Performance limits of a quantum receiver for detecting phase-modulated communication signals

Quantum sensors are an ideal candidate for detecting weak electromagnetic signals because of their exceptional sensitivity and compact form factor. In this work, we analyze the performance of a quantum-sensor-based receive chain for demodulating information encoded in phase-modulated electromagnetic waves. We introduce a generalized cumulant expansion to model a noisy quantum receiver and use it to compare the performance of various quantum demodulation protocols. Employing bit error probability (BEP) and channel capacity as quantitative performance metrics, we compare the capabilities of ensembles of quantum sensors - both unentangled and entangled - using Binary Phase-Shift Keying (BPSK) as a representative example of phase modulation. We identify conditions when the channel capacity of an ensemble of quantum sensors may surpass the limits of a classical electrically small antenna. Additionally, we discuss modifications to the quantum protocol that enables high-fidelity data recovery even in the presence of sensor noise and channel distortions. Finally, we explore practical performance limits of such a quantum receive chain, with a focus on NV-diamond as the quantum sensor platform.

💡 Research Summary

The paper investigates a quantum‑sensor‑based receiver architecture for demodulating phase‑modulated (specifically binary phase‑shift keying, BPSK) electromagnetic signals. The authors focus on ensembles of nitrogen‑vacancy (NV) centers in diamond, but the analysis applies to any two‑level quantum system. The work proceeds in several logical stages.

First, the classical communication background is reviewed. A BPSK signal is written as (S(t)=\sqrt{2P_s}\cos(2\pi f_s t+\phi(t))) with (\phi(t)\in{0,\pi}). For an electrically small antenna the Chu limit imposes a minimum quality factor (Q_{\text{Chu}}=1/(ka)^3+1/(ka)), which in turn limits the bandwidth and the Shannon‑Hartley channel capacity. This sets a benchmark that the quantum receiver must surpass.

Second, the authors recast the detection problem as a binary quantum state discrimination (QSD) task. The two possible phases correspond to two quantum states (\rho_0) and (\rho_1) of the sensor ensemble after interaction with the signal. The optimal measurement is described by a two‑outcome POVM ({E_0,E_1}) that minimizes the single‑shot bit‑error probability (BEP). Using the Helstrom bound, the BEP satisfies
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