Very few people know that all watches and clocks in the world are tuned to the oscillation of electrons in caesium-133 atoms, the only stable isotope of the metal caesium.
Caesium is a soft, silvery-golden alkali metal and one of only five metals that are liquid at or near room temperature.
The world time standard of one second is defined as “the duration of 9 192 631 770 cycles (Hz) of microwave light absorbed or emitted by the hyperfine transition of caesium-133 atoms in their ground state undisturbed by external fields.”
Since Einstein’s proof in 1967 that the speed of light is the most constant dimension in the universe, the International System of Units used two specific wave counts from an emission spectrum of caesium-133 to define a second and a metre. Since then, caesium has been commonly used in extremely accurate atomic clocks to keep the world on time.
It is exactly this utilisation of the nature of matter, such as using the difference between electrons in different energy states as a base unit, which enables quantum sensors to display the utmost levels of precision such as found in atomic clocks. Other quantum sensors use atomic transitions to detect minute changes in movement and infinitesimal differences in electric, magnetic and gravitational fields.
In solid-state physics, a quantum sensor is a quantum device that responds to a stimulus.
Usually this refers to a sensor that, which has quantised energy levels, uses quantum coherence to measure a physical quantity, or uses entanglement to improve measurements beyond what can be done with classical sensors. Quantised energy entails quantum mechanical systems or particles that is bound or spatially confined and can only take on certain discrete values of energy or energy levels in contrast with classical particles, which can have any amount of energy. Quantum coherence means that the frequency and waveform are identical and the phase difference is constant.
According to leading experts in the report “Top 10 Emerging Technologies of 2020,” published in November 2020 by The World Economic Forum, Quantum sensors as a transformative technology will play an increasingly important role in our future, allowing autonomous vehicles to “see” around corners or into rooms, and will detect precisely what lies under our feet, as well as enable underwater navigation systems, advance warning systems for earthquakes and volcanic activity, and compact scanners that monitor a person’s brain activity during everyday life and detect initial signs of multiple sclerosis.
Quantum sensors are so sensitive they can detect a wide range of very tiny signals from the world around us, such as the gravitational pull of buried objects or the magnetic field from the human brain.
Gravimeters map variations in density below the ground by recording gravitational fluctuations in time and space. Gravitational quantum sensors are typically used by geoscientists to monitor volcanic hazards by measuring density changes caused by the rising magma, or to gauge water resources by measuring the extent of aquifers. Energy companies uses gravimeters to measure variations in gravity over large areas to find oil and gas.
Recently, researchers at the University of Birmingham, in the UK, found a new possibility to build quantum sensors. They are working on the development of free-falling, supercooled atoms to detect tiny changes in local gravity. This new discovery and type of quantum gravimeter could prove very valuable in the detection of buried pipes, cables and other objects. This could be extremely valuable in South Africa where municipalities and other role players often lose their concealed pipes and cables and then have to dig up the whole road with great cost and to the inconvenience of all.
The same gravity sensors are also used by construction companies and civil engineers for underground surveys to determine old mineshafts, sewers, sinkholes or other dangerous structures. Ocean-going ships could use this same quantum sensor technology to detect submerged objects.
Glasgow University researchers are using tiny gravimeters to monitor one of the most active volcanoes in the world, namely Mount Etna in Sicily. When magna chambers below ground fill up, their gravity readings change, thus giving advanced warning of volcanic activity.
Although seismometers, ground deformation recorders, gas monitors, infrared cameras and satellite imagers are already used to monitor volcanoes, the cheap and permanent gravimeters is a potential game changer due to their low cost, high accuracy and because there is no need to move them around under dangerous conditions.
One of the biggest problems with current navigation systems is when contact with the GPS satellites is lost while travelling in a tunnel or when signals are deliberately jammed. A better approach is therefore to use atom interferometers to build navigation devices that have the ability to work even when contact with the GPS satellites is lost. Staying on course in this case relies on dead reckoning, the use of accelerometers and gyroscopes to continuously update a vehicle’s position, orientation and velocity with respect to a known starting point.
3D lidar seeing around the corner
Glasgow University researchers are also working on a special 3D type of lidar that will enable people and cars to “see” around the corners or into a room. Traditional lidar typically measures the distance to an object by illuminating it with pulsed laser light and then measuring the reflected pulses. However, quantum sensor technology enables scientists to measure the arrival time of every single photon with extremely high accuracy, in trillionths of a second.
Often when you shout in a canyon, you will hear the echo of your voice. Exactly the same can be done with light or a laser beam. The light will bounce off the walls, as long as you have the geometry right. It is then easy to build a 3D image using this data. The researchers from Glasgow University thus want to develop a next generation lidar for self-driving cars to give them enhanced awareness in fog, smoke and over longer distances. The prototype sensor from Glasgow can already detect moving people 100 meters away, even when they are still a few meters around a corner.
Quantum sensing is also making inroads in the health care industry, in particular with regard to the detection of degenerative diseases. The well-known Magnetic Resonance Imaging (MRI) has proven valuable over time, but quantum sensors are much simpler, cheaper and have a better resolution. It is known that in diseases such as multiple sclerosis, the processing speed of the spinal cord to the brain changes. Quantum sensors are able to detect these minute changes, which existing diagnostic tools such as MRI sensors cannot do.
Without doubt the military all over the world is also very interested in quantum sensing technology. Gravimeters, for instance, offer the potential to detect the submarines of an opponent. Gravity may be a weak force, but we are not able to shield from it. So, while modern stealth technology may hide the submarine’s radar signature, it would not hide it from an ultra-sensitive quantum gravity sensor.
Smaller and affordable quantum sensors
Unfortunately, most quantum sensing systems are currently very costly, complex and too big in size, but a new and promising generation of smaller and more affordable quantum sensors could bring numerous new applications in future. Scientists at the Massachusetts Institute of Technology (MIT) in the US, last year used traditional manufacturing methodology to put a diamond-based quantum sensor on a silicon chip, thus succeeding in packing multiple and usually bulky components on a square of a few tenths of a millimetre wide. The MIT quantum sensor prototype is an important step towards affordable, mass-produced quantum sensors that work at room temperature and that could be used for any application that involves taking extremely fine measurements of weak magnetic fields.
The future of quantum sensing
Because of the promising contribution of quantum sensor technology, governments and private investors are making significant investments in this technology, in particular to resolve the challenges of cost, scale and complexity. According to industry analysts, quantum sensors should be available on the market in the next five years, with medical and defence applications leading the way.
The subatomic realm and quantum technology are not only leading to high-precision metrology, but many other applications. The human capability to manipulate individual atoms and electrons are transforming industries ranging from communications and energy to medicine and defence. But although many scientists working on quantum sensors have formed companies to commercialise their technology, only a few have actual products on the market.
Professor Louis C H Fourie is a futurist and technology strategist