Synchronizing Two Superconducting Qubits through a Dissipating Resonator

A system consisting of two qubits and a resonator is considered in the presence of different sources of noise, bringing to light the possibility of making the two qubits evolve in a synchronized way. A direct qubit–qubit interaction turns out to be a crucial ingredient, as well as the dissipation processes involving the resonator. The detrimental role of the local dephasing of the qubits is also taken into account.     

Selective measurements of superconducting qubit states by a nonlinear Josephson oscillator

Selective measurements of the states are studied for a single quantum system, viz, a Josephson qubit, by a nonlinear oscillator operating in the mesoscopic regime in which the number of quanta during measurements is varied from a few dozen to several hundreds. The quantum Monte Carlo method is used to simulate the dissipative dynamics of the qubit–oscillator system and the process of measurements of the qubit states from the change in the number of oscillator quanta. It is shown that for π-pulses in the recording of qubit states, discrimination of states is possible in single realizations (measurements), while statistical projective measurements can be made for the prepared superposition state.     

Generating Squeezed States of Nanomechanical Resonator via a Flux Qubit in a Hybrid System

We propose a scheme for generating squeezed states based on a superconducting hybrid system.Our system consists of a nanomechanical resonator,a superconducting flux qubit,and a superconducting transmission line resonator.Using our proposal,one can easily generate the squeezed states of the nanomechanical resonator.In our scheme,the nonlinear interaction between the nanomechanical resonator and the superconducting transmission line resonator can be implemented by the flux qubit as 'nonlinear media' with a tunable Josephson energy.The realization of the nonlinearity does not need any operations on the flux qubit and just needs to adiabatically keep it at the ground state,which can greatly decrease the effect of the decoherence of the flux qubit on the squeezed efficiency.     

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.     

Approaching unit visibility for control of a superconducting qubit with dispersive readout

In a Rabi oscillation experiment with a superconducting qubit we show that a visibility in the qubit excited state population of more than 95% can be attained. We perform a dispersive measurement of the qubit state by coupling the qubit non-resonantly to a transmission line resonator and probing the resonator transmission spectrum. The measurement process is well characterized and quantitatively understood. In a measurement of Ramsey fringes, the qubit coherence time is larger than 500 ns.     

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.     

Driven dynamics and rotary echo of a qubit tunably coupled to a harmonic oscillator

We have investigated the driven dynamics of a superconducting flux qubit that is tunably coupled to a microwave resonator. We find that the qubit experiences an oscillating field mediated by off-resonant driving of the resonator, leading to strong modifications of the qubit Rabi frequency. This opens an additional noise channel, and we find that low-frequency noise in the coupling parameter causes a reduction of the coherence time during driven evolution. The noise can be mitigated with the rotary-echo pulse sequence, which, for driven systems, is analogous to the Hahn-echo sequence.     

Observation of a collective mode of an array of transmon qubits

Arrays of transmon qubits coupled to a λ/2 superconducting coplanar waveguide resonator have been studied by microwave spectroscopy. The emergence of a collective mode has been discovered for a cluster of N > 5 qubits, whose coupling constant to the electromagnetic field in the resonator is √N times greater compared to a single qubit. In addition, the emergence of collective multiphoton transitions exciting higher levels of a qubit cluster has been demonstrated and the interaction of an individual qubit with such a cluster has been investigated.     

Vacuum Rabi oscillations in a macroscopic superconducting qubit oscillator system

We have observed the coherent exchange of a single energy quantum between a flux qubit and a superconducting LC circuit acting as a quantum harmonic oscillator. The exchange of an energy quantum is known as the vacuum Rabi oscillation: the qubit is oscillating between the excited state and the ground state and the oscillator between the vacuum state and the first excited state. We also show that we can detect the state of the oscillator with the qubit and thereby obtained evidence of level quantization of the LC circuit. Our results support the idea of using oscillators as couplers of solid-state qubits.     

Controllable strong coupling between individual spin qubits and a transmission line resonator via nanomechanical resonators

We investigate a hybrid quantum system where an individual electronic spin qubit (EQ) and a transmission line resonator (TLR) are connected by a nanomechanical resonator (NAMR). We analyze the possibility of realizing a strong coupling between the EQ and the TLR. Compared with a direct coupling between an EQ and a TLR, the achieved coupling can be stronger and controllable. The proposal might be used to implement a high-fidelity quantum state transfer between the spin qubit and the TLR, and is scalable to involve several individual EQ-NAMR coupled systems with a TLR.     

High-contrast dispersive readout of a superconducting flux qubit using a nonlinear resonator

We demonstrate high-contrast state detection of a superconducting flux qubit. Detection is realized by probing the microwave transmission of a nonlinear resonator, based on a SQUID. Depending on the driving strength of the resonator, the detector can be operated in the monostable or the bistable mode. The bistable operation combines high-sensitivity with intrinsic latching. The measured contrast of Rabi oscillations is as high as 87%; of the missing 13%, only 3% of the loss of contrast is unaccounted for. Experiments involving two consecutive detection pulses are consistent with preparation of the qubit state by the first measurement.     

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.     

Hybrid quantum circuit with a superconducting qubit coupled to a spin ensemble

We report the experimental realization of a hybrid quantum circuit combining a superconducting qubit and an ensemble of electronic spins. The qubit, of the transmon type, is coherently coupled to the spin ensemble consisting of nitrogen-vacancy centers in a diamond crystal via a frequency-tunable superconducting resonator acting as a quantum bus. Using this circuit, we prepare a superposition of the qubit states that we store into collective excitations of the spin ensemble and retrieve back into the qubit later on. These results constitute a proof of concept of spin-ensemble based quantum memory for superconducting qubits.     

Measurement of Spin Singlet-Triplet Qubit in Quantum Dots Using Superconducting Resonator

The spin qubit in quantum dots is one of the leading platforms for quantum computation.A crucial requirement for scalable quantum information processing is the high efficient measurement.Here we analyze the measurement process of a quantum-dot spin qubit coupled to a superconducting transmission line resonator.Especially,the phase shift of the resonator is sensitive to the spin states and the gate operations.The response of the resonator can be used to measure the spin qubit efficiently,which can be extend to read out the multiple spin qubits in a scalable solid-state quantum processor.     

Modified SQUID Operator Equation for a Single-Qubit Structure Coupled to a Quantum Resonator

Role of self-inductance in superconducting quantum interference device （SQUID） charge qubit is considered. It is found that when an SQUID charge qubit is coupled to a quantum LC resonator, the SQUID voltage operator equation is modified in accompanying with the modification of operator Faraday equation describing the inductance. It is shown that when the extra energy is applied to the junction, the mean phase will be squeezed according to a damping factor.     

Quantum nondemolition measurements of a flux qubit coupled to a noisy detector

We theoretically study the quantum nondemolition measurements of a flux qubit coupled to a noisy superconduct-ing quantum interference device (SQUID).The obtained analytical results indicate that the measurement probability is frequency-dependent in a short time scale and has a close relationship with the measurement-induced dephasing.Furthermore,when the detuning between the driven and bare resonator equals the coupling strength,we can obtain the maximum measurement rate that is determined by the character of the noise in the SQUID.Finally,we analysed the mixed effect caused by coupling between the non-diagonal term and the external variable.It is found that the initial information of the qubit is destroyed due to quantum tunneling between the qubit states.     

Single-qubit lasing and cooling at the Rabi frequency

For a superconducting qubit driven to perform Rabi oscillations and coupled to a slow electromagnetic or nanomechanical oscillator we describe previously unexplored quantum optics effects. When the Rabi frequency is tuned to resonance with the oscillator, the latter can be driven far from equilibrium. Blue detuned driving leads to a population inversion in the qubit and a bistability with lasing behavior of the oscillator; for red detuning the qubit cools the oscillator. This behavior persists at the symmetry point where the qubit-oscillator coupling is quadratic and decoherence effects are minimized. There the system realizes a "single-atom-two-photon laser."     

Determination of the composition and content of pulsed cluster beams from time-of-flight mass spectra of cluster fragments

The properties of a superconducting flux quantum bit (qubit) in the quasidispersive mode, where the frequency of a probe signal is lower than the qubit excitation frequency but is close to it, have been experimentally studied. It has been shown that all parameters of the qubit inductively coupled to a coplanar resonator can be determined at the single-frequency excitation from the analysis of the frequency responses of the transmission of the probe signal at the output of this resonator. Under the additional excitation of the qubit by the signal at the second harmonic of the cavity, resonance dips have been observed because of resonance between the probe signal and induced Rabi splitting. It has been shown that the positions of these dips are in good agreement with the parameters of the qubit that are obtained by analyzing the amplitude–frequency response within the width of the fundamental resonance frequency.     

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.