Session D16: Fluxonium and Novel Superconducting Qubits II

Tunable Current Mirror Qubits: Experimental Status

In the last few years significant progress has been made in the development of tunable current-mirror qubits. This novel qubit type is projected to exhibit high coherence due to its insensitivity to environmental noise. However, as a consequence of their noise immunity, tunable current-mirror qubits are also insensitive to control signals, requiring novel control approaches. This talk reviews the successful demonstration of current-mirror test structures that share the same ‘Mobius’ capacitance connectivity as current-mirror qubits. The collective modes of these test structures exhibit a mode frequency parity effect such that even modes are much higher in frequency than odd modes—an indicator that such circuits can generate a parity valve effect—strong double-vortex hopping relative to single-vortex hopping—a key requirement for generating noise insensitivity. This talk also describes design optimizations of current-mirror qubits, and gives a status report on coherence tests, including tests that utilize adiabatic protection sweeps.   

Phase-shift flux qubit with a ferromagnetic π junction

A flux qubit with high anharmonicity is an attractive choice for large-scale superconducting quantum circuits. However, one of the biggest challenges is the necessity of an external flux bias corresponding to half flux quantum to achieve flux-insensitive point with the longest coherence time for each flux qubit.
In light of this, we developed phase-shift flux qubits by integrating a ferromagnetic π junction in the qubit loop providing spontaneous π phase shift, enabling flux-bias-free operations [1]. The developed qubit consists of a superconducting loop with three Al/AlO_x/Al junctions and an NbN/CuNi/NbN π junction [2] on an Si substrate. The qubit is shunted to a large capacitor and is capacitively coupled to a coplanar waveguide resonator made of NbN/TiN. Our results indicate that the phase-shift qubits are indeed at their flux-insensitive point without any external flux, with a relatively long coherence. In this talk, we will discuss the NbN-Al hybrid fabrication process and the results of the spectroscopy and time-domain measurements.
[1] T. Yamashita et al., Phys. Rev. Lett. 95, 097001 (2005).
[2] T. Yamashita et al., Phys. Rev. Appl. 8, 054028 (2017).   

Electrically-tunable phase-slip qubits

The simplest form of a flux qubit is constructed by interrupting a superconducting loop (e.g., a ring) with an insulating barrier, thus forming a Josephson junction. The tunneling of Cooper pairs across the insulating barrier allows for a coherent coupling of two macroscopic flux states with oppositely circulating currents. In a similar manner, one can interrupt the superconducting loop with a nanowire over which superconducting vortices are coherently exchanged, a mechanism referred to as quantum phase slips (QPSs), giving rise to phase-slip qubits. Here, we investigate the possibility of using a voltage-biased superconducting ring to realize a phase-slip qubit. Using time-dependent Ginzburg-Landau equations, we show that the bias voltage modulates the free energy barrier between subsequent flux states of the ring. For a small but non-zero barrier, we calculate the rate of QPSs as a function of bias voltage, and investigate the possibility of realizing electrically-tunable phase-slip qubits.   

Influence of charge fluctuations on Josephson phase-slip qubits

The Josephson phase-slip qubit (JPSQ) [1] is a superconducting circuit designed to emulate a quantum S=1/2, with a vector dipole moment that is nearly independent of applied effective field, even near zero. This property should enable the realization of full quantum vector spin interactions, including non-Stoquastic interactions that are of interest for quantum annealing and Hamiltonian quantum computing. We characterize the influence of charge fluctuations on the JPSQ, examining both discrete quasiparticle tunneling as well as drifts and jumps in the background charge offset. In addition, we describe methods for mitigating the influence of these effects to enhance operational robustness.

[1] A. J. Kerman, New J. Phys. 21, 073030 (2019)   

Bifluxon: Fluxon-Parity-Protected Superconducting Qubit

We have developed and characterized a symmetry-protected superconducting qubit that offers simultaneous exponential suppression of energy decay from both the charge and flux noise, and dephasing from flux noise [1]. The qubit is implemented as a superconducting loop formed by a Cooper-pair box (CPB) and a superinductor. Provided the offset charge on the CPB island is an odd number of electrons, the qubit potential corresponds to that of a cos (φ/2) Josephson element, preserving the parity of fluxons in the loop via Aharonov-Casher interference. Importantly, the protection can be turned on and off by controlling the CPB charge. In the protected state, the logical-state wavefunctions reside in disjoint regions of phase space, thereby ensuring protection against energy decay. By turning the protection on, we observed a ten-fold increase of the decay time, up to 100 μs. We will discuss strategies for mitigation of the charge noise effects by designing small arrays of cos (φ/2) elements.

1. K. Kalashnikov et al., Bifluxon: Fluxon-Parity-Protected Superconducting Qubit, arXiv:1910.03769.   

Blochnium: A flux-tunable qubit with flux-insensitive coherence times

We introduce a flux-insensitive superconducting artificial atom nicknamed ``blochnium", which is dual to a transmon. A blochnium circuit consists of a small-area Josephson junction shunted by a hyperinductance -- a micro-Henry range linear inductance whose impedance reaches above 160 kΩ. Using innovative circuit design and fabrication tricks, we significantly reduced the parasitic capacitance previously associated with such large inductances. The flux dispersion of the qubit transition is reduced to about 100 MHz while transitions to non-computational states can be tuned in the usual few-GHz range. Such a unique spectrum eliminates flux-noise induced dephasing during flux-controlled logical gates on coupled blochnium qubits, which in theory allows for gate fidelities better than 0.999.
  

Novel Qubit Designs

Lev Ioffe has pioneered the field of topologically protected superconducting qubuts, including the development of the rhombus qubit. This is a growing area within the superconducting community.   

Rabi Oscillations in a Superconducting Nanowire Circuit

At feature sizes of nanometer scale, superconducting wires made from a material with high normal state resistance show a pronounced nonlinear microwave response.
Disordered oxidized (granular) aluminum is a new material for superconducting quantum circuits, featuring not only a very high kinetic inductance but also microwave resonators with high quality factors [1,2]. Microscopically, it can be described as a disordered network of nano-scale aluminum grains, coupled via the Josephson effect [3].
We investigate the circuit quantum electrodynamics of superconducting nanowire oscillators. The sample circuit consists of a capacitively shunted nanowire with a width of about 20 nm and a varying length up to 350 nm, capacitively coupled to an on-chip resonator. By applying microwave pulses we observe Rabi oscillations, measure coherence times and the anharmonicity of the circuit. Despite the very compact design, simple top-down fabrication and high degree of disorder in the granular aluminum material, we observe lifetimes in the microsecond range. [4]

[1] H. Rotzinger, et al., Supercond. Sci. Technol. 30, 025002 (2016).
[2] L. Grünhaupt, et al., Nat. Mater. 18, 816 (2019).
[3] N. Maleeva, et al., Nat. Commun. 9, 3889 (2018).
[4] Y. Schön, et al., (2019), arXiv:1907.04107 [cond-mat.supr-con].   

Ultrastrong coupling to parasitic modes in superconducting circuits with hyperinductors

Parasitic modes are inevitable in any real-world circuit. The hope of any practical circuit design is that these modes are sufficiently above the frequency band of interest and do not participate in the low-frequency dynamics of the circuit. A notorious example is a large-value inductor. As the value of the inductance increases with the inductor length, so does the parasitic capacitance. Even in the simplest qubit circuit, in which a Josephson junction is shunted by a hyperinductor, the distributed nature of parasitic capacitance leads to multiple parasitic modes that couple ultrastrongly to the qubit. This ultrastrong coupling of the parasitic modes prevents a perturbation theory treatment of the qubit excitation spectra, while a complete quantum description of the underlying qubit circuit is computationally prohibitive. Progress can be made using an effective multi-mode Hamiltonian that fully reproduces the experimental data both below and above the lowest parasitic modes. This method allows the extraction of the bare qubit circuit parameters that are not renormalized by the presence of parasitic modes.   

Relaxation and decoherence of 2D fluxonium

High anharmonicity and wide frequency tunability of the Fluxonium qubit make it an indispensable candidate for emerging quantum computers. Moreover fluxonium qubit in a 3D cavity or environment, when biased at sweet spot, shows very high relaxation time well above 1ms with a potential to achieve higher magnitude [1]. However, 2D and on-chip qubits are more favorable to scale up. The transition from 3D to 2D is not trivial as the coupling to the unwanted degrees of freedom increases a lot. In this work we coupled the fluxonium qubit to on-chip lumped element and distributed resonators. We have studied effect of different qubit parameters on the coherence times T1 and T2.

[1] Ioan M. Pop et al., Nature 508, 369-372 (2014)   

A Rotary Echo Flux Qubit

Flux qubits can have long T1 decay times due to nearly disjoint support of their flux basis wavefunctions. However the coupling of flux noise to circulating currents can lead to short dephasing times. This talk describes a qubit that utilizes a rotary echo control technique to maintain long T1 times yet also generating insensitivity to flux noise below a cutoff frequency. As the majority of the flux noise is low frequency in nature, this generates a qubit design capable of achieving high coherence.   

Characterization and control of topologically protected charge-parity qubits: Part 1

Superconducting qubits with topological protection against local noise hold the promise of significantly enhanced coherence times and higher gate fidelities than is possible with conventional qubits. We are developing one such protected design — the hybrid charge-parity qubit — that combines arrays of compact, high inductances and conventional Josephson junctions with individual flux control for tuning each plaquette to a regime with a pi-periodic Josephson energy. With this scheme, concatenating multiple pi-periodic elements further enhances the degree of protection. Here, we will describe our experimental characterization of one- and two-plaquette devices through spectroscopy and time-domain measurements.   

Characterization of topologically protected charge-parity qubits: Part 2

Decoherence poses a major impediment to the implementation of large-scale quantum processing with superconducting qubits. There have been tremendous improvements in superconducting qubit coherence times over the past two decades, with current state-of-the-art coherence levels approaching the threshold for fault-tolerant quantum computing. Nevertheless, for scalability, high-fidelity qubit control and qubits protected from sources of noise at the hardware level are needed. We implement qubit protection with pi-periodic superconducting elements made from flux-biased plaquettes of kinetic inductors and Josephson junctions that allow only double Cooper pair tunneling. The amount of protection from noise in such plaquette-based qubits can be further increased by concatenating plaquettes. We describe the design and characterization of a plaquette-based qubit with an embedded SQUID switch.