Transcript The Dark Energy Atom Interferometer Experiment

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The Dark Energy Atom
Interferometer Experiment
To detect and measure effects of
Dark Energy density or any dark
contents of the vacuum (DCV)
Jon Coleman, Royal Society Research Fellow
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• Many thanks to the support of the:
– Liverpool Particle Physics Group
– Cockcroft Institute
– Physics Dept Technical & Workshop Support
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•
Looking for a new way to penetrate
the mystery of Dark Energy
– more generally to penetrate the puzzles
associated with the vacuum.
• For a more details:
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M. L. Perl and H. Mueller, “Exploring the possibility of detecting Dark Energy in a
terrestrial experiment using atom interferometry” arXiv 1001.4061v1 (2010).
M. L. Perl,” The Possible Detection of Dark Energy on Earth Using Atom
Interferometry”, arXiv 1007.1622v1 (2010).
R. J. Adler, H. Mueller and M. L. Perl, “A terrestrial search for dark contents of the
vacuum, such as Dark Energy, using atom interferometry”, arXiv1101.5626v1
(2011). To be submitted to Physical Review D.; To be published in International
Journal of Modern Physics A.
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Present
Observations
• Dark Energy has been a
dominant question in
both cosmology and
fundamental physics for
the past decade.
• The study of Dark Energy
using earth and space
based telescopes is continuing with more precise
measurement using existing and revised instruments
and will substantially improve with instruments now
under construction.
• Can it only be studied indirectly through observation
of the structure and motions of galaxies?
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• The average observed Dark Energy
density, assuming uniform distribution in
the universe, is rDE = 6.3x10-10 J/m3.
• This is the same energy density possessed
by a static electric field of 12 V/m, using
rEF =e0E2/2.
a SQUID in a Undergrad Lab…
• Such electric fields
easily measured in
the lab.
• DE may be small, but
non-zero: an area for
experiment!
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• Although it is easy to sense and measure
tiny electromagnetic fields due to the
relatively strong electromagnetic coupling.
• Obviously severe experimental problems in
detecting Dark Energy or DCV density:
• Unlike an electric field in the laboratory,
we cannot turn Dark Energy on and off.
• We do not expect there is a zero Dark
Energy field that could be used as an
experimental reference.
• Even if the Dark Energy density should
have a gradient, we do not know what
force it exerts on a material object.
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Atom interferometry - 101
• For a typical atom interferometer.
• Cesium atoms vertically drop from
rest in vacuum under the influence
of the earth’s gravitational field Fg
with the acceleration g=9.8 m/s2.
• In our experiment the drop is about
2 meters. As the Cs atom falls it is
excited by laser beams that move
the atomic state between the
ground state S1/2 and a low lying
excited state P3/2.
• The wavelength of this transition is
λ=852 nm.
• The S1/2 state has two hyperfine
levels, F= 3,4, and the laser beams
put an atom in either of these
states.
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Atom interferometry - 2
• At times 0, T and 2T photon pulses are
applied to the atoms changing state (F=3 or
4) and momentum.
• An atom is placed into superposition of two
momentum eigenstates,
– by interaction with the photons of counterpropagating laser beams.
• The first component of the matter wave
receives zero momentum transfer while the
second is given a downward momentum
kick of Dp.
• Both matter wave packets fall freely until
time T, whereupon they are given an
opposite kick of Dp.
• At 2T the first is given an upward kick of Dp
and waves recombine into a modulated
single wave packet.
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Atom interferometry - 3
• Wave function of atom has phase factor exp(if)
• The phase f changes as atom falls
through changing gravitational potential.
• Probability P of detecting the atom at one
output of the interferometer is
1.0 m
Df = Df1 - Df2
Df1, Df2 are phases accumulated by
matter waves on the two paths
• The probability
P = (1 + cosDf) / 2
is measured by laser-driven fluorescence
at the detector.
vacuum envelope
Cs atom source
falling bunch of
Cs atoms
upward laser
beams
Cs atom state
detector
Simplified schematic of a single
interferometer.
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Principle of the Experiment
• Use two identical, adjacent
interferometers, A and B, assume
that Fg is the same everywhere and
perpendicular to the earth’s surface.
Then ΔφΑ= ΔφΒ and the difference
in phase shift is
Δφ = ΔφΑ−ΔφΒ = 0.
• But suppose, there is an additional
force on the atoms caused by dark
energy, FDE in the vicinity of
interferometer A but not in the
vicinity of interferometer B.
• Then
Δφ = ΔDE
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Based upon two assumptions:
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First, that dark energy possesses some
space inhomogeneity in density.
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If the cosmological constant explanation of dark
energy is correct, this experiment will give a null
signal.
Second that it exerts a sufficiently strong
non-gravitational force on matter.
If we find such a noise signal, we can and
must show it is not Instrumental noise.
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NATURE OF THE SOUGHT SIGNAL
• We do not record Δφ which will average to zero,
we record the root mean square Δφ rms. We
can then determine the dark energy equivalent
acceleration, gDE.
• We expect to be able to detect the dark energy
equivalent acceleration, gDE with a precision of
10-15 m/s2.
• Apparatus sampling rate is
of the order of Hertz.
• Hence the dark energy
signal is a noise signal.
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Experimental Configuration
To cancel systematic effects:
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Incorporate the two
interferometers in one
vacuum envelope,
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reduce problems from common
mode noises such as
vibrations.
drop sources for simplicity.
Sources are staggered
vertically,
–
total phase change for each
atom is measured during the
same velocity period.
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Beginning Summer 2010
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Progress
University of Liverpool,
November 2011
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The next 12 Months
• Prototype Interferometer.
– Put in one arm, including a source, a beam splitter
using Raman pulses, and a detector.
– Take advantage of communications industry
revolution (i.e. low cost, high quality), and use
commercial laser, fibre, optical components, etc…
• Beyond 2012
– Refine design & Improve accuracy for
• Benchmark measurement of g @ 10-9
– Build double arm system
– Explore Parameter space
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Improving Sensitivity
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A crucial factor in the sensitivity of the
experiment depends upon having a large
phase shift f where
f = constant (gT2)
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Here g is the acceleration of gravity and T is
the time it takes for the atom cloud to fall
from the source at the top to the detector at
the bottom of the interferometer.
In the apparatus under construction the fall
height, h, is 1 m and f is about 107 radians.
Since T2 is proportional to h, f is proportional
to h. if h = 10 m would give 10 times the
present f, in principle the higher the better.
The Daresbury tower, is approx. 100
meters high, it would seem that the benefits
of exploiting this structure are obvious.
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Summary
• An experimental direct-detection effort is
underway.
• Using the principals of atom physics to
investigate cosmological questions.
• Even if this experiment fails to detect a signal
on the laboratory scale, it will, in and of itself,
be an extremely important constraint on the
nature of Dark Energy and more generally on
the Dark Content of the Vacuum.
CASCADE – SC cavity for HSP
Peter Williams et al
• Searching for hidden sector photons with superconducting cavities
brings the benefit of cavity quality factors ~109 – NC cavities only ~106
• Cryogenic temperatures improve S/N – near elimination of thermal
noise
• Address theoretically interesting region of 1’s – 100’s meV
• ILC crab cavities
pre-existing shielded cryostat
• Receiver under test
Expected limits:
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• The leading order phase difference between the
paths due to gravity is given by
Df = nkgT2drop
• where n is the number of photon momenta
transferred to the atom in each beam splitter, k
is the laser wave number, and g is the local
gravitational acceleration in the laboratory. The
sensitivity f of the interferometer depends
quadratically on the drop time, Tdrop.
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Magnitude of dark energy
density:
• Counting mass as energy via E=Mc2,the
average density of all energy is the
critical energy
• ρcrit=9 x10-10J/m3
• ρmass≈0.3 x ρcrit= 2.7 x10-10J/m3
• ρdark energy≈0.7 x ρcrit= 6.3 x10-10J/m3
• Use ρDE to denote ρdark energy
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• ρDE ≈ 6.3 x10-10J/m3 is a very small
energy density but as shown in the
next section we work with smaller
electric field densities in the laboratory
• ρDE is taken to be at least approximately
uniformly distributed in space