L–Quantum and Classical Sensors

By U.W. on 30 of June 2025 — as part of our Quantum Hardware Lecture

In a temperature-stabilized lab, the disturbing hum of the air-conditioning and other support equipment surrounds every element, including the Paul trap housing a single atomic ion. In a Ramsey sequence, the π/2 pulses prepare the ion in a superposition and analyse its coherence, and each fluorescence photon tells us how likely it is that it has felt a flicker of the magnetic flux around its set value or not. Within one second, about 1,000 repetitions on the trapped-ion sensor are measured, leveraging its phase coherence to attain sensitivities as low as pT/√Hz. By contrast, modern smartphones sample their Hall-effect magnetometers at rates up to a few hundred hertz, achieving field resolutions in the nT/√Hz range. Equally striking is the spatial scale: the ion’s sensing volume spans merely tens of nanometres, whereas Hall devices integrate over active areas of about 100×100 μm² or larger. Both instruments, however, share a similar anatomy: a transducer converting stimulus into a primary signal, a n integral detector registering events—whether scattered photons or charge carriers—followed by signal conditioning, digitization, and model-based interpretation to extract physical insights.

Classical sensors hinge on well-charted noise floors and calibration routines: thermal agitation and shot noise impose a standard limit (∼1/√N – here N equals to number of repetitions), bounding metrics like bandwidth, dynamic range, and precision. Quantum sensors, in contrast, exploit phase coherence and phase determination to expand these bounds. In the trapped-ion Ramsey sequence, the π/ 2 pulses serve as analogues of the beam splitters of a Mach–Zehnder interferometer, splitting and recombining the quantum “paths” to convert a phase shift into a population difference. Going further, entangled sensor entities boost performance: two entangled qubits yield interference fringes oscillating at twice the frequency of single-ion fringes, doubling phase sensitivity. Likewise, N00N states (in Mach–Zehnder interferometers) produce N-fold fringe frequency enhancement, approaching the Heisenberg limit ∼1/N, where here N denotes the Fock state number or the number of maximally entangled particles.

As we step back from these engineered marvels, we confront a deeper question: if every measurement depends on preserving—and then disturbing—quantum coherence, how much of the “measured” world is shaped by the very act of sensing? What limits does the measurement problem impose on our claims to knowledge, and how might the entanglement of detector and detected redefine the boundary between observer and system?

Special thanks to all my past, current, and future environments.

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References for Quantum Sensing

[1] V. Giovannetti, S. Lloyd, and L. Maccone, “Quantum-enhanced measurements: beating the standard quantum limit,” Nature Photonics 5, 222–229 (2011).

[2] C. L. Degen, F. Reinhard, and P. Cappellaro, “Quantum sensing,” Reviews of Modern Physics 89, 035002 (2017).