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Observing and Evading Quantum Backaction in an Interferometer

TYPECondensed Matter Seminar
Speaker:Nir Kampel
Location:Lidow Nathan Rosen (300)

In 1980, quantum mechanical backaction was shown to limit interferometric position measurement by C. Caves [PRL 45, 75]. Soon after, backaction evasion schemes were developed to surpass this limit. Only recently have the experimental conditions matured enough to enable reaching this regime with a macroscopic resonator. We use a silicon nitride membrane resonator in a high-finesse optical cavity to measure effects that arise in the backaction-dominated regime, for example, squeezing the probe light noise. Here I present initial experiments in which we avoid the backaction using two optical tones, thereby increasing the force (displacement) sensitivity.

Using Raman asymmetry thermometry we have shown that recent improvements allow us to reach a new regime. For example reaching (and surpassing) the force sensitivity limit given by the balance between the measurement imprecision and backaction (at zero temperature), known as the standard quantum limit. First, we have introduced a phononic crystal surrounding the membrane, which acoustically isolate the membrane by ~30dB. Second, we reduced the initial cryogenic temperature which enabled us to cool the mechanical mode to its ground state (mean occupation of 0.2).