We report on radio-frequency measurements of the charge-phase qubit being under continuous microwave irradiation in the state of weak coupling to a radio-frequency tank circuit. We studied the rf impedance dependence on the two important parameters such as power of microwave irradiation whose frequency is close to the gap between the two lowest qubit energy levels, and temperature of the internal heat bath. We have found that backaction effects of the qubit on the rf tank, and vice versa, tank on the qubit, lead to a negative as well as a positive real part of the qubit impedance Re$Z(ω)$ seen by the tank. We have implemented noise spectroscopy measurements for direct impedance readout at the extreme points corresponding to maximum voltage response and obtained absolute values of about 0.017 $Ω$ for the negative and positive Re$Z(ω)$. Our results demonstrate the existence and persistence of the coherent single- and multi-photon Rabi dynamics of the qubit with both negative and positive dynamic resistance inserted into the tank in the temperature range of 10 to 200 mK.
A junction with two superconductors coupled by a normal metal hosts Andreev bound states whose energy spectrum is phase-dependent and exhibits a minigap, resulting in a periodic supercurrent. Phase-dependent dissipation also appears at finite frequency due to relaxation of Andreev bound states. While dissipation and supercurrent versus phase have previously been measured near thermal equilibrium, their behavior in nonequilibrium is still elusive. By measuring the ac susceptibility of a graphene-superconductor junction under microwave irradiation, we find supercurrent response deviates from adi…
Irradiating a Josephson junction with microwaves can operate not only on the amplitude but also on the phase of the Josephson current. This requires breaking time inversion symmetry, which is achieved by introducing a phase lapse between the microwave components acting on the two† sides of the junction. General symmetry arguments and the solution of a specific single level quantum dot model show that this induces chirality in the Cooper pair dynamics, due to the topology of the Andreev bound state wavefunction. Another essential condition is to break electron-hole symmetry within the junction.…
Almost constant voltage plateaus on the V(I) curves of long quasi-one-dimensional superconducting aluminum wires placed in magnetic field at temperatures T slightly below the critical superconducting temperature Tc were found which were unexpected for the sample geometry and parameters of the experiment. The plateaus are assumed to be subharmonics of a superconducting gap and arise due to the multiple Andreev reflection and strong quasiparticle overheating in the wire nonequilibrium region. The plateaus are evidence of coexistence of superconductivity and dissipation in such wires. The results…
We address the problem of non-equilibrium superconductivity in the presence of microwave irradiation. We refine the old Eliashberg theory and generalize it to arbitrary temperatures $T$ and frequencies $ω$. Microwave radiation is shown to stimulate superconductivity in a bounded region in the $(ω,T)$ plane. In particular, for $T<0.47\, T_c$ and for $\hbarω>3.3\, k_BT_c$ superconductivity is always suppressed by a weak \emph{ac} driving. We also study the supercurrent in the presence of microwave irradiation and establish the criterion for the critical current enhancement. Our results can be qu…
Coherent control of a flux qubit by phase-shifted resonant microwave pulses
利用谐振微波成功地脉冲控制了磁通量子比特的量子态。我们在不引入失谐的情况下，通过施加一对相移π/2微波脉冲，观察到了Ramsey条纹。利用这种方法，量子比特态可以在旋转坐标系中在布洛赫球的x-y平面内的任意轴上旋转。我们从相干振荡中获得了量子比特信号，其角速度高达2 pi *11.4 Grad/s。结合拉比脉冲，这种方法使我们能够实现对单量子比特操作的完全控制。它还提供了数量级增加的可能性，在任意酉门操作的速度。
