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
28
Sep
2016
Flux-driven Josephson parametric amplifiers: Hysteretic flux response and nondegenerate gain measurements
Josephson parametric amplifiers (JPA) have become key devices in quantum science and technology with superconducting circuits. In particular, they can be utilized as quantum-limited
amplifiers or as a source of squeezed microwave fields. Here, we report on the detailed measurements of five flux-driven JPAs, three of them exhibiting a hysteretic dependence of the resonant frequency versus the applied magnetic flux. We model the measured characteristics by numerical simulations based on the two-dimensional potential landscape of the dc superconducting quantum interference devices (dc-SQUID), which provide the JPA nonlinearity, for a finite screening parameter βL>0 and demonstrate excellent agreement between the numerical results and the experimental data. Furthermore, we study the nondegenerate response of different JPAs and accurately describe the experimental results with our theory.
Generating Entangled Quantum Microwaves in a Josephson-Photonics Device
When connecting a voltage-biased Josephson junction in series to several microwave cavities, a Cooper-pair current across the junction gives rise to a continuous emission of strongly
correlated photons into the cavity modes. Tuning the bias voltage to the resonance where a single Cooper pair provides the energy to create an additional photon in each of the cavities, we demonstrate the entangling nature of these creation processes by simple witnesses in terms of experimentally accessible observables. To characterize the entanglement properties of the such created quantum states of light to the fullest possible extent, we then proceed to more elaborate entanglement criteria based on the knowledge of the full density matrix and provide a detailed study of bi- and multipartite entanglement. In particular, we illustrate how simple changes of experimental parameters allow to access a wide variety of entangled states differing, e.g., in the number of entangled parties or the dimension of state space. Such devices, besides their promising potential to act as a highly versatile source of entangled quantum microwaves, may thus represent an excellent natural testbed for classification and quantification schemes developed in quantum information theory.
Optimizing single microwave-photon detection: Input-Output theory
High fidelity microwave photon counting is an important tool for various areas from background radiation analysis in astronomy to the implementation of circuit QED architectures for
the realization of a scalable quantum information processor. In this work we describe a microwave photon counter coupled to a semi-infinite transmission line. We employ input-output theory to examine a continuously driven transmission line as well as traveling photon wave packets. Using analytic and numerical methods, we calculate the conditions on the system parameters necessary to optimize measurement and achieve high detection efficiency.
23
Sep
2016
Second-order decoherence mechanisms of a transmon qubit probed with thermal microwave states
Thermal microwave states are omnipresent noise sources in superconducting quantum circuits covering all relevant frequency regimes. We use them as a probe to identify three second-order
decoherence mechanisms of a superconducting transmon. First, we quantify the efficiency of a resonator filter in the dispersive Jaynes-Cummings regime and find evidence for parasitic loss channels. Second, we probe second-order noise in the low-frequency regime and demonstrate the expected T3 temperature dependence of the qubit dephasing rate. Finally, we show that qubit parameter fluctuations due to two-level states are enhanced under the influence of thermal microwave states. In particular, we experimentally confirm the T2-dependence of the fluctuation spectrum expected for noninteracting two-level states.
Photon Statistics of Propagating Thermal Microwaves
In experiments with superconducting quantum circuits, characterizing the photon statistics of propagating microwave fields is a fundamental task. We quantify the n2+n photon number
variance of thermal microwave photons emitted from a black-body radiator for mean photon numbers 0.05≲n≲1.5. We probe the fields using either correlation measurements or a transmon qubit coupled to a microwave resonator. Our experiments provide a precise quantitative characterization of weak microwave states and information on the noise emitted by a Josephson parametric amplifier.
22
Sep
2016
Quantum annealing with a network of all-to-all connected, two-photon driven Kerr nonlinear oscillators
Quantum annealing aims to solve combinatorial optimization problems mapped on to Ising interactions between quantum spins. A critical factor that limits the success of a quantum annealer
is its sensitivity to noise, and intensive research is consequently focussed towards developing noise-resilient annealers. Here we propose a new paradigm for quantum annealing with a scalable network of all-to-all connected, two-photon driven Kerr-nonlinear resonators. Each of these resonators encode an Ising spin in a robust degenerate subspace formed by two coherent states of opposite phases. The fully-connected optimization problem is mapped onto local fields driving the resonators, which are themselves connected by local four-body interactions. We describe an adiabatic annealing protocol in this system and analyze its performance in the presence of photon loss. Numerical simulations indicate substantial resilience to this noise channel, making it a promising platform for implementing a large scale quantum Ising machine. Finally, we propose a realistic implementation of this scheme in circuit QED.
Multiplexing Superconducting Qubit Circuit for Single Microwave Photon Generation
We report on a device that integrates eight superconducting transmon qubits in lambda/4 superconducting coplanar waveguide resonators fed from a common feedline. Using this multiplexing
architecture, each resonator and qubit can be addressed individually thus reducing the required hardware resources and allowing their individual characterisation by spectroscopic methods. The measured device parameters agree with the designed values and the resonators and qubits exhibit excellent coherence properties and strong coupling, with the qubit relaxation rate dominated by the Purcell effect when brought in resonance with the resonator. Our analysis shows that the circuit is suitable for generation of single microwave photons on demand with an efficiency exceeding 80%.
20
Sep
2016
Cat codes with optimal decoherence suppression for a lossy bosonic channel
We investigate cat codes that can correct multiple excitation losses and identify two types of logical errors: bit-flip errors due to excessive excitation loss and dephasing errors
due to quantum back-action from the environment. We show that selected choices of logical subspace and coherent amplitude can efficiently reduce dephasing errors. The trade-off between the two major errors enables optimized performance of cat codes in terms of minimized decoherence. With high coupling efficiency, we show that one-way quantum repeaters with cat codes feature drastically boosted secure communication rate per mode compared with conventional encoding schemes, and thus showcase the promising potential of quantum information processing with continuous variable quantum codes.
19
Sep
2016
Leggett-Garg test of superconducting qubit addressing the clumsiness loophole
The Leggett-Garg inequality holds for any macrorealistic system that is being measured noninvasively. A violation of the inequality can signal that a system does not conform to our
primal intuition about the physical world. Alternatively, a violation can simply indicate that „clumsy“ experimental technique led to invasive measurements. Here, we consider a recent Leggett-Garg test designed to try to rule out the mundane second possibility. We tailor this Leggett-Garg test to the IBM 5Q Quantum Experience system and find compelling evidence that qubit Q2 of the system cannot be described by noninvasive macrorealism.
15
Sep
2016
Direct identification of dilute surface spins on Al2O3: Origin of flux noise in quantum circuits
It is universally accepted that noise and decoherence affecting the performance of superconducting quantum circuits are consistent with the presence of spurious two-level systems (TLS).
In recent years bulk defects have been generally ruled out as the dominant source, and the search has focused on surfaces and interfaces. Despite a wide range of theoretical models and experimental efforts, the origin of these surface TLS still remains largely unknown, making further mitigation of TLS induced decoherence extremely challenging. Here we use a recently developed on-chip electron spin resonance (ESR) technique that allows us to detect spins with a very low surface coverage. We combine this technique with various surface treatments specifically to reveal the nature of native surface spins on Al2O3 — the mainstay of almost all solid state quantum devices. On a large number of samples we resolve three ESR peaks with the measured total paramagnetic spin density n=2.2×1017m−2, which matches the density inferred from the flux noise in SQUIDs. We show that two of these peaks originate from physisorbed atomic hydrogen which appears on the surface as a by-product of water dissociation. We suggest that the third peak is due to molecular oxygen on the Al2O3 surface captured at strong Lewis base defect sites, producing charged O−2. These results provide important information towards the origin of charge and flux noise in quantum circuits. Our findings open up a whole new approach to identification and controlled reduction of paramagnetic sources of noise in solid state quantum devices.