Our
digital society is highly dependent on secure information. But with the
progress of quantum computers that potentially can break today’s encryptions,
the security risks are rapidly increasing. Quantum communication – based on the
laws of physics – provides intercept-proof solutions.
There are
many reasons – political, military as well as commercial – to keep information
secret. Individuals are also highly dependent on information security in
today’s digital society, for example in e-banking, e-health and e-business.
The
encryption used today is based on mathematical problems presumed to take an
enormous amount of time to calculate, such as finding the prime number factors
of a specific very large number. However, no one has yet managed to
mathematically prove that there is no quicker way to solve these problems. And
as quantum computers are making progress, new computational methods will
develop. For example, it has been shown that future quantum computers have the
potential to very quickly find the factors of large numbers.
A
solution – already in use – is to use encryption based on quantum particles.
How does it work?
Encryption
relies on so called encryption keys – usually strings of ones and zeroes – used
to encrypt and decrypt information. If the receiver of an encrypted message has
the key, then he or she can decrypt and read the information. The problem is
generally to transfer the key without an adversary getting hold of it.
In
quantum communication, the encryption key is transferred using quantum
particles, so called Quantum Key Distribution (QKD). According to the laws of
quantum physics, it is impossible to measure or copy an unknown state of a quantum
particle without noticeably changing it. Therefore, one can always be sure to
detect interception. Once the key is safely transferred, the message encrypted
with the key can be sent in a conventional way, via a communication channel
that anyone can access.
The
quantum particles generally used in quantum key distribution are light
particles, photons. They have a quantum property called polarisation, which can
be manipulated and measured by letting the photon pass through a polarisation
filter. The most established scheme for quantum key distribution – the BB84
scheme – relies on the sender and the receiver to measure the polarisation of
the photons by randomly using different polarisation filters. This is nicely
explained in the Youtube video Quantum Cryptography in 6 Minutes.
Already available
Commercial
systems using the BB84 scheme are already on the market. The drawback of these
systems is that they require an unbroken optical fibre. This limits the distance
to 200–300 km, a physical limit set by the properties of optical fibres and
photons. Also, their cost-effectiveness can be questioned.
The most
advanced, known systems for quantum communication are found in China. The first
is a long-distance quantum encrypted link between Beijing and Shanghai, based
on the BB84 scheme and unbroken optical fibres. To cover the more than 1000 km
long distance between the cities, the signal is relayed by several nodes which
decrypts and re-encrypts the data before passing it on. The nodes are
susceptible to hacking, and therefore only security classed personnel have
access to the them.
In the
second system, satellites act as nodes. As the satellite passes over a ground
station, the encryption key is established by sending and receiving faint
pulses of few or single photons to the satellite. As it is both expensive and
difficult to travel to an orbiting satellite, one trusts that the satellite is
safe from local hacking attempts.
Coping with long distances
The
limited range over which it is possible to send quantum keys is a big hurdle.
For a global quantum communication network to become true, one must find a way
of amplifying and forwarding the signals, without having to decrypt and
re-encrypt the data along the way. A so-called quantum repeater could do the
job.
These are
very complex machines requiring many quantum devices and sub-systems to
function at high performance levels with extremely good timing. Unfortunately,
the existing quantum repeaters are quite poor. Developing satisfying quantum
repeaters is probably the most important and the most challenging task within
quantum communication research.
The lack
of good quantum repeaters is the reason for starting to use satellites as
nodes, as satellites in space are judged as more safe from hacking than
ground-based nodes.
At the front
Another
drawback with today’s BB84 systems is that they require trusted devices for
sending and receiving photons. If someone has installed spy equipment in your
photon source or receiver, they may be able to eavesdrop without you noticing.
Therefore,
scientists work on more advanced, device-independent quantum communication
schemes. By performing a quantum test, more specifically a so-called Bell test
experiment, on the received data, one can conclude if the data is secure or
not. The choice of equipment then becomes less important from a security point
of view – one could even buy equipment from an enemy. However, these
device-independent schemes are slower and more technologically challenging. For
example they require devices that can entangle photons, which is not a standard
capability of present photon sources.
Scientists
also work on increasing the transfer rate of quantum keys, as well as the
distance over which they can be sent. Large efforts are also put into the
important but difficult task of developing good quantum repeaters.