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
Aug
2019
Quantum bits with Josephson junctions
Already in the first edition of this book (Barone and Paterno, „Fundamentals and Physics and Applications of the Josephson Effect“, Wiley 1982), a great number of interesting
and important applications for Josephson junctions were discussed. In the decades that have passed since then, several new applications have emerged. This chapter treats one such new class of applications: quantum optics and quantum information processing (QIP) based on superconducting circuits with Josephson junctions. In this chapter, we aim to explain the basics of superconducting quantum circuits with Josephson junctions and demonstrate how these systems open up new prospects, both for QIP and for the study of quantum optics and atomic physics.
Amplitude and frequency sensing of microwave fields with a superconducting transmon qudit
Experiments with superconducting circuits require careful calibration of the applied pulses and fields over a large frequency range. This remains an ongoing challenge as commercial
semiconductor electronics are not able to probe signals arriving at the chip due to its cryogenic environment. Here, we demonstrate how the on-chip amplitude and frequency of a microwave field can be inferred from the ac Stark shifts of higher transmon levels. In our time-resolved measurements, we employ a simple quantum sensing protocol, i.e. Ramsey fringes, allowing us to detect the amplitude of the systems transfer function over a range of several hundreds of MHz with an energy sensitivity on the order of 10−4. Combined with similar measurements for the phase of the transfer function, our sensing method can facilitate the microwave calibration of high fidelity quantum gates necessary for working with superconducting quantum circuits. Additionally, the potential to characterize arbitrary microwave fields promotes applications in related areas of research, such as quantum optics or hybrid microwave systems including photonic, mechanical or magnonic subsystems.
20
Aug
2019
Universal gates for protected superconducting qubits using optimal control
We employ quantum optimal control theory to realize quantum gates for two protected superconducting circuits: the heavy-fluxonium qubit and the 0-π qubit. Utilizing automatic differentiation
facilitates the simultaneous inclusion of multiple optimization targets, allowing one to obtain high-fidelity gates with realistic pulse shapes. For both qubits, disjoint support of low-lying wave functions prevents direct population transfer between the computational-basis states. Instead, optimal control favors dynamics involving higher-lying levels, effectively lifting the protection for a fraction of the gate duration. For the 0-π qubit, offset-charge dependence of matrix elements among higher levels poses an additional challenge for gate protocols. To mitigate this issue, we randomize the offset charge during the optimization process, steering the system towards pulse shapes insensitive to charge variations. Closed-system fidelities obtained are 99% or higher, and show slight reductions in open-system simulations.
19
Aug
2019
Superconducting Josephson-based metamaterials for quantum-limited parametric amplification: a review
In the last few years, several groups have proposed and developed their own platforms demonstrating quantum-limited linear parametric amplification, with evident applications in quantum
information and computation, electrical and optical metrology, radio astronomy and basic physics concerning axion detection. Here we propose a short review on the physics behind parametric amplification via metamaterials composed by coplanar wave-guides embedding several Josephson junctions. We present and compare different schemes that exploit the nonlinearity of the Josephson current-phase relation to mix the so-called signal, idler and pump tones. The chapter then presents and compares three different theoretical models, developed in the last few years, to predict the dynamics of these nonlinear systems in the particular case of a 4-Wave Mixing process and under the degenerate undepleted pump assumption. We will demonstrate that, under the same assumption, all the results are comparable in terms of amplification of the output fields.
15
Aug
2019
Heat rectification via a superconducting artificial atom
In miniaturising electrical devices down to nanoscales, heat transfer has turned into a serious obstacle but also potential resource for future developments, both for conventional and
quantum computing architectures. Controlling heat transport in superconducting circuits has thus received increasing attention in engineering microwave environments for circuit quantum electrodynamics (cQED) and circuit quantum thermodynamics experiments (cQTD). While theoretical proposals for cQTD devices are numerous, the experimental situation is much less advanced. There exist only relatively few experimental realisations, mostly due to the difficulties in developing the hybrid devices and in interfacing these often technologically contrasting components. Here we show a realisation of a quantum heat rectifier, a thermal equivalent to the electronic diode, utilising a superconducting transmon qubit coupled to two strongly unequal resonators terminated by mesoscopic heat baths. Our work is the experimental realisation of the spin-boson rectifier proposed by Segal and Nitzan.
Parametric effects in circuit quantum electrodynamics
We review recent advances in the research on quantum parametric phenomena in superconducting circuits with Josephson junctions. We discuss physical processes in parametrically driven
tunable cavity and outline theoretical foundations for their description. Amplification and frequency conversion are discussed in detail for degenerate and non-degenerate parametric resonance, including quantum noise squeezing and photon entanglement. Experimental advances in this area played decisive role in successful development of quantum limited parametric amplifiers for superconducting quantum information technology. We also discuss nonlinear down-conversion processes and experiments on self-sustained parametric and subharmonic oscillations.
13
Aug
2019
Spectrum and Coherence Properties of the Current-Mirror Qubit
exhibits a robust ground-state degeneracy and wave functions with disjoint support for appropriate circuit parameters."]In this protected regime, Cooper-pair excitons form the relevant low-energy excitations. Based on a full circuit analysis of the current-mirror device, we introduce an effective model that systematically captures the relevant low-energy degrees of freedom, and is amenable to diagonalization using Density Matrix Renormalization Group (DMRG) methods. We find excellent agreement between DMRG and exact diagonalization, and can push DMRG simulations to much larger circuit sizes than feasible for exact diagonalization. We discuss the spectral properties of the current-mirror circuit, and predict coherence times exceeding 1 ms in parameter regimes believed to be within reach of experiments.
12
Aug
2019
Phonon traps reduce the quasiparticle density in superconducting circuits
Out of equilibrium quasiparticles (QPs) are one of the main sources of decoherence in superconducting quantum circuits, and are particularly detrimental in devices with high kinetic
inductance, such as high impedance resonators, qubits, and detectors. Despite significant progress in the understanding of QP dynamics, pinpointing their origin and decreasing their density remain outstanding tasks. The cyclic process of recombination and generation of QPs implies the exchange of phonons between the superconducting thin film and the underlying substrate. Reducing the number of substrate phonons with frequencies exceeding the spectral gap of the superconductor should result in a reduction of QPs. Indeed, we demonstrate that surrounding high impedance resonators made of granular aluminum (grAl) with lower gapped thin film aluminum islands increases the internal quality factors of the resonators in the single photon regime, suppresses the noise, and reduces the rate of observed QP bursts. The aluminum islands are positioned far enough from the resonators to be electromagnetically decoupled, thus not changing the resonator frequency, nor the loading. We therefore attribute the improvements observed in grAl resonators to phonon trapping at frequencies close to the spectral gap of aluminum, well below the grAl gap.
09
Aug
2019
Quantum simulation of molecular vibronic spectra on a superconducting bosonic processor
The efficient simulation of quantum systems is a primary motivating factor for developing controllable quantum machines. A controllable bosonic machine is naturally suited for simulating
systems with underlying bosonic structure, exploiting both quantum interference and an intrinsically large Hilbert space. Here, we experimentally realize a bosonic superconducting processor that combines arbitrary state preparation, a complete set of Gaussian operations, plus an essential non-Gaussian resource – a novel single-shot photon number resolving measurement scheme – all in one device. We utilize these controls to simulate the bosonic problem of molecular vibronic spectra, extracting the corresponding Franck-Condon factors for photoelectron processes in H2O, O3, NO2, and SO2. Our results demonstrate the versatile capabilities of the circuit QED platform, which can be extended to include non-Gaussian operations for simulating an even wider class of bosonic systems.
08
Aug
2019
Experimental Microwave Quantum Illumination
Quantum illumination is a powerful sensing technique which employs entangled photons to boost the detection of low-reflectivity objects in environments with bright thermal noise. The
promised advantage over classical strategies is particularly evident at low signal photon flux, a feature which makes the protocol an ideal prototype for non-invasive biomedical scanning or low-power short-range radar detection. In this work we experimentally demonstrate quantum illumination at microwave frequencies. We generate entangled fields using a Josephson parametric converter at millikelvin temperatures to illuminate a room-temperature object at a distance of 1 meter in a proof of principle bistatic radar setup. Using heterodyne detection and suitable data-processing at the receiver we observe an up to three times improved signal-to-noise ratio compared to the classical benchmark, the coherent-state transmitter, outperforming any classically-correlated radar source at the same signal power and bandwidth. Quantum illumination is a first room-temperature application of microwave quantum circuits demonstrating quantum supremacy in detection and sensing.