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
09
Jan
2021
Circulator function in a Josephson junction circuit and braiding of Majorana zero modes
We propose a scheme for the circulator function in a superconducting circuit consisting of a three-Josephson junction loop and a trijunction. In this study we obtain the exact Lagrangian
of the system by deriving the effective potential from the fundamental boundary conditions. We subsequently show that we can selectively choose the direction of current flowing through the branches connected at the trijunction, which performs a circulator function. Further, we use this circulator function for a non-Abelian braiding of Majorana zero modes (MZMs). In the branches of the system we introduce pairs of MZMs which interact with each other through the phases of trijunction. The circulator function determines the phases of the trijunction and thus the coupling between the MZMs to gives rise to the braiding operation. We modify the system so that MZMs might be coupled to the external ones to perform qubit operations in a scalable design.
06
Jan
2021
Perturbation impact of spectators and spurious qubit interactions on a cross-resonance gate in a tunable coupling superconducting circuit
Cross-resonance (CR) gate has proved to be a promising scheme for implementing fault-tolerant quantum computation with fixed-frequency qubits. In this work, we experimentally implement
an entangling cross-resonance gate by using a microwave-only control in a tunable coupling superconducting circuit. The flux-controlled tunable coupler allows us to continuously vary adjacent qubit coupling from positive to negative values, and thus providing an extra degree of freedom to verify optimal condition for constructing the CR gate. Based on three-qubit CR Hamiltonian tomography, we systematically investigate the dependency of gate fidelities on spurious interaction components and present the first experimental approach to evaluate the perturbation impact arising from the spectator qubits. Our results reveal that the spectator qubits can lead to reductions in the CR gate fidelity relying on the particular frequency resonance poles and the induced ZZ interaction between the spectator and gate qubits, while an improvement in the gate fidelity to 98.5% can be achieved by optimally tuning the inter-qubit detuning and flux bias on the coupler. Our experiments uncover an optimal CR operation regime and provide a guiding principle to improve the CR gate fidelity by suppression of unwanted qubit interactions.
05
Jan
2021
In-situ bandaged Josephson junctions for superconducting quantum processors
Shadow evaporation is commonly used to micro-fabricate the key element of superconducting qubits — the Josephson junction. However, in conventional two-angle deposition circuit
topology, unwanted stray Josephson junctions are created which contribute to dielectric loss. So far, this could be avoided by shorting the stray junctions with a so-called bandage layer deposited in an additional lithography step. Here, we present an improved shadow evaporation technique allowing one to deposit submicrometer-sized Josephson junctions together with bandage layers in a single lithography step. We also show that junction aging is signficantly reduced when junction electrodes and the bandage layers are oxidized in an oxygen atmosphere directly after deposition.
31
Dez
2020
RF mixing modules for superconducting qubit room temperature control systems
As the number of qubits in nascent quantum processing units increases, the connectorized RF (radio frequency) analog circuits used in first generation experiments become exceedingly
complex. The physical size, cost and electrical failure rate all become limiting factors in the extensibility of control systems. We have developed a series of compact RF mixing boards to adresss this challenge by integrating I/Q quadrature mixing, IF(intermediate frequency)/LO(local oscillator)/RF power level adjustments, and DC (direct current) bias fine tuning on a 40 mm × 80 mm 4-layer PCB (printed circuit board) board with EMI (electromagnetic interference) shielding. The RF mixing module is designed to work with RF and LO frequencies between 2.5 and 8.5 GHz. The typical image rejection and adjacent channel isolation are measured to be ∼27 dBc and ∼50 dB. By scanning the drive phase in a loopback test, the module short-term amplitude and phase stability are typically measured to be 5×10−4 (Vpp/Vmean) and 1×10−3 radian (pk-pk). The operation of RF mixing board was validated by integrating it into the room temperature control system of a superconducting quantum processor and executing randomized benchmarking characterization of single and two qubit gates. We measured a single-qubit process infidelity of 0.0020±0.0001 and a two-qubit process infidelity of 0.052±0.004.
QubiC: An open source FPGA-based control and measurement system for superconducting quantum information processors
As quantum information processors grow in quantum bit (qubit) count and functionality, the control and measurement system becomes a limiting factor to large scale extensibility. To
tackle this challenge and keep pace with rapidly evolving classical control requirements, full control stack access is essential to system level optimization. We design a modular FPGA (field-programmable gate array) based system called QubiC to control and measure a superconducting quantum processing unit. The system includes room temperature electronics hardware, FPGA gateware, and engineering software. A prototype hardware module is assembled from several commercial off-the-shelf evaluation boards and in-house developed circuit boards. Gateware and software are designed to implement basic qubit control and measurement protocols. System functionality and performance are demonstrated by performing qubit chip characterization, gate optimization, and randomized benchmarking sequences on a superconducting quantum processor operating at the Advanced Quantum Testbed at Lawrence Berkeley National Laboratory. The single-qubit and two-qubit process fidelities are measured to be 0.9980±0.0001 and 0.948±0.004 by randomized benchmarking. With fast circuit sequence loading capability, the QubiC performs randomized compiling experiments efficiently and improves the feasibility of executing more complex algorithms.
30
Dez
2020
Deterministic loading and phase shaping of microwaves onto a single artificial atom
Loading quantum information deterministically onto a quantum node is an important step towards a quantum network. Here, we demonstrate that coherent-state microwave photons, with anoptimal temporal waveform, can be efficiently loaded onto a single superconducting artificial atom in a semi-infinite one-dimensional (1D) transmission-line waveguide. Using a weak coherent state (average photon number N<<1 with an exponentially rising waveform, whose time constant matches the decoherence time of the artificial atom, we demonstrate a loading efficiency of above 94% from 1D semi-free space to the artificial atom. We also show that Fock-state microwave photons can be deterministically loaded with an efficiency of 98.5%. We further manipulate the phase of the coherent state exciting the atom, enabling coherent control of the loading process. Our results open up promising applications in realizing quantum networks based on waveguide quantum electrodynamics (QED).[/expand]
24
Dez
2020
Optically-Heralded Entanglement of Superconducting Systems in Quantum Networks
The typical approach to communication between two superconducting quantum computers is to transduce into the optical regime and then back into the microwave regime. However, direct
microwave-to-optical transduction has low fidelity due to the low microwave-optical coupling rates and added noise; these problems compound in the consecutive egress and ingress transduction steps. We break this rate-fidelity trade-off by heralding end-to-end entanglement with one detected photon and teleportation. In contrast to cascaded direct transduction, our scheme absorbs the low optical-microwave coupling efficiency into the entanglement heralding step. Our approach unifies and simplifies entanglement generation between superconducting devices and other physical modalities in quantum networks.
22
Dez
2020
Enhanced coherence in superconducting circuits via band engineering
In superconducting circuits interrupted by Josephson junctions, the dependence of the energy spectrum on offset charges on different islands is 2e periodic through the Aharonov-Casher
effect and resembles a crystal band structure that reflects the symmetries of the Josephson potential. We show that higher-harmonic Josephson elements described by a cos(2φ) energy-phase relation provide an increased freedom to tailor the shape of the Josephson potential and design spectra featuring multiplets of flat bands and Dirac points in the charge Brillouin zone. Flat bands provide noise-insensitive quantum states, and band engineering can help improve the coherence of the system. We discuss a modified version of a flux qubit that achieves in principle no decoherence from charge noise and introduce a flux qutrit that shows a spin-one Dirac spectrum and is simultaneously quote robust to both charge and flux noise.
19
Dez
2020
A „minimal“ topological quantum circuit
The outlook of protected quantum computing spurred enormous progress in the search for topological materials, sustaining a continued race to find the most experimentally feasible platform.
Here, we show that one of the simplest quantum circuits, the Cooper-pair transistor, exhibits a nontrivial Chern number which has not yet been discussed, in spite of the exhaustive existing literature. Surprisingly, the resulting quantized current response is robust with respect to a large number of external perturbations, most notably low-frequency charge noise and quasiparticle poisoning. Moreover, the fact that the higher bands experience crossings with higher topological charge leads to all the bands having the same Chern number, such that there is no restriction to stay close to the ground state. Remaining small perturbations are investigated based on a generic Master equation approach. Finally, we discuss feasible protocols to measure the quantized current.
Investigation of microwave loss induced by oxide regrowth in high-Q Nb resonators
The coherence of state-of-the-art superconducting qubit devices is predominantly limited by two-level-system defects, found primarily at amorphous interface layers. Reducing microwave
loss from these interfaces by proper surface treatments is key to push the device performance forward. Here, we study niobium resonators after removing the native oxides with a hydrofluoric acid etch. We investigate the reappearance of microwave losses introduced by surface oxides that grow after exposure to the ambient environment. We find that losses in quantum devices are reduced by an order of magnitude, with internal Q-factors reaching up to 7 ⋅ 106 in the single photon regime, when devices are exposed to ambient conditions for 16 min. Furthermore, we observe that Nb2O5 is the only surface oxide that grows significantly within the first 200 hours, following the extended Cabrera-Mott growth model. In this time, microwave losses scale linearly with the Nb2O5 thickness, with an extracted loss tangent tanδ = 9.9 ⋅ 10−3. Our findings are of particular interest for devices spanning from superconducting qubits, quantum-limited amplifiers, microwave kinetic inductance detectors to single photon detectors.