# Welcome

The QI group is lead by Prof. Dr. Roman Schnabel and is part of the Institute for Gravitational Physics of the Leibniz Universität Hannover and the Max-Planck-Institute for Gravitational Physics (Albert-Einstein-Institute Hannover). The research fields of the QI group are laser interferometry, nonlinear optics, quantum optics, gravitational wave detection and opto-mechanics, all matching to the research areas of QUEST, the Centre for Quantum Engineering and Space-Time Research.

## Research Highlights

**Metrology with entangled light**

Entanglement is useful for improving measurement devices. Up to now the focus has been on the measurement of just one out of two non-commuting observables. In our work we demonstrate a laser interferometer that provides information about two non-commuting observables, with uncertainties below the meter's quantum ground state. We propose to use the additional information to distinguish between the actual measurement signal and a parasitic signal due to scattered and frequency shifted photons. Our approach can be readily applied to further improve squeezed-light enhanced gravitational-wave detectors at non-quantum noise limited detection frequencies in terms of a sub shot-noise veto-channel. The figure shows our setup together with phase space cartoons of the entangled and squeezed quantum noise.

[Steinlechner, S. *et al.* Quantum-dense metrology. *Nature Photonics* **7,** 626–630 (2013)]

http://dx.doi.org/10.1038/nphoton.2013.150

http://arxiv.org/abs/1211.3570

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GEO600: 200 days of observation with squeezed light

GEO600: 200 days of observation with squeezed light

Our work reports the first long-term application of squeezed vacuum states of light to improve the sensitivity of a gravitational-wave observatory. The improvement corresponds to a factor of 2 increase in the observed volume of the Universe for gravitational-wave sources in the kHz region (e.g., supernovae, magnetars). We introduced three new techniques to actually enable the long-term application of squeezed light, and showed that the glitch rate of the detector did not increase from squeezing application. Squeezed vacuum states of light have arrived as a permanent application, capable of increasing the astrophysical reach of gravitational-wave detectors.

[H. Grote, K. Danzmann, K. L. Dooley, R. Schnabel, J. Slutsky, H. Vahlbruch, *First Long-Term Application of Squeezed States of Light in a Gravitational-Wave Observatory*, Phys. Rev. Lett. 110, 181101 (2013)]

http://prl.aps.org/abstract/PRL/v110/i18/e181101

http://arxiv.org/abs/1302.2188

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Strongest Einstein-Podolsky-Rosen (EPR) entanglement

Continuous variable entanglement is a fundamental resource for many quantum information tasks. Important protocols like superactivation of zero-capacity channels and finite-key length quantum cryptography that provides security against most general attacks, require about 10 dB of two-mode squeezing. Our work demonstrates the strongest EPR entangled light so far, in a fully phase-controlled long-term stable fashion. The entanglement corresponds to 10.45 dB of two-mode squeezing. The figure shows our measurement statistics of the EPR-Reid value (*x*-axis). A value below 1 certifies EPR entanglement.

[T. Eberle, V. Händchen, R. Schnabel, *Stable control of 10 dB two-mode squeezed vacuum states of light*, Opt. Exp. 21, 11546 (2013)]

http://dx.doi.org/10.1364/OE.21.011546

http://arxiv.org/abs/1305.3383

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One-way Einstein–Podolsky–Rosen steering

The distinctive non-classical features of quantum physics were first discussed in the seminal paper by A. Einstein, B. Podolsky and N. Rosen (EPR) in 1935. In his immediate response, E. Schrödinger introduced the notion of entanglement, now seen as the essential resource in quantum information as well as in quantum metrology. Furthermore, he showed that at the core of the EPR argument is a phenomenon that he called “steering”. In contrast to entanglement and violations of Bell’s inequalities, steering implies a direction between the parties involved. The Figure shows implications of inseparability criteria. Our work presents an experimental realization of two entangled Gaussian modes of light that in fact shows the steering effect in one direction but not in the other.

[Händchen, V. *et al.* Observation of one-way Einstein–Podolsky–Rosen steering. *Nature Photonics* **6,** 598–601 (2012)]

http://dx.doi.org/10.1038/nphoton.2012.202

http://arxiv.org/abs/1206.4446

### Squeezed light implemented in GEO600!

GEO600 now uses squeezed light for its searches for gravitational waves. After the Hannover squeezed-light laser was finished in 2010, its implementation and all tests were passed with flying colours. Now, GEO600 uses two lasers: its standard laser of about 10 W power, and the new squeezed-light laser (see Figure) that just adds a few entangled photons per second but significantly improves the sensitivity of GEO600. The concept was proposed 30 years ago, and has been now realized in Hannover for the first time. Our work was published online in September 2011 in the “Nature Physics” journal.

Read Nature Physics article or at arXiv.

[The LIGO Scientific Collaboration, Enhanced sensitivity of the LIGO gravitational wave detector by using squeezed states of light, Nature Photonics 7, 613–619 (2013)]

**Review: Squeezed Light / Quantum Metrology for Gravitational Wave Astronomy**

In 1981 C. M. Caves proposed the use of squeezed quantum states of light in order to increase the sensitivity of laser-interferometric gravitational wave detectors. As it turned out, squeezed vacuum states of light injected into a laser interferometer (in addition to the bright coherent laser field) leads to quantum entanglement of the light fields in the interferometer arms. Thus, this proposal was the first to use entanglement for metrology (quantum metrology). This review summarizes the experimental progress in squeezed light generation for gravitational wave detectors over the past years.

[R. Schnabel *et al.*, Quantum metrology for gravitational wave astronomy. Nat. Commun. 1:121 doi: 10.1038/ncomms1122 (2010)].

**The GEO 600 Squeezed Light Source**

The next upgrade of the GEO 600 gravitational-wave detector is scheduled for 2010 and will, in particular, involve the implementation of squeezed light. The required non-classical light source is assembled on a 1.5 m^{2} breadboard and includes a full coherent control system and a diagnostic balanced homodyne detector. Here, we present the first experimental characterization of this setup as well as a detailed description of its optical layout. A squeezed quantum noise of up to 9 dB below the shot-noise level was observed in the detection band between 10 Hz and 10 kHz. We also present an analysis of the optical loss in our experiment and provide an estimation of the possible non-classical sensitivity improvement of the future squeezed light enhanced GEO 600 detector.

[H. Vahlbruch *et al.*, Class. Quantum Grav. 27, 084027 (2010)]

This article has been selected for the Highlights of Classical and Quantum Gravity.

**The World's Strongest Squeezing in a Sagnac Interferometer **

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. Measurements performed directly on our squeezed-light laser output revealed squeezing of 12.7 dB. We discuss the Sagnac topology in view of future gravitational-wave (GW) detectors, such as the Einstein Telescope, whose design is currently being studied.

[T. Eberle *et al.*, Phys. Rev. Lett. 104, 251102 (2010)]

**High Reflectivity without Coating: Nano-Structuring Silicon Crystal Surfaces**

Our work experimentally demonstrates for the first time a monolithic surface mirror, i.e., a single piece of monocrystalline silicon with a reflectivity high enough to form a laser cavity with a finesse of almost 3000. The achieved high reflectivity relies on resonant coupling to a guided optical mode of a surface nanostructure (Figure left). Since no material is added to the silicon substrate, coating Brownian thermal noise is avoided.

[F. Brückner *et al.*,Phys. Rev. Lett. 104, 163903 (2010)], press release.

**Entanglement Distillation**

The distribution of entangled states of light over long distances is a major challenge in the field of quantum information. Decoherence destroys the non-classical states after some finite transmission-line length. In two articles we demonstrated the first two-copy and iterative three-copy entanglement distillation to overcome decoherence.

[B. Hage *et al.*, Nature Physics 4, 915 (2008)];

[B. Hage *et al.*, Phys. Rev. Lett. 105, 230502 (2010)].

**Entangled Mirrors**

Our theoretical analysis has proven a connection between (i) reaching the standard quantum limit (SQL) of a position measurement and (ii) the possibility of creating entanglement between the centre of mass motions of two suspended mirrors. We proposed to use a Michelson interferometer and two balanced homodyne readouts in order to create position/momentum entanglement between two macroscopic mirrors. Since the already planed gravitational wave detector Advanced LIGO is designed in order to reach the SQL, entanglement of macroscopic mirrors of several kg mass might be feasible in future experiments.

[H. Müller-Ebhardt *et al.*, Phys. Rev. Lett. 100, 013601 (2008)]

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