A Quantum Sensor Remembers Weak Signals That Classical Memory Forgets

Many sensors measure each weak signal and store a classical result. This paper asks what changes when the receiver preserves quantum coherence across an entire coded sequence instead. The sender represents a classical codeword as sequential weak phase rotations. The comparison keeps the signal budget fixed, so the claimed advantage comes from what the receiver retains between arrivals rather than from receiving a stronger signal.^[1][2]

Memory becomes part of the receiver

The proposed receiver accumulates weak phase rotations in persistent quantum memory before decoding. In the weak-signal limit, the authors prove separations from receivers with the same signal budget but only classical memory between incoming bits. The result turns memory itself into a sensing resource.^[1][2]

Where the separation appears

For even-k simplex codes, the protocol exactly corrects the stated bounded-error regime using Q that scales as (t+1) log n and a phase angle proportional to 1 divided by k times the square root of n+1. Under independent flips with fixed p below 1/16, the paper also gives a regime where the quantum-memory decoder's error vanishes while the compared classical-memory receiver's average success vanishes.^[1][2]

A theorem, not a device

This is a theoretical and asymptotic separation, not an experimental sensor. It relies on structured code families, weak phase rotations and specified noise. The paper does not show a practical device, a universal advantage for every signal, or a finished architecture for long-lived fault-tolerant memory.^[1][2]

The laboratory test

The useful next test is experimental: implement a small codeword instance with realistic memory decoherence, compare it with the strongest adaptive classical strategy, and report total hardware, calibration and readout costs. That would show whether the theorem's memory advantage survives laboratory overhead.^[1][2]