Finland Researchers Develop Robust Qubit Control for Quantum Computing Advancement

Researchers from Aalto University in Finland have developed robust, high-fidelity pulses for quantum control, based on phase modulation of the control field. The team demonstrated the resilience of these operations against deviations in drive amplitude and detuning from the qubit transition frequency. The modulation scheme can be implemented in any qubit-based experimental platform. The research addresses challenges in superconducting qubits, proposing a solution by chirping the frequency of the pulses. The techniques are validated in a transmon qubit setup, but are hardware-agnostic, making them versatile for quantum computing and simulation.

What is the Significance of High-Fidelity Robust Qubit Control by Phase-Modulated Pulses?

A team of researchers from the InstituteQ and QTF Centre of Excellence Department of Applied Physics School of Science at Aalto University in Finland have developed a set of robust and high-fidelity pulses that can perform paradigmatic operations such as the transfer of the ground state population into the excited state and arbitrary XY-rotations on the Bloch sphere. These pulses are based on the phase modulation of the control field. The team demonstrated the resilience of these operations against deviations in the drive amplitude of more than 20% and/or detuning from the qubit transition frequency in the order of 10 MHz. This modulation scheme is straightforward to implement in practice and can be deployed to any other qubit-based experimental platform.

The researchers’ work is a significant contribution to the field of quantum control, a toolbox of techniques enabling high fidelity dynamical operations, which is an essential tool in modern quantum technologies. The first example of quantum control is the 1932 Rosen-Zener sech-function design of the shape of the rate of rotation of the magnetic field in a double Stern-Gerlach experiment. However, changing the shape of the pulse envelope, its phase or frequency is a less explored avenue. The team’s work addresses this gap by demonstrating the precise manipulation of the pulse phases in the time domain, which allows the formulation of quantum control schemes where the phase is an externally controlled parameter.

How Does This Research Address Challenges in Superconducting Qubits?

Superconducting qubits, one of the most promising platforms for quantum computing and simulation, present a specific set of challenges. These artificial atoms, comprising several materials and complex geometries, are difficult to model accurately and completely. Losses and unaccounted-for interactions inevitably lead to errors when using simplified models. To combat this, several concepts have been proposed and applied recently, such as error mitigation, extrapolation to zero-noise limit, probabilistic error cancellation, Pauli and Clifford twirling, derivative removal by adiabatic gate, dynamical decoupling, counter-diabatic methods, composite pulses, and more recently reinforcement learning.

However, each of these methods comes with its own disadvantages. For example, machine-learning techniques typically require discretized forms of multiple control parameters, which complicates their synthesis by standard control systems. While this problem may be alleviated in the future by the use of cryogenic control systems, such as Josephson arbitrary waveform synthesizers, such pulses may still have a power spectrum leading to spurious excitations in a larger frequency-crowded device.

What is the Solution Proposed by the Researchers?

The researchers propose a solution to these challenges by chirping the frequency of the pulses according to relatively simple and smooth functions. This can achieve gates that are robust against both amplitude and frequency errors. The power of the microwave pulse is restricted to avoid exciting other modes due to frequency crowding and to limit the effect of nonlinearities in microwave components. This inevitably leads to pulses that are close to rectangular.

The team demonstrated this by implementing two paradigmatic operations: transfer of population from one level to another and arbitrary rotations on the Bloch sphere. In both cases, the experimental data are supported by simple theoretical models and by numerical simulations that include only parameters extracted from independent characterization measurements.

How Can This Research be Applied in Practice?

The control techniques developed by the researchers are validated in a setup comprising a transmon qubit, although the methods developed here are in principle hardware-agnostic. This means that they can be applied to any hardware setup, making them versatile and widely applicable in the field of quantum computing and simulation.

The robustness and high fidelity of these techniques make them a valuable tool in the toolbox of quantum control techniques. Their straightforward implementation and resilience against deviations in the drive amplitude and detuning from the qubit transition frequency make them a promising solution to the challenges faced in superconducting qubits. This research opens up new avenues for the exploration and application of phase-modulated pulses in quantum control.