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
22
Mä
2021
Random-access quantum memory using chirped pulse phase encoding
and quantum information"]processors [arXiv:1109.3743]. As in conventional computing, key attributes of such memories are high storage density and, crucially, random access, or the ability to read from or write to an arbitrarily chosen register. However, achieving such random access with quantum memories [arXiv:1904.09643] in a dense, hardware-efficient manner remains a challenge, for example requiring dedicated cavities per qubit [arXiv:1109.3743] or pulsed field gradients [arXiv:0908.0101]. Here we introduce a protocol using chirped pulses to encode qubits within an ensemble of quantum two-level systems, offering both random access and naturally supporting dynamical decoupling to enhance the memory lifetime. We demonstrate the protocol in the microwave regime using donor spins in silicon coupled to a superconducting cavity, storing up to four multi-photon microwave pulses and retrieving them on-demand up to 2~ms later. A further advantage is the natural suppression of superradiant echo emission, which we show is critical when approaching unit cooperativity. This approach offers the potential for microwave random access quantum memories with lifetimes exceeding seconds [arXiv:1301.6567, arXiv:2005.09275], while the chirped pulse phase encoding could also be applied in the optical regime to enhance quantum repeaters and networks.
21
Mä
2021
Metrological characterisation of non-Gaussian entangled states of superconducting qubits
Multipartite entangled states are significant resources for both quantum information processing and quantum metrology. In particular, non-Gaussian entangled states are predicted to
achieve a higher sensitivity of precision measurements than Gaussian states. On the basis of metrological sensitivity, the conventional linear Ramsey squeezing parameter (RSP) efficiently characterises the Gaussian entangled atomic states but fails for much wider classes of highly sensitive non-Gaussian states. These complex non-Gaussian entangled states can be classified by the nonlinear squeezing parameter (NLSP), as a generalisation of the RSP with respect to nonlinear observables, and identified via the Fisher information. However, the NLSP has never been measured experimentally. Using a 19-qubit programmable superconducting processor, here we report the characterisation of multiparticle entangled states generated during its nonlinear dynamics. First, selecting 10 qubits, we measure the RSP and the NLSP by single-shot readouts of collective spin operators in several different directions. Then, by extracting the Fisher information of the time-evolved state of all 19 qubits, we observe a large metrological gain of 9.89[Math Processing Error] dB over the standard quantum limit, indicating a high level of multiparticle entanglement for quantum-enhanced phase sensitivity. Benefiting from high-fidelity full controls and addressable single-shot readouts, the superconducting processor with interconnected qubits provides an ideal platform for engineering and benchmarking non-Gaussian entangled states that are useful for quantum-enhanced metrology.
Rapid and Unconditional Parametric Reset Protocol for Tunable Superconducting Qubits
Qubit initialization is critical for many quantum algorithms and error correction schemes, and extensive efforts have been made to achieve this with high speed and efficiency. Here
we experimentally demonstrate a fast and high fidelity reset scheme for tunable superconducting qubits. A rapid decay channel is constructed by modulating the flux through a transmon qubit and realizing a swap between the qubit and its readout resonator. The residual excited population can be suppressed to 0.08% ± 0.08% within 34 ns, and the scheme requires no additional chip architecture, projective measurements, or feedback loops. In addition, the scheme has negligible effects on neighboring qubits, and is therefore suitable for large-scale multi-qubit systems. Our method also offers a way of entangling the qubit state with an itinerant single photon, particularly useful in quantum communication and quantum network applications.
Josephson dynamical simulation using the electronic circuit simulator APLAC: a tutorial
Analysis Program for Linear Active Circuits (APLAC) is a general-purpose electronics circuit simulator, which has included a built-in model of the Josephson junction (JJ) since late
80’s. It capabilities in simulating eg. noisy Superconducting Quantum Interference Devices (SQUIDs), Rapid Single Flux Quantum (RSFQ) logic circuits, or Superconducting Transition Edge Sensors (TESes) are relatively unknown within the superconducting electronics community. Here we give a brief step-to-step tutorial for APLAC users to unleash those capabilities.
19
Mä
2021
Quantum sensing with superconducting circuits
Sensing and metrology play an important role in fundamental science and applications, by fulfilling the ever-present need for more precise data sets, and by allowing to make more reliable
conclusions on the validity of theoretical models. Sensors are ubiquitous, they are used in applications across a diverse range of fields including gravity imaging, geology, navigation, security, timekeeping, spectroscopy, chemistry, magnetometry, healthcare, and medicine. Current progress in quantum technologies inevitably triggers the exploration of quantum systems to be used as sensors with new and improved capabilities. This perspective initially provides a brief review of existing and tested quantum sensing systems, before discussing future possible directions of superconducting quantum circuits use for sensing and metrology: superconducting sensors including many entangled qubits and schemes employing Quantum Error Correction. The perspective also lists future research directions that could be of great value beyond quantum sensing, e.g. for applications in quantum computation and simulation.
The Anomalous Resonant Frequency Variation of Microwave Superconducting Niobium Cavities Near Tc
Superconducting radio-frequency (SRF) niobium cavities are the modern means of particle acceleration and an enabling technology for record coherence superconducting quantum systems
and ultra-sensitive searches for new physics. Here we report a systematic effect observed on a large set of bulk SRF cavities – an anomalous decrease of the resonant frequency at temperatures just below the superconducting transition temperature – which opens up a new means of understanding the physics behind nitrogen doping and other modern cavity surface treatments relevant for future quality factor and coherence improvements. The magnitude of the frequency change correlates systematically with the near-surface impurity distribution in studied cavities and with the observed Tc variation. We also present the first demonstration of the coherence peak in the real part of the AC complex conductivity in Nb SRF cavities and show that its magnitude varies with impurity distribution.
18
Mä
2021
Circuit quantum electrodynamics (cQED) with modular quasi-lumped models
Extracting the Hamiltonian of interacting quantum-information processing systems is a keystone problem in the realization of complex phenomena and large-scale quantum computers. The
remarkable growth of the field increasingly requires precise, widely-applicable, and modular methods that can model the quantum electrodynamics of the physical circuits, and even of their more-subtle renormalization effects. Here, we present a computationally-efficient method satisfying these criteria. The method partitions a quantum device into compact lumped or quasi-distributed cells. Each is first simulated individually. The composite system is then reduced and mapped to a set of simple subsystem building blocks and their pairwise interactions. The method operates within the quasi-lumped approximation and, with no further approximation, systematically accounts for constraints, couplings, parameter renormalizations, and non-perturbative loading effects. We experimentally validate the method on large-scale, state-of-the-art superconducting quantum processors. We find that the full method improves the experimental agreement by a factor of two over taking standard coupling approximations when tested on the most sensitive and dressed Hamiltonian parameters of the measured devices.
17
Mä
2021
Experimental Characterization of Crosstalk Errors with Simultaneous Gate Set Tomography
Crosstalk is a leading source of failure in multiqubit quantum information processors. It can arise from a wide range of disparate physical phenomena, and can introduce subtle correlations
in the errors experienced by a device. Several hardware characterization protocols are able to detect the presence of crosstalk, but few provide sufficient information to distinguish various crosstalk errors from one another. In this article we describe how gate set tomography, a protocol for detailed characterization of quantum operations, can be used to identify and characterize crosstalk errors in quantum information processors. We demonstrate our methods on a two-qubit trapped-ion processor and a two-qubit subsystem of a superconducting transmon processor.
A context-aware gate set tomography characterization of superconducting qubits
The efficiency of Quantum Characterisation, Verification, and Validation (QCVV) protocols highly hinges on the agreement between the assumed noise model and the underlying error mechanisms.
As a matter of fact, errors in Quantum Processing Units (QPUs) incorporate various aspects of context-dependability which are overlooked by the majority of the commonly used QCVV protocols. As QCVV protocols are indispensable when it comes to characterizing and evaluating quantum operations, there is a serious need for a detailed characterization taking into account such aspects. In this work, we address these shortcomings by designing a context-aware version of the gate set tomography (GST) protocol. Our experiment selection approach is based on a polynomial quantification of the accumulation of errors within the designed circuits. Using simulated QPUs, we show that this technique enables a characterization with an inaccuracy reaching 10−5. Furthermore, we use our proposed protocol to experimentally infer context-dependent errors, namely crosstalk and memory effects, in a publicly accessible cloud-based superconducting qubits platform. Our results show that when the GST is upgraded to include such features of context-awareness, a large coherence in the errors is observed. These findings open up possibilities of drastically reducing the errors within the currently demonstrated QPUs.
16
Mä
2021
Strong parametric dispersive shifts in a statically decoupled multi-qubit cavity QED system
Cavity quantum electrodynamics (QED) with in-situ tunable interactions is important for developing novel systems for quantum simulation and computing. The ability to tune the dispersive
shifts of a cavity QED system provides more functionality for performing either quantum measurements or logical manipulations. Here, we couple two transmon qubits to a lumped-element cavity through a shared dc-SQUID. Our design balances the mutual capacitive and inductive circuit components so that both qubits are highly decoupled from the cavity, offering protection from decoherence processes. We show that by parametrically driving the SQUID with an oscillating flux it is possible to independently tune the interactions between either of the qubits and the cavity dynamically. The strength and detuning of this cavity QED interaction can be fully controlled through the choice of the parametric pump frequency and amplitude. As a practical demonstration, we perform pulsed parametric dispersive readout of both qubits while statically decoupled from the cavity. The dispersive frequency shifts of the cavity mode follow the expected magnitude and sign based on simple theory that is supported by a more thorough theoretical investigation. This parametric approach creates a new tunable cavity QED framework for developing quantum information systems with various future applications, such as entanglement and error correction via multi-qubit parity readout, state and entanglement stabilization, and parametric logical gates.