基于量子点接触的开放双量子点系统电子转移特性

基于量子点接触探测器(QPC)理论上研究了双量子点(DQD)系统在耗散环境和纯退相环境影响下的电子转移特性.结果表明,耗散环境中探测器导致的退相干会增大平均电流和Fano factor随时间演化的值,并观察到量子芝诺效应的存在.在对称的DQD情况下,弛豫减小了平均电流随时间演化的震荡振幅.在非对称的DQD情况下,弛豫降低了Fano factor随时间演化的峰值.纯退相环境中测量会阻碍共隧穿过程中不同电流通道之间的转换,导致Fano factor的极高值.在对称的DQD情况下,增大纯退相速率会提高Fano factor.在非对称的DQD情况下,动力学随时间的演化对纯退相环境不敏感.另外,还发现探测器内n个电子的转移几率只受QPC与DQD耦合的影响.我们的结论可以为实验工作者研究电子输运特性提供理论参考.     

A double quantum dot defined by top gates in a single crystalline InSb nanosheet

We report on the transport study of a double quantum dot (DQD) device made from a freestanding, single crystalline InSb nanosheet. The freestanding nanosheet is grown by molecular beam epitaxy and the DQD is defined by the top gate technique. Through the transport measurements, we demonstrate how a single quantum dot (QD) and a DQD can be defined in an InSb nanosheet by tuning voltages applied to the top gates. We also measure the charge stability diagrams of the DQD and show that the charge states and the inter-dot coupling between the two individual QDs in the DQD can be efficiently regulated by the top gates. Numerical simulations for the potential profile and charge density distribution in the DQD have been performed and the results support the experimental findings and provide a better understanding of fabrication and transport characteristics of the DQD in the InSb nanosheet. The achieved DQD in the two-dimensional InSb nanosheet possesses pronounced benefits in lateral scaling and can thus serve as a new building block for the developments of quantum computation and quantum simulation technologies.     

ac Stark shift and dephasing of a superconducting qubit strongly coupled to a cavity field

We have performed spectroscopy of a superconducting charge qubit coupled nonresonantly to a single mode of an on-chip resonator. The strong coupling induces a large ac Stark shift in the energy levels of both the qubit and the resonator. The dispersive shift of the resonator frequency is used to nondestructively determine the qubit state. Photon shot noise in the measurement field induces qubit level fluctuations leading to dephasing which is characteristic for the measurement backaction. A crossover in line shape with measurement power is observed and theoretically explained. For weak measurement a long intrinsic dephasing time of T2>200 ns of the qubit is found.     

Nanomechanical resonator coupling with a double quantum dot: quantum state engineering

We propose a theoretical scheme to realize quantum state engineering of a nanomechanical resonator (NAMR) through the coupling between the NAMR and a double quantum dot (DQD). Hybrid entangled states between the NAMR and the DQD and superposed coherent states of the NAMR are created explicitly. It is shown that quantum state tomography for the NAMR can be implemented through carrying out unitary operations on the NAMR and the DQD. It is indicated that the scheme is feasible at the reach of the present technology.     

Quantum speed limit of the double quantum dot in pure dephasing environment under measurement

The quantum speed limit (QSL) of the double quantum dot (DQD) system has been theoretically investigated by adopting the detection of the quantum point contact (QPC) in the pure dephasing environment. The Mandelstam-Tamm (MT) type of the QSL bound which is based on the trace distance has been extended to the DQD system for calculating the shortest evolving time. The increase of decoherence rate can weaken the capacity for potential speedup (CPS) and delay the evolving process due to the frequently measurement localizing the electron in the DQD system. The system needs longer time to evolve to the target state as the enhancement of dephasing rate, because the strong interaction between pure dephasing environment and the DQD system could vary the oscillation of the electron. Increasing the dephasing rate can sharp the QSL bound, but the decoherence rate would weaken the former effect and vice versa. Moreover, the CPS would be raised by increasing the energy displacement, while the enhancement of the coupling strength between two quantum dots can diminish it. It is interesting that there has an inflection point, when the coupling strength is less than the value of the point, the increasing effect of the CPS from the energy displacement is dominant, otherwise the decreasing tendency of the CPS is determined by the coupling strength and suppress the action of the energy displacement if the coupling strength is greater than the point. Our results provide theoretical reference for studying the QSL time in a semiconductor device affected by numerous factors.     

Microwave Frequency Graphene Optomechanics. Coherent Propagation Properties and Nonlinear Responses

We demonstrate theoretically the slow and fast light effects based on the coupled graphene nanomechanical resonator–microwave cavity system. The numerical results show that the maximum group delay and advancing microwave signals can reach 0.4 and 0.12 ms, respectively, in the graphene resonator–microwave cavity system. In addition, the system can also behave as an optical transistor to amplify a weak microwave field through manipulating the pump field. Further we investigate the nonlinear effects of four-wave mixing (FWM) and show that the FWM intensity can be efficiently controlled and modulated by the pump power. The graphene optomechanics provides a good medium for controlling microwave photons at different frequencies and may indicate applications in quantum information processing.     

Dipole coupling of a double quantum dot to a microwave resonator

We demonstrate the realization of a hybrid solid-state quantum device, in which a semiconductor double quantum dot is dipole coupled to the microwave field of a superconducting coplanar waveguide resonator. The double dot charge stability diagram extracted from measurements of the amplitude and phase of a microwave tone transmitted through the resonator is in good agreement with that obtained from transport measurements. Both the observed frequency shift and linewidth broadening of the resonator are explained considering the double dot as a charge qubit coupled with a strength of several tens of MHz to the resonator.     

Photon-assisted Shot noise of the double quantum dot interferometer in weak Kondo regime

The shot noise of a parallel double quantum dot (DQD) system under the perturbation of microwave fields is investigated in the weak Kondo regime. Peak-valley structures exhibit in the differential conductance and shot noise, and side resonant peaks emerge around the Kondo peak due to the absorption and emission of photons. The shot noise is sensitively dependent on the adjusting approach through changing the gate voltages. Large resonant Fano factor accompanying photon-induced side peaks appear by simultaneously varying the two gate voltages. The photon suppression and enhancement of shot noise have been evaluated corresponding to the coherent and incoherent current correlation. The destructive interference causes the suppression of shot noise by changing the Aharonov–Bohm phase.     

Controllable coupling and quantum correlation dynamics of two double quantum dots coupled via a transmission line resonator

We propose a theoretical scheme to generate a controllable and switchable coupling between two double-quantum-dot (DQD) spin qubits by using a transmission line resonator (TLR) as a bus system. We study dynamical behaviors of quantum correlations described by entanglement correlation (EC) and discord correlation (DC) between two DQD spin qubits when the two spin qubits and the TLR are initially prepared in X-type quantum states and a coherent state, respectively. We demonstrate that in the EC death regions there exist DC stationary states in which the stable DC amplification or degradation can be generated during the dynamical evolution. It is shown that these DC stationary states can be controlled by initial-state parameters, the coupling, and detuning between qubits and the TLR. We reveal the full synchronization and anti-synchronization phenomena in the EC and DC time evolution, and show that the EC and DC synchronization and anti-synchronization depends on the initial-state parameters of the two DQD spin qubits. It is shown that the initial quantum correlation may be suppressed completely when the evolution time approaches to the infinity in the presence of dissipation. These results shed new light on dynamics of quantum correlations.     

Microwave-induced dephasing in one-dimensional metal wires

We report on the effect of monochromatic microwave (MW) radiation on the weak-localization corrections to the conductivity of quasi-one-dimensional silver wires. Because of the improved electron cooling in the wires, the MW-induced dephasing is observed without a concomitant overheating of electrons over wide ranges of the MW power P(MW) and frequency f. The observed dependences of the conductivity and MW-induced dephasing rate on P(MW) and f are in agreement with the theory by Altshuler, Aronov, and Khmelnitsky [Solid State Commun. 39, 619 (1981)]. Our results suggest that in the low-temperature experiments with 1D wires, saturation of the temperature dependence of the dephasing time can be caused by an MW electromagnetic noise with a sub-pW power.     

Nonlinear quantum dynamics of two BEC modes dispersively coupled by an optical cavity

We derive a quantum master equation for a single mode excitation of a Bose-Einstein condensate by a high-finesse optical cavity. This system is formally analogous to a broad class of opto-mechanical systems comprising vibrating mirrors and resonator modes coupled by radiation pressure. The presented equation accounts for the dissipative part of the dynamics due to the coupling of a driven, lossy optical mode of a resonator. This allows for exploring the quantum limit of opto-mechanical systems in the presence of dissipation in a classically bistable regime. We find that the measurement-induced back-action noise impedes the observation of quantum tunneling and leads to a non-exponential dephasing of coherent matter wave oscillations.     

Continuous measurement of a microwave-driven solid state qubit

We analyze the dynamics of a continuously observed, damped, microwave-driven solid state charge qubit, consisting of a single electron in a double well potential. The microwave field induces transitions between the qubit eigenstates, which have a profound effect on the detector output current. Useful information about the qubit dynamics, such as dephasing and relaxation rates, and the Rabi frequency, can be extracted from the detector conductance and output noise power spectrum. We also propose a technique for single-shot electron spin readout, for spin based quantum information processing, which has a number of practical advantages over existing schemes.     

Circuit QED with a nonlinear resonator: ac-Stark shift and dephasing

We have performed spectroscopic measurements of a superconducting qubit dispersively coupled to a nonlinear resonator driven by a pump microwave field. Measurements of the qubit frequency shift provide a sensitive probe of the intracavity field, yielding a precise characterization of the resonator nonlinearity. The qubit linewidth has a complex dependence on the pump frequency and amplitude, which is correlated with the gain of the nonlinear resonator operated as a small-signal amplifier. The corresponding dephasing rate is found to be close to the quantum limit in the low-gain limit of the amplifier.     

Valley-based noise-resistant quantum computation using Si quantum dots

We devise a platform for noise-resistant quantum computing using the valley degree of freedom of Si quantum dots. The qubit is encoded in two polarized (1,1) spin-triplet states with different valley compositions in a double quantum dot, with a Zeeman field enabling unambiguous initialization. A top gate gives a difference in the valley splitting between the dots, allowing controllable interdot tunneling between opposite valley eigenstates, which enables one-qubit rotations. Two-qubit operations rely on a stripline resonator, and readout on charge sensing. Sensitivity to charge and spin fluctuations is determined by intervalley processes and is greatly reduced as compared to conventional spin and charge qubits. We describe a valley echo for further noise suppression.     

Quantum spin and charge pumping through double quantum dots with ferromagnetic leads

The pumping of electrons through double quantum dots (DQDs) attached to ferromagnetic leads have been theoretically investigated by using the nonequilibrium Green?s function method. It is found that an oscillating electric field applied to the quantum dot may give rise to the pumped charge and spin currents. In the case that both leads are ferromagnet, a pure spin current can be generated in the antiparallel magnetization configuration, where no net charge current exists. The possibility of manipulating the pumped spin current is explored by tuning the dot level and the ac field. By making use of various tunings, the magnitude and direction of the pumped spin current can be well controlled. For the case that only one lead is ferromagnetic, both of the charge and spin currents can be pumped and flow in opposite directions on the average. The control of the magnitude and direction of the pumped charge and spin currents is also discussed by means of the magnetic flux threading through the DQD ring.     

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.     

Noise dephasing in edge states of the integer quantum Hall regime

An electronic Mach-Zehnder interferometer is used in the integer quantum Hall regime at a filling factor 2 to study the dephasing of the interferences. This is found to be induced by the electrical noise existing in the edge states capacitively coupled to each other. Electrical shot noise created in one channel leads to phase randomization in the other, which destroys the interference pattern. These findings are extended to the dephasing induced by thermal noise instead of shot noise: it explains the underlying mechanism responsible for the finite temperature coherence time tau_{phi}(T) of the edge states at filling factor 2, measured in a recent experiment. Finally, we present here a theory of the dephasing based on Gaussian noise, which is found to be in excellent agreement with our experimental results.     

Double-dot quantum ratchet driven by an independently biased quantum point contact

We study a double quantum dot (DQD) coupled to a strongly biased quantum point contact (QPC), each embedded in independent electric circuits. For weak interdot tunneling we observe a finite current flowing through the Coulomb blockaded DQD in response to a strong bias on the QPC. The direction of the current through the DQD is determined by the relative detuning of the energy levels of the two quantum dots. The results are interpreted in terms of a quantum ratchet phenomenon in a DQD energized by a nearby QPC.     

Dephasing effect on transport of a graphene p-n junction in a quantum Hall regime

The influence of the dephasing effect on the conductance distribution of disordered graphene p-n junctions is studied. Without the dephasing, the conductance distribution has a very wide range and the conductance fluctuation is large. In this case, the conductance plateaus cannot be obtained in a single sample with the fixed disorder configuration. However, by introducing the dephasing, we find that the distribution becomes narrow dramatically and the fluctuation is suppressed strongly, so that the conductance plateaus are obtained clearly for one single sample, which is consistent with experimental measurements. Furthermore, we also investigate the scaling feature of the conductance distribution and find that it has good scaling behavior in the strong dephasing case.     

Cavity-assisted quantum computing in a silicon nanostructure

We present a scheme of quantum computing with charge qubits corresponding to one excess electron shared between dangling-bond pairs of surface silicon atoms that couple to a microwave stripline resonator on a chip. By choosing a certain evolution time, we propose the realization of a set of universal single-and two-qubit logical gates. Due to its intrinsic stability and scalability, the silicon dangling-bond charge qubit can be regarded as one of the most promising candidates for quantum computation. Compared to the previous schemes on quantum computing with silicon bulk systems, our scheme shows such advantages as a long coherent time and direct control and readout.