A quantum computer has the potential to solve some specific problems that take huge amounts of time in conventional classical computers. Quantum bits (qubits), which can express the superposition of two quantum states, are used in the quantum computer. Superconducting qubits using Josephson junctions have excellent controllability and integration in a variety of qubits [1]. In order to realize a large-scale quantum computer, it is highly demanded to integrate a qubit control system as close as possible to qubits that operate at cryogenic temperature to reduce the heat load, latency, power consumption, and the overall system foot-print [2]. Generally, superconducting qubits are controlled by an irradiation of a ns-width microwave pulse with a frequency of several GHz. In this study, we propose a microwave pulse generator with variable amplitude to control the qubit using single flux quantum (SFQ) circuits [3].

The proposed microwave pulse generator can generate a number of microwave pulses whose envelopes are arbitrarily varied to perform the desired operations of qubits. The data to determine the envelope of the microwave pulses are stored in registers in advance. By selecting an appropriate register by a decoder, a microwave pulse for the desired operation of qubits is generated.
The basic part of the microwave pulse generator is composed of circular shift register (CSR), a low-pass filter, and an impedance matching circuit. The CSR is a shift register with a feedback loop for the non-destructive redout of an SFQ pulse train. A microwave pulse of the frequency around several GHz are obtained by removing the higher harmonics of an SFQ pulse train by using a low-pass filter. The amplitude of microwaves can be modulated by changing the density of SFQ pulses. To increase the precision of the microwave amplitude, the higher the frequency of the SFQ pulse train, the better. Therefore, the operating frequency of the CSR needs to be high enough. In addition, the bit length of CSR should be large enough because the required microwave pulse width is several ns.
In this study, we designed and implemented a 64-bit CSR using the AIST-ADP2 process with the critical current density of 10 kA/cm2. The functional test of the 64-bit CSR is performed at high speed using on-chip clock generators by shifting the stored data in the CSR at high speed. We confirmed the correct operation of the CSR up to 68 GHz, which is limited by the on-chip clock generator.

Reference
[1] J. Clarke and F. K. Wilhelm, “Superconducting quantum bits,” Nature (London) 453, 1031, 2008.
[2] R. McDermott and M. G. Vavilov, “Accurate Qubit Control with Single Flux Quantum Pulses,” Phys. Rev. Appl. 2, 014007, 2014.
[3] K. K. Likharev and V. K. Semenov, “RSFQ logic/memory family: a new Josephson-junction technology for sub-terahertz-clock-frequency digital systems”, IEEE Trans. Appl. Supercond., 1991.

Keywords: single flux quantum, qubit