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
02
Mai
2014
Mitigating information leakage in a crowded spectrum of weakly anharmonic qubits
A challenge for scaling up quantum processors using frequency-crowded, weakly anharmonic qubits is to drive individual qubits without causing leakage into non-computational levels of
the others, while also minimizing the number of control lines. To address this, we implement single-qubit Wah-Wah control in a circuit QED processor with a single feedline for all transmon qubits, operating at the maximum gate speed achievable given the frequency crowding. Randomized benchmarking and quantum process tomography confirm alternating qubit control with ≤1 average error per computational step and decoherence-limited idling of one qubit while driving another with a Wah-Wah pulse train.
Fabrication and measurements of hybrid Nb/Al Josephson junctions and flux qubits with pi-shifters
We describe fabrication and testing of composite flux qubits combining Nb- and Al-based superconducting circuit technology. This hybrid approach to making qubits allows for employing
pi-phase shifters fabricated using well-established Nb-based technology of superconductor-ferromagnet-superconductor Josephson junctions. The important feature here is to obtain high interface transparency between Nb and Al layers without degrading sub-micron shadow mask. We achieve this by in-situ Ar etching using e-beam gun. Shadow-evaporated Al/AlOx/Al Josephson junctions with Nb bias pads show the expected current-voltage characteristics with reproducible critical currents. Using this technique, we fabricated composite Nb/Al flux qubits with Nb/CuNi/Nb pi-shifters and measured their magnetic field response. The observed offset between the field responses of the qubits with and without pi-junction is attributed to the pi phase shift. The reported approach can be used for implementing a variety of hybrid Nb/Al superconducting quantum circuits.
25
Apr
2014
Comment on „Distinguishing Classical and Quantum Models for the D-Wave Device“
The SSSV model is a simple classical model that achieves excellent correlation with published experimental data on the D-Wave machine’s behavior on random instances of its native
problem, thus raising questions about how „quantum“ the D-Wave machine is at large scales. In response, a recent preprint by Vinci et al. proposes a particular set of instances on which the D-Wave machine behaves differently from the SSSV model. In this short note, we explain how a simple modeling of systematic errors in the machine allows the SSSV model to reproduce the behavior reported in the experiments of Vinci et al.
24
Apr
2014
Steady-state phase diagram of a driven QED-cavity array with cross-Kerr nonlinearities
We study the properties of an array of QED-cavities coupled by nonlinear elements in the presence of photon leakage and driven by a coherent source. The main effect of the nonlinear
couplings is to provide an effective cross-Kerr interaction between nearest-neighbor cavities. Additionally correlated photon hopping between neighboring cavities arises. We provide a detailed mean-field analysis of the steady-state phase diagram as a function of the system parameters, the leakage and the external driving, and show the emergence of a number of different quantum phases. A photon crystal associated to a spatial modulation of the photon blockade appears. The steady state can also display oscillating behavior and bi-stability. In some regions the crystalline ordering may coexist with the oscillating behavior. Furthermore we study the effect of short-range quantum fluctuations by employing a cluster mean-field analysis. Focusing on the corrections to the photon crystal boundaries, we show that, apart for some quantitative differences, the cluster mean field supports the findings of the simple single-site analysis. In the last part of the paper we concentrate on the possibility to build up the class of arrays introduced here, by means of superconducting circuits of existing technology. We consider a realistic choice of the parameters for this specific implementation and discuss some properties of the steady-state phase diagram.
23
Apr
2014
Scalable quantum random-access memory with superconducting circuits
Quantum networks play an important role in the implementation of quantum computing, communication and metrology. Circuit quantum electrodynamics (QED), consisting of superconducting
artificial atoms coupled to on-chip resonators, provides a prime candidate to implement these networks due to their controllability and scalability. Furthermore, recent advances have also pushed the technology to the ultrastrong coupling (USC) regime of light-matter interaction, where the qubit-cavity coupling strength reaches a considerable fraction of the cavity frequency. Here, we propose the implementation of a scalable quantum random-access memory (QRAM) architecture based on a circuit QED network, whose edges operate in the USC regime. In particular, we study the storage and retrieval of quantum information in a parity-protected quantum memory and propose quantum interconnects in experimentally feasible schemes. Our proposal may pave the way for novel quantum memory applications ranging from entangled-state cryptography, teleportation, purification, fault-tolerant quantum computation, to quantum simulations.
19
Apr
2014
Wireless Josephson Amplifier
Josephson junction parametric amplifiers are playing a crucial role in the readout chain in superconducting quantum information experiments. However, their integration with current
3D cavity implementations poses the problem of transitioning between waveguide, coax cables and planar circuits. Moreover, Josephson amplifiers require auxiliary microwave components, like directional couplers and/or hybrids, that are sources of spurious losses and impedance mismatches that limit measurement efficiency and amplifier tunability. We have developed a new wireless architecture for these parametric amplifiers that eliminates superfluous microwave components and interconnects. This greatly simplifies their assembly and integration into experiments. We present an experimental realization of such a device operating in the 9−11 GHz band with about 100 MHz of amplitude gain-bandwidth product, on par with devices mounted in conventional sample holders. The simpler impedance environment presented to the amplifier also results in increased amplifier tunability.
17
Apr
2014
Quantum limited amplification and entanglement in coupled nonlinear resonators
We demonstrate a coupled cavity realization of a Bose Hubbard dimer to achieve quantum limited amplification and to generate frequency entangled microwave fields with squeezing parameters
well below -12 dB. In contrast to previous implementations of parametric amplifiers our dimer can be operated both as a degenerate and as a nondegenerate amplifier. The large measured gain-bandwidth product of more than 250 MHz for nondegenerate operation and the saturation at input photon numbers as high as 2000 per us are both expected to be improvable even further, while maintaining wide frequency tunability of about 2 GHz. Featuring flexible control over all relevant system parameters, the presented Bose-Hubbard dimer based on lumped element circuits has significant potential as an elementary cell in nonlinear cavity arrays for quantum simulation.
16
Apr
2014
Quantum Bayesian rule for weak measurements of qubits in superconducting circuit QED
There exist two scenarios of quantum weak measurement theories. One is the well-known quantum trajectory theory which, in terms of continuous differential equation, has been broadly
applied in quantum optics and quantum control problems. Another is the relatively newer quantum Bayesian approach, which has the advantage of being more efficient to infer the state of the measured quantum system merely based on certain integrated output of measurements. In this work, we aim to develop a quantum Bayesian rule for weak measurements of qubits in circuit quantum electrodynamics (QED). Starting with the optical quantum trajectory equation, our analysis pays particular attention to the nature of the cavity field under continuous quadrature monitoring. This allows our treatment unrestricted to the „bad-cavity“ and weak-response limits, thus making the obtained rule applicable to general setup parameters. With accuracy well proven numerically in this work, we expect this proposed approach to be useful for future circuit-QED measurement and control experiments.
11
Apr
2014
Fermion-fermion scattering with superconducting circuits
Quantum field theories (QFTs) are among the deepest descriptions of nature. In this sense, different computing approaches have been developed, as Feynman diagrams or lattice gauge theories.
In general, the numerical simulations of QFTs are computationally hard, with the processing time growing exponentially with the system size. Nevertheless, a quantum simulator could provide an efficient way to emulate these theories in polynomial time. Here, we propose the quantum simulation of fermionic field modes interacting via a continuum of bosonic modes with superconducting circuits, which are among the most advanced quantum technologies in terms of quantum control and scalability. An important feature of superconducting devices is that, unlike other quantum platforms, they offer naturally a strong coupling of qubits to a continuum of bosonic modes. Therefore, this system is a specially suited platform to realize quantum simulations of scattering processes involving interacting fermionic and bosonic quantum field theories, where access to the continuum of modes is required.
07
Apr
2014
Tunable microwave impedance matching to a high impedance source using a Josephson metamaterial
We report the efficient coupling of a 50Ω microwave circuit to a high impedance conductor. We use an impedance transformer consisting of a λ/4 co-planar resonator whose inner conductor
contains an array of superconducting quantum interference devices (SQUIDs), providing the resonator with a large and tunable lineic inductance ∼80μ0, resulting in a large characteristic impedance ZC∼1kΩ. The impedance matching efficiency is characterized by measuring the shot noise power emitted by a dc biased high resistance tunnel junction connected to the resonator. We demonstrate matching to impedances in the 15 to 35kΩ range with bandwidths above 100MHz around a resonant frequency tunable in the 4 to 6GHz range.