Quantum technology is based on the ability to precisely control
individual quantum systems in order to make use of the phenomena
described above. There are applications in secure communications, highly
sensitive measurement methods and in creating computing power that far
exceeds today’s supercomputers.

Quantum technology is often divided into four areas: quantum computing, quantum simulation, quantum communication and quantum sensing. The last two are very close to commercialisation and products have already started to appear.

Quantum technology is often divided into four areas: quantum computing, quantum simulation, quantum communication and quantum sensing. The last two are very close to commercialisation and products have already started to appear.

##### Quantum computing

In today’s computers the smallest data carrier is the bit, which
can have only the value 0 or 1. But the quantum equivalent, known as a
quantum bit or qubit, can have both the value 0 and 1 at the same time
as a result of superposition. Two qubits can have four values at the
same time – 00, 01, 10 and 11 – and each additional qubit doubles the
number of possible states. This means that a quantum computer with 300
qubits would be able to represent more values than there are particles
in the entire universe. And it only takes 50-60 qubits to exceed the
computing power in today’s supercomputers.

The first ideas involving the use of quantum systems for
calculations came in the 1980s, but at the outset they were not
considered to have any practical significance. There was both a lack of
usable algorithms (quantum computers cannot be programmed in the same
way as normal computers) and nobody knew how to correct the errors that
would inevitably arise in a quantum computer.

The situation changed drastically in 1994 when Peter Shor published
a quantum algorithm which rapidly finds the prime number factors that
make up a given large number, which is the key to cracking today’s
encryption codes (see 3.3 Quantum communications). A year later he
showed how a special error correction code can deal with the errors that
occur in a quantum computer. This sparked an interest in building a
quantum computer among researchers around the world. A quantum computer
with many qubits is the ultimate goal in quantum technology according to
many.

The most promising technologies for building a quantum computer are
ion traps and superconducting circuits. In an ion trap, qubits are made
up of floating ions which are held in place by electric and magnetic
fields, and are manipulated by laser light. The ion trap record is
currently 14 fully controlled qubits.

Superconducting qubits consist of electric circuits without any
electrical resistance (= superconducting), where the energy switches
between being electric and magnetic. The circuits can be manipulated by
microwaves. Since the circuits are placed on a microchip, it is
relatively straightforward to scale up to a large number of qubits. The
short decoherence time (see section Decoherence) has been a major
limitation, but following diligent development work it has been possible
to increase it dramatically in the past 20 years. IT companies such as
Google, IBM and Intel have launched research projects into
superconducting quantum computers. In autumn 2017 IBM launched the
largest processor so far with 16 qubits, and Google has announced that
it is working on a 49-qubit processor.

An important but difficult element is to implement codes that limit
the effect of the errors that inevitably occur, just as they do in all
computers. One option is to allow each logical qubit to be represented
by several physical qubits which are read out four at a time to check
whether an error has occurred. Another option is also to code the
quantum information in microwaves.

A quantum computer can also be used to solve problems involving a
large number of different possibilities, such as optimisation problems
in machine learning and artificial intelligence. It is also suited to
calculating the characteristics of large molecules – for example, for
developing new pharmaceutical products or materials.

##### Quantum simulation

A quantum simulator is a specially designed quantum computer
constructed to simulate a certain process. It can therefore only solve a
limited number of problems. If you want to solve other problems, you
need to build a new quantum simulator, designed to solve those specific
problems.

A number of fairly simple examples of quantum simulation have
already been demonstrated, but have not yet surpassed classical
computers. Rapid progress is, however, being made and researchers are
preparing to scale up to the level required to demonstrate what is known
as quantum supremacy, which means solving a problem that is beyond the
reach of even the most powerful classical computer. Usable applications
for quantum simulation are expected within five years.

##### Quantum communication

Our internet-based society with internet banks, digital medical
records, web-based commerce etc, is based on the secure transmission of
information. Today encryption is used, which is based on problems which
are presumed to be difficult to calculate such as finding the prime
number factors which have created a specific very large number.

But when the quantum computer makes its appearance, cracking
today’s encryption will be child’s play. However, quantum technology
also offers a solution – the secure transmission of encryption keys via
quantum communication. This is the only known solution which can
guarantee that an outsider cannot read the encrypted message.

The encryption key is the code that the recipient needs in order to
decode the encrypted message. The sender uses individual photons to
send the encryption key to the recipient. Since it is not possible to
measure a photon without it being affected, you can be sure of detecting
whether an outsider has tried to steal the encryption key.

Nowadays there are commercial systems which can transfer quantum
encryption keys via an unbroken optical fibre over a distance of
approximately 100 kilometres, but at a fairly low speed.

In order to guarantee the security of the next generation of
communications systems, a global quantum network must be developed,
which can rapidly and securely transmit encryption keys between many
different points. The quantum phenomenon of entanglement (see section
about Entanglement) play a key role when it comes to strengthening and
transmitting quantum signals in a large network.

##### Quantum sensing

Human knowledge of the world and our technological progress is
limited by what we can measure and how precisely we can do so.
Researchers are now in the process of learning to use individual
particles such as photons and electrons as sensors in measurements of
forces, gravitation, electric fields etc. Measurement capability is thus
pushed far beyond what had previously been possible. For example,
researchers have demonstrated measuring techniques which can measure
forces as weak as the gravitational force between two people on either
side of the American continent.

Heisenberg’s uncertainty principle (see section Squeezed states)
limits how precisely measurements can be made. In most cases the
uncertainty is so incredibly small that it can be disregarded. But on a
very small scale, such as when measuring a single electron, the
limitations of the uncertainty principle are significant. Quantum
technology provides tools for enhancing the precision in those
quantities we want to measure by shifting the uncertainty to another
variable (known as a squeezed state). It also provides the opportunity
to create specially designed quantum states for specific measurement
tasks that make the measurement insensitive to the strongest sources of
noise.

The development of quantum sensors will lead to more powerful
instruments for measuring electric and magnetic fields both in our
environment and inside our bodies. We can also expect to have
instruments which can measure local variations in gravitation to find
minerals, water or embedded pipelines and advanced warning systems for
earthquakes and volcanic eruptions.