Welcome to the Greiner group website! We study ultracold gases loaded into artificial crystals of light known as optical lattices. The behavior of ultracold atoms in optical lattices is similar to that of electrons in solids. Because of that, ultracold atoms can provide clean realizations of models from condensed matter which can be studied in a highly controlled environment.
We developed a novel microscopy technique that allows us to image atoms in optical lattices with submicron and single-site optical resolution. Our experiments with bosonic rubidium and fermionic lithium are at the interface of atomic physics, condensed matter physics and quantum information. Find out more about our current work by following the links below.
Site-resolved imaging of a fermionic Mott insulator
Conventional band theory predicts an insulating behavior of a solid if
the freely moving electrons occupy every possible quantum state in the
highest energy band, whereas the state is conducing otherwise. This
simple picture of band insulating and conducting metallic states is
altered in the presence of strong interactions, which can lead to an
insulating behavior even in the presence of a half-filled energy band.
These Mott insulators, named after the British physicist Sir Nevill F.
Mott, are one of the conceptually simplest examples of many-body
systems, where strong correlations lead to surprising phenomena.
We have observed fermionic Mott insulators, metals and band insulators
with single-site resolution by trapping a repulsively interacting,
two-component mixture of Li-6 atoms in a square optical lattice. We
observe large, defect-free 2D Mott insulators containing more than 400
atoms with an average entropy per particle of 1.0kB.
The images above show single-shot picture of the atomic distribution for
varying interactions U/t, along with the deconvolved images in the
bottom row. The metallic state (left picture) is characterized by a
large occupation variance, the band insulating region appears as a core
of empty sites (middle picture) and the Mott insulator is signaled by an
extended region of constant occupation with one particle per site and
strongly reduced occupation variance (right picture and ring in middle
Using a novel single-site addressing scheme, we study the quantum walks of individual and interacting bosonic atoms in an optical lattice. Top: An individual particle, initially localized to one lattice site, performs a free quantum walk and delocalizes over 20 lattice sites (left). An applied gradient reveals the excellent coherence of this motion, forcing the particle to undergo Bloch oscillations in position space (right).
Bottom: We deterministically add more particles to the system, and observe their dynamics with single-particle resolution. We directly detect bosonic bunching in a Hanbury Brown-Twiss experiment of two bosons, and observe the emergence of fermion-type correlations when the repulsive interactions become strong. Our work opens the door to the study of condensed matter problems using quantum walks of interacting particles.
"Probing the superfluid to Mott insulator transition at the single atom level" wins the 2011 AAAS Newcomb Cleveland Prize!
Quantum Magnetism in an Optical lattice. (a) Two spin chains, one
paramagnetically ordered (back), and one antiferromagnetically ordered
(front). The background is a single-shot image from the quantum gas
microscope, demonstrating the scalability of this approach. (b)
Strong magnetic interactions and fast dynamics are realized by mapping
the position degree of freedom of each atom onto the state of a single
spin 1/2 particle. By adiabatically tuning the tilt (E/U) of the
lattice, we drive a phase transition from a paramagnetically ordered
(Mott) state, to an antiferromagnetically ordered state with
density-wave (DW) ordering. Because the DW ordered state is contains
sites with occupations of only n=2,0, it appears dark (podd~0), while
the Mott state appears bright (podd~1). This first demonstration of
quantum magnetism in a lattice should open new perspectives for
studies of criticality, and high temperature superconductivity, with
cold atoms. The work uses a theoretical proposal by Prof. Subir Sachdev
and collaborators, PRB 66, 075128 (2002).
News: Single atom imaging of Mott insulator selected as a top breakthrough of this year by Science
Mott insulator (MI) in a Quantum Gas Microscope. (a) Sketch of Quantum Gas Microscope, enabling high fidelity single lattice site imaging. (b) Mott insulator shell structure with n=1 MI (bright ring), surrounding a n=2 MI core (dark). (c) near perfect n=1 Mott insulator.
Experimentally observed Mott shells (averaged) for increasing atom number.
The Quantum Gas Microscope enables high fidelity detection of single atoms in a Hubbard-regime optical lattice, bringing ultracold atom research to a new, microscopic level. We investigate the Bose-Hubbard model using space- and time-resolved characterization of the number statistics across the superfluid - Mott insulator quantum phase transition. Site-resolved probing of fluctuations provides us with a sensitive local thermometer, allows us to identify microscopic heterostructures of low entropy Mott domains, and enables us to measure local quantum dynamics, revealing surprisingly fast transition timescales. Our results may serve as a benchmark for theoretical studies of quantum dynamics, and open new possibilities for realizing and probing quantum magnetism.