Even though the world is focused on the next development in quantum computing, it is quantum sensors that are making big waves after measuring fields and forces that were previously impossible to even detect.

From brain waves to gravitational waves, quantum sensors have successfully detected both and are now being prepared to operate outside laboratories as well. 

How do quantum sensors work? 

Physical sensors usually use an engineered part, such as a spring, a coil, or even a computer chip, to convert a parameter into a number. So, whether you are looking at temperature or pressure, light or magnetic fields, a sensor can give you a measure of their presence within a limited area. 

A quantum sensor also works in a similar way, but instead of using an engineered part, it uses either an atom, an electron’s spin, or a superconducting qubit to measure a physical quantity. Most quantum sensors follow a three-step loop. First, they prepare a known quantum state, then let the physical parameter change it, and then measure the change in the third step. 

Depending on whether the sensor uses an atom, an electron, or a qubit, the original quantum state can be a known energy level, or an electron spin, or an electrical loop, respectively.

Unlike physical sensors whose readings become inaccurate as a result of temperature or prolonged usage, quantum measurements are much more uniform due to the consistency of the material used and are sensitive to even the tiniest of nudges in the parameter being monitored. 

Where quantum sensors are being used

Modern medicine allows imaging of the brain using magnetic fields produced by brain activity. Typically, this imaging uses sensors that can pick up femtotesla or picatesla range of magnetic fields, weaker than even refrigerator magnets. This is achieved by shielding the sensors from other magnetic fields. 

However, a new atomic-scale magnetometer developed by the National Institute of Standards and Technology (NIST) not only operates at room temperature but can also measure magnetic fields from the heart. In an experiment, researchers at NIST used their atomic-scale device to measure fetal heart measurements, too. 

The world we know today is heavily dependent on GPS signals for navigation. From international travel to local food delivery, GPS is being used everywhere. The fallout from  GPS signals failing or being blocked is increasing every day, and scientists want to use accelerometers and gyroscopes as a backup instead. 

Although these sensors are now present even in our smartphones, they are error-prone and, over time, build up errors. The solution to the problem is an atom interferometer, in which a cloud of laser-cooled atoms helps reduce these errors. While the technology is still being developed, the UK and Europe have included it in their resilience plans in case GPS becomes unavailable. 

Although the use of quantum sensors is increasing, quantum states are delicate and can be swayed easily. For instance, quantum noise can impact how well the LIGO instrument works when detecting gravitational waves.

So scientists use frequency-dependent squeezing to reduce quantum noise. In other sensors, vacuum chambers, shielding, and other lasers are deployed to keep quantum sensors stable. 

Research is ongoing to make quantum sensors smaller, cheaper, and also tough enough that they can be deployed in everyday environments without needing special protection.