Session N48: Unconventional Superconducting Qubits

Microwave characterization of hybrid superconductor-semiconductor junction arrays

Arrays of Josephson junctions can achieve a nondissipative impedance much larger than the impedance of free space. These devices have largely been limited to Al/AlOx tunnel junction arrays, but a superconductor-semiconductor (S-Sm) junction array could be of interest for its application in a voltage tunable superconducting quantum circuit architecture. We show microwave measurements on an S-Sm array to study microwave loss, nonlinearity, and phase slip dynamics in S-Sm junctions. We also show preliminary results with a S-SM array incorporated in a fluxonium style qubit.   

Toward implementation of protected charge-parity qubits

Topologically protected qubits hold the potential for orders-of-magnitude improvements in coherence compared to conventional qubits. One such protected qubit is the charge-parity (C-parity) qubit, which consists of a pi-periodic Josephson element shunted by a large capacitance. Here, we implement imperfect pi-elements as "plaquettes" consisting of two arms in parallel, each arm incorporating a small-area Josephson junction in series with a large inductor. When the plaquette is biased at half a flux quantum, the first harmonic of the Josephson energy is suppressed and the second harmonic, which is proportional to cos(2φ), remains. While individual plaquettes are not protected against dephasing from flux noise in the loop, protection scales exponentially with the number of plaquettes concatenated in series. In this talk, we describe the design, fabrication, and characterization of multi-plaquette devices that aspire to protection.   

Electronic capacitance in tunnel junctions for protected charge-parity qubits

The prospects of implementing the charge-parity topologically protected qubit rely on the hybridization of states resulting from concatenated cos(2φ) elements known as plaquettes, implemented with dc SQUIDs with non-negligible inductances. The required junction parameters for optimal protection with these plaquettes fall outside the range of standard transmon junctions, where both the large Josephson and charging energy required can push the plasma frequency close to the superconductor gap frequency. As a result, a renormalization occurs in the form of an extra electronic capacitance that acts to push down the effective plasma frequency. This electronic capacitance contributes to the overall junction capacitance and poses a significant obstacle in creating junctions for new charge-parity qubits with optimal protection. This talk will describe measurements of electronic capacitance in small tunnel junctions and strategies for mitigating this in future charge-parity topologically protected devices.   

Geometrical approach for designing novel superconducting qubits

To unlock the potential of quantum computers, one of the key challenges that the field has to overcome is to preserve the coherence of a quantum superposition over extended times. Besides implementing quantum error correction schemes, a complementary approach to prolong the coherence of quantum processors is to develop qubits that are intrinsically protected against decoherence. In this talk, first, we present a universal method to quantize circuits containing both Josephson junctions and quantum phase slip elements [1]. Relying on the connection between symplectic geometry, graph theory, and circuit theory, we describe a Hamiltonian formulation and quantization of non-dissipative electrodynamic circuits. In the second part of the talk, we present experimental results on how to use disordered superconductors to build superconducting qubits with protection against information loss. In particular, we focus on elements built from disordered WSi which has shown excellent properties in single photon detectors.

[1] Osborne et al., arXiv:2304.08531 (2023)   

Design and Characterization of the Heavy Double Cooper-Pair Tunneling Element

Symmetry plays an important role in various physical systems, giving rise to parity, selection rules, and interference effects. Notably, single-Cooper-pair tunneling across a symmetric Josephson rhombus biased at its frustration point is forbidden, allowing the exploration of multi-Cooper-pair tunneling processes. Here, we report the implementation of an inductively shunted double-Cooper-pair junction in the heavy regime, with circuit parameters tailored to enhance the novel effect. Specifically, we discuss fabrication optimization, control and readout strategy, and decoherence mechanisms of the circuit, establishing the framework for building other types of superconducting circuits with similar topologies. The circuit can also be broadly adapted for potential applications in nano-fabrication testing, quantum metrology, and quantum sensing.   

Exploring the superconducting grid-states qubit

Quantum information is inevitably susceptible to decoherence mechanisms imposed by the environment, leading to imprecision that hinders the progress in quantum computing. One promising approach to correct information loss is to encode a finite-dimensional subspace non-locally into the infinite-dimensional Hilbert space of a continuous-variable system, forming a periodic grid where error can be efficiently detected and suppressed. Alternatively, the information can be protected if it is hardware-encoded into the grid-like eigenstates of a superconducting circuit with dual nonlinearity in charge and phase. Here, we present an approach to implement such a qubit by combining the quasicharge element with the double-Cooper-pair tunneling element. We discuss the concepts, design principles, and expected behaviors of the circuit. We then showcase the experimental progress toward implementing the qubit.   

Multimode effects in a unimon circuit

Unimon is a novel superconducting qubit that exhibits high anharmonicity and is fully protected from low-frequency charge noise due to its island-free structure [1]. It consists of a grounded half-wavelength coplanar waveguide (CPW) resonator, with the central conductor interrupted by a Josephson junction. Due to the distributed nature of the CPW, unimon circuits are inherently multimode systems. Previous models for unimon have been insufficient in accurately and efficiently treating the multimode effects. We introduce a new numerical model that efficiently incorporates the multimode effects for a typical unimon circuit [2]. Provided that the junction is positioned away from the center of the circuit, we find significant cross-Kerr interactions between modes. This observation paves the way for the utilization of the multimode structure, for example, to encode multiple qubits in a single unimon circuit.

[1] Hyyppä, E., Kundu, S., Chan, C.F. et al. Unimon qubit. Nat Commun 13, 6895 (2022).

[2] S. Tuohino, V. Vadimov, W.S. Teixeira et al., Multimode physics of the unimon circuit, arXiv:2309.09732 (2023).   

High-gap Josephson junctions for single-film superconducting qubits

This talk reports recent experiments aimed at creating Josephson junctions and their associated superconducting qubits based on constrictions in high-gap superconducting thin films rather than conventional barrier oxides. Our goal is to create fabrication-friendly superconducting qubits from a single material, harnessing the properties of its intrinsic electronic phases, and operating at elevated temperature. The talk will discuss design considerations for constriction-based qubits and present preliminary measurements on junctions fabricated in a single lithographic step without shadow evaporation. These results indicate a promising path towards high-gap single-film superconducting qubits for operation at super-Kelvin temperature and frequencies above 20GHz.   

Metrology to Enable Higher Temperature/Frequency Qubits

Typical quantum information systems using superconducting qubits operate in the (4-8) GHz frequency range and at temperatures below 50 mK. Operating superconducting qubits at temperatures above ~0.5 K is a likely essential  requirement in order to scale system sizes to larger and larger numbers of qubits. To operate at higher temperatures and remain above the thermal background, qubits must operate at higher frequencies where the appropriate materials, designs, and metrology need to be developed and characterized. In this presentation, we discuss the measurement science tools we are developing and plan to use to quantify sources of loss and decoherence in resonators and qubits at frequencies above ten gigahertz. Specifically, we demonstrate cryogenic high-Q resonators, parametric amplifiers, and harmonic generating devices using kinetic inductance and present our measured characterizations of these devices up to 30 GHz freq and 0.5 K temperature.

A Millimeter-wave Qubit Cooled with Helium-4

Superconducting qubits are a promising platform for quantum experiments studying light-matter interactions in the strong coupling regime. Current microwave qubits are cooled to extremely low temperatures through dilution of helium-3 and helium-4 in order to reduce sources of decoherence. Scaling these superconducting quantum devices to millimeter-wave frequencies (near 100 GHz) can greatly reduce their sensitivity to thermal noise. This enables operation at significantly higher temperatures near 1 K: achievable using simpler methods such as helium-4 evaporation, which provides orders of magnitude higher cooling power. Combining low-loss niobium trilayer junctions with a ground-shielded millimeter-wave coupling design, we realize a qubit at 72 GHz with resolved energy level transitions in a cryogenic system cooled solely with Helium-4. Using high-speed millimeter-wave pulse spectroscopy, we measure decoherence and dephasing times of 15.9 and 17.4 ns respectively, corresponding to qubit quality factors of 7.2x10³. This demonstration of a millimeter-wave quantum emitter enables new opportunities for quantum transduction, integrated experiments with high heat dissipation requirements, and opens the door to higher-frequency, higher-energy quantum experiments.   

Superconducting qubits at elevated frequencies

Quantum processors based on superconducting qubits typically operate in a frequency range between 4-8 GHz. Raising the qubit frequencies beyond this well-established frequency range could result in significant advantages including reduced residual mode populations, enhanced anharmonicities and the potential for operation at elevated cryogenic temperatures. In this work, we study the properties of transmon-type superconducting qubits based on niobium electrodes and standard Al/AlOx/Al junctions with resonance frequencies up to 20GHz. We observe a reduction in qubit lifetimes by two orders of magnitude for high-frequency qubits compared to standard devices operating around 5 GHz. To investigate the source of the additional losses we measure the behavior of niobium (Nb) and aluminum (Al) resonators in the same range. We find that the quality factors of the Nb resonators are nearly constant as a function of frequency, while the losses of Al resonators significantly increase at higher frequencies, suggesting that the lower qubit lifetimes are caused by losses from the Al of the junctions. Finally, we propose strategies to transition to niobium-based junctions to overcome these limitations and explore qubit properties at higher temperatures.   

Tunable on-chip readout resonator in the W-band

Readout resonators are an integral part of all circuit QED (cQED) setups as it enables QND measurement on the qubit state. In this work, we demonstrate superconducting “Ouroboros” resonators in the W-band (75-110 GHz). Previously we were able to design and tune quarter-wavelength NbTiN resonators with DC bias lines and radial stub filters to optimize the RF response [1]. But rescaling devices with different sheet inductances require complete redesigning of all circuit components which is labor intensive. Hence, the goal of this work is to demonstrate easy tunability of the resonator using the Meissner effect. We will present finite element electromagnetic simulations alongside measurement results on fabricated devices. Achieving this takes us one-step closer to implementing a cQED system at the millimeter-wave frequencies.

[1] D. Das, et al., "Superconducting on-chip tunable mm-wave resonator" , doi: 10.1109/IRMMW-THz50927.2022.9895550.