[cryptography] Quantum cryptography conquers noise problem

Eugen Leitl eugen at leitl.org
Fri Nov 23 07:50:46 EST 2012

(ob caveat snake oil crypto)


Quantum cryptography conquers noise problem

Encoded photons sent a record distance along busy optical fibres.

Zeeya Merali

20 November 2012

Quantum cryptography could keep messages ultra-secure — if the right detector
can be developed.

N. Gregory/Alamy

It’s hard to stand out from the crowd — particularly if you are a single
photon in a sea of millions in an optical fibre. Because of that,
ultra-secure quantum-encryption systems that encode signals into a series of
single photons have so far been unable to piggyback on existing
telecommunications lines. But now, physicists using a technique for detecting
dim light signals have transmitted a quantum key along 90 kilometres of noisy
optical fibre1. The feat could see quantum cryptography finally enter the

You cannot measure a quantum system without noticeably disrupting it. That
means that two people can encode an encryption key — for bank transfers, for
instance — into a series of photons and share it, safe in the knowledge that
any eavesdropper will trip the system’s alarms. But such systems have not
been able to transmit keys along telecommunications lines, because other data
traffic swamps the encoded signal. As a result, quantum cryptography has had
only niche applications, such as connecting offices to nearby back-up sites
using expensive 'dark' fibres that carry no other signals. “This is really
the bottleneck for quantum cryptography,” says physicist Nicolas Gisin, a
scientific adviser at quantum-cryptography company ID Quantique in Geneva,

Physicists have attempted to solve the problem by sending photons through a
shared fibre along a 'quantum channel' at one characteristic wavelength. The
trouble is that the fibre scatters light from the normal data traffic into
that wavelength, polluting the quantum channel with stray photons. Andrew
Shields, a physicist at the Toshiba Cambridge Research Laboratory, UK, and
his colleagues have now developed a detector that picks out photons from this
channel only if they strike it at a precise instant, calculated on the basis
of when the encoded photons were sent. The team publishes its results in
Physics Review X.

Designing a detector with such a sharp time focus was tough, explains
Shields. Standard detectors use semiconducting devices that create an
avalanche of electrical charge when struck by a single photon. But it usually
takes more than one nanosecond (10−9 seconds) for the avalanche to grow large
enough to stand out against the detector’s internal electrical hiss — much
longer than the narrow window of 100 picoseconds (10−10 seconds) needed to
filter a single photon from a crowd.

The team’s ‘self-differentiating’ detector activates for 100 picoseconds,
every nanosecond. The weak charge triggered by a photon strike in this short
interval would not normally stand out, but the detector measures the
difference between the signal recorded during one operational cycle and the
signal from the preceding cycle — when no matching photon was likely to be
detected. This cancels out the background hum. Using this device, the team
has transmitted a quantum key along a 90-kilometre fibre, which also carried
noisy data at 1 billion bits per second in both directions — a rate typical
of a telecommunications fibre. The team now intends to test the technique on
a real telecommunications line.

Gisin’s team has independently developed a photon detector with a similar
time window, which they presented at the QCrypt 2012 meeting at the Centre
for Quantum Technologies in Singapore in September. However, Gisin has
calculated that such a technique cannot be used to transmit quantum signals
beyond the range of a large city of 100 kilometres2. Scattering accumulates
over distance, so there would eventually be so many stray photons that it
would be impossible to filter them out, even with a precisely timed detector.

Still, 90 kilometres is a “world record that is a big step forward in
demonstrating the applicability of quantum cryptography in real-world
telecommunications infrastructures”, says Vicente Martín, a physicist at the
Technical University of Madrid.

    Nature doi:10.1038/nature.2012.11849


    Patel, K. A. et al. Phys. Rev. X 2, 041010 (2012).  Article Show context

    Eraerds, P., Walenta, N., Legré, M., Gisin, N. & Zbinden, H. N. J. Phys.
12, 063027 (2010).

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