MassQ - quantum physics of massive objects

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.

In the lab: Together with Torben Sobottke, Jan Petermann is working on the implementation of MassQ.

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.

In a first stage of the project, measurements are performed only 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 have to be decoupled from their environment as good as possible. Both will be suspended by thin tungsten wires.

As a next step, square test masses with sharp edges will be manufactured from a cylindrical test mass of high purity sapphire. The new shape shall reduce the mechanical losses of the pendulum.

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.

Daniel Hartwig's experiment researches parametric cooling of a pendulum: the square shaped mirror of polished aluminum can be seen right in the center, with its thin tungsten wire suspension barely visible above it. The suspension height varies with twice the pendulum frequency, reducing the pendulum energy.

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.