Physics of Quantum Sensors

Quantum sensing

In physics, a quantum is the minimum amount of any physical entity involved in an interaction.

Quantum sensors can detect magnetic fields with the highest spatial resolution and sensitivity so far. New possibilities to study neuronal circuits and do rapid clinical testing arise with the Nitrogen-vacancy (NV) – center – based magnetometry, SQUID-MEG (superconducting quantum interference device Magnetoencephalography) and OPM-MEG (Optically pumped magnetometer MEG).

Quantum gravitometer - general applications

A recent example is the quantum gravitometer, it can measure very small changes in acceleration due to gravity. Here, to create a quantum, lasers are used. One uses a laser to create a cloud of very cold atoms, that approach the temperature of absolute 0 (principle of cryogenics). 

As the temperature is so low, it becomes a quantum cloud, and is now allowed to interact freely with itself. These movements create unique patterns, that can be analysed to determine how strong the gravity field is in a specific location. This can be applied to detect underground pipelines, chambers, cables, mineral deposits or volcanic activity. Another example is the satellite Sentinel-5P. It was made using a quantum gravitometer and is now measuring the different polluting gases contained in the atmosphere.

Metals and superconductors 

In the early 19th century, Kamerling Onnes experimented with cryogenics. When cooling metals at really cold temperatures, like 4K, their resistance lowered to 0 ohms. This means that at very low temperatures, close to the absolute zero, metals become superconductors. This is one of the first demonstration of quantum effects: superconductivity is therefore a macroscopic proof of how electrons behave in a metal. Another property demonstrated was how superconductors can be considered diamagnetic. In this case, superconductors prevent magnetic fields at their interior and so they create an electric current on their surface which is needed to expel the interior magnetic field. This results in a field of the opposite direction, making the object levitate. 

Laser cooling

As already mentioned in gravitometers, another way to cool atoms is by using lasers. Usually, atoms that move towards a laser beam experience a frequency shift, giving them enough energy to absorb a photon and get excited. This photon is then emitted in a random direction, and the excited particle decays back down to the ground state. On average, this means that atoms which move towards a laser beam are slowed down, whilst when it moves away from the laser beam, it accelerates again. If you shine lasers from all directions, you give the atom only the possibility to slow down. This makes it cool down efficiently to very low temperatures, close to the absolute Zero. An analogy to this is throwing many balls at an airplane: this slows it down if we have sufficient balls. Likewise, when a laser emits many photons, it slows the atoms down. 

Quantum electrical metrology (R, V, I)

 Here three methods are explained to do precise measurements and also the efefcts and physical principles used in quantum metrology (metrology = the scientific study of measurement)

This behavior is determined by fundamental constants of nature, namely Planck's constant (h) and the elementary charge (e), and is independent of the material being studied. As a result, this effect enables the creation of a natural standard for resistance that is consistent and reproducible anywhere:

R= h/e^2 * 1/n 

where n is an integer or fractional value representing the quantised Hall conductance.

Because the Quantum Hall Effect depends solely on universal constants, it is essential for defining precise electrical standards in metrology.

Josephson Effect

The Josephson Effect provides a natural standard for voltage. This effect occurs in a Josephson Junction, where two superconductors are separated by a very thin insulating barrier. Classically, electrons shouldn't pass through the barrier because the resistance is effectively infinite. However, due to quantum tunneling, electrons can behave as waves, penetrating the barrier and creating a supercurrent without any voltage drop.

When electromagnetic waves, like microwaves, are applied to the junction, a voltage is generated, given by:

V= n⋅h⋅ f / 2e

where n is an integer, f is the frequency of the applied radiation, h is Planck's constant, and e is the elementary charge.

This property allows precise voltage measurements and has been critical in establishing voltage standards globally.

The current produced is described by the formula:

I = n⋅e⋅f

where n is the number of electrons, e is the elementary charge, and f is the frequency at which electrons are pumped.

These pumps create highly accurate current standards, complementing the Quantum Hall Effect and Josephson Effect to establish a universal system for electrical measurements.