Usually used to describe a highly sensitive sensor that has quantized energy levels, the term “quantum sensor” is usually used to refer to a range of superconducting sensors that use quantum coherence to measure a physical quantity, or use quantum entanglement to improve on the accuracy available from more traditional sensors.
Researchers from ETH Zurich, Technische Universität München, and MIT (one of whom has also written a fun game of quantum mini-golf) describes the following requirements for a quantum system to function as a quantum sensor, which are similar to the DiVincenzo requirements for quantum computing:
The quantum system has discrete, resolvable energy levels.
It must be possible to initialize the quantum system into a well-known state, and to read out its state.
The quantum system can be coherently manipulated, typically by time-dependent fields.
The quantum system interacts with a relevant physical quantity V(t), like an electric or magnetic field.
Though the first rule focusing on two-level systems may at first seem quite limiting, in fact many systems can be modelled using a qubit sensor. A combination of different sensors can thus be used to detect a wide range of conditions to within extremely small tolerances.
What Are Some Examples of Quantum Sensors in Action?
In some ways, we have been using quantum sensors for years. Atomic clocks, based on the decay of Caesium or Rubidium atoms, are one example of quantum sensors in action. A Caesium-based atomic clock is used to define the second in the International System of Units (SI), and thus the definition of the second for the entirety of timekeeping mankind.
The atoms of these two elements are also good for highly accurate detection of position and acceleration. This can be used for highly accurate positioning technology, useful for many different scientific and military applications.
After this, quantum sensors start to become a little more interesting, with the most unusual of them being a thermal vapor of atoms which is used to detect tiny fluctuations in magnetic fields. The sensing of magnetic fields relies on neutral atoms, usually those of an alkali.
A quantum sensor designed to measure PAR (Photosynthically Active Radiation) in plants. Image Credit: LI-COR.
Laser cooling has meant the sensing of magnetic fields can also be achieved using cold atomic clouds. The coldest and most dense of these used to date has been a Bose-Einstein condensate. Trapped ions and Rydberg atoms have both been used to great effect to monitor tiny fluctuations in electrical fields, and detectors have been developed that use more than one atomic state, called ensemble spin detectors.
These ensemble spin detectors use solid state spins, as do the related single state detectors. Ensemble spin sensors are very robust, and have found uses in space, and in archeology and geology. Single spin sensors have been used to tag individual cells in living organisms, making them hugely beneficial to the biosciences. Superconducting qubits have proved to be extremely versatile, and elementary particle qubits using muons and neutrons have also been developed and used.
There is also some research into optomechanical sensors and lots of work has been done with photonic sensors. Both of these fields are extremely promising, and we can expect to see great things in the future from quantum sensors using this technology.
Ultimately, the main uses so far for quantum sensors are:
Highly accurate measurement of position and acceleration
Accurate measurement of time
Tagging and monitoring the location of tiny things as small as individual cells or molecules
Accurate measurement of tiny fluctuations in electrical and magnetic fields
The LI190SB quantum sensor which monitors PPFD (Photosynthetic Photon Flux Density) in natural and artificial light. Image Credit: Campbell Scientific.
How Useful Are Quantum Sensors?
Looking at this list, it’s easy to see how quantum sensor technology has multiple applications in electrical and electronics engineering. The electromagnetic applications alone should instantly pique the interest of any EE engineer.
Quantum Sensor Research for Aerospace and Defense Industries
Much of the current research into quantum sensor technology is in the defense and aerospace fields. Rydberg atoms are one area with aerospace and defense applications. Rydberg receivers can replace traditional antennas, a discovery made when US Army scientists were looking into ways to transimit quantum data over long distances. A Rydberg receiver has several advantages over traditional antennas, including being able to operate on any frequency from DC to THZ (0 to 1,000,000,000,000 Hz), naturally integrating with optical technologies, and detecting a field without absorbing the energy.
Rydberg-based electric field sensors aren’t likely to replace regular antennas in the near future, but their capacities open up avenues for further research in directions that haven’t been on the map before now.
Fraunhofer has a number of quantum sensor projects on the go at the moment, including the development of highly accurate diamond-based sensors for measuring the fluctuation in magnetic fields. This will give us more accurate MRIs, GPS-independent navigation for autonomous vehicles, and improved data density for magnetic storage media.
United Kingdom University Research Projects
The Cavendish Laboratory at Cambridge University is working on a number of quantum sensors that will revolutionise ground-based astronomy. In particular, they are developing Superconducting Quantum Mixers for ground-based high-resolution spectroscopy, Kinetic Inductance Detectors for far-infrared wavelengths, and millimetre-wave and submillimetre-wave Transition Edge Sensors for ground-based astrophysics, amongst other things.
The University of Birmingham is developing atom interferometers that can measure and detect the weakest of forces, gravity. Quantum sensors are paving the way for better optical computing, more accurate uses of magnetism, and optimization of electronic chips and circuits. Radiation detection is another strong area for quantum sensors, with commercial detectors for photosynthetic photon flux density (PPFD) sensors that detect light levels in aquariums already available for hobbyists and commercial aquariums.
The Future of Quantum Sensors
Quantum sensor technology is starting to make a move from the lab to real-world use in industry and the military. We’re starting to get access to technology that can do things like use gravity sensing to help detect oil and minerals, improve telecoms signal sensing, and more.
What’s more, an increasing number of quantum sensor types now have room-temperature alternatives, making tiny, accurate quantum sensors more practical than ever to deploy. If you think you might benefit from its numerous advances, keep an eye on this fascinating field.