Quantum Sensing Demonstrates Order-of-magnitude Differences in Robustness to Pulse Errors and Qubit Leakage

Quantum sensors promise unprecedented precision in measuring magnetic fields, but their performance relies on maintaining the delicate quantum states of individual qubits. David M. Lancaster, Muhammad Ali Shahbaz, and Hamed Goli Yousefabad, alongside colleagues at the University of Nevada, Reno, now demonstrate that these sensors are surprisingly vulnerable to common experimental imperfections. The team investigates how realistic errors, such as imprecise control pulses and unwanted leakage of quantum information, impact the effectiveness of dynamical decoupling sequences, techniques used to shield qubits from noise. Their results reveal substantial differences in performance between commonly used sequences, with some proving far more robust to these errors than others, and represent a crucial step towards building practical and reliable quantum sensors.

Using both computer simulations and laboratory experiments, they assessed the robustness of dynamical decoupling sequences, techniques designed to protect quantum information, under these conditions. They discovered that these sequences exhibit varying sensitivities to different types of errors, impacting their ability to detect weak signals, crucial for practical applications. The results demonstrate a trade-off between the length of the decoupling sequence and its susceptibility to errors, providing valuable insights for optimising quantum sensing protocols. The study considered the effects of qubit leakage, where quantum information escapes the sensing qubit, and showed how this further degrades performance, contributing to a more complete understanding of the challenges facing quantum sensing in noisy environments and paving the way for more reliable and robust quantum sensors.

MLEV Pulse Sequence Enhances Spin Sensing

This research details an investigation into methods for enhancing the sensitivity of magnetic resonance sensing, particularly for detecting nearby spins, relevant to nanoscale sensing applications. The authors compared several pulse sequences, including Carr-Purcell-Meiboom-Gill (CPMG) and its variations, to optimise performance in the presence of both leakage and pulse errors. They found that a sequence called MLEV, a modified version of CPMG, offers a good compromise between robustness to these two error sources. Magnetic resonance sensing uses the magnetic properties of materials to detect and measure physical quantities, with applications ranging from medical imaging to materials science and nanoscale sensing.

Pulse sequences are series of precisely timed radiofrequency pulses and magnetic field gradients used to manipulate the spins of atomic nuclei or electrons. Different pulse sequences are designed to achieve specific goals, such as suppressing unwanted signals, enhancing sensitivity, or extracting information about the sample. Leakage refers to signal loss due to imperfections in the experimental setup or the sample, while pulse errors arise from imprecise control of the radiofrequency pulses or magnetic field gradients. The research question addressed was: which pulse sequence provides the best sensitivity for magnetic resonance sensing in the presence of both leakage and pulse errors?

The approach involved theoretical analysis using mathematical modelling and simulations, experimental validation to verify the theoretical predictions, and a comparison of the performance of CPMG, MLEV, and other sequences under varying conditions. The key finding was that MLEV offers a good balance between robustness to leakage and pulse errors, meaning it performs well even when the experimental setup is not perfect. These findings could lead to more sensitive magnetic resonance sensors with applications in biomedical imaging, materials science, and quantum information processing, and could also inform the design of more robust and reliable magnetic resonance sensors and contribute to the development of new nanoscale sensing techniques.

Decoupling Sequence Performance Under Realistic Errors

This research demonstrates that dynamical decoupling sequences, used to protect quantum information, respond differently to common experimental errors like imprecise pulse durations and unintended interactions with other quantum levels, known as leakage. Simulations and experiments reveal significant variations in performance between commonly used sequences, APCP, CPMG, XY16, and MLEV32Y, under these realistic conditions. Specifically, the team found that APCP and CPMG are more robust against leakage, maintaining longer coherence times, while XY and MLEV32Y exhibit greater sensitivity to pulse errors. Notably, the study highlights a trade-off between these sensitivities; APCP and CPMG excel when leakage is dominant, but struggle with pulse errors, and vice versa for XY and MLEV32Y. Under conditions of both leakage and pulse errors, MLEV appears to offer the best overall compromise. The authors acknowledge that no single protocol currently provides complete protection against both types of errors simultaneously, suggesting a need for further development of protocols that can mitigate both leakage and pulse errors effectively.