quantum noise-beating technique with entangled photons...
TRANSCRIPT
Using quantum twins of photons from a laser
(pictured here) improv es the detection of an
object in a noisy env ironment.
Image: Giorgio Brida
Technologies that rely on ‘quantum-
weirdness’ phenomena, such as electrons
being in two places at the same time, are
inherently delicate: the smallest disruption
can make such uncertain states ‘collapse’
into well-defined outcomes. Now, however,
physicists have shown that quantum effects
do not always succumb entirely to
disruptions — at least not those from
electromagnetic noise.
Their technique, called quantum
illumination, could enable schemes based on
quantum effects to work in much noisier
environments than they can now. It could
even make quantum physics useful in
applications such as radar, which now relies
on classical physics.
Quantum illumination was first proposed in
2008 by Seth Lloyd, a physicist at the
Massachusetts Institute of Technology in
Cambridge, as a way of improving the
sensitivity of radar and other methods of
detecting distant objects. The principle of
radar is simple: a device sends
electromagnetic waves into the environment
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and then listens for echoes that would signify
the presence of an object. The trouble arises when there is a lot of electromagnetic
noise — either from heat in the environment, or the radar’s own circuits, which can
obscure the fainter echoes from small or very distant objects.
Buddy system
Lloyd’s idea was to ‘entangle’ the beam sent out from a device with a reference beam,
so that each photon in one beam would be matched to a photon in the other, like
soldiers in two marching lines. Any noise would involve photons coming in at random,
which could be told apart and ignored because they lack a correlated twin. By filtering
out noise in this way, Lloyd suggested, quantum illumination offered a means to detect
objects that would be invisible using non-entangled beams.
Physicists were surprised by Lloyd's theoretical result. Noise is normally thought to
destroy entanglement. This is why quantum cryptography — a highly secure scheme
for sharing information — is usually done inside fiber-optic cables. But Lloyd
calculated that a trace of entanglement can remain despite the presence of noise.
Now, for the first time, researchers have experimentally demonstrated a version of
Lloyd’s proposal. The result, by Marco Genovese, a physicist at the National Institute
for Metrological Research in Turin, Italy, and his colleagues, is published on the
preprint server arXiv and is forthcoming in Physical Review Letters.
To generate a pair of entangled beams, the researchers sent laser light through a
‘nonlinear’ crystal, which split the beam of incoming photons into two beams of
correlated, lower-energy photons. They sent one of these beams to a reference
detector and the other through a space, next to which they placed a primary detector.
Nearby they added a light-scattering device, similar to a disco ball, to generate noise.
When no object was in the space, the beam just went through and Genovese and his
colleagues recorded only a few random photon correlations typical of background
noise. When they placed a small piece of glass in the space, however, photons
scattered off it and the number of correlated hits on the detectors rose by 10 times.
To check that this signal was indeed caused by quantum illumination, the researchers
repeated the experiment with non-entangled light. They recorded few photon
correlations, both when the object was present and when it was not.
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3 Comments Add Comment
View Oldest to Newest1. enbuenora_sci
06:07 PM 4/5/13
A bit like analog XLR audio connections. Well, only in the
sense of a vague analogy, but still...
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