1982/10/15 by Carlton M. Caves · 1 voice · 23 citations
Computer Science · Engineering · Physics and Astronomy · #Analog and Mixed-Signal Circuit Design #Quantum Information and Cryptography #Quantum Mechanics and Applications
paper · doi:10.1103/physrevd.26.1817
openalex publication_date 1982/10/15 · openalex created_date 2025/10/10 · openalex updated_date 2026/07/29
How much noise does quantum mechanics require a linear amplifier to add to a signal it processes? An analysis of narrow-band amplifiers (single-mode input and output) yields a fundamental theorem for phase-insensitive linear amplifiers; it requires such an amplifier, in the limit of high gain, to add noise which, referred to the input, is at least as large as the half-quantum of zero-point fluctuations. For phase-sensitive linear amplifiers, which can respond differently to the two quadrature phases ("cos\ensuremathωt" and "sin\ensuremathωt"), the single-mode analysis yields an amplifier uncertainty principle---a lower limit on the product of the noises added to the two phases. A multimode treatment of linear amplifiers generalizes the single-mode analysis to amplifiers with nonzero bandwidth. The results for phase-insensitive amplifiers remain the same, but for phase-sensitive amplifiers there emerge bandwidth-dependent corrections to the single-mode results. Specifically, there is a bandwidth-dependent lower limit on the noise carried by one quadrature phase of a signal and a corresponding lower limit on the noise a high-gain linear amplifier must add to one quadrature phase. Particular attention is focused on developing a multimode description of signals with unequal noise in the two quadrature phases.