This is neat. The photons are sent and the extra photon acts as
confirmation with zero possibility of copying the information. It
also can be tuned over a range of wavelengths to provide even more
utility.
Copying this signal can only destroy the data and unless it is
mirrored somehow it can not be sent on. I will not call it
impossible, but anyone trying deserves to pay the big budgets needed
to make that happen.
Obviously manipulating a photon is the lower limit when it comes to
Moore's Law. It now appears that we will actually reach that limit
with this type of work. I always thought we would, but it is nice to
see the barriers evaporate.
Quantum Rainbow
Photon Gun Unveiled
A photon gun capable
of reliably producing single photons of different colours could
become an important building block of a quantum internet
We've heard much
about the possibility of a quantum internet which uses single photons
to encode and send information protected by the emerging technology
of quantum cryptography.
The main advantage is
of such a system is perfect security, the kind of thing that
governments, the military, banks and assorted other groups would pay
handsomely to achieve.
One of the enabling
technologies for a quantum internet is a reliable photon gun that can
fire single photons on demand. That's not easy.
One of the significant
weaknesses of current quantum cryptographic systems is the finite
possibility that today's lasers emit photons in bunches rather than
one at a time. When this happens, an eavesdropper can use these extra
photons to extract information about the data being transmitted.
So there's no shortage
of interest in developing photon guns that emit single photons and
indeed various groups have made significant progress towards this.
Against this
background, Michael Fortsch at the Max Planck Institute for the
Science of Light in Erlangen, Germany, and a few pals today say
they've made a significant breakthrough. These guys reckon they've
built a photon emitter with a range of properties that make it far
more flexible, efficient and useful than any before--a kind of photon
supergun.
The gun is a
disc-shaped crystal of lithium niobate zapped with 582nm light from a
neodymium-doped yttrium aluminium garnet (Nd:YAG) laser. Lithium
niobate is a nonlinear material that causes single photons to
spontaneously convert into photon pairs.
So the 582nm photons
ricochet around inside the disc and eventually emerge either as
unchanged 582nm photons or as a pair of entangled photons with about
twice the wavelength (about 1060nm). This entangled pair don't have
quite the same wavelength and so all three types of photon can be
easily separated
The 582 nm photons are
ignored. Of the other pair, one is used to transmit information and
the other is picked up by a detector to confirm that the other photon
is ready form transmission.
So what's so special
about this photon gun? First and most important is that the gun emits
photons in pairs. That's significant because the detection of one
photon is an unambiguous sign that another has also been emitted.
It's like a time stamp that says a photon is on its way.
This so-called photon
herald means that there can be no confusion over whether the gun is
secretly leaking information to a potential eavesdropper.
This gun is also fast,
emitting some 10 million pairs of photons per second per mW and also
two orders of magnitude more efficient than other photon guns.
These guys can also
change the wavelength of the photons the gun emits by heating or
cooling the crystal and thereby changing its size. This rainbow of
colours stretches over 100nm (OK, not quite a rainbow but you get the
picture).
That's important
because it means the gun can be tuned to various different atomic
transitions allowing physicists and engineers to play with a variety
of different atoms for quantum information storage.
All in all, an
impressive feat and clearly an enabling step along the way to more
powerful quantum information processing tools.
Ref: arxiv.org/abs/1204.3056:
A Versatile Source of Single Photons for Quantum Information
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