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
28
Nov
2019
Verification of a resetting protocol for an uncontrolled superconducting qubit
We experimentally verify the simplest non-trivial case of a quantum resetting protocol with five superconducting qubits, testing it with different types of free evolutions and target-probe
interactions. After post-selection, we obtained a reset state fidelity as high as 0.951, and the process fidelity was found to be 0.792. We also implemented 100 randomly-chosen interactions and demonstrated an average success probability of 0.323, experimentally confirmed the non-zeros probability of success for unknown interactions; the numerical simulated value is 0.384. We anticipate this protocol will have widespread applications in quantum information processing science, since it is able to combat any form of free evolution.
27
Nov
2019
Path-Independent Quantum Gates with Noisy Ancilla
Ancilla systems are often indispensable to universal control of a nearly isolated quantum system. However, ancilla systems are typically more vulnerable to environmental noise, which
limits the performance of such ancilla-assisted quantum control. To address this challenge of ancilla-induced decoherence, we propose a general framework that integrates quantum control and quantum error correction, so that we can achieve robust quantum gates resilient to ancilla noise. We introduce the path independence criterion for fault-tolerant quantum gates against ancilla errors. As an example, a path-independent gate is provided for superconducting circuits with a hardware-efficient design.
25
Nov
2019
Optimizing Josephson-Ring-Modulator-based Josephson Parametric Amplifiers via full Hamiltonian control
Josephson Parametric Amplifiers (JPA) are nonlinear devices that are used for quantum sensing and qubit readout in the microwave regime. While JPAs regularly operate in the quantum
limit, their gain saturates for very small (few photon) input power. In a previous work, we showed that the saturation power of JPAs is not limited by pump depletion, but instead by the high-order nonlinearity of Josephson junctions, the nonlinear circuit elements that enables amplification in JPAs. Here, we present a systematic study of the nonlinearities in JPAs, we show which nonlinearities limit the saturation power, and present a strategy for optimizing the circuit parameters for achieving the best possible JPA. For concreteness, we focus on JPAs that are constructed around a Josephson Ring Modulator (JRM). We show that by tuning the external and shunt inductors, we should be able to take the best experimentally available JPAs and improve their saturation power by ∼15 dB. Finally, we argue that our methods and qualitative results are applicable to a broad range of cavity based JPAs.
22
Nov
2019
Suppressed Charge Dispersion via Resonant Tunneling in a Single-Channel Transmon
We demonstrate strong suppression of charge dispersion in a semiconductor-based transmon qubit across Josephson resonances associated with a quantum dot in the junction. On resonance,
dispersion is drastically reduced compared to conventional transmons with corresponding Josephson and charging energies. We develop a model of qubit dispersion for a single-channel resonance, which is in quantitative agreement with experimental data.
Controlling the charge dispersion of a nearly-open superconducting island
Isolation from the environment determines the extent to which charge is confined on an island. This confinement leads to an energy cost for adding an electron onto the island, which
manifests experimentally through Coulomb oscillations such as charge dispersion. In superconducting circuits, the link to the environment has typically been formed from tunnel junctions where the charge dispersion can be tuned by changing the ratio between the Josephson energy EJ and the charging energy Ec. If, instead, a transparent ballistic junction forms the link between the superconducting island and the environment, the charge dispersion is predicted to be suppressed far beyond the dependence on the EJ/Ec ratio due to imaginary-time Landau-Zener tunneling between Andreev bound states. Here we investigate the charge dispersion of a nanowire transmon hosting a quantum dot in the junction. We observe rapid suppression of the charge dispersion consistent with the predicted scaling law incorporating diabatic transitions between Andreev bound states. We also observe greatly improved qubit coherence times at the point of highest charge dispersion suppression. Our observations further our fundamental understanding of charging effects in superconductors and suggest novel approaches for building charge-insensitive qubits.
21
Nov
2019
Superconducting Edge Contact and Quantum Interference Between Two-Dimensional van der Waals and Three-Dimensional Conventional Superconductors
Two-dimensional (2D) transition-metal dichalcogenide (TMD) superconductors have unique and desirable properties for integration with conventional superconducting circuits. These include
the ability to form atomically-flat and clean interfaces with stable tunnel barriers, increased kinetic inductance due to the atomically-thin geometry, and resilience to very high in-plane magnetic fields. However, integration of 2D TMD superconductors in conventional superconducting circuits, particularly those employing microwave drive and readout of qubits, requires that a fully superconducting contact be made between the 2D material and a three-dimensional (3D) superconductor. Here, we present an edge contact method for creating zero-resistance contacts between 2D \nbse and 3D aluminum. These hybrid Al-NbSe_2 Josephson junctions (JJs) display a Fraunhofer response to magnetic field with micron2-scale effective areas as the thin NbSe_2 allows field to uniformly penetrate the flake. We present a model for the supercurrent flow in a 2D-3D superconducting structure by numerical solution of the Ginzburg-Landau equations and find good agreement with experiment. The devices formed from 2D TMD superconductors are strongly influenced by the geometry of the flakes themselves as well as the placement of the contacts to bulk 3D superconducting leads. These results demonstrate our ability to graft 2D TMD superconductors and nano-devices onto conventional 3D superconducting materials, opening the way to a new generation of hybrid superconducting quantum circuits.
20
Nov
2019
Generating NOON states in circuit QED using multi-photon resonance in the presence of counter-rotating interactions
The NOON states are valuable quantum resources, which have a wide range of applications in quantum communication, quantum metrology, and quantum information processing. Here we propose
a fast, concise and reliable protocol for deterministically generating the NOON states of two resonators coupled to a single △-type superconducting qutrit. In particular, we derive the effective Hamiltonians at the multi-photon resonances by virtue of the strong counter-rotating interaction between the resonator modes and the qutrit. Based on these crucial effective Hamiltonians, our protocol simplifies the previous ones using the single-photon resonance and consequently reduces the number of operations for state preparation. To test the robustness of this protocol, we analyze the effects from both the decoherence including dissipation and dephasing and the crosstalk of resonator modes on the state fidelity through a Lindblad master equation in the eigenstates of the full Hamiltonian.
19
Nov
2019
Resolving the positions of defects in superconducting quantum bits
Solid-state quantum coherent devices are quickly progressing. Superconducting circuits, for instance, have already been used to demonstrate prototype quantum processors comprising a
few tens of quantum bits. This development also revealed that a major part of decoherence and energy loss in such devices originates from a bath of parasitic material defects. However, neither the microscopic structure of defects nor the mechanisms by which they emerge during sample fabrication are understood. Here, we present a technique to obtain information on locations of defects relative to the thin film edge of the qubit circuit. Resonance frequencies of defects are tuned by exposing the qubit sample to electric fields generated by electrodes surrounding the chip. By determining the defect’s coupling strength to each electrode and comparing it to a simulation of the field distribution, we obtain the probability at which location and at which interface the defect resides. This method is applicable to already existing samples of various qubit types, without further on-chip design changes. It provides a valuable tool for improving the material quality and nano-fabrication procedures towards more coherent quantum circuits.
18
Nov
2019
Coupling spin ‚clock states‘ to superconducting circuits
A central goal in quantum technologies is to maximize GT2, where G stands for the rate at which each qubit can be coherently driven and T2 is the qubit’s phase coherence time.
This is challenging, as increasing G (e.g. by coupling the qubit more strongly to external stimuli) often leads to deleterious effects on T2. Here, we study a physical situation in which both G and T2 can be simultaneously optimized. We measure the coupling to microwave superconducting coplanar waveguides of pure (i.e. non magnetically diluted) crystals of HoW10 magnetic clusters, which show level anticrossings, or spin clock transitions, at equidistant magnetic fields. The absorption lines give a complete picture of the magnetic energy level scheme and, in particular, confirm the existence of such clock transitions. The quantitative analysis of the microwave transmission allows monitoring the overlap between spin wave functions and gives information about their coupling to the environment and to the propagating photons. The formation of quantum superpositions of spin-up and spin-down states at the clock transitions allows simultaneously maximizing the spin-photon coupling and minimizing environmental spin perturbations. Using the same experimental device, we also explore the coupling of these qubits to a 11.7 GHz cavity mode, arising from a nonperfect microwave propagation at the chip boundaries and find a collective spin to single photon coupling GN = 100 MHz. The engineering of spin states in molecular systems offers a promising strategy to combine sizeable photon-mediated interactions, thus scalability, with a sufficient isolation from unwanted magnetic noise sources.
17
Nov
2019
Observation of enhanced coherence in Josephson SQUID cavities using a hybrid fabrication approach
We study the coherence of flux-tunable Josephson junction resonators made with two different fabrication processes. In the first process, devices are made using a single step of evaporation
in which the resonator and the junctions of the SQUID are made at the same time. In the second process, devices are made with an identical geometry, but in which the resonators are made from a MoRe superconding layer to which an the junctions are added later in a second step. To characterize the coherence of the two types of SQUID cavities, we observe and analyze the quality factor of their resonances as a function of flux and photon number. Despite a detailed cleaning process applied during fabrication, the single-step Al devices show significantly worse quality factor than the hybrid devices, and conclude that a the hybrid technique provides a much more reliable approach for fabricating high-Q flux-tunable resonators.