Temperature dependence of coherent oscillations in Josephson phase qubits

We experimentally investigate the temperature dependence of Rabi oscillations and Ramsey fringes in superconducting phase qubits. In a wide range of temperatures, we find that both the decay time and the amplitude of these coherent oscillations remain nearly unaffected by thermal fluctuations. In the two-level limit, coherent qubit response rapidly vanishes as soon as the energy of thermal fluctuations k(B)T becomes larger than the energy level spacing variant Planck's over h omega of the qubit. In contrast, a sample of much shorter coherence times displayed semiclassical oscillations very similar to Rabi oscillation, but showing a qualitatively different temperature dependence. Our observations shed new light on the origin of decoherence in superconducting qubits. The experimental data suggest that, without degrading already achieved coherence times, phase qubits can be operated at temperatures much higher than those reported till now.     

费米超流气体的非线性Landau-Zener 隧穿

在平均场近似下,通过对相平面和不动点的分析, 研究了非线性两能级系统中费米超流气体的Landau-Zener 隧穿现象. 研究发现,费米子间的相互作用能够显著地影响量子隧穿. 当相互作用参数c小于临界值c^*时,在绝热极限下隧穿仍然满足量子绝热定理, 而大于这一临界值时,量子绝热定理不再满足. 最后通过和线性情况比较,得到了c*时隧穿率与扫描速率间满足的指数关系.     

Rabi oscillations in a partially coherent field

Both, phase and amplitude fluctuations of a laser field are taken into account. A gaussian stochastic process describing the latter one is not assumed to be delta correlated. Apart from the modification of the damping constant, known before, a significant modification of the Rabi frequency is found.     

Spin-orbit-driven coherent oscillations in a few-electron quantum dot

We propose an experiment to observe coherent oscillations in a single quantum dot with the oscillations driven by spin-orbit interaction. This is achieved without spin-polarized leads, and relies on changing the strength of the spin-orbit coupling via an applied gate pulse. We derive an effective model of this system which is formally equivalent to the Jaynes-Cummings model of quantum optics. For parameters relevant to an InGaAs dot, we calculate a Rabi frequency of 2 GHz.     

Damping of coherent oscillations in a quantum dot photodiode

In this letter, we develop a model to describe the Rabi oscillations observed in a quantum-dot photodiode. Using a multi-level density matrix formulation, which includes multi-exciton and single particle states, we show that the damping observed in recent experiments is the result of a non-resonant excitation from or to the continuum of the wetting layer states.     

Observation of coherent oscillations in a single electron spin

Rabi nutations and Hahn echo modulation of a single electron spin in a single defect center have been observed. The coherent evolution of the spin quantum state is followed via optical detection of the spin state. Coherence times up to several microseconds at room temperature have been measured. Optical excitation of the spin states leads to decoherence. Quantum beats between electron spin transitions in a single spin Hahn echo experiment are observed. A closer analysis reveals that beats also result from the hyperfine coupling of the electron spin to a single 14N nuclear spin. The results are analyzed in terms of a density matrix approach of an electron spin interacting with two oscillating fields.     

Many-body aspects of coherent atom-molecule oscillations

We study the many-body effects on coherent atom-molecule oscillations by means of an effective quantum field theory that describes Feshbach-resonant interactions in Bose gases in terms of an atom-molecule Hamiltonian. We determine numerically the many-body corrections to the oscillation frequency for various densities of the atomic condensate. We also derive an analytic expression that approximately describes both the density and magnetic-field dependence of this frequency near the resonance. We find excellent agreement with experiment.     

Landau-Zener transition of a two-level system driven by spin chains near their critical points

The Landau-Zener (LZ) transition of a two-level system coupling to spin chains near their critical points is studied in this paper. Two kinds of spin chains, the Ising spin chain and XY spin chain, are considered. We calculate and analyze the effects of system-chain coupling on the LZ transition. A relation between the LZ transition and the critical points of the spin chain is established. These results suggest that LZ transitions may serve as the witnesses of criticality of the spin chain. This may provide a new way to study quantum phase transitions as well as LZ transitions.     

Combined effects of particle interaction and nonlinear sweep on Landau-Zener transition

We investigate the Landau-Zener transition (LZT) of a two-level system with two types of nonlinearity: particle interaction and nonlinear time-dependent energy sweep, both numerically and analytically. Due to the interplay between these two types of nonlinearity, many interesting phenomena in different parameter regions occur, including breakdown of adiabaticity, fade-out of interference phenomenon, and asymmetry of transition probability. The potential experimental observation of transition dynamics in this system is suggested.     

Impurity-induced coherent current oscillations in one-dimensional conductors

The electronic transport through a single impurity in a repulsive Luttinger liquid (LL) has been theoretically studied. It has been found that the direct current Ī above the threshold voltage related to the strength of the impurity potential is accompanied by coherent oscillations with frequency f= Ī/e. There is an analogy with the Josephson junctions: the well-known regime of the power-law I−V curves in the LL corresponds to the damping of the Josephson current below the critical one, while the oscillatory regime in the LL can be compared with the Josephson oscillations above the critical current. The text was submitted by the authors in English.     

Evidence for a percolation-driven transition to coherent surface superconductivity

Above the upper critical field we have investigated the field dependences of the surface conductance, G'-iG" and the critical current J(c) of an electropolished pure niobium cylinder. The low frequency limits of G', G", and J(c) display power-law singularities, defining a transition to coherent surface superconductivity at H(c)(c3). The critical exponents as well as the dynamical scaling of G'-iG" are consistent with predictions for a two-dimensional percolation transition. Relating H(c)(c3) to the conventional onset field, we find H(c)(c3)/H(c3)=0.81, and, surprisingly, this ratio turns out to be independent of significant variations of H(c3) due to differently structured surfaces.     

Period dependence of coherent oscillations in manganite superlattices

In three-component superlattices of [(NdMnO3)_n/(SrMnO3)_n/(LaMnO3)_n]_m consisting of nonferroelectric and antiferromagnetic layers, the generation of coherent phonons in the system is investigated by the relaxation process of the superlattice using a pump–probe technique. The coherent phonons are related to MnO₆ octahedra in the layers. While the frequency of the phonons is 49 GHz without regard to the period of each layer, the amplitude of the phonons is influenced by the period of the superlattices. The superlattices have the maximum amplitude of the acoustic phonons at a period of (5, 12) together with strong emergent interfacial effects such as ferromagnetism and ferroelectricity, which are not present in bulk materials. This implies a possible correlation of the lattice structure with interface-induced properties.     

两耦合玻色-爱因斯坦凝聚体间的量子相干振荡

理论上考察了两耦合玻色-爱因斯坦凝聚体间的相干原子振荡,我们用时空不能完全分离的波函数去描述囚禁在双磁阱中的玻色-爱因斯坦凝聚体,根据托马斯-费米近似,得到两凝聚体的相位差和布局数随时间的演化方程,应用数值计算的方法,考察了相干原子遂穿和宏观量子自囚禁效应.这些研究结果和采用双模时空分离波函数近似法得到的结果进行了比较.