# Freiburg 2019 – wissenschaftliches Programm

## Sitzungen | Tage | Auswahl | Suche | Aktualisierungen | Downloads | Hilfe

# FM: Fall Meeting

## FM 24: Quantum Sensing: Entanglement and Beyond Shot Noise

### FM 24.5: Talk

### Montag, 23. September 2019, 17:30–17:45, 3044

**Approximate quantum non-demolition measurements** — Sami Boulebnane, Mischa P. Woods, and •Joseph M. Renes — Institute of Theoretical Physics, ETH Zürich

With the advent of gravitational wave detectors employing squeezed light, quantum waveform estimation---estimating a time-dependent signal by means of a quantum-mechanical probe---is of increasing importance. As is well known, backaction of quantum measurement limits the precision with which the waveform can be estimated, though these limits can in principle be overcome by ``quantum nondemolition'' (QND) measurement setups found in the literature. Strictly speak- ing, however, their implementation would require infinite energy, as their mathematical description involves Hamiltonians unbounded from below. This raises the question of how well one may approx- imate nondemolition setups with finite energy or finite-dimensional realizations. Here we consider a finite-dimensional waveform estimation setup based on the ``quasi-ideal clock'' and show that the estimation errors due to approximating the QND condition decrease slowly, as a power law, with increasing dimension. As a result, we find that good QND approximations require large energy or dimensionality. We argue that this result can be expected to also hold for setups based on truncated oscillators or spin systems.