Cryogenic Digital Flux Control of Superconducting Qubits

To achieve scalable superconducting quantum computers, qubit control electronics need to be integrated at cryogenic temperatures, ideally at the same stage as qubits. Although cryogenic digital charge control was shown recently [1], magnetic flux control for implementing single- and two- qubit gates has yet to be demonstrated at the mK stage. Dynamic flux control is most commonly implemented by applying voltage pulses synthesized by an AWG at room temperature. These analog pulses are susceptible to distortions that form as they traverse the electrical lines connecting the control hardware to the qubits at cryogenic temperatures [2]. The pulse distortions limit qubit gate fidelities. Furthermore, the application of the voltage pulses can induce crosstalk to unaddressed qubits. In this talk, we present cryogenically integrated digital flux control of tunable transmon qubits using energy-efficient Rapid Single Flux Quantum (RSFQ) technology [3] at 10 mK. Here, the transmon is inductively coupled to a current loop, where fluxons are injected or drained by triggering an on-chip dc-to-SFQ converter. With this method, we demonstrate controllable two-qubit interactions without the need for complex pulse-shaping, as well as an order of magnitude reduction in the flux crosstalk to spectator qubits.



[1] C.H. Liu et al., PRX Quantum 4, 030310 (2023)

[2] M.A. Rol et al., Appl. Phys. Lett. 116, 054001 (2020)

[3] D.E. Kirichenko et al., IEEE Trans. Appl. Supercond. 21, 776 (2011)