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
08
Sep
2020
Demonstration of a non-Abelian geometric controlled-Not gate in a superconducting circuit
Holonomies, arising from non-Abelian geometric transformations of quantum states in Hilbert space, offer a promising way for quantum computation. The non-community of these holonomies
renders them suitable for realization of a universal set of quantum logic gates, while the global geometric feature may result in some noise-resilient advantages. Here we report on the first on-chip realization of the non-Abelian geometric controlled-Not gate, which is a buidling block for constructing a holonomic quantum computer. The conditional dynamics is achieved in an all-to-all connected architecture involving multiple frequency-tunable superconducting qubits controllably coupled to a resonator; a holonomic gate between any two qubits can be implemented by tuning their frequencies on resonance with the resonator and applying a two-tone drive to one of them. The combination of the present gate and previously demonstrated holonomic single-qubit operations represents an all-holonomic approach to scalable quantum computation on a superconducting platform.
07
Sep
2020
Characterizing cryogenic amplifiers with a matched temperature-variable noise source
We present a cryogenic microwave noise source with characteristic impedance of 50 Ω that can be installed in a coaxial line of a cryostat. The bath temperature of the noise source
is continuously variable between 0.1 K and 5 K without causing significant back-action heating on the sample space. As a proof-of-concept experiment, we perform Y-factor measurements of an amplifier cascade that includes a traveling wave parametric amplifier and a commercial high electron mobility transistor amplifier. We observe system noise temperatures as low as 680+20−200 mK at 5.7 GHz corresponding to 1.5+0.1−0.7 excess photons. The system we present has immediate applications in the validation of solid-state qubit readout lines.
06
Sep
2020
Time molecules with periodically driven interacting qubits
We provide numerical evidence for a temporal quantum-mechanical interference phenomenon: time molecules (TM). A variety of such stroboscopic states are observed in the dynamics of two
interacting qubits subject to a periodic sequence of π-pulses with the period T. The TMs appear periodically in time and have a large duration, δtTM≫T. All TMs demonstrate an almost zero value of the total polarization and a strong enhancement of the entanglement entropy S up to the maximum value S=ln2 of a corresponding Bell state. The TMs are generated by the commensurability of the Floquet eigenvalues and the presence of maximally entangled Floquet eigenstates. The TMs remain stable with detuned system parameters and with an increased number of qubits. The TMs can be observed in microwave experiments with an array of superconducting qubits.
02
Sep
2020
Control and readout of a superconducting qubit using a photonic link
Delivering on the revolutionary promise of a universal quantum computer will require processors with millions of quantum bits (qubits). In superconducting quantum processors, each qubit
is individually addressed with microwave signal lines that connect room temperature electronics to the cryogenic environment of the quantum circuit. The complexity and heat load associated with the multiple coaxial lines per qubit limits the possible size of a processor to a few thousand qubits. Here we introduce a photonic link employing an optical fiber to guide modulated laser light from room temperature to a cryogenic photodetector, capable of delivering shot-noise limited microwave signals directly at millikelvin temperatures. By demonstrating high-fidelity control and readout of a superconducting qubit, we show that this photonic link can meet the stringent requirements of superconducting quantum information processing. Leveraging the low thermal conductivity and large intrinsic bandwidth of optical fiber enables efficient and massively multiplexed delivery of coherent microwave control pulses, providing a path towards a million-qubit universal quantum computer.
Laser-annealing Josephson junctions for yielding scaled-up superconducting quantum processors
As superconducting quantum circuits scale to larger sizes, the problem of frequency crowding proves a formidable task. Here we present a solution for this problem in fixed-frequency
qubit architectures. By systematically adjusting qubit frequencies post-fabrication, we show a nearly ten-fold improvement in the precision of setting qubit frequencies. To assess scalability, we identify the types of ‚frequency collisions‘ that will impair a transmon qubit and cross-resonance gate architecture. Using statistical modeling, we compute the probability of evading all such conditions, as a function of qubit frequency precision. We find that without post-fabrication tuning, the probability of finding a workable lattice quickly approaches 0. However with the demonstrated precisions it is possible to find collision-free lattices with favorable yield. These techniques and models are currently employed in available quantum systems and will be indispensable as systems continue to scale to larger sizes.
A quantum model for rf-SQUIDs based metamaterials enabling 3WM and 4WM Travelling Wave Parametric Amplification
A quantum model for Josephson-based metamaterials working in the Three-Wave Mixing (3WM) and Four-Wave Mixing (4WM) regimes at the single photon level is presented. The transmission
line taken into account, namely Traveling Wave Josephson Parametric Amplifier (TWJPA), is a bipole composed by a chain of rf-SQUIDs which can be biased by a DC current or a magnetic field in order to activate the 3WM or 4WM nonlinearities. The model exploits a Hamiltonian approach to analytically work out the time evolution both in the Heisenberg and interaction pictures. The former returns the analytic form of the gain of the amplifier, while the latter allows to recover the probability distributions vs. time of the photonic populations, for multimodal Fock and coherent input states. The dependence of the metamaterial’s nonlinearities is presented in terms of circuit parameters in a lumped model framework while evaluating the experimental conditions effects on the model validity.
01
Sep
2020
Entanglement dynamics of a dc SQUID interacting with a single mode radiation field
In this work, we study the entanglement dynamics in the coupling between a superconducting device as a dc SQUID and a single-mode radiation field for the information transference process.
We show that a pair of photons can be generated through the entanglement dynamics which emerge from the atomic population transfer. In this regard, we also demonstrate the transfer of entanglement and quantum coherence from the superconducting device to the radiation field modes, where entanglement is transferred as the quantum coherence is transmitted, since both stem from the quantum superposition principle, encapsulating the quantumness of the system.
Single-photon switch controlled by a qubit embedded in an engineered electromagnetic environment
A single-photon switch is an important element for the building of scalable quantum networks. In the paper, we propose a feasible scheme for efficient single-photon switching. The proposed
switch is controlled by a state of a qubit formed by the pair of the lowest levels of a three-level system (qutrit) coupled to a resonator. This resonator-qutrit system comprises a switching unit of the considered setup. For suppression of the Purcell relaxation of the control qubit, the switching unit is embedded into a coupled-resonator array serving as an engineered electromagnetic environment with a bandgap on a qubit transition frequency. We discuss the possible implementation of the considered single-photon switch on the microwave circuit QED architecture. We demonstrate that high switching contrasts can be attained for the parameters achievable for the state-of-the-art superconducting circuit QED setups.
31
Aug
2020
Weyl Josephson Circuits
We introduce Weyl Josephson circuits: small Josephson junction circuits that simulate Weyl band structures. We first formulate a general approach to design circuits that are analogous
to Bloch Hamiltonians of a desired dimensionality and symmetry class. We then construct and analyze a six-junction device that produces a 3D Weyl Hamiltonian with broken inversion symmetry and in which topological phase transitions can be triggered \emph{in situ}. We argue that currently available superconducting circuit technology allows experiments that probe topological properties inaccessible in condensed matter systems.
Supercomputer simulations of transmon quantum computers
We develop a simulator for quantum computers composed of superconducting transmon qubits. The simulation model supports an arbitrary number of transmons and resonators. Quantum gates
are implemented by time-dependent pulses. Nontrivial effects such as crosstalk, leakage to non-computational states, entanglement between transmons and resonators, and control errors due to the pulses are inherently included. The time evolution of the quantum computer is obtained by solving the time-dependent Schrödinger equation. The simulation algorithm shows excellent scalability on high-performance supercomputers. We present results for the simulation of up to 16 transmons and resonators. Additionally, the model can be used to simulate environments, and we demonstrate the transition from an isolated system to an open quantum system governed by a Lindblad master equation. We also describe a procedure to extract model parameters from electromagnetic simulations or experiments. We compare simulation results to experiments on several NISQ processors of the IBM Q Experience. We find nearly perfect agreement between simulation and experiment for quantum circuits designed to probe crosstalk in transmon systems. By studying common gate metrics such as the fidelity or the diamond distance, we find that they cannot reliably predict the performance of repeated gate applications or practical quantum algorithms. As an alternative, we find that the results from two-transmon gate set tomography have an exceptional predictive power. Finally, we test a protocol from the theory of quantum error correction and fault tolerance. We find that the protocol systematically improves the performance of transmon quantum computers in the presence of characteristic control and measurement errors.