Researchers reframe qubit errors as solvable with software compensation

Researchers at the University of Waterloo have demonstrated a software-based method to correct for coherent errors in qubits and qudits, achieving a reduction in the contribution of AC mains interference to oscillator detuning. The team verified their protocol using a trapped 137Ba+ ion, recovering an average single-qubit gate fidelity of 99.93(1)% on a magnetic-field-sensitive qubit after compensation.

This work demonstrates that reproducible, trigger-synchronized errors can be reframed as deterministic control errors and corrected through software-defined updates to control signals, and extends the framework to 16-level qudits, increasing the success probability of the Bernstein-Vazirani algorithm from 10(7)% to 70(9)%.

Trigger-Synchronous Errors in Qubits and Qudits Defined

This substantial reduction demonstrates the efficacy of reframing reproducible, time-dependent disturbances as deterministic control-frame errors, allowing for correction via software-defined updates to control frequencies and phases. The team’s approach directly addresses coherent, time-dependent disturbances that commonly affect quantum control experiments, particularly those occurring on timescales comparable to the experiment’s duration. The framework developed centers on measuring the effect of disturbances relative to a trigger signal and then compensating for resulting detuning and phase evolution directly within the control software.

This method bypasses the need to suppress the disturbance at its source, offering a practical solution to a pervasive problem in quantum computing environments. Specifically, the researchers focused on magnetic-field fluctuations synchronized with the AC mains line signal, which induce predictable frequency shifts and accumulated phase errors when experiments are triggered by the line signal.

Experimental verification involved a trapped 137Ba+ ion, where the protocol successfully suppressed both instantaneous detuning errors and the resulting phase accumulation. Extending the validation beyond single qubits, the researchers applied the compensation framework to multi-level qudit control using the Bernstein-Vazirani algorithm as a benchmark, increasing the algorithm’s success probability from 10(7)% to 70(9)% when the compensation was enabled.

Software Compensation of Oscillator Detuning & Phase

The calibrated contribution of standard AC mains interference to instantaneous oscillator detuning was reduced by a factor of 21(9), demonstrating a substantial improvement in qubit control achieved through software compensation. This reduction, alongside a corresponding decrease in AC-induced phase amplitude below measurement uncertainty, highlights the efficacy of a new approach to mitigating coherent errors in quantum systems. The technique doesn’t rely on physical shielding or active cancellation, but instead reframes reproducible disturbances as deterministic control-frame errors correctable through software-defined updates to oscillator frequency and phase.

The core of this method lies in recognizing that time-dependent disturbances, when synchronized with a trigger signal, manifest as predictable errors in both frequency and phase. By measuring these disturbances in a line-trigger-referenced time coordinate, researchers can calculate the resulting phase evolution and apply compensating adjustments to the control oscillator on a pulse-by-pulse basis.

The system employs an RFSoC-based control system, enabling independent programming of the frequency and phase of each optical tone used for modulation, a key element in achieving precise compensation. These results demonstrate the protocol’s ability to address a common source of coherent errors, one often encountered in standard laboratory settings. The approach is not limited to magnetic-field fluctuations; it is applicable regardless of the physical origin of the disturbance, provided the resulting detuning waveform can be determined.

Randomized benchmarking confirmed a 99.93(1)% average single-qubit gate fidelity on a qubit particularly susceptible to magnetic fields following implementation of the compensation protocol, a substantial improvement over uncompensated performance.

Bernstein-Vazirani Algorithm Demonstrates Qudit Control

The Bernstein-Vazirani algorithm, an oracle-identification task, relies on preserving coherent phase information throughout a quantum circuit to accurately determine an unknown hidden value. Without compensation, the algorithm yielded an average success probability of 10(7)%, indicating significant errors in phase preservation. Enabling the compensation framework increased this success probability to 70(9)%, demonstrating its ability to maintain coherence across a more complex quantum operation. The researchers note that the framework extends naturally to qudits demonstrating its adaptability to more general multilevel quantum systems.

Measuring & Correcting Coherent Errors in Software

This approach reframes these previously problematic, time-dependent errors as deterministic control-frame errors, allowing for precise software correction of both instantaneous frequency errors and accumulated phase shifts during pulse sequences. The compensation framework extends beyond single-qubit operations to multi-level qudit control, enabling more complex quantum computations, and offers a promising avenue for improving the stability and scalability of quantum systems.

More like this

Quantum Research NewsNambu’s fluid notes suggest a new path to quantum coherence

Quantum Research NewsResearchers estimate quantum computing resources for lattice field theories

Quantum PhysicsPhysicists simulate magnetic monopoles on a quantum spin chain

Quantum PhysicsRubidium atoms let physicists retrodict quantum spin beyond uncertainty