Quantum Enhancement of Electric Field Measurement Beyond Classical Limits.

Measurements of electric field frequency now surpass the standard quantum limit using classical states and subharmonic excitation. This technique, demonstrated with a single calcium ion, extends measurement precision beyond decoherence times and is applicable to diverse quantum platforms across radio frequency, microwave and optical spectra.

The pursuit of increasingly precise measurement techniques underpins advances across diverse scientific and technological fields. A limitation frequently encountered is the trade-off between measurement duration and the preservation of quantum states; longer measurements, while potentially yielding greater precision, are susceptible to decoherence – the loss of quantum information. Researchers at the University of California, Los Angeles, have developed a method to circumvent this constraint, achieving enhanced measurement resolution even with extended measurement times. Hao Wu, Clayton Z. C. Ho, Grant D. Mitts, Joshua A. Rabinowitz, and Eric R. Hudson detail their findings in a new study titled ‘Floquet-engineered decoherence-resilient protocols for wideband sensing beyond the linear standard quantum limit’, demonstrating a technique utilising precisely timed, subharmonic excitation to measure electric field frequencies with resolution exceeding the conventional limitations of classical measurement.

Subharmonic Excitation Enhances Frequency Measurement Precision

A novel quantum sensing technique utilising subharmonic excitation of a trapped ion achieves frequency resolution beyond the standard quantum limit. The method centres on Raman excitation of a harmonic oscillator – specifically, the motional state of a single ⁴⁰Ca⁺ ion – through engineered Floquet states, resulting in enhanced sensitivity and accuracy.

Experimental validation confirms the sensor’s operation across a broad frequency range, demonstrating its potential for wideband applications. Characterisation of the K=6 subharmonic resonance at 70, 80, and 200 MHz reveals overlapping lineshapes, indicating minimal sensitivity to signal frequency variations and suitability for broadband operation. The achieved frequency resolution reaches 0.56(32) Hz for an 80 MHz signal tone, measured over a 5 ms interrogation period and averaged across 9600 repetitions, representing a substantial improvement in measurement precision.

Motional coherence sustains for 20 ms, enabling extended measurement times and facilitating more accurate data acquisition. Analysis identifies the 5 Watt damage threshold of the trap electronics as the primary constraint on further resolution improvements, providing a clear pathway for future development.

Surprisingly, researchers demonstrate that coherent states provide a more robust and sensitive readout mechanism for this system, offering a practical advantage over more complex quantum states. The study compares the sensitivity of phonon readout using coherent and squeezed states, revealing that coherent states consistently yield superior results. Calculations of Fisher information – a metric quantifying the amount of information a random variable carries about an unknown parameter – consistently favour coherent states, demonstrating enhanced detection sensitivity and establishing their suitability for precision measurement in this context. This finding challenges the assumption that non-classical states invariably improve measurement precision and highlights the importance of careful state selection for optimal performance.

Researchers anticipate that this technique can be extended to other quantum platforms, including nitrogen-vacancy (NV) centres in diamond, solid-state qubits, and neutral atoms, expanding its versatility. This scalability opens up the possibility of developing highly sensitive sensors across a wide range of frequencies, from radio waves to optical light, with potential applications in diverse fields such as precision spectroscopy, materials science, and fundamental physics tests. Further improvements are possible, notably through upgrading the electronics controlling the ion trap, which currently limits the achievable signal amplification.

The demonstrated technique exhibits potential for extension to diverse platforms, offering a versatile approach to precision sensing across radio frequency, microwave, and optical domains. This scalability suggests the potential for deploying high-precision frequency measurements across a broad spectrum of frequencies, encompassing diverse applications.