A paper recently published in Nature Communications by researchers from the University of Southern California displayed a novel quantum sensing technique that addresses decoherence limitations and significantly surpasses conventional methods. This method could hasten advances in diverse fields, including foundational physics research and medical imaging.1-3
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Context and Background
Quantum sensing involves using quantum systems like light particles, atoms, or qubits as sensors to measure physical quantities like ultra-precise clocks, brain activity, or gravity anomalies with exceptional precision, surpassing the classical sensors’ capabilities. Quantum sensing devices use quantum properties like entanglement, superposition, and coherence to detect tiny signals otherwise obscured by noise. The quantum sensor performance has been limited by decoherence for decades, which is unpredictable behavior caused by environmental noise. Decoherence causes a quantum system’s state to become randomly scrambled, erasing the quantum sensing signal.2,3
Ramsey interferometry has been established as the most sensitive qubit frequency measure. A qubit is prepared in a superposition of energy states, allowed to freely evolve and acquire phase, and then measured along a certain axis in a Ramsey measurement. The phase acquired/the measured state probability relies on the qubit frequency. This protocol is used for magnetic field and other continuous variables’ quantum sensing, for biomedical applications, for foundational physics, for non-equilibrium quasiparticle density detection, and for rapid qubit frequency recalibration.1
Decoherence limits the of quantum sensors’ signal-to-noise ratio (SNR). Until now, most studies have focused on SNR scaling beyond the √N standard quantum limit, where N represents the number of independent qubits/measurements, using dynamical decoupling to improve frequency discrimination and characterize/reduce non-Markovian decoherence using measurement-based feedback to rapidly lock in on time-varying signals, large signals, or an unknown signal axis to compensate for measurement errors. Additionally, researchers also developed sensors that couple strongly to desired signals and are less prone to decoherence. Yet, no results displayed improvement over Ramsey interferometry to increase the SNR from a single qubit measuring a static/zero frequency, small signal.1
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Key Research Findings
In this study, researchers addressed the issue of decoherence using a novel pre-determined coherence-stabilized protocol on their experiment's qubit, stabilizing one key property of the quantum state. Specifically, the qubit frequency measurement’s sensitivity in the presence of decoherence was improved by applying a continuous drive to stabilize one Bloch vector’s component.1-3
The objective of this novel protocol was to enhance quantum sensing of static fields. The protocol is based on a recent quantum property preservation theory4, showing how a quantum state’s certain scalar functions can be stabilized using purely Hamiltonian control. Deterministic Hamiltonian control of a single qubit was utilized to stabilize one Bloch vector component, allowing greater phase accumulation in the orthogonal component and, resultantly, enhanced sensitivity.1
Researchers derived analytical expressions in the small-signal regime for the signal enhancement, and displayed simulations regarding the protocol’s robustness to miscalibrations. The protocol was demonstrated on a superconducting qubit, improving sensitivity per qubit evolution time by 1.09× and sensitivity per measurement shot by 1.65× compared to Ramsey. Significant signal enhancement was achieved using the proposed protocol over standard Ramsey interferometry, up to a factor of 1.18 per qubit evolution time or 1.96 per measurement shot, during theoretical investigations of the protocol. Additionally, the protocol was robust to parameter miscalibrations and requires no feedback, extra control, or measurement resources.1
Thus, the experiment improved the small frequency shift measurement in quantum systems significantly. The work’s coherence-stabilized sensing protocol enabled the sensing signal to grow larger than it would with the standard protocol sensing measurement.1-3
Specifically, this stabilization could be critical for applications where subtle signal detection is necessary. The protocol can be applied immediately in different quantum sensors and quantum computing technologies. Additionally, the experimental demonstration of sensing with a stabilized state demonstrated the existence of approaches to improve quantum sensors without depending on complicated techniques like entangling many sensors or real-time feedback.2,3
Methodology Summary
The device used in this work was a standard grounded superconducting transmon qubit coupled to a quarter-wave transmission line cavity. The cavity and qubit were far off resonance. While there was approximately zero energy exchange between cavity and qubit, the cavity frequency shifted by χ/2π = 150 kHz when the qubit changed state in this dispersive regime.1
The qubit was measured by driving the cavity using an on-resonant pulse generated by combining a carrier at the cavity frequency with a Gaussian envelope. The pulse transmitted through the device, interacted with the cavity as it passed, then passed through an amplification chain up to room temperature, where it was mixed back down to direct current (DC) with an in-phase and quadrature (IQ) mixer, giving a two-channel DC voltage signal that was then digitized.1
The measured two-channel voltage was projected onto an axis that gives maximum discrimination between the signals for the qubit excited and ground states. Qubit state rotations were driven by driving the qubit state with an on-resonance microwave pulse. The duration and amplitude of the pulse determined the total rotation angle, while the phase determined the rotation axis in the xy plane.1
This novel coherence-stabilization protocol offers an alternative to traditional decoherence suppression techniques like spin locking and dynamical decoupling. While those methods use continuous or pulsed control to average out slowly varying noise like noisy Hamiltonian terms along orthogonal axes or quasi-static noise, they typically eliminate sensitivity to static signals and enhance detection only at specific oscillatory frequencies. In contrast, the coherence-stabilization approach is effective for broadband Markovian decoherence and preserves sensitivity to static Hamiltonian terms.1
Industry Implications
This broadly applicable, resource-efficient technique is suitable for speeding calibration of qubit parameters and unconditionally enhancing the SNR from qubit-based sensors. It is particularly useful for sensing magnetic fields or measuring the field-to-frequency transduction function of a spin species. While these measurements are often done in ambient conditions far from the low-temperature limit, which would seem to reduce the benefit of the proposed protocol, the system is initialized in a thermal state, ensuring performance gains over traditional methods like Ramsey.1
Future Outlook
This novel decoherence-mitigating technique significantly boosts SNR over Ramsey interferometry but is not fully optimal. For any given environmental condition, a Bloch trajectory may exist that yields a stronger signal than coherence stabilization alone, framing the challenge as one of optimal control. This problem is relatively unconstrained, the initial state, final state, and total evolution time can all vary, necessitating numerical solutions.1
Similar optimal control methods have been applied in quantum sensing of time-varying and large signals, offering valuable insights. Future work could extend this sensitivity enhancement to entangled states. However, like Ramsey, the current protocol cannot reach the Heisenberg limit, where SNR scales linearly with experiment time. Combining this method with entanglement-based sensing, continuous weak-measurement feedback, and advanced quantum control techniques could potentially unlock further improvements in quantum sensor performance.1
References and Further Reading
- Hecht, M. O., Saurav, K., Vlachos, E., Lidar, D. A., Levenson-Falk, E, M. (2025). Beating the Ramsey limit on sensing with deterministic qubit control. Nature Communications, 16(1), 1-8. DOI: 10.1038/s41467-025-58947-4, https://www.nature.com/articles/s41467-025-58947-4
- Overcoming the quantum sensing barrier: New protocol counteracts the limitation of decoherence [Online] Available at https://phys.org/news/2025-04-quantum-barrier-protocol-counteracts-limitation.html (Accessed on 13 May 2025)
- Joosting, J, P. (2025) Protocol overcomes the quantum sensing decoherence barrier [Online] Available at https://www.eenewseurope.com/en/protocol-overcomes-the-quantum-sensing-decoherence-barrier/ (Accessed on 13 May 2025)
- Saurav, K., Lidar, D. A. (2025). Quantum property preservation. PRX Quantum, 6(1), 010335. DOI: 10.1103/PRXQuantum.6.010335, https://journals.aps.org/prxquantum/abstract/10.1103/PRXQuantum.6.010335
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