Quantum physics for a long time seemed like a field existing somewhere beyond the everyday world. Superposition, entanglement, and other effects of the microworld sounded more like fundamental riddles of nature than a basis for practical technologies. But today, it is precisely these properties that are beginning to turn into tools capable of measuring what conventional instruments find extremely difficult to see.

We are talking about quantum sensors — devices that utilize the sensitivity of atoms, electrons, and other quantum systems to changes in their environment. They make it possible to measure magnetic and gravitational fields with high precision, which means obtaining information about hidden objects, the structure of terrestrial rocks, and even the electrical activity of the human brain.

Seeing what is hidden underground

Imagine a construction site where, even before excavation work begins, it is necessary to understand what lies beneath the surface. Pipes, old foundations, or other objects might be passing through there.

Conventional methods of geophysical exploration have limitations. For instance, ground-penetrating radars do not work in all types of soil, and certain underground objects become practically impossible to detect at great depths. Quantum gravimeters offer a fundamentally different approach. They measure extremely small changes in the gravitational field.

Gravity depends on the distribution of mass. If an object lies underground whose density differs from the surrounding rock, it slightly alters the local gravitational field. A void, a tunnel, or a section with less dense material, for example, creates a tiny decrease in gravity. It is so small that capturing it with a conventional device is difficult, but for a quantum sensor, such a difference can become a measurable signal.

How atoms turn into a gravimeter

At the core of some of these devices lie clouds of ultracold atoms, such as rubidium. They are placed in a vacuum chamber and, using lasers and magnetic fields, are held in a strictly controlled state. This is where quantum mechanics enters the picture. A laser can place an atom into a state of superposition. In such a state, a quantum system is described by several possible states at once until the moment of measurement. Researchers use this property to compare how atoms behave at different heights.

The atomic cloud is effectively split into two quantum components. One of them ends up slightly higher than the other. Because the gravitational potential varies at different heights, the quantum states begin to evolve at different rates. When the two components recombine, they interfere with each other. By the nature of this interference, scientists can determine how much the gravitational field has changed. This resembles a very precise quantum clock: instead of directly measuring gravity, the device tracks how gravity has altered the quantum state of the atoms.

Why two clouds of atoms are needed

One of the main challenges in such measurements is vibration. If the device sits on the Earth's surface, it can be shaken by traffic movement, operating machinery, human footsteps, and even natural soil oscillations. Therefore, researchers use gravity gradiometers. They simultaneously measure the gravitational field at two points located at different heights. Common vibrations affect both measurements, allowing them to be partially canceled out, leaving the difference specifically associated with the mass distribution underground.

In the long run, such devices will be able to search for underground pipes and voids, study geological structures, detect aquifers, assist in locating mineral resources, and monitor changes in volcanic systems. There is also another potential application — archaeology. A quantum gravimeter could theoretically detect structures hidden underground without requiring excavation.

From the Earth's crust to the human brain

However, gravity is only one of the quantities that can be measured using quantum technologies. An equally interesting direction involves magnetic fields. Any electric current creates a magnetic field. In the human body, electrical currents arise constantly, including during the transmission of signals between nerve cells.

Consequently, brain activity is accompanied by extremely weak magnetic signals. If we learn to register them, we can literally observe the activity of various regions of the brain. This is precisely what magnetoencephalography, or MEG, does.

A quantum sensor hears the brain

Modern quantum magnetometers can use vapors of alkali metals — for example, cesium or rubidium atoms in vapor form. Such an atom has an electron with spin. It can be pictured as a tiny magnet. A laser allows lining up many such atomic "magnets" in a specific direction.

When an external magnetic field arises nearby, the orientation of the atoms begins to change. They effectively begin to precess — perform a characteristic rotational movement. This change can be detected by how laser light interacts with the atoms. This results in a highly sensitive magnetic sensor. And its sensitivity is sufficient to register the weak magnetic fields generated during brain function.

Why this could change brain research

Before the advent of such technologies, MEG required bulky superconducting sensors that had to be cooled to extremely low temperatures, including through the use of liquid helium. Such systems are expensive, complex, and have another drawback: the sensors cannot be placed directly against the patient's head.

For brain research, this is critically important. Magnetic signals from neuronal activity are very weak and decay rapidly with distance. Quantum magnetometers allow making sensors much more compact and positioning them literally a few millimeters from the surface of the head. Researchers have already created prototypes of MEG systems where dozens of small magnetometers were installed in a special helmet that conforms to the shape of the human head.

This approach provides two advantages at once. First, the signal becomes stronger due to the sensor's proximity to the brain. Second, the person gains the ability to move a bit more freely than in traditional systems.

This can prove especially important when working with children. It is difficult to compel a child to remain still inside a large medical scanner for a long time, and their head dimensions differ from those of an adult. A flexible sensor helmet is capable of solving both problems.

What can be seen with such a system

Quantum magnetometers make it possible to record changes in brain activity with very high temporal precision. In experiments, researchers were able to observe how activity in the motor cortex changes when a person moves their fingers.

This provides an unusual combination of spatial and temporal resolution. Unlike electroencephalography, which registers electrical signals via electrodes on the surface of the head, magnetometric systems can localize the source of activity more accurately. And unlike functional MRI, which shows well which brain regions are activated but works relatively slowly, MEG allows tracking ongoing processes in near real time.

Therefore, quantum magnetometers are viewed as a promising tool for studying epilepsy, neurodegenerative diseases, and brain development in children.

Based on materials from PubMed Central.