Specifically, Miciusaims to distribute quantum keys between
the satellite and pairs of ground stations in China. The objec-
tive of this quantum key distribution (QKD) demonstration
is to transfer quantum key bits at a rate of 10 kb/s over much
longer distances than current optical fibre links, and some 1000
times faster.
A long-range QKD demonstration between China and
Austria is also planned, foreshadowing a global satellite-based
network. Not only does the success of this demonstration and
its global extension depend on technological issues but also
critically upon Einstein’s spooky action at a distance, which
still remains to be demonstrated on a global scales.
This is a major question to be addressed by the QUESS
team. Is quantum entanglement preserved over long distances,
such as the 7500 km between Vienna and Beijing, as is predicted
by quantum mechanics and deemed essential for the realisa-
tion of worldwide QKD? If this is not the case it would signal
a fundamental failure of quantum mechanics, with immense
theoretical and conceptual consequences.
Quantum Cryptography
QUESS plans to distribute quantum keys by generating polarised
photon pairs on board the satellite and then transmitting these
by line-of-sight to selected pairs of ground stations to two recip-
ients usually referred to as Alice and Bob (see box: Qubits,
Entanglement and Quantum Keys ). Their shared key then serves
as a private one-time pad. One-time pads have been used for a
century by secret agents and clandestine groups, most notably
by Ché Guevara to communicate with fellow Latin American
revolutionary Fidel Castro.
A truly random one-time pad is a “perfect cipher” because
it enables Alice to encrypt a plain text message in a provably
secure manner so that it can only be deciphered by Bob using
his shared copy of the secret key. Maintaining absolute security
then becomes the task of distributing the key so that it cannot
be covertly intercepted by Eve.
This is the unique advantage of quantum key distribution:
any eavesdropping by Eve will be detected by Alice and Bob
because of the peculiarly elusive and fragile character of qubits.
Historical Background
The idea that secure cryptographic keys constructed from qubits
could be distributed worldwide by satellite is not new. More
than two decades ago, quantum physics and engineering research
groups in the UK, Austria, the US (at Los Alamos National
Laboratory) and elsewhere, including The University of Canberra,
competed to demonstrate quantum key distribution over increas-
ingly long “free space” atmospheric links. Some, including the
Canberra group, investigated the limits imposed by the turbulent
atmosphere on simulated satellite “free space” links.
In the early work, cryptographic keys were usually distributed
by Alice, who transmitted sequences of weak, heavily attenu-
ated pulses of laser light to Bob. Each pulse contained, on
average, much less than one photon in order to suppress the
emission of pulses containing two (or more) photons as this
could give the game away to Eve. Efficient single-photon sources
had not been developed, and a major advance occurred when
French, Swiss and Austrian groups successfully distributed
entangled photon keys over optical fibre and short-range atmo-
spheric links.Australian Research
In 1999 the Centre for Advanced Telecommunications and
Quantum Electronics Research at The University of CanberraMAY/JUNE 2017 | | 23Qubits, Entanglement
and Quantum Keys
Quantum information is quite unlike the classical
information routinely exchanged for everyday purposes as
sequences of the ordinary binary digits “0” and “1”. The
unit of quantum information is called the qubit. A single
photon of light can carry a qubit of information in the form
of the state of its unknown polarisation. This is
mathematically described as the superposition of two
equally likely polarisation states; for example, horizontal
and vertical polarisations represent the two classical binary
digits “0” and “1”.
However, the classical value remains undetermined until
the measurement is actually made. This ambiguity makes
qubit sequences ideal for cryptography.
Polarised single photons are ideal for transmitting
quantum information through the atmosphere as they retain
their polarisation over very long distances.
Quantum-entangled pairs of polarised photons are
particularly well-suited for cryptography because if the
recipient of one photon of an entangled pair measures a
“1”, then the other recipient will measure a “0” providing
they have both set up their receivers appropriately.
The two recipients of a stream of entangled photon pairs
can therefore share a secret sequence of bits. Moreover, by
changing their receiver settings and privately comparing
sections of their respective bit streams, the two recipients
(usually referred to as Alice and Bob in the cryptographic
literature) can detect any loss of quantum correlation due to
an attempt (by Eve) to eavesdrop on the sharing of what
will become a secret quantum distributed key.Credit: Mart [email protected]