In characterization of quantum systems, adapting measurement settings based on data while it is collected can generally outperform in efficiency conventional measurements that are carriedout independently of data. The existing methods for choosing measurement settings adaptively assume that the model, or the number of unknown parameters, is known. We introduce simultaneous adaptive model selection and parameter estimation. We apply our technique for characterization of a superconducting qubit and a bath of incoherent two-level systems, a leading decoherence mechanism in the state-of-the-art superconducting qubits.
Single flux quantum (SFQ) pulses are a natural candidate for on-chip control of superconducting qubits. We perform single qubit gates at a constant gate time using trains of singleflux quantum pulses with fixed amplitudes. The pulse sequence is optimized by applying genetic algorithms, which decreases the gate error by two orders of magnitude compared to an evenly spaced pulse train. Hereby, we consider leakage transitions into a third energy level as well. Timing jitter of the pulses is taken into account, exploring the robustness of our optimized sequence. This takes us one step further to on-chip qubit controls.
Readout of the state of a superconducting qubit by homodyne detection of the output signal from a dispersively coupled microwave resonator is a common technique in circuit quantum electrodynamics,and is often claimed to be quantum non-demolition (QND) up to the same order of approximation as in the dispersive approximation. However, in this work we show that only in the limit of infinite measurement time is this protocol QND, as the formation of a dressed coherent state in the qubit-cavity system applies an effective rotation to the qubit state. We show how this rotation can be corrected by a coherent operation, leading to improved qubit initialization by measurement and coherent feedback.
We investigate the transient dynamics of a lumped-element oscillator based on a dc superconducting quantum interference device (SQUID). The SQUID is shunted with a capacitor forminga nonlinear oscillator with resonance frequency in the range of several GHz. The resonance frequency is varied by tuning the Josephson inductance of the SQUID with on-chip flux lines. We report measurements of decaying oscillations in the time domain following a brief excitation with a microwave pulse. The nonlinearity of the SQUID oscillator is probed by observing the ringdown response for different excitation amplitudes while the SQUID potential is varied by adjusting the flux bias. Simulations are performed on a model circuit by numerically solving the corresponding Langevin equations incorporating the SQUID potential at the experimental temperature and using parameters obtained from separate measurements characterizing the SQUID oscillator. Simulations are in good agreement with the experimental observations of the ringdowns as a function of applied magnetic flux and pulse amplitude. We observe a crossover between the occurrence of ringdowns close to resonance and adiabatic following at larger detuning from the resonance. We also discuss the occurrence of phase jumps at large amplitude drive. Finally, we briefly outline prospects for a readout scheme for superconducting flux qubits based on the discrimination between ringdown signals for different levels of magnetic flux coupled to the SQUID.
Parity measurement is a central tool to many quantum information processing tasks. In this Letter, we propose a method to directly measure two- and four-qubit parity with low overheadin hard- and software, while remaining robust to experimental imperfections. Our scheme relies on dispersive qubit-cavity coupling and photon counting that is sensitive only to intensity; both ingredients are widely realized in many different quantum computing modalities. For a leading technology in quantum computing, superconducting integrated circuits, we analyze the measurement contrast and the back action of the scheme and show that this measurement comes close enough to an ideal parity measurement to be applicable to quantum error correction.
High-fidelity, efficient quantum nondemolition readout of quantum bits is integral to the goal of quantum computation. As superconducting circuits approach the requirements of scalable,universal fault tolerance, qubit readout must also meet the demand of simplicity to scale with growing system size. Here we propose a fast, high-fidelity, scalable measurement scheme based on the state-selective ring-up of a cavity followed by photodetection with the recently introduced Josephson photomultiplier (JPM), a current-biased Josephson junction. This scheme maps qubit state information to the binary digital output of the JPM, circumventing the need for room-temperature heterodyne detection and offering the possibility of a cryogenic interface to superconducting digital control circuitry. Numerics show that measurement contrast in excess of 95% is achievable in a measurement time of 140 ns. We discuss perspectives to scale this scheme to enable readout of multiple qubit channels with a single JPM.
Pulses to steer the time evolution of quantum systems can be designed with optimal control theory. In most cases it is the coherent processes that can be controlled and one optimizesthe time evolution towards a target unitary process, sometimes also in the presence of non-controllable incoherent processes. Here we show how to extend the GRAPE algorithm in the case where the incoherent processes are controllable and the target time evolution is a non-unitary quantum channel. We perform a gradient search on a fidelity measure based on Choi matrices. We illustrate our algorithm by optimizing a phase qubit measurement pulse. We show how this technique can lead to large measurement contrast close to 99%. We also show, within the validity of our model, that this algorithm can produce short 1.4 ns pulses with 98.2% contrast.
Quantum transmission lines are a central to superconducting and hybrid
quantum computing. Parallel to these developments are those of left-handed
meta-materials. They have a wide varietyof applications in photonics from the
microwave to the visible range such as invisibility cloaks and perfect flat
lenses. For classical guided microwaves, left-handed transmission lines have
been proposed and studied on the macroscopic scale. We combine these ideas in
presenting a left-handed/right-handed hybrid transmission line for applications
in quantum optics on a chip. The resulting system allows circuit QED to reach a
new regime: multi-mode ultra-strong coupling. Out of the many potential
applications of this novel device, we discuss two; the preparation of
multipartite entangled states and its use as a quantum simulator for the
spin-boson model where a quantum phase transition is reached up to finite
size-effects.
We describe the back action of microwave-photon detection via a Josephson
photomultiplier (JPM), a superconducting qubit coupled strongly to a
high-quality microwave cavity. The backaction operator depends qualitatively
on the duration of the measurement interval, resembling the regular photon
annihilation operator at short interaction times and approaching a variant of
the photon subtraction operator at long times. The optimal operating conditions
of the JPM differ from those considered optimal for processing and storing of
quantum information, in that a short $T_2$ of the JPM suppresses the cavity
dephasing incurred during measurement. Understanding this back action opens the
possibility to perform multiple JPM measurements on the same state, hence
performing efficient state tomography.