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
Okt
2018
Superconducting qubit circuit emulation of a vector spin-1/2
We propose a superconducting qubit circuit that can fully emulate a quantum vector spin-1/2, having three independent physical dipole moments whose operators obey the commutation relations
of a vector angular momentum in the computational subspace. Each dipole moment couples to an independently-controllable external bias, emulating the Zeeman effect due to a fictitious, three-component vector magnetic field, and all three of these moments remain relatively constant over a broad range of emulated total fields around zero. This capability, combined with established techniques for two-qubit coupling, would enable for the first time the direct hardware emulation of nearly arbitrary quantum spin-1/2 systems, including the canonical Heisenberg model. In addition to its fundamental interest in condensed-matter physics, this would also constitute a crucial step towards important quantum information processing and quantum annealing capabilities that have so far been inaccessible to any available hardware. These include, for example, strong non-stoquastic interactions for quantum annealing, quantum error suppression, Hamiltonian and holonomic quantum computing, and adiabatic quantum chemistry.
01
Okt
2018
Probing the Tavis-Cummings level splitting with intermediate-scale superconducting circuits
We demonstrate the local control of up to eight two-level systems interacting strongly with a microwave cavity. Following calibration, the frequency of each individual two-level system
(qubit) is tunable without influencing the others. Bringing the qubits one by one on resonance with the cavity, we observe the collective coupling strength of the qubit ensemble. The splitting scales up with the square root of the number of the qubits, being the hallmark of the Tavis-Cummings model. The local control circuitry causes a bypass shunting the resonator, and a Fano interference in the microwave readout, whose contribution can be calibrated away to recover the pure cavity spectrum. The simulator’s attainable size of dressed states is limited by reduced signal visibility, and -if uncalibrated- by off-resonance shifts of sub-components. Our work demonstrates control and readout of quantum coherent mesoscopic multi-qubit system of intermediate scale under conditions of noise.
Intrinsic dissipation in superconducting junctions probed by qubit spectroscopy
We propose to study frequency dependent intrinsic dissipation in a highly transparent Josephson junction by means of qubit spectroscopy. The spectral density of the effective dissipative
bath may contain significant contributions from Andreev bound states coupled to fluctuations of the Josephson phase. Varying either the bias current applied to the junction or magnetic flux through a superconducting ring in the rf-SQUID setup, one can tune the level splitting value close to the bottom of the Josephson potential well. Monitoring the qubit energy relaxation time one can probe the spectral density of the effective dissipative bath and unambiguously identify the contribution emerging from Andreev levels.
28
Sep
2018
Kinetic Inductance Traveling Wave Amplifiers For Multiplexed Qubit Readout
We describe a kinetic inductance traveling-wave (KIT) amplifier suitable for superconducting quantum information measurements and characterize its wideband scattering and noise properties.
We use mechanical microwave switches to calibrate the four amplifier scattering parameters up to the device input and output connectors at the dilution refrigerator base temperature and a tunable temperature load to characterize the amplifier noise. Finally, we demonstrate the high fidelity simultaneous dispersive readout of two superconducting transmon qubits. The KIT amplifier provides low-noise amplification of both readout tones with readout fidelities in excess of 89% and negligible effect on qubit lifetime and coherence.
27
Sep
2018
Granular aluminum: A superconducting material for high impedance quantum circuits
Superconducting quantum information processing machines are predominantly based on microwave circuits with relatively low characteristic impedance, of about 100 Ohm, and small anharmonicity,
which can limit their coherence and logic gate fidelity. A promising alternative are circuits based on so-called superinductors, with characteristic impedances exceeding the resistance quantum RQ=6.4 kΩ. However, previous implementations of superinductors, consisting of mesoscopic Josephson junction arrays, can introduce unintended nonlinearity or parasitic resonant modes in the qubit vicinity, degrading its coherence. Here we present a fluxonium qubit design using a granular aluminum (grAl) superinductor strip. Granular aluminum is a particularly attractive material, as it self-assembles into an effective junction array with a remarkably high kinetic inductance, and its fabrication can be in-situ integrated with standard aluminum circuit processing. The measured qubit coherence time TR2 up to 30 μs illustrates the potential of grAl for applications ranging from protected qubit designs to quantum limited amplifiers and detectors.
Superstrong coupling in circuit quantum electrodynamics
Vacuum fluctuations fundamentally affect an atom by inducing a fnite excited state lifetime along with a Lamb shift of its transition frequency. Here we report the reverse effect: modifcation
of vacuum by a single atom in circuit quantum electrodynamics. Our one-dimensional vacuum is a long section of a high wave impedance (comparable to resistance quantum) superconducting transmission line. It is directly wired to a transmon qubit circuit. Owing to the combination of high impedance and galvanic connection, the transmon’s spontaneous emission linewidth can greatly exceed the discrete transmission line modes spacing. This condition defines a previously unexplored superstrong coupling regime of quantum electrodynamics where many vacuum modes hybridize with each other through interactions with a single atom. We explore this regime by spectroscopically measuring the positions of over 100 consecutive transmission line resonances. The atom reveals itself as a broad peak in the vacuum’s density of states (DOS) together with the Kerr and cross-Kerr interaction of photons at frequencies within the DOS peak. Both dispersive effects are well described by a dissipative Caldeira-Leggett model of our circuit, with the transmon’s quartic anharmonicity treated as a perturbation. Non-perturbative modifications of such a vacuum, including inelastic scattering of single photons, are expected upon replacing the transmon by more anharmonic circuits, with broad implications for simulating critical dynamics of quantum impurity models.
24
Sep
2018
Design of an on-chip superconducting microwave circulator with octave bandwidth
We present a design for a superconducting, on-chip circulator composed of dynamically modulated transfer switches and delays. Design goals are set for the multiplexed readout of superconducting
qubits. Simulations of the device show that it allows for low-loss circulation (insertion loss < 0.35 dB and isolation >20 dB) over an instantaneous bandwidth of 2.3 GHz. As the device is estimated to be linear for input powers up to -65 dBm, this design improves on the bandwidth and power-handling of previous superconducting circulators by over a factor of 50, making it ideal for integration with broadband quantum limited amplifiers.
22
Sep
2018
Understanding the saturation power of Josephson Parametric Amplifiers made from SQUIDs arrays
We report on the implementation and detailed modelling of a Josephson Parametric Amplifier (JPA) made from an array of eighty Superconducting QUantum Interference Devices (SQUIDs),
forming a non-linear quarter-wave resonator. This device was fabricated using a very simple single step fabrication process. It shows a large bandwidth (45 MHz), an operating frequency tunable between 5.9 GHz and 6.8 GHz and a large input saturation power (-117 dBm) when biased to obtain 20 dB of gain. Despite the length of the SQUID array being comparable to the wavelength, we present a model based on an effective non-linear LC series resonator that quantitatively describes these figures of merit without fitting parameters. Our work illustrates the advantage of using array-based JPA since a single-SQUID device showing the same bandwidth and resonant frequency would display a saturation power 15 dB lower.
18
Sep
2018
Gated conditional displacement readout of superconducting qubits
We have realized a new interaction between superconducting qubits and a readout cavity that results in the displacement of a coherent state in the cavity, conditioned on the state of
the qubit. This conditional state, when it reaches the cavity-following, phase-sensitive amplifier, matches its measured observable, namely the in-phase quadrature. In a setup where several qubits are coupled to the same readout resonator, we show it is possible to measure the state of a target qubit with minimal dephasing of the other qubits. Our results suggest novel directions for faster readout of superconducting qubits and implementations of bosonic quantum error-correcting codes.
Charge-Qubit-Resonator-Interface-Based Nonlinear Circuit QED
We explore applications of nonlinear circuit QED with a charge qubit inductively coupled to a microwave LC resonator in the photonic engineering and ultrastrong-coupling multiphoton
quantum optics. Simply sweeping the gate-voltage bias achieves arbitrary Fock-state pulsed maser, where the single qubit plays the role of artificial gain medium. Resonantly pumping the parametric qubit-resonator interface leads to the squeezing of resonator field, which is utilizable to the quantum-limited microwave amplification. Moreover, upwards and downwards multiphoton quantum jumps may be observed in the steady state of the driving-free system.