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
19
Mä
2020
Elimination of unwanted qubit interactions for parametric exchange two-qubit gates in a tunable coupling circuit
We experimentally demonstrate a simple-design tunable coupler, achieving a continuous tunability for eliminating unwanted qubit interactions. We implement two-qubit iSWAP gate by applying
a fast-flux bias modulation pulse on the coupler to turn on parametric exchange interaction between computational qubits. Aiming to fully investigate error sources on the two-qubit gates, we perform quantum process tomography measurements and numerical simulations as varying static ZZ coupling strength. Our results reveal that the change in the two-qubit gate error is mainly attributed to unwanted high-frequency oscillation error terms, while the dynamic ZZ coupling parasitising in two-qubit gate operation may also contribute to the dependency of the gate fidelity. This approach, which has not yet been previously explored, provides a guiding principle to improve gate fidelity of parametric iSWAP gate by the elimination of unwanted qubit interactions. This controllable interaction, together with the parametric architecture by using modulation techniques, is desirable for crosstalk free multiqubit quantum circuits and quantum simulation applications.
18
Mä
2020
A Phononic Bus for Coherent Interfaces Between a Superconducting Quantum Processor, Spin Memory, and Photonic Quantum Networks
We introduce a method for high-fidelity quantum state transduction between a superconducting microwave qubit and the ground state spin system of a solid-state artificial atom, mediated
via an acoustic bus connected by piezoelectric transducers. Applied to present-day experimental parameters for superconducting circuit qubits and diamond silicon vacancy centers in an optimized phononic cavity, we estimate quantum state transduction with fidelity exceeding 99\% at a MHz-scale bandwidth. By combining the complementary strengths of superconducting circuit quantum computing and artificial atoms, the hybrid architecture provides high-fidelity qubit gates with long-lived quantum memory, high-fidelity measurement, large qubit number, reconfigurable qubit connectivity, and high-fidelity state and gate teleportation through optical quantum networks.
Many-body quantum circuits for quantum simulation and computing
Quantum simulators are attractive as a means to study many-body quantum systems that are not amenable to classical numerical treatment. A versatile framework for quantum simulation
is offered by superconducting circuits. In this perspective, we discuss how superconducting circuits allow the engineering of a wide variety of interactions, which in turn allows the simulation of a wide variety of model Hamiltonians. In particular we focus on strong photon-photon interactions mediated by nonlinear elements. This includes on-site, nearest-neighbour and four-body interactions in lattice models, allowing the implementation of extended Bose-Hubbard models and the toric code. We discuss not only the present state in analogue quantum simulation, but also future perspectives of superconducting quantum simulation that open up when concatenating quantum gates in emerging quantum computing platforms.
17
Mä
2020
A femto-Tesla DC SQUID design for quantum-ready readouts
Among some of the current uses of the DC Superconducting QUantum Interference Devices (SQUIDs) are qubit-readouts and sensors for probing properties of quantum materials. We present
a rather unique gradiometric niobium SQUID design with state-of-the-art sensitivity in the femto-Tesla range which can be easily tuned to specific readout requirements. The sensor is a next generation of the fractional SQUIDs with tightly optimized input coil and a combination of all measures known for reducing parasitic resonances and other detrimental effects. In addition, our modeling predicts small dimensions for these planar sensors. This makes them of great interest for material studies and for detection of magnetic fields in small volumes, e.g. as part of a cryogenic scanning quantum imaging apparatus for efficient diagnostics and quantum device readouts. This manuscript will benefit scientists and engineers working on quantum computing technologies by clarifying potential general misconceptions about DC SQUID optimization alongside the introduction of the novel flexible compact DC SQUID design.
16
Mä
2020
Generating Spatially Entangled Itinerant Photons with Waveguide Quantum Electrodynamics
Realizing a fully connected network of quantum processors requires the ability to distribute quantum entanglement. For distant processing nodes, this can be achieved by generating,
routing, and capturing spatially entangled itinerant photons. In this work, we demonstrate deterministic generation of such photons using superconducting transmon qubits that are directly coupled to a waveguide. In particular, we generate two-photon N00N states and show that the state and spatial entanglement of the emitted photons can be tuned via the qubit frequencies. Using quadrature amplitude detection, we reconstruct the moments and correlations of the photonic modes and demonstrate state preparation fidelities of 84%. Our results provide a path towards realizing quantum communication and teleportation protocols using non-classical, spatially entangled itinerant photons.
12
Mä
2020
Leakage reduction in fast superconducting qubit gates via optimal control
Reaching high speed, high fidelity qubit operations requires precise control over the shape of the underlying pulses. For weakly anharmonic systems, such as superconducting transmon
qubits, short gates lead to leakage to states outside of the computational subspace. Control pulses designed with open-loop optimal control may reduce such leakage. However, model inaccuracies can severely limit the usability of such pulses. We implemented a closed-loop optimization that simultaneously adapts all control parameters based on measurements of a cost function built from Clifford gates. By parameterizing pulses with a piecewise-constant representation that matches the capabilities of the control hardware we create a 4.16 ns single-qubit pulse with 99.76% fidelity and 0.044% leakage. This is a seven-fold reduction of the leakage rate of the best DRAG pulse we have calibrated at such short durations on the same system.
10
Mä
2020
Flux noise in superconducting qubits and the gap states continuum
In the present study we investigate the selected local aspects of the metal-induced gap states (MIGSs), localized at the disordered metal-insulator interface, that were previously proposed
to produce magnetic moments responsible for the flux noise in superconducting qubits. Our analysis attempts to supplement the available studies and provide new theoretical contribution toward their validation. In particular, we explicitly discuss the behavior of the MIGSs in the momentum space as a function of the local onsite energy deviation, that mimics the random potential disorder at the interface. It is found, that when the difference between the characteristic electronic potentials in the insulator increases, the corresponding MIGSs become more localized. This effect is associated with the increasing degree of the potential disorder that was earlier observed to produce highly localized MIGSs in the superconducting qubits. At the same time, the presented findings show that the disorder-induced localization of the MIGSs can be related directly to the decay characteristics of these states as well as to the bulk electronic properties of the insulator. As a result, our study reinforces plausibility of the previous corresponding investigations on the origin of the flux noise, but also allows to draw future directions toward their better verification.
Transparent Gatable Superconducting Shadow Junctions
Gate tunable junctions are key elements in quantum devices based on hybrid semiconductor-superconductor materials. They serve multiple purposes ranging from tunnel spectroscopy probes
to voltage-controlled qubit operations in gatemon and topological qubits. Common to all is that junction transparency plays a critical role. In this study, we grow single crystalline InAs, InSb and InAs1−xSbx nanowires with epitaxial superconductors and in-situ shadowed junctions in a single-step molecular beam epitaxy process. We investigate correlations between fabrication parameters, junction morphologies, and electronic transport properties of the junctions and show that the examined in-situ shadowed junctions are of significantly higher quality than the etched junctions. By varying the edge sharpness of the shadow junctions we show that the sharpest edges yield the highest junction transparency for all three examined semiconductors. Further, critical supercurrent measurements reveal an extraordinarily high ICRN, close to the KO−2 limit. This study demonstrates a promising engineering path towards reliable gate-tunable superconducting qubits.
A superconducting detector that counts microwave photons up to two
We propose a detector of microwave photons which can distinguish the vacuum state, one-photon state, and the states with two or more photons. Its operation is based on the two-photon
transition in a biased Josephson junction and detection occurs when it switches from a superconducting to a normal state. We model the detector theoretically. The detector performs with more than 90% success probability in several microseconds. It is sensitive for the 8.2GHz photons. The working frequency could be set at the design stage in the range from about 1GHz to 20GHz.
Implementation of a Walsh-Hadamard gate in a superconducting qutrit
We have implemented a Walsh-Hadamard gate, which performs a quantum Fourier transform, in a superconducting qutrit. The qutrit is encoded in the lowest three energy levels of a capacitively
shunted flux device, operated at the optimal flux-symmetry point. We use an efficient decomposition of the Walsh-Hadamard gate into two unitaries, generated by off-diagonal and diagonal Hamiltonians respectively. The gate implementation utilizes simultaneous driving of all three transitions between the three pairs of energy levels of the qutrit, one of which is implemented with a two-photon process. The gate has a duration of 35 ns and an average fidelity over a representative set of states, including preparation and tomography errors, of 99.2%, characterized with quantum state tomography. Compensation of ac-Stark and Bloch-Siegert shifts is essential for reaching high gate fidelities.