QUANTUM-Seminar

I discuss the concept of boundary time crystals, where the continuous time-translation symmetry breaking occurs only in a macroscopic fraction of a many-body quantum system. After introducing its definition and properties, we discuss in detail a solvable model where an accurate scaling analysis can be performed. The existence of the boundary time crystals is intimately connected to the emergence of a time-periodic steady state in the thermodynamic limit of a many-body open quantum system. I will also discuss the spreading of genuine multipartite correlations (GMC's) on such phases, showing results both for the (i) the structure (orders) of GMC's among its subsystem constituents, as well as (ii) their build-up dynamics for an initially uncorrelated state.

The toolkit of quantum technologies developed in atomic, molecular and optical physics are ideally suited to enhance the search for dark matter axions with masses above ~40 µeV. I will present an overview of a new experimental effort under construction at Imperial College, developing technologies to detect DFSZ axions with masses 120-250 µeV. We plan to use a large mode area Fabry-Perot cavity to efficiently convert axions into microwave photons. Compared to other geometries, the Fabry-Perot cavity can present a large mode volume and high Q, and can be easily tuned. To detect the microwaves, we will use an electron in a Penning trap as a single photon counter. Individual microwave absorption events will change the cyclotron state of the electron, causing measurable shifts in the trapped particle’s oscillation frequencies. This versatile device will also open other possible detection routes for alternative dark matter candidates and cosmological phenomena.

Excellent experimental control over quantum systems is increasingly bringing applications in quan-tum technologies within reach. In my talk, I will present our work on quantum technology applica-tions in various physical systems. Ultracold atoms have proven to be excellent platforms for studying quantum effects. In recent years, we have succeeded in introducing single Cs atoms as controlled impurities in an ultracold gas of Rb atoms. Spin-exchange collisions allow a very controlled transfer of energy quanta, and we use this transfer to operate the single atom as a machine in a magnetic field gradient. In another exper-imental setup, we address whether the significant energy differences resulting from the Pauli prin-ciple between ensembles of different fermionic and bosonic quantum statistics can be used as a novel form of energy to drive a quantum machine. Finally, I will present two new projects in the field of quantum technology. First, we are studying nanocrystals of diamond with a large number of NV centers in terms of collective effects and track-ing how the typical signatures in fluorescence lifetime and photon statistics change when larger agglomerates of nanocrystals show the transition to a bulk-like material. Second, I report on a new BMBF collaborative project, the quantum computing demonstrator pro-ject Rymax, which will simulate optimization problems from logistics and industry expressed as gra-phene problems on an array of single Yb atoms with Rydberg excitations.

Light pulses are an excellent tool to manipulate atoms, so that they move in superposition of different trajectories through space and time. In analogy to the concept of an optical interferometer, these branches can be brought to interference and used as sensors for gravity and other inertial forces. As such, atom interferometry has become a versatile tool technique high-precision quantum metrology, which ranges from gyroscopes to gravitational wave detection, as well as a testbed for the interface of relativity and quantum mechanics. At the same time, atoms not only possess a center-of-mass motion, but also internal degrees of freedom that are the very basis for atomic clocks. Making use of this additional property, one can in principle generate atomic clocks that move in superposition of different branches, interfere with each other, and therefore constitute a different probe of relativistic effects linked to time. This colloquium gives an introduction into the main concepts of atom interferometry and the toolbox necessary to manipulate atoms. While we explain basic examples of atom interferometers and state-of-the-art experiments, we also discuss current and ambitious proposals for high-precision tests of fundamental physics.