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
17
Jun
2020
Simulating and mitigating crosstalk
We describe an efficient and scalable framework for modeling crosstalk effects on quantum information processors. By applying optimal control techniques, we show how to tuneup arbitrary
high-fidelity parallel operations on systems with substantial local and nonlocal crosstalk. Simulations show drastically lower error rates for a 2D square array of 100 superconducting transmon qubits. These results suggest that rather than striving to engineer away undesirable interactions during fabrication, we can largely mitigate their effects through careful characterization and control optimization.
A superconducting circuit realization of combinatorial gauge symmetry
We propose a superconducting wire array that realizes a family of quantum Hamiltonians that possess combinatorial gauge symmetry — a local symmetry where monomial transformations
play a central role. This physical system exhibits a rich structure. In the classical limit its ground state consists of two superimposed spin liquids; one is a crystal of small loops containing disordered U(1) degrees of freedom, and the other is a soup of loops of all sizes associated to Z2 topological order. We show that the classical results carry over to the quantum case when fluctuations are gradually tuned via the wire capacitances, yielding Z2 quantum topological order. In an extreme quantum limit where the capacitances are all small, we arrive at an effective quantum spin Hamiltonian that we conjecture would sustain Z2 quantum topological order with a gap of the order of the Josephson coupling in the array. The principles behind the construction for superconducting arrays extends to other bosonic and fermionic systems, and offers a promising path towards topological qubits and the study of other many-body systems.
15
Jun
2020
Voltage-tunable superconducting resonators: a platform for random access quantum memory
In quantum computing architectures, one important factor is the trade-off between the need to couple qubits to each other and to an external drive and the need to isolate them well
enough in order to protect the information for an extended period of time. In the case of superconducting circuits, one approach is to utilize fixed frequency qubits coupled to coplanar waveguide resonators such that the system can be kept in a configuration that is relatively insensitive to noise. Here, we propose a scalable voltage-tunable quantum memory (QuMem) design concept compatible with superconducting qubit platforms. Our design builds on the recent progress in fabrication of Josephson field effect transistors (JJ-FETs) which use InAs quantum wells. The JJ-FET is incorporated into a tunable coupler between a transmission line and a high-quality resonator in order to control the overall inductance of the coupler. A full isolation of the high-quality resonator can be achieved by turning off the JJ-FET. This could allow for long coherence times and protection of the quantum information inside the storage cavity. The proposed design would facilitate the implementation of random access memory for storage of quantum information in between computational gate operations.
09
Jun
2020
Superconducting granular aluminum resonators resilient to magnetic fields up to 1 Tesla
High kinetic inductance materials constitute a valuable resource for superconducting quantum circuits and hybrid architectures. Superconducting granular aluminum (grAl) reaches kinetic
sheet inductances in the nH/□ range, with proven applicability in superconducting quantum bits and microwave detectors. Here we show that the single photon internal quality factor Qi of grAl microwave resonators exceeds 105 in magnetic fields up to 1T, aligned in-plane to the grAl films. Small perpendicular magnetic fields, in the range of 0.5mT, enhance Qi by approximately 15%, possibly due to the introduction of quasiparticle traps in the form of fluxons. Further increasing the perpendicular field deteriorates the resonators‘ quality factor. These results open the door for the use of high kinetic inductance grAl structures in circuit quantum electrodynamics and hybrid architectures with magnetic field requirements.
08
Jun
2020
Materials loss measurements using superconducting microwave resonators
The performance of superconducting circuits for quantum computing is limited by materials losses. In particular, coherence times are typically bounded by two-level system (TLS) losses
at single photon powers and millikelvin temperatures. The identification of low loss fabrication techniques, materials, and thin film dielectrics is critical to achieving scalable architectures for superconducting quantum computing. Superconducting microwave resonators provide a convenient qubit proxy for assessing performance and studying TLS loss and other mechanisms relevant to superconducting circuits such as non-equilibrium quasiparticles and magnetic flux vortices. In this review article, we provide an overview of considerations for designing accurate resonator experiments to characterize loss, including applicable types of loss, cryogenic setup, device design, and methods for extracting material and interface losses, summarizing techniques that have been evolving for over two decades. Results from measurements of a wide variety of materials and processes are also summarized. Lastly, we present recommendations for the reporting of loss data from superconducting microwave resonators to facilitate materials comparisons across the field.
07
Jun
2020
Engineering Framework for Optimizing Superconducting Qubit Designs
Superconducting quantum technologies require qubit systems whose properties meet several often conflicting requirements, such as long coherence times and high anharmonicity. Here, we
provide an engineering framework based on a generalized superconducting qubit model in the flux regime, which abstracts multiple circuit design parameters and thereby supports design optimization across multiple qubit properties. We experimentally investigate a special parameter regime which has both high anharmonicity (∼1GHz) and long quantum coherence times (T1=40−80μs and T2Echo=2T1).
05
Jun
2020
Waveguide Bandgap Engineering with an Array of Superconducting Qubits
In this work, we experimentally study a metamaterial made of eight superconducting transmon qubits with local frequency control coupled to the mode continuum of a superconducting waveguide.
By consecutively tuning the qubits to a common resonance frequency we observe the formation of super- and subradiant states as well as the emergence of a polaritonic bandgap. Making use of the qubits strong intrinsic quantum nonlinearity we study the saturation of the collective modes with increasing photon number and electromagnetically induce a transparency window in the bandgap region of the ensemble, allowing to directly control the band structure of the array. The moderately scaled circuit of this work extends experiments with one and two qubits towards a full-blown quantum metamaterial, thus paving the way for large-scale applications in superconducting waveguide quantum electrodynamics.
02
Jun
2020
On-chip single-pump interferometric Josephson isolator for quantum measurements
Nonreciprocal microwave devices, such as circulators and isolators, are critical in high-fidelity qubit readout schemes. They unidirectionally route the readout signals and protect
the qubits against noise coming from the output chain. However, cryogenic circulators and isolators are prohibitive in scalable superconducting architectures because they rely on magneto-optical effects. Here, we realize an on-chip, single-microwave-pump Josephson ISolator (JIS), formed by coupling two nondegenerate Josephson mixers in an interferometric scheme. We unravel the interplay between the orientation parity of the magnetic fluxes, biasing the mixers, and the JIS directionality. Furthermore, we build a motherboard, which integrates the JIS and other superconducting components, including a Josephson directional amplifier, into a printed circuit and use it to read out a qubit with 92% fidelity, while maintaining 75% of its T2E. Improved versions of this motherboard could replace magnetic circulators and isolators in large superconducting quantum processors.
27
Mai
2020
High-fidelity software-defined quantum logic on a superconducting qudit
Nearly all modern solid-state quantum processors approach quantum computation with a set of discrete qubit operations (gates) that can achieve universal quantum control with only a
handful of primitive gates. In principle, this approach is highly flexible, allowing full control over the qubits‘ Hilbert space without necessitating the development of specific control protocols for each application. However, current error rates on quantum hardware place harsh limits on the number of primitive gates that can be concatenated together (with compounding error rates) and remain viable. Here, we report our efforts at implementing a software-defined 0↔2 SWAP gate that does not rely on a primitive gate set and achieves an average gate fidelity of 99.4%. Our work represents an alternative, fully generalizable route towards achieving nontrivial quantum control through the use of optimal control techniques. We describe our procedure for computing optimal control solutions, calibrating the quantum and classical hardware chain, and characterizing the fidelity of the optimal control gate.
21
Mai
2020
Overlap junctions for superconducting quantum electronics and amplifiers
Due to their unique properties as lossless, nonlinear circuit elements, Josephson junctions lie at the heart of superconducting quantum information processing. Previously, we demonstrated
a two-layer, submicrometer-scale overlap junction fabrication process suitable for qubits with long coherence times. Here, we extend the overlap junction fabrication process to micrometer-scale junctions. This allows us to fabricate other superconducting quantum devices. For example, we demonstrate an overlap-junction-based Josephson parametric amplifier that uses only 2 layers. This efficient fabrication process yields frequency-tunable devices with negligible insertion loss and a gain of ~ 30 dB. Compared to other processes, the overlap junction allows for fabrication with minimal infrastructure, high yield, and state-of-the-art device performance.