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
10
Nov
2020
Quantum versus Classical Regime in Circuit Quantum Acoustodynamics
We experimentally study a circuit quantum acoustodynamics system, which consists of a superconducting artificial atom, coupled to both a two-dimensional surface acoustic wave resonator
and a one-dimensional microwave transmission line. The strong coupling between the artificial atom and the acoustic wave resonator is confirmed by the observation of the vacuum Rabi splitting at the base temperature of dilution refrigerator. We show that the propagation of microwave photons in the microwave transmission line can be controlled by a few phonons in the acoustic wave resonator. Furthermore, we demonstrate the temperature effect on the measurements of the Rabi splitting and temperature induced transitions from high excited dressed states. We find that the spectrum structure of two-peak for the Rabi splitting becomes into those of several peaks, and gradually disappears with the increase of the environmental temperature T. The quantum-to-classical transition is observed around the crossover temperature Tc, which is determined via the thermal fluctuation energy kBT and the characteristic energy level spacing of the coupled system. Experimental results agree well with the theoretical simulations via the master equation of the coupled system at different effective temperatures.
08
Nov
2020
Suppression of static ZZ interaction in an all-transmon quantum processor
The superconducting transmon qubit is currently a leading qubit modality for quantum computing, but gate performance in quantum processor with transmons is often insufficient to support
running complex algorithms for practical applications. It is thus highly desirable to further improve gate performance. Due to the weak anharmonicity of transmon, a static ZZ interaction between coupled transmons commonly exists, undermining the gate performance, and in long term, it can become performance limiting. Here we theoretically explore a previously unexplored parameter region in an all-transmon system to address this issue. We show that an experimentally feasible parameter region, where the ZZ interaction is heavily suppressed while leaving XY interaction with an adequate strength to implement two-qubit gates, can be found in an all-transmon system. Thus, two-qubit gates, such as cross-resonance gate or iSWAP gate, can be realized without the detrimental effect from static ZZ interaction. To illustrate this, we show that an iSWAP gate with fast gate speed and dramatically lower conditional phase error can be achieved. Scaling up to large-scale transmon quantum processor, especially the cases with fixed coupling, addressing error, idling error, and crosstalk that arises from static ZZ interaction could also be heavily suppressed.
07
Nov
2020
Efficient simulation of so-called non-stoquastic superconducting flux circuits
There is a tremendous interest in fabricating superconducting flux circuits that are nonstoquastic—i.e., have positive off-diagonal matrix elements—in their qubit representation,
as these circuits are thought to be unsimulable by classical approaches and thus could play a key role in the demonstration of speedups in quantum annealing protocols. We show however that the efficient simulation of these systems is possible by the direct simulation of the flux circuits. Our approach not only obviates the reduction to a qubit representation but also produces results that are more in the spirit of the experimental setup. We discuss the implications of our work. Specifically we argue that our results cast doubt on the conception that superconducting flux circuits represent the correct avenue for universal adiabatic quantum computers.
Double Resonance Landau-Zener-Stückelburg-Majorana Interference in Circuit QED
We report on Floquet spectroscopy in a cavity-coupled double quantum dot system. By applying microwave induced consecutive passages, we observe Landau-Zener-Stückelberg-Majorana fringes
which are split by holes with the shape of crescents. We demonstrate that these crescents represent a universal feature that stems from a depletion of the predominantly occupied Floquet state at avoided crossings of the Floquet spectrum. The emergence of crescents can be controlled electrically via drive frequency and amplitude, which is perfectly consistent with the simulations based on our theoretical model. These results provide insight to the nonequilibrium population of Floquet states.
06
Nov
2020
Perfect state transfer on hypercubes and its implementation using superconducting qubits
We propose a protocol for perfect state transfer between any pair of vertices in a hypercube. Given a pair of distinct vertices in the hypercube we determine a sub-hypercube that contains
the pair of vertices as antipodal vertices. Then a switching process is introduced for determining the sub-hypercube of a memory enhanced hypercube that facilitates perfect state transfer between the desired pair of vertices. Furthermore, we propose a physical architecture for the pretty good state transfer implementation of our switching protocol with fidelity arbitrary close to unity, using superconducting transmon qubits with tunable couplings. The switching is realised by the control over the effective coupling between the qubits resulting from the effect of ancilla qubit couplers for the graph edges. We also report an error bound on the fidelity of state transfer due to faulty implementation of our protocol.
05
Nov
2020
Fast logic with slow qubits: microwave-activated controlled-Z gate on low-frequency fluxoniums
We demonstrate a controlled-Z gate between capacitively coupled fluxonium qubits with transition frequencies 72.3 MHz and 136.3 MHz. The gate is activated by a 61.6 ns long pulse at
the frequency between non-computational transitions |10⟩−|20⟩ and |11⟩−|21⟩, during which the qubits complete only 4 and 8 Larmor periods, respectively. The measured gate error of (8±1)×10−3 is limited by decoherence in the non-computational subspace, which will likely improve in the next generation devices. Although our qubits are about fifty times slower than transmons, the two-qubit gate is faster than microwave-activated gates on transmons, and the gate error is on par with the lowest reported. Architectural advantages of low-frequency fluxoniums include long qubit coherence time, weak hybridization in the computational subspace, suppressed residual ZZ-coupling rate (here 46 kHz), and absence of either excessive parameter matching or complex pulse shaping requirements.
04
Nov
2020
Nonlinear quantum processes in superconducting resonators terminated by neon-focused-ion-beam-fabricated superconducting nanowires
We have used a neon focused-ion-beam to fabricate both nanoscale Nb Dayem bridges and NbN phase-slip nanowires located at the short-circuited end of quarter-wavelength coplanar waveguide
resonators. The Dayem bridge devices show flux-tunability and intrinsic quality factor exceeding 10,000 at 300 mK up to local fields of at least 60 mT. The NbN nanowires show signatures of incoherent quantum tunnelling of flux at 300 mK.
03
Nov
2020
Microwaves in Quantum Computing
Quantum information processing systems rely on a broad range of microwave technologies and have spurred development of microwave devices and methods in new operating regimes. Here we
review the use of microwave signals and systems in quantum computing, with specific reference to three leading quantum computing platforms: trapped atomic ion qubits, spin qubits in semiconductors, and superconducting qubits. We highlight some key results and progress in quantum computing achieved through the use of microwave systems, and discuss how quantum computing applications have pushed the frontiers of microwave technology in some areas. We also describe open microwave engineering challenges for the construction of large-scale, fault-tolerant quantum computers.
02
Nov
2020
Stoquasticity in circuit-QED
We analyze whether circuit-QED Hamiltonians are stoquastic focusing on systems of coupled flux qubits: we show that scalable sign-problem free path integral Monte Carlo simulations
can typically be performed for such systems. Despite this, we corroborate the recent finding [1] that an effective, non-stoquastic qubit Hamiltonian can emerge in a system of capacitively coupled flux qubits. We find that if the capacitive coupling is sufficiently small, this non-stoquasticity of the effective qubit Hamiltonian can be avoided if we perform a canonical transformation prior to projecting onto an effective qubit Hamiltonian. Our results shed light on the power of circuit-QED Hamiltonians for the use of quantum adiabatic computation and the subtlety of finding a representation which cures the sign problem in these systems
Beyond the standard quantum limit of parametric amplification
The low-noise amplification of weak microwave signals is crucial for countless protocols in quantum information processing. Quantum mechanics sets an ultimate lower limit of half a
photon to the added input noise for phase-preserving amplification of narrowband signals, also known as the standard quantum limit (SQL). This limit, which is equivalent to a maximum quantum efficiency of 0.5, can be overcome by employing nondegenerate parametric amplification of broadband signals. We show that, in principle, a maximum quantum efficiency of 1 can be reached. Experimentally, we find a quantum efficiency of 0.69±0.02, well beyond the SQL, by employing a flux-driven Josephson parametric amplifier and broadband thermal signals. We expect that our results allow for fundamental improvements in the detection of ultraweak microwave signals.