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Quantum frontiers: Communication in space
52
physicsworld.com
The quantum space race
Sending satellites equipped with quantum technologies into space will be the first step towards a
global quantum-communication network. As Thomas Jennewein and Brendon Higgins explain, these
systems will also enable physicists to test fundamental physics in new regimes
Thomas Jennewein
and Brendon
Higgins are at the
Institute for
Quantum Computing
at the University of
Waterloo, Canada,
e-mail thomas.
jennewein@
uwaterloo.ca
Suppose you have a photon – a single particle of
light. This is a quantum system by nature, so it
exists in a particular quantum state. The photon
could, for example, be vertically polarized, horizontally
polarized or even something in-between: a
quantum superposition.
So what happens when you send your photon to
a receiver at some other location? This question
sounds simple but the answer can tell us some quite
fundamental and startling truths about nature. In
fact, when a pair of photons possesses correlations
much stronger than classically allowed – entangled
quantum states – the implications of what we observe
in so-called “Bell tests” are enough to have spooked
even Albert Einstein, and many people thereafter.
The consequences of these experimental and theoretical
insights are profound as they conflict with
our intuitive understanding of how the world works
(see box on p54). As Richard Feynman wryly concluded:
“after people read [Einstein’s paper on relativity],
a lot of people understood [it] in some way or
other, certainly more than 12. On the other hand,
I think I can safely say that nobody understands
quantum mechanics”.
Beyond these fundamental interests is the field of
quantum communication – the science of transmitting
quantum states from one place to another. Information
is often transmitted by using the aforementioned
vertically polarized light to represent the “0”, horizontally
polarized to represent the “1” and a quantum
superposition to represent a combination of “0”
and “1” simultaneously. Quantum communication
has received significant attention in the last few years
owing to the discovery of quantum cryptography.
Quantum-assured security
Quantum cryptography, or, more correctly, quantum
key distribution (QKD), exploits the fundamental
nature of quantum systems to change state
upon measurement, allowing you to establish a common
encryption key between yourself and a distant
partner with the absolute certainty that if someone
is eavesdropping, you will know about it. An eavesdropper
would leave a trace and, if none is found, the
key can be safely used to securely encode messages.
These messages are sent using ordinary, classical
communication channels, before being decoded by
the distant party using their copy of the key. Traditional
encryption techniques, in contrast, either rely
on assumptions that certain mathematical operations
are difficult to invert, or require the effort of a
trusted courier to physically carry the key from one
location to the other.
Because of this obvious and significant application,
it is not just researchers tucked away in university
laboratories who are interested. Several
quantum-communication companies have also
emerged over the last few years seeking to exploit
the secure messaging that QKD allows, including
ID Quantique in Switzerland, MagiQ in the US and
QuintessenceLabs in Australia. Their efforts come
on top of established programmes by the likes of HP,
IBM, Mitsubishi, NEC, NTT and Toshiba. All of
these companies – and more – are looking to develop
real-world-applicable QKD devices for governments,
banks and other security-focused clients.
The devices that are being built and implemented
today form the seeds of what could one day become
a grander quantum internet – interconnected networks
of quantum-communication channels. These
networks would permit not just quantum-secured
communications, but also distributed quantum computation
(several quantum computers working on
the same problem in tandem) and other quantumenhanced
information technologies.
While the possibilities are exciting, quantum communication
over long distances turns out to be really
difficult. The culprit: transmission loss. Signals
weaken in intensity when they travel long distances
because photons get absorbed or scatter off molecules,
with the transmission loss getting exponentially
worse with distance. Classical communication
can cope with the high losses experienced over long
distances by using repeater devices to boost the signal.
But for quantum signals this approach does not
work. Quantum signals cannot be perfectly cloned,
which rules out standard repeaters, and tricks such as
boosting the signals by transmitting many duplicates
of each quantum state at the same time would defeat
the purpose of encoding information into individual
quantum systems in the first place: an eavesdropper
could simply pick off and examine a subset of
those duplicates, which outwardly would appear to
be nothing more than regular loss. As it stands, the
furthest that quantum-communication signals have
been sent is only a few hundred kilometres.
For applications such as quantum cryptography,
that distance restriction means that your QKD system
can at best allow you to securely communicate
with someone just one or two cities away, which is
hardly ideal in an increasingly globalized society.
Moreover, in terms of physics research, it means that
Physics World March 2013