​The central phenomena of quantum technology

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.
Squeezed states
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 controlled well.
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, and sensing.

Published: Mon 27 Nov 2017.