Quantum Enhancement of the Zero-Area Sagnac Interferometer Topology for Gravitational Wave Detection

Only a few years ago, it was realized that the zero-area Sagnac interferometer topology is able to perform quantum nondemolition measurements of position changes of a mechanical oscillator. Here, we experimentally show that such an interferometer can also be efficiently enhanced by squeezed light. We achieved a nonclassical sensitivity improvement of up to 8.2 dB, limited by optical loss inside our interferometer. Measurements performed directly on our squeezed-light laser output revealed squeezing of 12.7 dB. We show that the sensitivity of a squeezed-light enhanced Sagnac interferometer can surpass the standard quantum limit for a broad spectrum of signal frequencies without the need for filter cavities as required for Michelson interferometers. The Sagnac topology is therefore a powerful option for future gravitational-wave detectors, such as the Einstein Telescope, whose design is currently being studied.

Figure 1

Schematic of the experiment. A continuous-wave laser beam at 1064 nm was used to produce squeezed light, operate a zero-area Sagnac interferometer, and perform the interferometer readout with balanced homodyne detection. The squeezed field was injected into the interferometer through its dark port by means of a polarizing beam splitter and a Faraday rotator.

Figure 2

10 MHz phase modulation measured with the Sagnac interferometer without [trace ($a$)] and with [trace ($b$)] squeezed-light input. Trace ($a$) corresponds to the vacuum noise, is normalized to unity, and completely buries the signal. Both traces are averaged twice, each measured with a resolution bandwidth of 300 kHz and a video bandwidth of 300 Hz. The homodyne detector dark noise was 22–25 dB below the vacuum noise and was subtracted from the data. The solid line shows a model for the squeezing strength fitted to the data.

Figure 3

Characterization of our squeezed-light laser at a sideband frequency of 5 MHz. All traces are normalized to the vacuum-noise reference of the homodyne detector’s local oscillator beam ($a$). Trace ($b$) shows the noise in the squeezed field quadrature; trace ($c$) shows the antisqueezing in the orthogonal quadrature. Detector dark noise is at $-25\mathrm{dB}$ and, here, not subtracted from the data.

Figure 4

Total quantum noise for a displacement measurement of a zero-area Sagnac interferometer with (lower trace) and without (upper trace) squeezed-light injection. Our model assumes optical losses to the squeezed light of 9% in total, keeping 8.2 dB of detected squeezing similar to our experiment. The model is further based upon 40 kg test mass mirrors and arm cavities of 10 km length, 80 Hz linewidth, and 10 kW intracavity power. Dividing the linear noise spectral density on the $Y$ axis by 10 km yields the noise spectral density for a gravitational-wave strain measurement.