We demonstrate the efficient modulation of flux-tunable superconducting resonators (FTRs) using flip-chip or on-chip-based input coils. The FTRs we use are aluminum-based quarter-wave coplanar waveguide resonators terminated with 100um or 200um-wide square loop dc superconducting quantum interference devices (SQUIDs) employing 1um-sized Josephson junctions. We employ SQUIDs with a geometric loop inductance of up to 0.7nH to increase the flux transfer efficiency. The geometric inductance of the SQUID results in a non-zero screening parameter
βL, whose branch switching effect is mitigated by using asymmetric junctions. We achieve flux modulation of the FTRs by more than one GHz and flux responsivities of up to tens of GHz/
Φ0 with uA-scale on-chip currents. We compare flip-chip with on-chip input-coil-based flux modulation, where the former is realized through galvanically connected and closely spaced chips, while the latter is achieved through superconducting air-bridge connections. We achieve a flux-transfer efficiency from the input coil to the SQUID loop of up to 20%. Our work paves the way for efficient low current flux modulation of FTRs and sensitive measurement of flux signals.
The spin qubit in quantum dots is one of the leading platforms for quantum computation. A crucial requirement for scalable quantum information processing is the high efficient measurement. Here we analyze the measurement process of a quantum-dot spin qubit coupled to a superconducting transmission line resonator. Especially, the phase shift of the resonator is sensitive to the spin states and the gate operations. The response of the resonator can be used to measure the spin qubit efficiently, which can be extend to read out the multiple spin qubits in a scalable solid-state quantum processor.
Abstract An algorithm for estimating the energy of a radio pulse when measuring a superconducting system qubit–resonator consisting of a superconducting resonator and a coupled qubit is proposed. The algorithm is based on the use of complete sufficient statistics and does not require a priori information about the level of additive white Gaussian noise in the process, as well as about the amplitude, initial phase, and temporal position of the radio pulse. The estimates are effective and consistent. The efficiency of the algorithm is estimated by means of computer simulation.