Squeezed States of Light for Future Gravitational Wave Detectors at a Wavelength of 1550 nm

The generation of strongly squeezed vacuum states of light is a key technology for future ground-based gravitational wave detectors (GWDs) to reach sensitivities beyond their quantum noise limit. For some proposed observatory designs, an operating laser wavelength of 1550 nm or around $2\mu \mathrm{m}$ is required to enable the use of cryogenically cooled silicon test masses for thermal noise reduction. Here, we present for the first time the direct measurement of up to 11.5 dB squeezing at 1550 nm over the complete detection bandwidth of future ground-based GWDs ranging from 10 kHz down to below 1 Hz. Furthermore, we directly observe a quantum shot-noise reduction of up to $(13.5\pm 0.1)\mathrm{dB}$ at megahertz frequencies. This allows us to derive a precise constraint on the absolute quantum efficiency of the photodiode used for balanced homodyne detection. These results hold important insight regarding the quantum noise reduction efficiency in future GWDs, as well as for quantum information and cryptography, where low decoherence of nonclassical states of light is also of high relevance.

Figure 1

Simplified schematic of the experimental setup.

Figure 2

Homodyne measurements with the signal input from the linear OPA. Left: noise spectra of the (anti)squeezing using three different OPA pump powers normalized to the vacuum noise reference for 20 mW local oscillator power at frequencies between 1 and 10 MHz. The measurements were modeled to estimate the optical loss and phase noise for the squeezed states. The best fits (see black dashed curves) were obtained with an optical loss of $(3.5\pm 0.2)\%$ and a phase noise of $(5\pm 1)\mathrm{mrad}$. All spectra are captured with a resolution bandwidth (RBW) of 300 kHz and a video bandwidth (VBW) of 100 Hz (left and right). Center: a zero span measurement at 2 MHz over 200 ms time confirms the detection of $(13.5\pm 0.1)\mathrm{dB}$ squeezing (RBW 100 kHz, VBW 30 Hz). Right: enlargement of the measured quantum noise reduction of the left-hand graph. The uncertainties of our model fits are illustrated with dotted black lines.

Figure 3

Homodyne measurements with the signal input from the bowtie OPA. Left: noise spectra of the (anti)squeezing using three different OPA pump powers normalized to the vacuum noise reference for 20 mW local oscillator power at frequencies between 200 kHz and 5 MHz. Given the high electronic dark noise clearance a noise subtraction has only minor impact on the model (black dashed curves), which was found to fit best with $(4.0\pm 0.2)\%$ optical loss and $(4\pm 1)\mathrm{mrad}$ phase noise. All spectra are captured with a RBW 100 kHz and VBW 100 Hz (left and right). Center: zero span measurement at 500 kHz over 200 ms time confirms the detection of $(13.2\pm 0.1)\mathrm{dB}$ squeezing (RBW 50 kHz, VBW 30 Hz). Right: enlargement of the measured quantum noise reduction of the left-hand graph. The uncertainties of our model fit are illustrated with the dotted black lines.

Figure 4

Quantum noise measurements from 10 kHz down to 0.5 Hz obtained with the bowtie OPA. The results correspond to a harmonic OPA pump power of 98 mW and are plotted relative to the vacuum noise reference. The dashed curve illustrates the squeezing level after the electronic dark noise was subtracted.