We demonstrate K-band nondegenerate three-wave-mixing amplification in a multimode resonator comprising an array of 500 SNAILs. Pairing K-band signal modes with lower-frequency internalidler modes enables 20 dB of gain continuously from 19.6 to 25.6 GHz with pump frequencies below 32.2 GHz. At 20 dB gain, the 3-dB bandwidth ranges from 2.6 to 11.7 MHz, and the input-referred 1-dB compression power reaches -100.9 dBm. In the 20-22 GHz band, the average added noise is 0.73 quanta. A two-mode nondegenerate parametric amplification model describes the measured gain response and gain-bandwidth behavior, while a multimode Kerr model captures the overall compression scale. Our amplifier provides a practical solution for implementing a state-of-the-art readout of superconducting qubits in the K-band.
The gatemon qubits, made of transparent super-semi Josephson junctions, typically have even weaker anharmonicity than the opaque AlOx-junction transmons. However, flux-frustrated gatemonscan acquire a much stronger anharmonicity, originating from the interference of the higher-order harmonics of the supercurrent. Here we investigate this effect of enhanced anharmonicity in split-junction gatemon devices based on InAs/Al 2D heterostructure. We find that anharmonicity in excess of 100% can be routinely achieved at the half-integer flux sweet-spot without any need for electrical gating or excessive sensitivity to the offset charge noise. We verified that such „gateless gatemon“ qubits can be driven with Rabi frequencies more than 100 MHz, enabling gate operations much faster than what is possible with traditional gatemons and transmons. Furthermore, by analyzing a relatively high-resolution spectroscopy of the device transitions as a function of flux, we were able to extract fine details of the current-phase relation, to which transport measurements would hardly be sensitive. The strong anharmonicity of our gateless gatemons, along with their bare-bones design, can prove to be a precious resource that transparent super-semi junctions bring to quantum information processing.
We propose to use an ancilla fluxonium qubit to interact with a Majorana
qubit hosted by a topological 1D wire. The coupling is obtained using the
Majorana qubit controlled $4pi$ Josephsoneffect to flux bias the fluxonium
qubit. We demonstrate how this coupling can be used to sensitively identify
topological superconductivity, to measure the state of the Majorana qubit, to
construct 2-qubit operations, and to implement quantum memories with
topological protection.