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Modular mechanical-atomic quantum systems

Periodic Reporting for period 3 - MODULAR (Modular mechanical-atomic quantum systems)

Reporting period: 2019-01-01 to 2020-06-30

Mechanical oscillators are widely used as force or acceleration sensors and signal transducers, with applications ranging from atomic force microscopy and gravitational wave detection to consumer electronics. In today’s devices, the vibrations of these oscillators are governed by classical physics and the readout sensitivity is limited by thermal noise. To reach the quantum limits of operation in such devices will likely have a strong impact on technology, enabling sensors with improved precision as well as conceptually new quantum technology.

In this project, we explore the new conceptual and experimental possibilities offered by hybrid systems in which the vibrations of a mechanical oscillator are coupled to an ensemble of ultracold atoms. The coupling will be realized in a modular way by connecting the two systems with laser light. The coupled mechanical-atomic system will be used for a range of experiments on quantum control and quantum metrology of mechanical vibrations. Besides the interesting perspective of observing quantum phenomena in engineered mechanical devices that are visible to the bare eye, the project will open up new avenues for quantum measurement of mechanical vibrations with potential impact on the development of mechanical quantum sensors and transducers for accelerations, forces and fields.
The project started by building the experimental setup for the planned experiments. This included building a new membrane optomechanics setup in a cryostat and the further development of an ultracold atom setup. While the cryogenic setup was being assembled, experiments on atom-membrane coupling were carried out at room temperature. At large coupling strengths, light-mediated collective motion of atoms in the optical lattice was observed. Such collective optomechanical phenomena currently receive great scientific interest. They were observed here for the first time with atoms in a free-space optical lattice, without placing the atoms in an optical cavity.

In parallel, a general method to generate light-mediated Hamiltonian interactions between distant quantum systems was developed. It relies on cascading quantum systems in a loop to realize bidirectional interactions. The method is rather general and relevant for various applications in the fields of optomechanics and atomic ensembles, such as sympathetic cooling, hybrid entanglement generation and unconditional squeezing of such systems. This method was implemented in the experimental setup to strongly couple the vibrations of the membrane to the collective spin of the atoms. The observation of light-mediated strong coupling between the two systems over a distance of 1 meter represents a major result of this project and a breakthrough in the field. The coupling is highly tunable and allowed us to carry out a variety of interesting experiments, observing normal-mode splitting, coherent energy exchange oscillations, two-mode thermal noise squeezing and dissipative coupling of the two systems. Our approach to engineer coherent long-distance interactions with light makes it possible to couple very different systems in a modular way, opening up a range of further opportunities for quantum control and coherent feedback networks.
The successful implementation of the project will establish a new approach to quantum control of mechanical vibrations, with features and functionality that have not been achieved in purely optomechanical systems. It will lead to new insights into remote control of quantum systems with light and quantum metrology. Exploring the fundamental limits of control and measurement imposed on us by nature has important conceptual, philosophical and technological implications. The project will have a significant impact on the development of quantum metrology with mechanical quantum sensors, whose classical counterparts already find widespread use as sensors for accelerations, forces and fields.
Schematic picture of the hybrid mechanical-atomic system