Anharmonicity of a superconducting qubit with a few-mode Josephson junction

Abstract

Coherent operation of gate-voltage-controlled hybrid transmon qubits (gatemons) based on semiconductor nanowires was recently demonstrated. Here we experimentally investigate the anharmonicity in epitaxial InAs-Al Josephson junctions, a key parameter for their use as a qubit. Anharmonicity is found to be reduced by roughly a factor of two compared to conventional metallic junctions, and dependent on gate voltage. Experimental results are consistent with a theoretical model, indicating that Josephson coupling is mediated by a small number of highly transmitting modes in the semiconductor junction.

Current-phase relations of InAs nanowire Josephson junctions: From interacting to multimode regimes

“…For example, a recent work examined multiple Andreev reflections as a probe of transparency in few-mode junctions, but ignored interaction effects which are known to influence IV characteristics [18,53]. Spectroscopy experiments also studied anharmonicity and Zeeman-induced spin-splitting of Andreev levels in the few-mode regime, but may also be affected by splitting effects from interactions as examined in our work [16,19]. Overall our ability to accurately model device behavior, as well as to understand and control both interactions and anharmonicity, will elucidate the interpretation of current experiments and provide a method for device design.…”

“…Gate-tunable nanowire Josephson junctions have also been integrated into a cavity-QED architecture, providing electrostatic control of the qubit transition frequency [14,15]. Improving the design and operation of these devices will rely on understanding their electrostatic environment and electronic structure, with several recent works examining mode behavior in wire devices [16,17,18,19,20,21]. Moreover, electron-electron interactions, often overlooked in both experiment and theory, should also be considered as they are expected to play an important role, particularly in one or fewer dimensions.…”

Coherent control of a hybrid superconducting circuit made with graphene-based van der Waals heterostructures

“…With E C /h = 100 MHz used for this circuit, the critical Josephson current I c of the S-G-S junction is estimated to be 90 nA ~ 360 nA, consistent with results reported in previous DC-transport studies 3 . In contrast to superconductor -insulator -superconductor tunnel junctions, where E J is generally modified using a current to induce a magnetic field in a SQUID-type configuration, in superconductor-normal conductor-superconductor devices the value of E J can be tuned with V g due to two effects: (i) V g changes the Fermi energy and thereby directly modifies the density of states, and therefore the total number of transmission channels 6,23,29 , and (ii) V g modifies the transmission probability for a number of high-transmission conduction channels 7,8,30 . Overall this results in an electrostatic control of the Andreev spectrum in graphene 6 shows clear asymmetry in f qb with respect to V CNP, manifested as a lower f qb value in the P-doped region for a given gate voltage -as compared to its equal carrier density (same |V g -V CNP |) counterpart in the N-doped region -and pronounced f qb oscillations observed in the P-doped region.…”

Josephson currents in chaotic quantum dots

“…This motivated us to perform a detailed understanding of their CPRs, which is crucial in modeling of qubit nonlinearity. In light of the recent nanofabrication advances and development of gatemon qubits [25][26][27] we focus our study on quantum dot Josephson junctions with multi-mode transparent interfaces. Coincidently, in this regime the effect of the secondary gap is the most pronounced.…”