What are quantum technologies?

“Quantum technologies” is a name for all the methods used to produce tools, the functioning principle of which is essentially based on one of the following quantum properties: The quantum superposition of states of a physical object, or the quantum entanglement of several subparts of that object.

1) Quantum superposition

The world is made such that if an object can exist in several distinct states, it is a priori possible to prepare it in several of these states simultaneously. The object is said to be in a quantum superposition of states.

The illustration on the left shows, as an example, a ball that can be either outside or inside a box, or in a quantum superposition of the two states simultaneously (with the usual notations used by the physicists).

Quantum technologies are based on this kind of superposition of states with atoms, ions, photons, particles with spins, or even with specific electrical circuits.

2) Quantum entanglement

By applying the principle of superposition to a system composed of several objects, each of which can be found in several possible states, one can obtain very strange quantum states called entangled states: In the example on the left the system consists of a blue ball and a red ball each being able to be outside (0) or inside (1) a box. The superposed state in which the balls are simultaneously both outside and both inside the box is difficult to represent: it is at the same time very indeterminate (each of the balls can be found either inside or outside) but at the same time very determined because the balls are for sure at the same place. This kind of entangled state is a resource widely used in quantum technologies.

3) Quantum sensors

Quantum superposition of states are very sensitive to the environment and provide sensors of high precision: atomic clocks, accelerometers, gyrometers or gravimeters based on atomic interferometry, sensitive and compact magnetometers based on natural or artificial atoms . The ever-increasing advances in the control and reduction of classical noise sources lead to the sensitivity of these sensors to a frontier called the “quantum limit”.

The picture on the left shows an absolute quantum gravimeter of the μquans company.

4) Quantum simulators

These are particularly simple (pure), adjustable, easy to control, and easy to observe quantum systems that can mimic and thus reveal the unknown and incalculable behavior of real quantum systems with large numbers of particles. They allow to gradually introduce the disorder that exists in the real system and even to explore parameters that cannot be realized otherwise. They find their application in solid state physics, quantum chemistry, and even astrophysics!

The photo on the left shows a quantum simulator of NIST (USA) consisting of a plan of electromagnetically trapped Berylium atoms. These atoms are crystallized and simulate a magnetic material (here without defects).

5) Quantum communications

They are based on the propagation of photons (light particles) in a quantum superposition of states, or on pairs of entangled photons. Applications are (i) securing communications by quantum cryptography, (ii) interfacing remote quantum systems to share a certain degree of entanglement, or (iii) transferring a quantum state between two physical systems of different nature.

6) Quantum Computing

Quantum computing consists in processing information in a massively parallel way using superposed and entangled states within quantum computers running quantum algorithms. The most common quantum processor is modeled on classical logic gate processor. The information is stored in two-state elementary memory cells, the quantum its or qubits, grouped into multiqubits registers. A minimal set of single- and two-qubit logic gates allows to implement any algorithm. The technical challenge is to succeed in maintaining the quantum coherence of quantum register during the calculation or to correct precisely the errors that occur.

The picture on the left shows a 5-qubit superconducting quantum processor prototype made by IBM (USA).

QUANTUM TECHNOLOGIES IN BRIEF

Technologies involving quantum superpositions of states and/or entangled states of various physical systems are numerous. They can be classified by the type of quantum object they are built on: quantum optics systems involving photons (light particles) trapped in cavities or propagating in vacuum or optical fibers, systems with room temperature or cold atoms, systems of electromagnetically trapped ions, systems involving electronic or nuclear spins, or systems based on superconducting quantum circuits or mechanical oscillators.

Within SIRTEQ, quantum technologies are classified into four applicative domains: (1) the field of “quantum sensors and metrology“, with for example, atomic clocks of GPS satellites or atomic quantum gravimeters to probe the underground with unsurpassed sensitivity; 2) The field of “quantum simulators“, able of modelling in a pure and controlled way the behavior of quantum systems that cannot be calculated; 3) the fields of “quantum communications” allowing, for example, the inviolability of information communicated along optical fibers; 4) Finally, the field of “quantum computing” which should lead to computers able to perform certain calculations much more efficiently than with our current computers.