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Ultracold atoms have become a powerful tool to study many-body quantum systems. They enable quantum simulation of complex condensed matter models, as well as studies of nonequilibrium quantum dynamics in regimes that are inaccessible to classical computers. However, the interactions between ultracold atoms are simple. They dominantly interact via contact interactions, such that atoms need to be in the same place in order to interact.
Ultracold dipolar molecules offer a route to introduce long-range interactions into ultracold quantum gas systems. Dipole-dipole interactions will give access to novel many-body quantum phases:
In this project, we are creating two-dimensional systems of dipolar molecules. Confined into a single plane, we work towards creating a dipolar crystal - a quantum phase in which the dipolar molecules enter a self-organized crystal phase without the need of an external optical lattice. We envision direct imaging of the crystal structure via a molecular quantum gas microscope. This self-organized crystal will have phonons, which will allow us to study the propagation of phonons and crystal defects in the quantum regime. The phase transitions of the cyrstal are of special interest. When it get heated, the crystal is expected to melt into a normal phase and may cross a hexatic phase as conjectured by Kosterlitz, Thouless, Nelson, Halperin, and Young.
We will carry out these experiments with NaCs molecules. Ultracold NaCs molecules in their strongly dipolar ground state have not been created so far; we will create them from quantum gas mixtures of ultracold sodium and cesium atoms. NaCs has one of the largest dipole moments among the bialkali molecules (4.6 Debye). The effective range of dipole-dipole interactions will be up to 10 micrometers, compared to a typical interparticle spacing of about 1 micrometer. This will enable ideal conditions to observe strongly correlated dipolar quantum phases.
Ultracold Dipolar Molecules
Programmable Atomic Tweezer Arrays
Quantum systems with robust coherence are essential in the quest for controllable and scalable quantum technologies. For example, highly coherent quantum systems are needed for the construction of quantum sensors, which can measure time, electromagnetic forces, and gravity with the highest precision, and for a future quantum computer, which promises to speed up data processing to levels unachievable with conventional silicon-based computer technology. However, most of today's quantum applications rely on the limited coherence of individual quantum systems, such as nuclear spins, electron spins, or electronic excitations.
In this project we will demonstrate that coherence can be boosted beyond the limitations of individual quantum systems. We will exploit collective effects in ordered atomic arrays, in which individual atoms will be trapped in close proximity. In this arrangement the atoms can interact and are expected to display subradiance - a quantum mechanical effect that prevents the atoms from losing internal quantum excitations - which will boost the coherence of atomic quantum systems.
We will develop and implement a novel nanophotonic platform to trap and position individual atoms in a programmable array. The setup will rely on laser-cooled strontium atoms trapped by optical tweezers with sub-micrometer spacings. The tweezer array will be generated via projection through a holographic spatial light modulator that will allow for independent control of both the intensity and phase of the trapping light field and enable the generation of optical arrays with unprecedented accuracy and high-speed tunability. With interatomic distances comparable to the atomic resonance wavelength, interference in photon emission gives rise to strongly correlated atomic states that are protected from decay, and thus have substantially longer coherence times than a single atom in free space. The researchers will develop protocols, both theoretically and experimentally, that enable accessing and exploiting the unconventional physical properties of these exotic quantum states.
Second-scale coherence time in molecular nuclear spin states
In collaboration with the Zwierlein group, we demonstrated that pure nuclear spin states in ultracold molecules can display coherence times of about one second. This comparably long coherence time makes ultracold dipolar molecules an interesting candidate for the implementation of qubits. It may be possible ot use molecules to fulfill two roles at the same time: Molecular nuclear spin states can serve as a quantum memory for qubits, while dipole-dipole interactions may be used to implement to perform two-qubit quantum gates.
Quantum control over the internal states of molecules
Complex internal structure of molecules can be controlled by via microwave radiation
Being able to create ensembles of molecules at temperatures of less than a microkelvin, gives us a new level of control over molecular states. At higher temperatures the molecules can be randomly set into rotation or vibration, while at ultracold temperatures we can prepare molecules in perfectly defined internal quantum states - a feat that was unthinkable just a few years ago. In collaboration with the Zwierlein group, we demonstrated rotational and hyperfine states can be efficiently controlled via microwave radiation. We do this for thousands of molecules an ensemble in parallel. For example, we prepare every molecule in the ensemble in the first excited rotational quantum state. We find that also molecular ensembles in rotationally excited quantum states are similarly long-lived as ground state ensembles. This may open pathways to use molecules as quantum bits.
Creation of chemically stable fermionic ground state molecules
At temperatures below a microkelvin, the motion of atoms almost comes to a rest. This gives us enormous control over the quantum properties of atoms - their internal quantum states and their position. Furthermore, we can use Feshbach resonances to tune their interactions and assemble ultracold molecules atom-by-atom.
In collaboration with the Zwierlein group, we have created the first ultracold gases of dipolar sodium-potassium (NaK) molecules. This involved molecule association close to a Feshbach resonance, followed by a so-called stimulated Raman adiabatic passage (STIRAP) for coherent transfer the to the rovibrational ground state, the most strongly bound state of a molecule. This technique enables a special feat: We remove an internal energy form each molecule that corresponds to a thermal excitation energy of 7000 Kelvin - while the overall molecular ensemble stays at a temperature of a few hundred nanokelvin.
These findings had significant impact. From submission to print it took 16 days. It is one of the fastest papers ever published in Physical Review Letters.
Artistic rendition of an ensemble of ultracold molecules (top). In-situ image of 5000 molecules at few 100 nK temperature (bottom)
Quantum quenches and collapse and revival dynamics
Long-lived collapse and revival dynamics of atomic
In collaboration with the Bloch group, we observed long-lived collapse and revival dynamics following a quantum quench. We prepared Glauber coherent states of bosonic atoms on individual sites of a an optical lattice. By abruptly changing the interactions between the atoms, we induced nonlinear quantum dynamics of the atomic coherent states on individual lattice sites - reminiscent of the quantum Rabi dynamics of coherent photon fields. The dynamics happens in parallel in 10,000s of lattice sites, which act like micro-beakers filled with identical coherent states. In a follow-up study, we were able to demonstrate similar collapse and revival dynamics in metallic states of fermionic quantum gases.
Thanks to the long coherence time of the dynamics, we were able to turn this into a precision tool to measure atomic interactions. In particular, we were able to measure the interactions in the Hubbard model and found effective multi-body interactions - strong higher-order interactions that had not been observed before.
Bose-Fermi mixtures and impurity interactions
What is the impact of interacting fermionic impurities on the physics of degenerate Bose gases in an optical lattice? Controlling the interaction between bosonic rubidium and fermionic potassium with a Feshbach resonances, we identified that the superfluid to Mott insulator transition of the Bose gas gets modified through a renormalization of interactions and tunneling that is induced by the fermionic impurities. Furthermore, we demonstrated that the interaction of a fermionic impurity with a Glauber coherent state of bosonic atoms gives rise to pristine quantum dynamics with allowed to extract the interaction strength between bosons and fermions on a 10 Hz scale, which is on the sub-percent level.
Quantum dynamics revealing impurity interactions
Quantum simulation with ultracold atoms in optical lattices
Illustration of Mott insulator
Using ultracold fermionic potassium atoms in an optical lattice, we implemented one of the first demonstrations of the Fermi-Hubbard model with ultracold atomic systems in the Bloch group. This laid the groundwork for quantum simulation of condensed matter models with ultracold fermionic quantum gases - emulating the movement of electrons through the crystal structure of solid materials. We observed under which conditions the atomic gas takes on metallic, band insulating and Mott insulating phases. On the technical side, we pioneered optical lattices with tunable harmonic confinement, later called "compensated optical lattice," which allowed us to identify quantum phases by measuring their compressibility via phase-contrast in-situ imaging.
Interferometry with trapped Bose-Einstein condensates on a chip
In collaboration with the Ketterle group, we coherently split Bose-Einstein condensates (BECs) on an atom chip. The atoms were trapped in a magnetic single wire trap and the splitting was achieved via radiofrequency-dressed potentials. We used this setup for trapped atom interferometry, demonstrated coherence times of more than 100 ms between the separated condensates, and implemented a proof-of-concept demonstration of a trapped-atom gyroscope - splitting a condensate into to halves, letting them propagate, and recombining them.
Atom chip and interference pattern of overlapping BECs
Ultracold atoms trapped in a hollow photonic crystal fiber
Hollow photonic fiber and atomic BEC attracted into the fiber
In collaboration with the Ketterle group, we demonstrated in 2006 that ultracold atoms can be trapped in a hollow photonic bandgap fiber and retrieved after hold times of several 10s of ms. This constituted one of the first demonstrations that ultracold atoms can be interfaced with nanophotonic structures.