Squeezed light at 2128 nm for future gravitational-wave observatories.

All gravitational-wave observatories (GWOs) have been using the laser wavelength of 1064 nm. Ultra-stable laser devices are at the sites of GEO 600, Kagra, LIGO, and Virgo. Since 2019, not only GEO 600, but also LIGO and Virgo have been using separate devices for squeezing the uncertainty of the light, so-called squeeze lasers. The sensitivities of future GWOs will strongly gain from reducing the thermal noise of the suspended mirrors, which involves shifting the wavelength into the 2 µm region. This Letter aims to reuse the existing high-performance lasers at 1064 nm. Here we report the realization of a squeeze laser at 2128 nm that uses pump light at 1064 nm. We achieve the direct observation of 7.2 dB of squeezing as the first step at megahertz sideband frequencies. The squeeze factor achieved is mainly limited by the photodiode's quantum efficiency, which we estimated to (92±3)%. Reaching larger squeeze factors seems feasible also in the required audio and sub-audio sideband, provided photo diodes with sufficiently low dark noise will be available. Our result promotes 2128 nm as the new, to the best of our knowledge, cost-efficient wavelength of GWOs.

Highly efficient generation of coherent light at 2128 nm via degenerate optical-parametric oscillation.

Cryogenic operation, in conjunction with new test-mass materials, promises to reduce the sensitivity limitations from thermal noise in gravitational-wave detectors. Currently, the most advanced materials under discussion are crystalline silicon as a substrate with amorphous silicon-based coatings. However, they require operational wavelengths around 2 µm to avoid laser absorption. Here we present a light source at 2128 nm based on a degenerate optical parametric oscillator to convert light from a 1064 nm nonplanar ring-oscillator. We achieve an external conversion efficiency of (87.1±0.4)% at a pump power of 52 mW in periodically poled potassium titanyl phosphate (internal efficiency was 93%). With our approach, light from the established and existing laser sources can be efficiently converted to the 2 µm regime while retaining the excellent stability properties.

Demonstration of interferometer enhancement through EPR entanglement.

The recent series of gravitational-wave (GW) detections by the Advanced LIGO and Advanced Virgo observatories launched the new field of GW astronomy. As the sensitivity of GW detectors is limited by quantum noise of light, concepts from quantum metrology have been adapted to increase the observational range. Since 2010, squeezed light with reduced quantum noise has been used for improved sensitivity at signal frequencies above 100 Hz. However, 100 m long optical filter resonators would be required to also improve the sensitivity at lower frequencies, adding significant cost and complexity. Here we report on a proof-of-principle setup of an alternative concept that achieves the broadband noise reduction by using Einstein-Podolsky-Rosen (EPR) entangled states instead. We show that the desired sensitivity improvement can then be obtained with the signal-recycling resonator that is already part of current observatories, providing the viable alternative to high-cost filter cavities.

NQontrol: An open-source platform for digital control-loops in quantum-optical experiments.

In this paper, we present NQontrol, a digital feedback-control solution based on the ADwin platform that delivers eight simultaneous feedback loops running with 200 kHz sampling frequency and offers five second-order filtering sections per channel for flexible shaping of the feedback loop. With this system, we demonstrate a Pound-Drever-Hall lock of an optical resonator and compare its performance to an analog reference implementation. A comprehensive support package written in Python, together with a web-based graphical user interface, makes the system quick to setup and easy to use, while maintaining the full flexibility of open-source platforms.

Mitigating Mode-Matching Loss in Nonclassical Laser Interferometry.

Strongly squeezed states of light are a key technology in boosting the sensitivity of interferometric setups, such as in gravitational-wave detectors. However, the practical use of squeezed states is limited by optical loss, which reduces the observable squeeze factor. Here, we experimentally demonstrate that introducing squeezed states in additional, higher-order spatial modes can significantly improve the observed nonclassical sensitivity improvement when the loss is due to mode-matching deficiencies. Our results could be directly applied to gravitational-wave detectors, where this type of loss is a major contribution.

A new quantum speed-meter interferometer: measuring speed to search for intermediate mass black holes.

The recent discovery of gravitational waves (GW) by Advanced LIGO (Laser Interferometric Gravitational-wave Observatory) has impressively launched the novel field of gravitational astronomy and allowed us to glimpse exciting objects about which we could previously only speculate. Further sensitivity improvements at the low-frequency end of the detection band of future GW observatories must rely on quantum non-demolition (QND) methods to suppress fundamental quantum fluctuations of the light fields used to readout the GW signal. Here we present a novel concept of how to turn a conventional Michelson interferometer into a QND speed-meter interferometer with coherently suppressed quantum back-action noise. We use two orthogonal polarizations of light and an optical circulator to couple them. We carry out a detailed analysis of how imperfections and optical loss influence the achievable sensitivity. We find that the proposed configuration significantly enhances the low-frequency sensitivity and increases the observable event rate of binary black-hole coalescences in the range of 1 0 2 - 1 0 3 M ⊙ by a factor of up to ~300.

Reduction of Classical Measurement Noise via Quantum-Dense Metrology.

Quantum-dense metrology constitutes a special case of quantum metrology in which two orthogonal phase space projections of a signal are simultaneously sensed beyond the shot-noise limit. Previously, it was shown that the additional sensing channel that is provided by quantum-dense metrology contains information that can be used to identify and to discard corrupted segments from the measurement data. Here, we propose and demonstrate a new method in which this information is used for improving the sensitivity without discarding any measurement segments. Our measurement reached sub-shot-noise performance, although initially strong classical noise polluted the data. The new method has high potential for improving the noise spectral density of gravitational-wave detectors at signal frequencies of high astrophysical relevance.

Continuous-wave nonclassical light with gigahertz squeezing bandwidth.

Squeezed states can be employed for entanglement-based continuous-variable quantum key distribution, where the secure key rate is proportional to the bandwidth of the squeezing. We produced a nonclassical cw laser field at the telecommunication wavelength of 1550 nm, which showed squeezing over a bandwidth of more than 2 GHz. The experimental setup used parametric downconversion via a periodically poled potassium titanyl phosphate crystal. We did not use any resonant enhancement for the fundamental wavelength, which should in principle allow a production of squeezed light over the full phase-matching bandwidth of several nanometers. We measured the squeezing to be up to 0.3 dB below the vacuum noise from 50 MHz to 2 GHz limited by the measuring bandwidth of the homodyne detector. The squeezing strength was possibly limited by thermal lensing inside the nonlinear crystal.

Photothermal self-phase-modulation technique for absorption measurements on high-reflective coatings.

We propose and demonstrate a new measurement technique for the optical absorption of high-reflection coatings. Our technique is based on photothermal self-phase modulation and exploits the deformation of cavity Airy peaks that occurs due to coating absorption of intracavity light. The mirror whose coating is under investigation needs to be the input mirror of a high-finesse cavity. Our example measurements were performed on a high-reflection SiO2-Ta2O5 coating in a three-mirror ring-cavity setup at a wavelength of 1064 nm. The optical absorption of the coating was determined to be α=(23.9±2.0)·10(-6) per coating. Our result is in excellent agreement with an independently performed laser calorimetry measurement that gave a value of α=(24.4±3.2)·10(-6) per coating. Since the self-phase modulation in our coating-absorption measurement affects mainly the propagation through the cavity input mirror, our measurement result is practically uninfluenced by the optical absorption of the other cavity mirrors.

High-efficiency frequency doubling of continuous-wave laser light.

We report on the observation of high-efficiency frequency doubling of 1550 nm continuous-wave laser light in a nonlinear cavity containing a periodically poled potassium titanyl phosphate crystal (PPKTP). The fundamental field had a power of 1.10 W and was converted into 1.05 W at 775 nm, yielding a total external conversion efficiency of 95±1%. The latter value is based on the measured depletion of the fundamental field being consistent with the absolute values derived from numerical simulations. According to our model, the conversion efficiency achieved was limited by the nonperfect mode matching into the nonlinear cavity and by the nonperfect impedance matching for the maximum input power available. Our result shows that cavity-assisted frequency conversion based on PPKTP is well suited for low-decoherence frequency conversion of quantum states of light.

Squeezed light at 1550 nm with a quantum noise reduction of 12.3 dB.

Continuous-wave squeezed states of light at the wavelength of 1550 nm have recently been demonstrated, but so far the obtained factors of noise suppression still lag behind today's best squeezing values demonstrated at 1064 nm. Here we report on the realization of a half-monolithic nonlinear resonator based on periodically-poled potassium titanyl phosphate which enabled the direct detection of up to 12.3 dB of squeezing at 5 MHz. Squeezing was observed down to a frequency of 2 kHz which is well within the detection band of gravitational wave interferometers. Our results suggest that a long-term stable 1550 nm squeezed light source can be realized with strong squeezing covering the entire detection band of a 3rd generation gravitational-wave detector such as the Einstein Telescope.

Measuring small absorptions by exploiting photothermal self-phase modulation.

We present a method for the measurement of small optical absorption coefficients. The method exploits the deformation of cavity Airy peaks that occur if the cavity contains an absorbing material with a nonzero thermorefractive coefficient dn/dT or a nonzero expansion coefficient α(th). Light absorption leads to a local temperature change and to an intensity-dependent phase shift, i.e., to a photothermal self-phase modulation. The absorption coefficient is derived from a comparison of time-resolved measurements with a numerical time-domain simulation applying a Markov-chain Monte Carlo algorithm. We apply our method to the absorption coefficient of lithium niobate doped with 7 mol. % magnesium oxide and derive a value of α(LN) = (5.9 ± 0.9) × 10(-4)/cm. Our method should also apply to materials with much lower absorption coefficients. Based on our modeling, we estimate that, with cavity finesse values of the order of 10(4), absorption coefficients of as low as 10(-8)/cm can be measured.

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.

Demonstration of a quantum-enhanced fiber Sagnac interferometer.

The injection of squeezed light can be used to improve the sensitivity of an interferometer beyond the limit imposed by the zero-point fluctuation of the electromagnetic field. Here, we report on the realization of such a quantum-enhanced interferometer with a fiber-based Sagnac topology. Continuous wave squeezed states at 1550 nm with a noise reduction of 6.4 dB below shot noise were produced by type I optical parametric amplification and subsequently injected into the dark port of the interferometer. A reduction of the interferometer shot noise by 4.5 dB was observed, and the enhancement of the signal-to-noise ratio for a phase modulation signal generated within the interferometer could be demonstrated. We achieved a 95% fiber transmission for the squeezed states, which suggests that corresponding fiber-based quantum metrology and communication systems are feasible.

Observation of cw squeezed light at 1550 nm.

We report on the generation of cw squeezed vacuum states of light at the telecommunication wavelength of 1550 nm. The squeezed vacuum states were produced by type I optical parametric amplification in a standing-wave cavity built around a periodically poled potassium titanyl phosphate crystal. A nonclassical noise reduction of 5.3 dB below the shot noise was observed by means of balanced homodyne detection.