Two-Qubit Gate Performance Now Optimises Via Just Two Measured States

Scientists Alessandro Marcomini from the Peter Grünberg Institute – Quantum Control (PGI-8), Forschungszentrum Jülich GmbH, and Philipp J. Vetter from Ulm University, with colleagues at the Peter Grünberg Institute – Quantum Control (PGI-8), Forschungszentrum Jülich GmbH, and the Institute for Quantum Optics, Ulm University, have developed a new method for optimising quantum gates using efficient two-qubit benchmarking. The research centres on nitrogen-vacancy centres in diamond and introduces a closed-loop optimisation technique that incorporates experimental feedback, a feature previously considered impractical due to the substantial demands placed on measurement resources. This innovative approach sharply reduces the number of measurements needed for gate optimisation by two orders of magnitude compared to conventional process tomography, thereby enabling faster and more practical quantum control experiments, as highlighted by Tommaso Calarco and Felix Motzoi. The development addresses a critical need in the field of quantum information processing, where achieving high-fidelity control over qubits is paramount for building functional quantum computers and advanced quantum sensors.

Two-state evaluation delivers substantial gains in quantum gate fidelity

Error rates were reduced to 0.6 percent, representing a reduction of two orders of magnitude compared to conventional optimisation techniques such as process tomography. Precise calibration of two-qubit gates previously demanded an extensive number of measurements, significantly hindering practical implementation and limiting the scalability of quantum systems. Achieving high-fidelity gate operation necessitates complex calculations involving the precise shaping of control pulses and exhaustive testing of their effects on the quantum system, thereby limiting the speed and scalability of quantum experiments. Process tomography, a standard method for characterising quantum gates, requires a complete mapping of the gate’s behaviour across all possible input and output states, which becomes computationally expensive and time-consuming as the number of qubits increases.

A long-standing barrier in quantum control has now been surpassed. The method simplifies this process by evaluating gate performance using data from only two quantum states, enabling real-time adjustments via closed-loop optimisation. This streamlined approach enables faster and more reliable control of nitrogen-vacancy centres in diamond, which are defects in the diamond lattice exhibiting quantum mechanical properties, making them important for advancing quantum computing and sensing technologies. Nitrogen-vacancy centres possess a spin that can be manipulated and used as a qubit, offering relatively long coherence times compared to other qubit modalities. The ability to precisely control these spins is crucial for performing complex quantum operations.

The protocol was tested on sample systems with unknown parameters, confirming its adaptability and identifying key calibration factors. Control pulses can now be tested and refined more rapidly, accelerating progress towards practical quantum computers and opening possibilities for more complex pulse sequences. A two-qubit gate optimisation achieved a figure of merit exceeding 0.99, a substantial improvement over typical values obtained with conventional methods. This figure of merit represents a quantitative measure of the gate’s accuracy and reliability, with values closer to 1 indicating higher fidelity.

No prior method matched this level of efficiency. Numerical simulations revealed a reduction of two orders of magnitude in the number of measurements needed for calibration, streamlining the process sharply. These simulations currently assume ideal conditions and do not yet account for the complexities of long-term coherence – the duration for which a qubit maintains its quantum state – or the impact of manufacturing imperfections on real devices. Maintaining coherence is a significant challenge, as qubits are susceptible to environmental noise that can cause them to decohere and lose information. Imperfections in the diamond lattice or variations in the nitrogen-vacancy centre’s properties can also affect gate performance.

The team applied this method to nitrogen-vacancy centres in diamond, a promising platform for quantum technologies, and successfully optimised gate performance under realistic experimental conditions. While validating improvements via simulation differs from achieving them in a real quantum device, the technique offers a pathway to faster development of more stable and scalable quantum computing systems. The ability to rapidly calibrate and optimise gates is essential for building larger quantum processors, where the cumulative effect of errors can quickly degrade performance. This research contributes to the ongoing effort to overcome these challenges and realise the full potential of quantum computing.

Reduced measurement requirements for improved diamond qubit gate fidelity

Optimising qubits—the fundamental units of quantum information—demands increasingly precise control over their delicate states. This research offers a streamlined method for calibrating two-qubit gates within nitrogen-vacancy centres in diamond, utilising a closed-loop optimisation technique. Naren Manjunath from the Perimeter Institute and colleagues acknowledge a key tension: can this computationally-validated improvement translate to real-world performance given the inherent complexities of maintaining quantum coherence and accounting for manufacturing imperfections in physical devices. The fidelity of quantum gates is directly linked to the accuracy with which quantum information can be processed, and even small errors can accumulate and lead to incorrect results.

Imperfections in diamond and maintaining the fragile quantum coherence of qubits present ongoing challenges. The new calibration method sharply reduces the measurement burden needed to optimise two-qubit gates, enabling the use of more complex pulse sequences. Real-time adjustments to quantum systems are now possible, previously limited to theoretical, open-loop control scenarios. Open-loop control relies on pre-calculated control pulses without incorporating feedback from measurements, making it susceptible to errors caused by uncertainties in the system parameters.

This advance cuts the measurement workload for optimising two-qubit gates in diamond by a hundredfold. A new calibration method for two-qubit gates—the fundamental building blocks of quantum computation—has been introduced within nitrogen-vacancy centres in diamond. Experimental feedback drastically reduced the complexity of assessing gate performance. The closed-loop approach allows the system to adapt to variations in the experimental conditions and compensate for imperfections in the control pulses.

Closed-loop optimisation of a two-qubit gate is enabled by preparing and measuring only two quantum states. The approach was tailored for nitrogen-vacancy centres in diamond and, via numerical simulations, can reduce required measurements by two orders of magnitude compared to standard process tomography. Speed doubled, offering a significant advantage for complex quantum systems. By minimising the number of measurements, the technique reduces the time required for calibration and allows for more frequent adjustments, improving the overall stability and performance of the quantum system.