Cryogenic Digital-Analog Converter for Two-Qubit Gates in Spin Qubits

Abstract

As the number of qubits increases, controlling qubits at cryogenic temperatures with electronics at room temperature becomes infeasible due to the vast number of cabling. This necessitates controlling quantum operations on the qubits with a cryogenic interface, which is low power, scalable and maintains qubit fidelity.

In pursuit of this interface, this thesis investigates the two-qubit DAC specifications of both the adiabatic and nonadiabatic CPHASE gate. The DAC specifications are identified using simulation models of spin qubits. The adiabatic signal requirements are found to be lower than the nonadiabatic signals in both sampling frequency and quantization resolution. A new method using spectral analysis on the unitary of the adiabatic operation is used to infer the minimum sampling frequency by applying the Nyquist Criterion and shows why they lose their fidelity as gate time decreases.

A novel two-stage current-switching DAC architecture is proposed, which maintains fidelity operating at 140MHz sampling rate with less than 75 uW of power consumption. The first 6-bit stage maintains CPHASE fidelity for different qubit pairs, while the second 5-bit stage creates the adiabatic pulse. Finally, the DAC is implemented in the Intel 16-nm finFET process.