of an arbitrary degree of decoherence on the cumulants of the current. We recover known results for the limiting cases of fully coherent and totally incoherent transport and are able to obtain detailed information on the intermediate regime of partial coherence for a varying number of open channels. We show that dephasing affects the average current, shot noise, and higher order cumulants in a quantitatively and qualitatively similar way, and that consequently shot noise or higher order cumulants of the current do not provide information on decoherence additional or complementary to what can be already obtained from the average current.     

Non-Gaussianity dynamics of two-mode squeezed number states subject to different types of noise based on cumulant theory

We provide a measure to characterize the non-Gaussianity of phase-space function of bosonic quantum states based on the cumulant theory. We study the non-Gaussianity dynamics of two-mode squeezed number states by analyzing the phase-averaged kurtosis for two different models of decoherence: amplitude damping model and phase damping model.For the amplitude damping model, the non-Gaussianity is very fragile and completely vanishes at a finite time. For the phase damping model, such states exhibit rich non-Gaussian characters. In particular, we obtain a transition time that such states can transform from sub-Gaussianity into super-Gaussianity during the evolution. Finally, we compare our measure with the existing measures of non-Gaussianity under the independent dephasing environment.     

Dephasing processes in a single semiconductor quantum dot

We discuss the decoherence dynamics in a single semiconductor quantum dot and analyze two dephasing mechanisms. In the first part of the review, we examine the intrinsic source of dephasing provided by the coupling to acoustic phonons. We show that the non-perturbative reaction of the lattice to the interband optical transition results in a composite optical spectrum with a central zero-phonon line and lateral side-bands. In fact, these acoustic phonon side-bands completely dominate the quantum dot optical response at room temperature. In the second part of the article, we focus on the extrinsic dephasing mechanism of spectral diffusion that determines the quantum dot decoherence at low temperatures. We interpret the variations of both width and shape of the zero-phonon line as due to the fluctuating electrostatic environment. In particular, we demonstrate the existence of a motional narrowing regime in the limit of low incident power or low temperature, thus revealing an unconventional phenomenology compared to nuclear magnetic resonance. To cite this article: G. Cassabois, R. Ferreira, C. R. Physique 9 (2008).     

Spin squeezing in Bose—Einstein condensates: Limits imposed by decoherence and non-zero temperature

We consider dynamically generated spin squeezing in interacting bimodal condensates. We show that particle losses and non-zero temperature effects in a multimode theory completely change the scaling of the best squeezing for large atom numbers. We present the new scalings and we give approximate analytical expressions for the squeezing in the thermodynamic limit. Besides reviewing our recent theoretical results, we give here a simple physical picture of how decoherence acts to limit the squeezing. We show in particular that under certain conditions the decoherence due to losses and non-zero temperature acts as a simple dephasing.     

Property of Quantum Correlations with Correlated Noise

We analyze the classical and quantum correlation properties of the standard and so-called quasiclassical depolarizing channel with correlated noise and non-Markovian dephasing channel, specifically we use the quantum discord, entanglement, and measurement-induced disturbance （MID） to measure the quantum correlations. For the depolarizing channel, we find that the memory effect has more influence on the MID and quantum discord than entanglement. For the dephasing channel, we show that the non-Markovian dephasing channel is more robust than Markovian dephasing channel against deeoherence. We also find that at first MID and quantum discord take different values, and then after a specific time they will take almost the same value and both decay monotonically in the same way.     

Entanglement Dynamics of Two Qubits Coupled to a Noise Environment

We study the time evolution of two two-state systems （two qubits） initially in the pure entangled states or the maximally entangled mixed states interacting with the individual environmental noise. It is shown that due to environment noise, all quantum entangled states are very fragile and become a classical mixed state in a short-time limit. But the environment can affect entanglement in very different ways. The type of decoherence process for certain entangled states belongs to amplitude damping, while the others belong to dephasing decoherenee.     

Decoherence of flux qubits due to 1/f flux noise

We have investigated decoherence in Josephson-junction flux qubits. Based on the measurements of decoherence at various bias conditions, we discriminate contributions of different noise sources. We present a Gaussian decay function extracted from the echo signal as evidence of dephasing due to 1/f flux noise whose spectral density is evaluated to be about (10(-6)Phi0)2/Hz at 1 Hz. We also demonstrate that, at an optimal bias condition where the noise sources are well decoupled, the coherence observed in the echo measurement is limited mainly by energy relaxation of the qubit.     

Deterministic geometric quantum phase gates for two atoms in decoherence-free subspace

We propose a scheme for realizing conventional geometric quantum phase gates in the context of cavity QED. During the operation neither the atomic system nor the cavity mode is excited, which is important in view of decoherence. The scheme does not require detection of photons, so the gate operation is deterministic and the influence of photodetection imperfection is eliminated. Taking advantage of the geometric manipulation, the phase gate is resilient to fluctuations of experimental parameters.     

量子纠缠消相干对确定型远程制备的影响

研究了两种典型的量子纠缠消相干现象对确定型量子态远程制备方案的影响.首先对该确定型远程制备方案进行了分析,得到该方案确定性和比特消耗情况; 然后通过分析制备过程中纠缠消相干现象对系统的影响得出: 在极化消相干过程中,该系统保真度与目标量子比特在Bloch球上的经度选择无关,仅与目标比特的纬度和消相干的大小有关;在相位消相干中,该系统的保真度不会受到消相干的影响,仅与目标量子态的纬度相关. 关键词： 远程制备 纠缠消相干 通信消耗 保真度