MassQ - Massive object Quantum physics
ERC funding 2014 - 2020
There are currently two fundamental theories successfully describing our world: Quantum theory e.g. accurately predicts the absorption and emission of light by atoms, while the general theory of relativity is concerned with gravitation, the force emanating from masses which governs the dynamics of planets, galaxies and black holes.
One challenge of modern physics is the unification of the two in one overarching theory. To do so, we deem experiments dealing with both gravitation and quantum mechanics an important step. The aim of MassQ is the quantum mechanical entanglement of two massive mirrors which are heavy enough such that in principle their gravitational forces can be observed. The proposed experiment could potentially lead to surprising observations right at the interface of quantum mechanics and general relativity.
Daniel Hartwig and Jan Petermann.
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This project received funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation programme under grant agreement No 339897 (2014 - 2020)
Entanglement of two mirrors
The experiment uses techniques originally developed for the detection of gravitational waves. Two mirrors, of approximately 100 g, are suspended by thin wires as a pendulum, so they may oscillate undisturbed by the environment. Laser light is split into two paths on a beam splitter, of which each single beam is then reflected by one of the mirrors and reunited again, so as to interfere with the other. The intensities at both exits of this laser interferometer are measured using photodiodes.
This way, two precise measurements can be undertaken continuously: one detector measures the difference in distance to the beamsplitter, while a second one measures the sum of mirror velocities (meaning the changerate of distance over time). It is crucial to use light of a sufficiently high intensity such that the quantum mechanical uncertainty of the radiation pressure affects the movement of the mirrors. If this is the case, the movement of the mirrors will be entangled.
MassQ aims to demonstrate the Einstein-Podolsky-Rosen paradox based on the positions and velocities of the mirrors. Therefore, we have to show that their positions and velocities are well-defined relatively to each other while in relation to our laboratory frame the individual mirrors' uncertainties in position and momentum are much higher.
The mirrors and optics of our interferometer must not be affected by their environment. In order to avoid momentum transfers through gas molecules, the experiment is situated in an ultra-high vacuum (<10-8 bar). Both mirrors as well as the rest of our interferometer are suspended on thin tungsten wires with a diameter of a few micro meters. One of the difficulties is to finely tune all parts to each other so as to maximize interference contrast of the reflected light.
To minimize Brownian motion of the mirror surface, the final setup is operated at temperatures below 1K. The laser light also has to be free of intensity and frequency fluctuations and thus needs to be stabilized. In order to reach sufficient optical power of multiple kilowatts, optical resonators are built around the setup, amplifying light intensity according to their finesse.
As well as in future gravitational wave detectors, a major challenge is avoiding the heating of the mirrors caused by light absorption. The resulting Brownian motion of the mirror surface would make entanglement impossible. Technologies developed for MassQ could thus as well allow an improvement in sensitivity of future gravitational wave detectors.
Measurements on one mirror
The project starts with measurements on a single mirror of 100 g. The aim is to realize a quantum mechanically limited state of motion, e.g. a coherent state or a state of squeezed quantum noise. To avoid a quick decoherence of these states, the mirror and interferometer haveto be decoupled from their environment as good as possible. Both will be suspended by thin tungsten wires.
A specially coated mirror as used in the first MassQ setup.
The pendulums have a natural frequency of about one Hertz. The oscillation mode has to be parametrically attenuated, i.e. height of the suspension point varies with twice the pendulum frequency, depriving the pendulum of a little energy with every oscillation („feedback cooling“). For practical reasons it cannot be brought down to its quantum mechanical ground state this way, but overtones at around 300 Hertz can. One should be able to observe a rather pure quantum state propagating through phase space – propelled by the remaining coupling to the thermal bath.
MassQ relies on various preliminary works in the context of gravitational wave detection by our and by other groups. Directly connected to MassQ, we have studied the Michelson-Sagnac interferometer and its opto-mechanical coupling with a membrane at cryogenic temperatures with both coherent and squeezed states of light.
The parametric cooling of our pendulum is currently researched and optimized in a separate experiment as part of a master thesis. Simultaneously, our group is theoretically and experimentally researching new mirror coatings with promise of reduced Brownian noise.
13dB Squeezed Vacuum States at 1550nm from 12mW external pump power at 775nm
A. Schönbeck, F. Thies, R. Schnabel; Opt. Lett. 43, 1 (2018)
Squeezed states of light and their applications in laser interferometers
R. Schnabel; Physics Reports 684, 1–51 (2017)
Proposal for gravitational-wave detection beyond the standard quantum limit through EPR entanglement
Y. Ma, H. Miao, B.H. Pang, M. Evans, C. Zhao, J. Harms, R. Schnabel and Y. Chen; Nature Phys. (2017)
Beating the Standard Sensitivity-Bandwidth Limit of Cavity-Enhanced Interferometers with Internal Squeezed-Light Generation
M. Korobko, L. Kleybolte, S. Ast, H. Miao, Y. Chen, and R. Schnabel; Phys. Rev. Lett. 118, 143601 (2017)
Generalized analysis of quantum noise and dynamic back-action in signal-recycled Michelson-type laser interferometers
F. Khalili, S. Tarabrin, R. Schnabel, K. Hammerer; Phys. Rev. A 94, 013844 (2016)
Einstein-Podolsky-Rosen-entangled motion of two massive objects
R. Schnabel; Phys. Rev. A 92, 012126 (2015)
Observation of generalized optomechanical coupling and cooling on cavity resonance
A. Sawadsky, H. Kaufer, R. Moghadas Nia, Sergey P. Tarabrin, F. Y. Khalili, K. Hammerer, R. Schnabel; Phys. Rev. Lett. 114, 043601 (2015)
Kühlen von großen Objekten mit Laserlicht
A. Sawadsky, K. Hammerer, R. Schnabel; Phys. Unserer Zeit 4, 162 (2015)
Interferometer readout-noise below the Standard Quantum Limit of a membrane
T. Westphal, D. Friedrich, H. Kaufer, K. Yamamoto, S. Goßler, H. Müller-Ebhardt, S. L. Danilishin, F. Ya. Khalili, K. Danzmann, R. Schnabel; Phys. Rev. A 85, 063806 (2012)
Entanglement of macroscopic test masses and the standard quantum limit in laser interferometry
H. Müller-Ebhardt, H. Rehbein, R. Schnabel, K. Danzmann, Y. Chen; Phys. Rev. Lett. 100, 013601 (2008)
Verschränkung zweier Spiegel
R. Schnabel; Spektrum der Wissenschaft, Juni (2008)