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
26
Jan
2020
10-GHz superconducting cavity piezo-optomechanics for microwave-optical photon conversion
Coherent photon conversion between microwave and optics holds promise for the realization of distributed quantum networks, in particular, the architecture that incorporates superconducting
quantum processors with optical telecommunication channels. High-frequency gigahertz piezo-mechanics featuring low thermal excitations offers an ideal platform to mediate microwave-optical coupling. However, integrating nanophotonic and superconducting circuits at cryogenic temperatures to simultaneously achieve strong photon-phonon interactions remains a tremendous challenge. Here, we report the first demonstration of an integrated superconducting cavity piezo-optomechanical converter where 10-GHz phonons are resonantly coupled with photons in a superconducting microwave and a nanophotonic cavities at the same time. Benefited from the cavity-enhanced interactions, efficient bidirectional microwave-optical photon conversion is realized with an on-chip efficiency of 0.07% and an internal efficiency of 5.8%. The demonstrated superconducting piezo-optomechanical interface makes a substantial step towards quantum communication at large scale, as well as novel explorations in hybrid quantum systems such as microwave-optical photon entanglement and quantum sensing.
24
Jan
2020
Impact of ionizing radiation on superconducting qubit coherence
The practical viability of any qubit technology stands on long coherence times and high-fidelity operations, with the superconducting qubit modality being a leading example. However,
superconducting qubit coherence is impacted by broken Cooper pairs, referred to as quasiparticles, with a density that is empirically observed to be orders of magnitude greater than the value predicted for thermal equilibrium by the Bardeen-Cooper-Schrieffer (BCS) theory of superconductivity. Previous work has shown that infrared photons significantly increase the quasiparticle density, yet even in the best isolated systems, it still remains higher than expected, suggesting that another generation mechanism exists. In this Letter, we provide evidence that ionizing radiation from environmental radioactive materials and cosmic rays contributes to this observed difference, leading to an elevated quasiparticle density that would ultimately limit superconducting qubits of the type measured here to coherence times in the millisecond regime. We further demonstrate that introducing radiation shielding reduces the flux of ionizing radiation and positively correlates with increased coherence time. Albeit a small effect for today’s qubits, reducing or otherwise mitigating the impact of ionizing radiation will be critical for realizing fault-tolerant superconducting quantum computers.
High coherence superconducting microwave cavities with indium bump bonding
Low-loss cavities are important in building high-coherence superconducting quantum computers. Generating high quality joints between parts is crucial to the realization of a scalable
quantum computer using the circuit quantum electrodynamics (cQED) framework. In this paper, we adapt the technique of indium bump bonding to the cQED architecture to realize high quality superconducting microwave joints between chips. We use this technique to fabricate compact superconducting cavities in the multilayer microwave integrated quantum circuits (MMIQC) architecture and achieve single photon quality factor over 300 million or single-photon lifetimes approaching 5 ms. To quantify the performance of the resulting seam, we fabricate microwave stripline resonators in multiple sections connected by different numbers of bonds, resulting in a wide range of seam admittances. The measured quality factors combined with the designed seam admittances allow us to bound the conductance of the seam at gseam≥2×1010/(Ωm). Such a conductance should enable construction of micromachined superconducting cavities with quality factor of at least a billion. These results demonstrate the capability to construct very high quality microwave structures within the MMIQC architecture.
23
Jan
2020
A Quantum Instruction Set Implemented on a Superconducting Quantum Processor
A quantum algorithm consists of a sequence of operations and measurements applied to a quantum processor. To date, the instruction set which defines this sequence has been provided
by a classical computer and passed via control hardware to the quantum processor. Here, we demonstrate the first experimental realization of a quantum instruction set, in which a fixed sequence of classically-defined gates perform an operation that is fully determined only by a quantum input to the fixed sequence. Specifically, we implement the density matrix exponentiation algorithm, which consumes N copies of the instruction state ρ to approximate the operation e−iρθ (θ an arbitrary angle). Our implementation relies on a 99.7\% fidelity controlled-phase gate between two superconducting transmon qubits. We achieve an average algorithmic fidelity ≈0.9, independent of the setting of ρ, to circuit depth nearly 90. This new paradigm for quantum instructions has applications to resource-efficient protocols for validating entanglement spectra, principal component analysis of large quantum states, and universal quantum emulation.
22
Jan
2020
Probing XY phase transitions in a Josephson junction array with tunable frustration
The seminal theoretical works of Berezinskii, Kosterlitz, and Thouless presented a new paradigm for phase transitions in condensed matter that are driven by topological excitations.
These transitions have been extensively studied in the context of two-dimensional XY models — coupled compasses — and have generated interest in the context of quantum simulation. Here, we use a circuit quantum-electrodynamics architecture to study the critical behavior of engineered XY models through their dynamical response. In particular, we examine not only the unfrustrated case but also the fully-frustrated case which leads to enhanced degeneracy associated with the spin rotational [U(1)] and discrete chiral (Z2) symmetries. The nature of the transition in the frustrated case has posed a challenge for theoretical studies while direct experimental probes remain elusive. Here we identify the transition temperatures for both the unfrustrated and fully-frustrated XY models by probing a Josephson junction array close to equilibrium using weak microwave excitations and measuring the temperature dependence of the effective damping obtained from the complex reflection coefficient. We argue that our probing technique is primarily sensitive to the dynamics of the U(1) part.
16
Jan
2020
Large flux-mediated coupling in hybrid electromechanical system with a transmon qubit
Control over the quantum states of a massive oscillator is important for several technological applications and to test the fundamental limits of quantum mechanics. Addition of an internal
degree of freedom to the oscillator could be a valuable resource for such control. Recently, hybrid electromechanical systems using superconducting qubits, based on electric-charge mediated coupling, have been quite successful. Here, we realize a hybrid device, consisting of a superconducting transmon qubit and a mechanical resonator coupled using the magnetic-flux. The coupling stems from the quantum interference of the superconducting phase across the tunnel junctions. We demonstrate a vacuum electromechanical coupling rate up to 4 kHz by making the transmon qubit resonant with the readout cavity. Consequently, thermal-motion of the mechanical resonator is detectable by driving the dressed-mode with mean-occupancy well below one photon. By tuning the qubit away from the cavity, electromechanical coupling between qubit and mechanical mode can be further enhanced to 40 kHz. In this limit, a small coherent drive of the mechanical resonator results into the splitting of qubit spectrum and we observe interference signature arising from the Landau-Zener-Stuckelberg effect. With further improvements in the qubit coherence, this system offers a novel platform to realize rich interactions and could potentially provide full control over the quantum motional states.
15
Jan
2020
Time-resolved tomography of a driven adiabatic quantum simulation
A typical goal of a quantum simulation is to find the energy levels and eigenstates of a given Hamiltonian. This can be realized by adiabatically varying the system control parameters
to steer an initial eigenstate into the eigenstate of the target Hamiltonian. Such an adiabatic quantum simulation is demonstrated by directly implementing a controllable and smoothly varying Hamiltonian in the rotating frame of two superconducting qubits, including longitudinal and transverse fields and iSWAP-type two-qubit interactions. The evolution of each eigenstate is tracked using time-resolved state tomography. The energy gaps between instantaneous eigenstates are chosen such that depending on the energy transition rate either diabatic or adiabatic passages are observed in the measured energies and correlators. Errors in the obtained energy values induced by finite T1 and T2 times of the qubits are mitigated by extrapolation to short protocol times.
12
Jan
2020
Simulation of Higher-Order Topological Phases and Related Topological Phase Transitions in a Superconducting Qubit
Higher-order topological insulators (TIs) and superconductors (TSCs) give rise to new bulk and boundary physics, as well as new classes of topological phase transitions. While higher-order
TIs have been actively studied on many platforms, the experimental study of higher-order TSCs has thus far been greatly hindered due to the scarcity of material realizations. To advance the study of higher-order TSCs, in this work we carry out the simulation of a two-dimensional spinless second-order TSC belonging to the symmetry class D in a superconducting qubit. Owing to the great flexibility and controllability of the quantum simulator, we observe the realization of higher-order topology directly through the measurement of the pseudo-spin texture in momentum space of the bulk for the first time, in sharp contrast to previous experiments based on the detection of gapless boundary modes in real space. Also through the measurement of the evolution of pseudo-spin texture with parameters, we further observe novel topological phase transitions from the second-order TSC to the trivial superconductor, as well as to the first-order TSC with nonzero Chern number. Our work sheds new light on the study of higher-order topological phases and topological phase transitions.
10
Jan
2020
A Josephson quantum phase battery
A battery is a classical apparatus which converts a chemical reaction into a persistent voltage bias able to power electronic circuits. Similarly, a phase battery is a quantum equipment
which provides a persistent phase bias to the wave function of a quantum circuit. It represents a key element for quantum technologies based on quantum coherence. Unlike the voltage batteries, a phase battery has not been implemented so far, mainly because of the natural rigidity of the quantum phase that, in typical quantum circuits, is imposed by the parity and time-reversal symmetry constrains. Here we report on the first experimental realization of a phase battery in a hybrid superconducting circuit. It consists of an n-doped InAs nanowire with unpaired-spin surface states and proximitized by Al superconducting leads. We find that the ferromagnetic polarization of the unpaired-spin states is efficiently converted into a persistent phase bias φ0 across the wire, leading to the anomalous Josephson effect. By applying an external in-plane magnetic field a continuous tuning of φ0 is achieved. This allows the charging and discharging of the quantum phase battery and reveals the symmetries of the anomalous Josephson effect predicted by our theoretical model. Our results demonstrate how the combined action of spin-orbit coupling and exchange interaction breaks the phase rigidity of the system inducing a strong coupling between charge, spin and superconducting phase. This interplay opens avenues for topological quantum technologies, superconducting circuitry and advanced schemes of circuit quantum electrodynamics.
09
Jan
2020
Collapse and Revival of an Artificial Atom Coupled to a Structured Photonic Reservoir
A structured electromagnetic reservoir can result in novel dynamics of quantum emitters. In particular, the reservoir can be tailored to have a memory of past interactions with emitters,
in contrast to memory-less Markovian dynamics of typical open systems. In this Article, we investigate the non-Markovian dynamics of a superconducting qubit strongly coupled to a superconducting slow-light waveguide reservoir. Tuning the qubit into the spectral vicinity of the passband of this waveguide, we find non-exponential energy relaxation as well as substantial changes to the qubit emission rate. Further, upon addition of a reflective boundary to one end of the waveguide, we observe revivals in the qubit population on a timescale 30 times longer than the inverse of the qubit’s emission rate, corresponding to the round-trip travel time of an emitted photon. By tuning of the qubit-waveguide interaction strength, we probe a crossover between Markovian and non-Markovian qubit emission dynamics. These attributes allow for future studies of multi-qubit circuits coupled to structured reservoirs, in addition to constituting the necessary resources for generation of multiphoton highly entangled states.