PW_mar13_sample_issue

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atoms share the same wavefunction, and thus behave
like a single “matter wave” – are first trapped in a 2D
plane. The atoms are then transferred into an eggcarton-shaped
lattice potential of light, which at the
same time helps to bring the atoms into an interesting,
strongly interacting quantum phase. Before the
atoms are imaged, the depth of the lattices is suddenly
increased to stop the atoms moving and hold
them tight in space. Then, a laser beam with a frequency
near resonance with an atomic transition is
switched on, causing the atoms to fluoresce like tiny
nanoscopic light bulbs. The resulting array of nanobulbs
can be imaged using a high-resolution microscope
objective (figure 1).
The precise moment when the first photons of the
imaging laser beam are scattered off the atoms is
crucial for the quantum-measurement process. Initially,
the atoms are in a complicated quantum superposition
state of different spatial configurations, with
a many-body wavefunction Ψ(x 1…x N). The measurement
process collapses this wavefunction, and the
atoms are observed to be in one of many possible
spatial configurations. Image-processing algorithms
allow us to reconstruct the position of each individual
atom in the lattice, but the particular configuration
of atoms observed in a single snapshot is, of
course, completely random. Only by repeating the
experiment many times does it become possible to
build up a histogram of the occurrences of different
spatial configurations. Examples of such reconstructed
particle positions for two distinct phases of
matter – a BEC and a Mott insulator – can be seen
in figure 2.
Mapping fluctuations
Just being able to image single atoms in a lattice
is pretty exciting in itself, but it gets better. If you
take a close look at the reconstructed images in figure
2 b–c, you will see some gaps, or defects, in the
lattice. These “missing atoms” represent individual
thermal fluctuations, and they are directly visible in
a single shot of the experiment. Being able to see single
thermal fluctuations in such a system gives us an
extremely precise thermometer, which has allowed
researchers to determine temperatures down to the
100 pK level using just a single image.
Perhaps more importantly, the “quantum gas
microscope” also enables us to directly observe the
zero-temperature quantum fluctuations of a manybody
system. Many readers will have encountered
such quantum fluctuations in the classic textbook
example of a single quantum particle in the absolute
ground state of a harmonic oscillator. In this simple
system, the position of the particle remains undetermined
within a region given by the extension of
the ground state wavefunction – in sharp contrast
with the behaviour of a classical particle at zero temperature,
for which position and momentum would
always be well defined.
A quantum many-body system also exhibits such
inherent quantum fluctuations, and when they
become very strong, these fluctuations can give
rise to a phase transition – specifically, to a quantum
phase transition that occurs even at zero tem-
1 Quantum gas microscope
a
high-resolution
objective
z
Quantum frontiers: Quantum simulation
y
16 µm
x
(a) Ultracold atoms trapped in a single plane of an optical lattice are imaged using a highresolution
microscope that detects the atoms’ laser-induced fluorescence. (b) A typical
image of atoms trapped in the lattice.
2 Imaging different phases of matter
perature. Quantum gas microscopes thus offer us
the chance to learn how the re-ordering of a system
takes place during a phase transition, and on what
timescales. This is an unparalleled glimpse into the
inner workings and dynamics of many-body systems,
and one that would simply not be possible in this
detail for a real material.
In addition to using the phenomenal spatial resolution
of quantum gas microscopes to observe single
atoms, one can also perform other experiments.
Physics World March 2013 49
b
optical lattice
laser beams
a b c
Atoms in a Bose–Einstein condensate (BEC) can be described by one macroscopic
wavefunction, but at the same time a BEC in a lattice exhibits large fluctuations in the
number of atoms per lattice site, n, thanks to an uncertainty-like relationship between n and
the wavefunction phase Φ (Δn ΔΦ > 1). In contrast, if the atoms are in a different phase of
matter, known as a Mott insulator, strong repulsive interactions between the atoms destroy
the coherent matter-wave field of the BEC, but they also suppress fluctuations in particle
number. The result is an almost perfect ordering of atoms in the lattice, with one atom
occupying every lattice site.
(a) Reconstructed atom positions in a BEC and (b–c) strongly interacting Mott
insulators. The hole in c actually corresponds to a region where two atoms are occupying
every central lattice site. This region appears dark because the presence of near-resonant
laser light induces interactions between pairs of atoms trapped in the same lattice site,
causing both atoms in the pair to escape the lattice. Hence, bright areas in the images
correspond to odd-numbered lattice occupancies.
Nature 467 68; reused with permission
I Bloch, MPQ