Adapting Microwave Control to Evolving Quantum Computers
Operating a quantum processor requires programming and generating a precisely timed sequence of pulses that are resonant with characteristic transition frequencies of the qubit device. For some of the most advanced platforms, comprised of trapped ions, spins, or superconducting quantum bits, these transitions lie in the microwave range (typically from S to K bands). In this talk, we will give an overview of the state-of-the-art in qubit control using microwaves, with focus on superconducting qubits, starting from the description of single-qubit operations all the way up to the current systems, capable of controlling tens of qubits simultaneously. To this day, the most common setup consists of a room-temperature transmitter circuit, including microwave sources, mixers, and DACs, which combine to deliver the desired pulse sequences to the qubits in a cryogenic setup. These microwave pulses — with duration varying from ~1e-8 to 1e-6 s — are calibrated to implement the quantum gates that compose the programmed algorithm, or the measurements that determine its outcome. A similar circuit is placed on the receiver side to capture the measurement pulses returning from the quantum processor and convert them into qubit states. Furthermore, closed-loop control following measurement requires low latency between receiver and transmitter to overcome the limited (1e-5 to 1e-4 s) lifetime of qubit states. As initial single-qubit demonstrations have given way to more complex experiments, including long sequences and multi-qubit operations, increasing attention has been paid to multi-channel properties, such as relative phase stability and suppression of crosstalk. With the quality and quantity of qubits in a processor continuously growing, the requirements on microwave properties, starting at room temperature and all the way down to the device, become more stringent and expose new bottlenecks to the quantum circuit performance. Challenges to scalability include per-qubit cost and size of the control hardware, signal delivery, and sub-nanosecond synchronization between transmitters. We present some of the current directions to address these issues, drawing on established microwave tools such as frequency multiplexing and direct digital synthesis.