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Journal of Applied Physics

Optimizing multi-tone microwave pulses via phase selection for quantum computing applications

Ryutaro Ohira, R. Matsuda, H. Shiomi, Kazuhisa Ogawa, Makoto Negoro

Published Sep 20, 2024• 6 citations
Publisher

Abstract

Multi-tone microwave signals are crucial for advanced quantum computing applications, including frequency-multiplexed qubit control and simultaneous two-qubit gate execution. However, interference among microwave signal components can cause signal amplitudes to surpass the output limits of an arbitrary waveform generator (AWG), hindering the generation of precise signals necessary for accurate qubit manipulation. To address this issue, we introduce a method that adjusts the phase of individual microwave signal components, effectively reducing interference and maintaining signal amplitude within the AWG’s operational range.

DOI
10.1063/5.0222724

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Multi-tone microwave signals are crucial for advanced quantum computing applications, including frequency-multiplexed qubit control and simultaneous two-qubit gate execution. However, interference among microwave signal components can cause signal amplitudes to surpass the output limits of an arbitrary waveform generator (AWG), hindering the generation of precise signals necessary for accurate qubit manipulation. To address this issue, we introduce a method that adjusts the phase of individual microwave signal components, effectively reducing interference and maintaining signal amplitude within the AWG’s operational range.
Current state of the art quantum computing experiments in the microwave regime use control pulses generated by modulating microwave tones with baseband signals generated by an arbitrary waveform generator (AWG). Recent advances in digital analog conversion technology have made it possible to directly synthesize arbitrary microwave pulses with sampling rates up to 92 gigasamples per second (GS/s). These new high bandwidth AWG's could dramatically simplify the classical control chain for quantum computing experiments, enabling more advanced pulse shaping and reducing the number of components that need to be carefully calibrated. Here we use a high speed AWG to study the viability of such a simplified scheme. We characterize the AWG and perform randomized benchmarking of a superconducting qubit, achieving average single qubit gate error rates below 5×10−45\times10^{-4}5×10−4.
The increasing number of control lines connecting quantum processors to external electronics constitutes a major bottleneck in the realization of large-scale quantum computers. Frequency-division multiplexing is expected to enable control of multiple qubits through a single microwave cable; however, interference from off-resonant microwave tones hinders precise qubit control. Here, we propose an active interference suppression method for frequency-division-multiplexed simultaneous gates on microwave-controlled qubits. We demonstrate that the deliberate incorporation of off-resonant microwave tones improves single-qubit gate fidelity. In particular, the gate infidelity scales inversely with the square of the number of microwave tones when off-resonant orthogonal or quasi-orthogonal tones are incorporated. Furthermore, we show that fast oscillations, neglected under the rotating wave approximation, degrade the gate fidelity, and that this degradation can be mitigated through optimized frequency allocation. The proposed approach is simple and effective for improving the performance of frequency-division-multiplexed quantum gates.
A register of quantum bits with fixed transition frequencies and weakly coupled to one another through simple linear circuit elements is an experimentally minimal architecture for a small-scale superconducting quantum information processor. Presently, the known schemes for implementing two-qubit gates in this system require microwave signals having amplitudes and frequencies precisely tuned to meet a resonance condition, leaving only the signal phases as free experimentally adjustable parameters. Here, we report a minimal and robust microwave scheme to generate fast, tunable universal two-qubit gates: simply irradiate one qubit (the ``control'') at the transition frequency of another (the ``target''). The effective coupling between them is then switched on by tuning only the frequency of this single drive tone; the drive amplitude adjusts the effective coupling strength; and the drive phase selects the particular two-qubit gate implemented. This cross-resonance effect turns on linearly with the ratio of the drive amplitude $\ensuremath{\Omega}$ to the qubit-qubit detuning $\ensuremath{\Delta}$, as compared with earlier proposals that turn on as ${(\ensuremath{\Omega}/\ensuremath{\Delta})}^{4}$.
Let's boot up a quantum computer. Far from being a simple push of a button, initializing a prototype quantum computer requires the precise tuning and calibration of many different parameters. Rather than switching transistors on and off, the controller of a quantum processor emits combinations of analog waveforms, each with a characteristic shape, frequency, and duration. These waveforms are used to either manipulate or read out the states of quantum bits (qubits), the basic units of quantum information in the processor. The analog nature of the system inherits many of the complexities of analog computing, including device parameter drift and offsets, system component tolerances and variabilities, and other well-known analog-circuit intricacies. Further, de livering these signals to the target qubits necessitates coordination between multiple low-noise and low-jitter instruments. In spite of these challenges, the quantum computing community has made tremendous progress toward useful quantum machines. We hope to provide an instructive introduction to the control, signal generation, and distribution principles currently used in small quantum systems that operate in the microwave frequency regime.
Scalable quantum information processing requires that modular gate operations can be executed in parallel. The presence of crosstalk decreases the individual addressability, causing erroneous results during simultaneous operations. For superconducting qubits which operate in the microwave regime, electromagnetic isolation is often limited due to design constraints, leading to signal crosstalk that can deteriorate the quality of simultaneous gate operations. Here, we propose and demonstrate a method based on the alternative-current Stark effect for calibrating the microwave signal crosstalk. The method is suitable for processors based on fixed-frequency qubits, which are known for high coherence and simple control. The optimal compensation parameters can be reliably identified from a well-defined interference pattern. We implement the method on an array of seven superconducting qubits and show its effectiveness in removing the majority of crosstalk errors.