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Quantum probe beats Heisenberg limit

Mar 24, 2011 4 comments

Werner
Heisenberg: King of uncertainty

A group of physicists in Spain has shown how to make a quantum measurement
that overcomes a limit related to Werner Heisenberg's uncertainty principle. The
researchers confirmed a theoretical prediction of how to beat the Heisenberg
limit by using interacting photons to measure atomic spin, and they say that
their approach could lead to more sensitive searches for the ripples in
space–time known as gravitational waves and perhaps also to improved brain
imaging.

The standard limit on the precision with which a quantum measurement can be
carried out is due to the statistical error associated with counting discrete
particles rather than continuous quantities. So, for example, when measuring the
phase difference between the waves sent down two arms of an interferometer, the
error in this quantity will scale with the square root of the total number of
photons measured, N. Since the signal scales with N, the
signal-to-noise ratio also scales in the same way. Or, put another way, the
sensitivity of the measurement, which is the minimum signal that can be measured
with a given level of noise, will scale with 1/N1/2.

It is possible to improve on this scaling, however, by entangling the
photons, because this correlates what would otherwise be independent sources of
noise from the individual particles. Such entanglement allows measurements to
approach the so-called Heisenberg limit, which means that sensitivity scales
with 1/N. Until recently it was thought that this scaling represented an
absolute limit on the sensitivity of quantum measurements.

Caught in a trap

However, in 2007 a group led by Carlton Caves at the University of New Mexico
in the US predicted that the Heisenberg limit could be beaten by introducing
nonlinear interactions between the measuring particles. That prediction has now
been shown to be true, thanks to an experiment carried out by Morgan Mitchell
and colleagues at the Institute of Photonic Sciences at Barcelona. Mitchell's
group fired laser pulses into a sample of ultracold rubidium atoms held in an
optical trap and measured how the atoms' spin angular momentum caused the
polarization axis of the photons to rotate.

In a linear measurement, each photon would interact separately with the
atoms, resulting in a relatively weak signal. But what the researchers did was
to carry out nonlinear measurements, ramping up the intensity of the laser
pulses enough so that each photon, as well as registering the magnetic state of
an atom also altered the electronic structure of that atom. This in turn left
its mark on the polarization of the next photon, so amplifying the signal. "We
have a signal that is not dependent just on the thing we are aiming at, but also
on what we send in," explains team member Mario Napolitano.

According to Napolitano, it wasn't clear that a signal could in practice be
amplified in this way because it was reckoned that the nonlinearity would
increase the noise as well as the signal. But his team was able to tailor the
nonlinearity accordingly, by concentrating the interaction between atoms and
photons to a very tiny region of space and by very precisely tuning the
frequency of the laser so that it was very well matched to the atoms’ electronic
structure. Then by measuring the rotation in the photons' polarization using an
interferometer, measuring the noise and measuring the number of photons, then
repeating this process for different photon numbers, the researchers were able
to show that the sensitivity scales with photon number better than the scaling
of the Heisenberg limit. In fact, they achieved a sensitivity that scaled with
1/N3/2.

Clocks and brains could benefit

Napolitano is keen to point out that this result does not imply that the
Heisenberg uncertainty principle is wrong, but rather it shows that we do not
properly understand how to scale that principle up to multiple-particle systems.
He also believes that the work could ultimately have significant practical
applications, such as improving atomic clocks, given that such devices rely on
interferometers. What's more, several research groups are investigating the
possibility of measuring electrical changes in the brain by using light to probe
the magnetic properties of atoms placed close to the brain, and the lastest work
could enhance this technique.

Jonathan Dowling, a theoretical physicist at Louisiana State University in
the US, says that the latest work could also help in the search for
gravitational waves. Researchers hope to register gravitational waves'
distortion of space time by measuring the difference in path length experienced
by laser beams travelling in the two orthogonal pipes of an interferometer.
Dowling says that if the American LIGO detector could operate with a sensitivity
that scales as 1/N3/2 rather than as 1/N1/2
then either its sensitivity could be greatly increased or its laser power
enormously reduced, which would avoid potential heating and deformation of the
facilities' optics. "This opens up a whole new ball game in nonlinear
interferometry," he adds.

However, Barry Sanders, a quantum physicist at the University of Calgary in
Canada, urges caution. "The experiment demonstrates that the Heisenberg limit
can be beaten in the real world," he says. "But practical applications are not
likely in the near future because of the technical challenges that need to be
overcome, especially noise. We are still exploring the basic physics of using
quantum resources for precise measurements."

The research is published in Nature.

About the author

Edwin Cartlidge is a science writer based in Rome

4 comments

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your comments on this article

1

liris77 Mar
24, 2011 10:12 PM

Transcending uncertainty

. . . the point of reality?

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2

vupinto Mar
25, 2011 5:42 AM

Quote:

Originally posted by liris77 View
comment
. . . the point of reality?
Simple, yet
endlessly delightful: Its enjoyment and the glorification of its
Creator.

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3

reader01 Mar
25, 2011 8:22 AM

I am a stranger so I do not quite right understand

According my understanding, polarizised and entangled photons
influens spin of electron one by the other, so these photons interact with
electron one after the other and that causes electron changing of spin and
this is measured on photons. So if we use more photons we receive precise of
measurment on photons 1/N(3/2 ), where N is nuber of entangled
photons????

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4

reader01 Mar
25, 2011 9:55 AM

Quote:

Originally posted by reader01 View
comment
According my understanding, polarizised and
entangled photons influens spin of electron one by the other, so these
photons interact with electron one after the other and that causes
electron changing of spin and this is measured on photons. So if we use
more photons we receive precise of measurment on photons 1/N(3/2 ), where
N is nuber of entangled photons????
Should not be Heisenberg
equations of uncertainty be changed according to latest measurments and the
change of the statistical precise?

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to this comment

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