Last week, I participated in Quantum Sensing Linz, a small-scale conference on Quantum Sensing. Since my own work centers mostly on Quantum Software Engineering, the conference was a great opportunity to broaden my horizons and realize once again how little I actually know.
But let’s start at the beginning.
What is Quantum Sensing?
Similar to Quantum Computing, Quantum Sensing uses the special properties of quantum mechanical objects to do something. In quantum computing, quantum mechanical phenomena are used to solve problems, such as Grover’s algorithm. In Quantum Sensing, these same phenomena are used to better understand the properties of a material or environment.
The overall objective of the field is to build better sensors by exploiting effects that only exist on the quantum mechanical level.
Magnetic Resonance Imaging
A common example of Quantum Sensing is Magnetic Resonance Imaging (MRI).
The protons within the water molecules of a body are aligned using an external magnetic field. Then, electromagnetic pulses change the alignment of individual atoms in the body. As these protons realign themselves with the external magnetic field, they induce a current which can be detected by the machine.
Because how the protons realign themselves with the external field depends on the context (e.g. tissue) they are in, the forces they produce can be used to detect, for example, the shape and constitution of certain organs.
If you are interested in a more comprehensible introduction to the physical mechanisms behind MRI:
Sensing with Undetected Photons
While Magnetic Resonance Imaging is among the most common quantum sensing technologies in the field, another technology took the spot as VIP during Quantum Sensing Linz: Quantum Sensing with undetected photons.
Just like protons, photons are quantum particles and thereby underlie the influence of quantum mechanical phenomena. In this case, the phenomenon of interest is quantum entanglement. Whenever two quantum objects become entangled, the properties of the system consisting of both objects cannot be described based on the properties of the individual objects alone. The quantum wave function only allows certain pairs of values to exist.
When sensing with undetected photons, a pair of entangled photons is created by sending a photon beam through a nonlinear crystal. The created photons have different wavelengths, which are interrelated through quantum entanglement. Only certain wavelength pairs/combinations are possible. If one photon is manipulated in a certain way, the wavelengths we can measure the other photon in can change due to entanglement. Even if the photons have been physically separated.
Of these photons, we send one towards the material/probe we wish to extract information from (i.e. the idler photon) and keep the other (i.e. the signal photon) as it is. After the idler photon has interacted with the probe, both photons interfere, and the signal photon is measured.
Because both photons have different wavelengths, we can choose to send a photon with a convenient wavelength towards the probe, have it affect the signal photon through entanglement, and finally measure the effect on the signal photon in a value range easy to detect. Through entanglement, we can separate the wavelength at which we want to produce an effect from the wavelength range in which we want to measure.
I am skipping a lot of quantum physics and experimental caveats here, but the core idea is the same as with MRI: We use effects that only exist for quantum mechanical objects to detect/measure/sense information.
In the end, that’s what Quantum Sensing is all about.