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

20
Jan
2021

# Observation of thermalization and information scrambling in a superconducting quantum processor

Understanding various phenomena in non-equilibrium dynamics of closed quantum many-body systems, such as quantum thermalization, information scrambling, and nonergodic dynamics, is

a crucial for modern physics. Using a ladder-type superconducting quantum processor, we perform analog quantum simulations of both the XX ladder and one-dimensional (1D) XX model. By measuring the dynamics of local observables, entanglement entropy and tripartite mutual information, we signal quantum thermalization and information scrambling in the XX ladder. In contrast, we show that the XX chain, as free fermions on a 1D lattice, fails to thermalize, and local information does not scramble in the integrable channel. Our experiments reveal ergodicity and scrambling in the controllable qubit ladder, and opens the door to further investigations on the thermodynamics and chaos in quantum many-body systems.

19
Jan
2021

# Tunable Coupling Architecture for Fixed-frequency Transmons

Implementation of high-fidelity two-qubit operations is a key ingredient for scalable quantum error correction. In superconducting qubit architectures tunable buses have been explored

as a means to higher fidelity gates. However, these buses introduce new pathways for leakage. Here we present a modified tunable bus architecture appropriate for fixed-frequency qubits in which the adiabaticity restrictions on gate speed are reduced. We characterize this coupler on a range of two-qubit devices achieving a maximum gate fidelity of 99.85%. We further show the calibration is stable over one day.

17
Jan
2021

# Coherent manipulation of an Andreev spin qubit

Two promising architectures for solid-state quantum information processing are electron spins in semiconductor quantum dots and the collective electromagnetic modes of superconducting

circuits. In some aspects, these two platforms are dual to one another: superconducting qubits are more easily coupled but are relatively large among quantum devices (∼mm), while electrostatically-confined electron spins are spatially compact (∼μm) but more complex to link. Here we combine beneficial aspects of both platforms in the Andreev spin qubit: the spin degree of freedom of an electronic quasiparticle trapped in the supercurrent-carrying Andreev levels of a Josephson semiconductor nanowire. We demonstrate coherent spin manipulation by combining single-shot circuit-QED readout and spin-flipping Raman transitions, finding a spin-flip time TS=17 μs and a spin coherence time T2E=52 ns. These results herald a new spin qubit with supercurrent-based circuit-QED integration and further our understanding and control of Andreev levels — the parent states of Majorana zero modes — in semiconductor-superconductor heterostructures.

14
Jan
2021

# A reversed Kerr traveling wave parametric amplifier

Traveling wave parametric amplification in a nonlinear medium provides broadband quantum-noise limited gain and is a remarkable resource for the detection of electromagnetic radiation.

This nonlinearity is at the same time the key to the amplification phenomenon but also the cause of a fundamental limitation: poor phase matching between the signal and the pump. Here we solve this issue with a new phase matching mechanism based on the sign reversal of the Kerr nonlinearity. We present a novel traveling wave parametric amplifier composed of a chain of superconducting nonlinear asymmetric inductive elements (SNAILs) which allows this sign reversal when biased with the proper magnetic flux. Compared to previous state of the art phase matching approaches, this reversed Kerr phase matching mechanism avoids the presence of gaps in transmission, reduces gain ripples, and allows in situ tunability of the amplification band over an unprecedented wide range. Besides such notable advancements in the amplification performance, with direct applications to superconducting quantum computing, the in-situ tunability of the nonlinearity in traveling wave structures, with no counterpart in optics to the best of our knowledge, opens exciting experimental possibilities in the general framework of microwave quantum optics and single-photon detection.

# Quantum computing with superconducting circuits in the picosecond regime

We discuss the realization of a universal set of ultrafast single- and two-qubit operations with superconducting quantum circuits and investigate the most relevant physical and technical

limitations that arise when pushing for faster and faster gates. With the help of numerical optimization techniques, we establish a fundamental bound on the minimal gate time, which is determined independently of the qubit design solely by its nonlinearity. In addition, important practical restrictions arise from the finite qubit transition frequency and the limited bandwidth of the control pulses. We show that for highly anharmonic flux qubits and commercially available control electronics, elementary single- and two-qubit operations can be implemented in about 100 picoseconds with residual gate errors below 10−4. Under the same conditions, we simulate the complete execution of a compressed version of Shor’s algorithm for factoring the number 15 in about one nanosecond. These results demonstrate that compared to state-of-the-art implementations with transmon qubits, a hundredfold increase in the speed of gate operations with superconducting circuits is still feasible.

13
Jan
2021

# Magnetic-Field-Compatible Superconducting Transmon Qubit

We present a hybrid semiconductor-based superconducting qubit device which remains coherent at magnetic fields up to 1 T. The qubit transition frequency exhibits periodic oscillations

with magnetic field, consistent with interference effects due to the magnetic flux threading the cross section of the proximitized semiconductor nanowire junction. As induced superconductivity revives, additional coherent modes emerge at high magnetic fields, which we attribute to the interaction of the qubit and low-energy Andreev states.

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.