I am going to post here all newly submitted articles on the arXiv related to superconducting circuits. If your article has been accidentally forgotten, feel free to contact me
12
Aug
2015
Stimulated Raman adiabatic passage in a three-level superconducting circuit
The adiabatic manipulation of quantum states is a powerful technique that has opened up new directions in quantum engineering, enabling tests of fundamental concepts such as the Berry
phase and its nonabelian generalization, the observation of topological transitions, and holds the promise of alternative models of quantum computation. Here we benchmark the stimulated Raman adiabatic passage process for circuit quantum electrodynamics, by using the first three levels of a transmon qubit. We demonstrate a population transfer efficiency above 80% between the ground state and the second excited state using two adiabatic Gaussian-shaped control microwave pulses coupled to the first and second transition. The advantage of this techniques is robustness against errors in the timing of the control pulses. By doing quantum tomography at successive moments during the Raman pulses, we investigate the transfer of the population in time-domain. We also show that this protocol can be reversed by applying a third adiabatic pulse on the first transition. Furthermore, we demonstrate a hybrid adiabatic-nonadiabatic gate using a fast pulse followed by the adiabatic Raman sequence, and we study experimentally the case of a quasi-degenerate intermediate level.
08
Aug
2015
Coherent population transfer between weakly-coupled states in a ladder-type superconducting qutrit
Stimulated Raman adiabatic passage (STIRAP) offers significant advantages for coherent population transfer between un- or weakly-coupled states and has the potential of realizing efficient
quantum gate, qubit entanglement, and quantum information transfer. Here we report on the realization of STIRAP in a superconducting phase qutrit – a ladder-type system in which the ground state population is coherently transferred to the second-excited state via the dark state subspace. The result agrees well with the numerical simulation of the master equation, which further demonstrates that with the state-of-the-art superconducting qutrits the transfer efficiency readily exceeds 99% while keeping the population in the first-excited state below 1%. We show that population transfer via STIRAP is significantly more robust against variations of the experimental parameters compared to that via the conventional resonant π pulse method. Our work opens up a new venue for exploring STIRAP for quantum information processing using the superconducting artificial atoms.
07
Aug
2015
Broadband Filters for Abatement of Spontaneous Emission for Superconducting Qubits
The ability to perform fast, high-fidelity readout of quantum bits (qubits) is essential for the goal of building a quantum computer. However, the parameters of a superconducting qubit
device necessary to achieve this typically enhance qubit relaxation by spontaneous emission through the measurement channel. Here we design a broadband filter using impedance engineering to allow photons to leave the resonator at the cavity frequency but not at the qubit frequency. This broadband filter is implemented both in an on-chip and off-chip configuration.
06
Aug
2015
Digital feedback in superconducting quantum circuits
This chapter covers the development of feedback control of superconducting qubits using projective measurement and a discrete set of conditional actions, here referred to as digital
feedback. We begin with an overview of the applications of digital feedback in quantum computing. We then introduce an implementation of high-fidelity projective measurement of superconducting qubits. This development lays the ground for closed-loop control based on the binary measurement result. A first application of digital feedback control is fast and deterministic qubit reset, allowing the repeated initialization of a qubit more than an order of magnitude faster than its relaxation rate. A second application employs feedback in a multi-qubit setting to convert the generation of entanglement by parity measurement from probabilistic to deterministic, targeting an entangled state with the desired parity every time.
05
Aug
2015
State-Signal Correlations of a Continuously Monitored Superconducting Qubit
A superconducting transmon qubit undergoing driven unitary evolution is continuously monitored to observe the time evolution of its quantum state. If projective measurements are used
to herald a definite initial state, the average of many measurement records displays damped Rabi oscillations. If instead the average of many measurements is conditioned on the outcome of a final post-selection measurement, the result exhibits similar damped Rabi oscillations with the exception that the damping of the signal occurs backwards in time. Such pre- and post-selections are specific examples of qubit state and signal temporal correlations and stimulate a more general discussion of the temporal correlations in stochastic quantum trajectories associated with continuous quantum measurements.
03
Aug
2015
Coherent controlization using superconducting qubits
Coherent controlization, i.e., coherent conditioning of arbitrary single- or multi-qubit operations on the state of one or more control qubits, is an important ingredient for the flexible
implementation of many algorithms in quantum computation. This is of particular significance when certain subroutines are changing over time or when they are frequently modified, such as in decision-making algorithms for learning agents. We propose a scheme to realize coherent controlization for any number of superconducting qubits coupled to a microwave resonator. For two and three qubits, we present an explicit construction that is of high relevance for quantum learning agents. We demonstrate the feasibility of our proposal, taking into account loss, dephasing, and the cavity self-Kerr effect.
01
Aug
2015
One-step implementation of entanglement generation on microwave photons in distant 1D superconducting resonators
We present a scalable quantum-bus-based device for generating the entanglement on microwave photons (MPs) in distant superconducting resonators (SRs). Different from the processors
in previous works with some resonators coupled to a superconducting qubit (SQ), our device is composed of some 1D SRs rj which are coupled to the quantum bus (another common resonator R) in its different positions simply, assisted by superconducting quantum interferometer devices. By using the technique for catching and releasing a MP state in a 1D SR, it can work as an entanglement generator or a node in quantum communication. To demonstrate the performance of this device, we propose a one-step scheme to generate high-fidelity Bell states on MPs in two distant SRs. It works in the dispersive regime of rj and R, which enables us to extend it to generate high-fidelity multi-Bell states on different resonator pairs simultaneously.
29
Jul
2015
Coupling a single Nitrogen-Vacancy center to a superconducting flux qubit in the far off resonance regime
We present a theoretical proposal to couple a single Nitrogen-Vacancy (NV) center to a superconducting flux qubit (FQ) in the regime where both systems are off resonance. The coupling
between both quantum devices is achieved through the strong driving of the flux qubit by a classical microwave field that creates dressed states with an experimentally controlled characteristic frequency. We discuss several applications such as controlling the NV center’s state by manipulation of the flux qubit, performing the NV center full tomography and using the NV center as a quantum memory. The effect of decoherence and its consequences to the proposed applications are also analyzed. Our results provide a theoretical framework describing a promising hybrid system for quantum information processing, which combines the advantages of fast manipulation and long coherence times.
28
Jul
2015
Exact quantum Bayesian rule for qubit measurements in circuit QED
Developing efficient and reliable schemes for practical quantum measurements is of essential importance to quantum information science and quantum metrology. In this work, for the increasingly
important superconducting circuit-QED setup, we present a rigorous approach starting with the quantum trajectory equation (QTE) to establish an {\it exact} quantum Bayesian rule. For the „realistic“ back-action (no qubit state information gain), we obtain important correction factors for arbitrary setup parameters. For the „spooky“ information gain back-action, we establish new prior distribution knowledge for the Bayesian inference, which differ from the standard Gaussian distribution and ensure to give strictly the same results as that by numerically integrating the QTE. Compared to the QTE approach, while keeping the same accuracy, the obtained quantum Bayesian rule has much higher efficiency to compute the stochastic change of the measured state. The generic method of this work opens also a new way to construct exact quantum Bayesian rules for quantum measurement in other systems.
Semiconductor-inspired superconducting quantum computing
Superconducting circuits offer tremendous design flexibility in the quantum regime culminating most recently in the demonstration of few qubit systems supposedly approaching the threshold
for fault-tolerant quantum information processing. Competition in the solid-state comes from semiconductor qubits, where nature has bestowed some almost magical and very useful properties which can be utilized for spin qubit based quantum computing. Here we begin to explore how selective design principles deduced from spin-based systems could be used to advance superconducting qubit science. We take an initial step along this path proposing an encoded qubit approach realizable with state-of-the-art tunable Josephson junction qubits. Our results show that this design philosophy holds promise, enables microwave-free control with minimal overhead (zero overhead in 2-qubit gates), and offers a pathway to future qubit designs with new capabilities such as with higher fidelity or, perhaps, operation at higher temperature. The approach is especially suited to qubits based on variable super-semi junctions.