Many of the predictions of quantum physics are counter-intuitive.
But so far quantum theory has proved to be correct on all points that
can be verified. Quantum technology aims to make use of the phenomena of
quantum physics to make completely new things possible. The most
important phenomena are described below.
In our everyday life things have specific characteristics – for
example, they can only be in one place at a given time. But the world of
quantum physics is governed by uncertainty and chance. Electrons which
spin round atomic nuclei can be both here and there at the same time,
and a light particle (photon) can travel along two different paths at
the same time. Such composite conditions are called superpositions. It
is only when a measurement is made that the particle is forced into one
of the possible alternatives; chance determines which one. The concept
of superposition is general and also applies to other properties such as
energy, electric charge and velocity.
Superposition makes it possible to store and process vast
quantities of information; read more in section 3.1 Quantum computers.
Entanglement is a superposition that extends between two or more
particles. Interestingly enough the entangled state of the particles
remains even when they are separated by a large distance.
Example: Light particles, photons, can be polarised either
horizontally or vertically. We place two photons in an entangled
condition, which is a superposition of a state in which both photons are
horizontally polarised and one in which both are vertically polarised.
The polarisation of the photons is therefore indeterminate, but they
always have the same polarisation. Then we send one photon off to the
moon. When we measure the polarisation of the second photon here on
earth, we randomly get a horizontal or vertical result. And the photon
on the moon immediately takes on the same polarisation, even though it
is so far away and has no communication channel to earth. Einstein was
extremely sceptical and called this “spooky action at a distance”, but
experiments have shown that it is correct.
Entangled states can be used to send completely intercept-proof messages.
One of the cornerstones of quantum physics is Heisenberg’s
uncertainty principle. This states that there is a limit to the
precision with which the position and velocity of an object can be known
at the same time. The same applies to other interlinked variables such
as frequency and time.
The uncertainty is normally split equally between the two
variables. But by manipulating the quantum system you can ensure that
the uncertainty mainly affects one variable. This is called a squeezed
state. In such a state, it is possible to measure the second variable
with extremely high precision, which can be used to design highly
sensitive measuring instruments.
States of superposition (see section 2.1) are very sensitive to
disturbances. Disturbances cause the superposition to diminish and
finally collapse – and the quantum characteristics then disappear. This
process is called decoherence and is one of the greatest challenges to
be faced in quantum technology. There is an inherent contradiction
between isolating the system from its surroundings to avoid decoherence
and the need to be able to manipulate the system.
The larger the system, the greater the problems with decoherence.
But significant progress has been made in the past 20 years and systems
with dozens of qubits (defined in 3.1 Quantum computers) can now be
Quantum physics has had an enormous impact on society. The
importance of devices based on quantum physics – for example the
transistor and the laser – cannot be overstated. This development is
sometimes called the first quantum revolution.
However, Quantum Technology (QT), a technology based on quantum
physics, can lead much further. A set of additional, subtle quantum
phenomena – notably superposition states, entanglement, and quantum
noise squeezing – were not exploited in the first quantum revolution;
they were harnessed only in the 80s and 90s with the help of modern
equipment and computers. A second quantum revolution is currently
underway, based on the exploitation of these phenomena and on the
precise control of individual quantum systems. This is expected to bring
disruptive change to important areas such as computing, communication,