Quantum sensors use “creepy” science to measure the world with unprecedented precision

When you stepped on your bathroom scale this morning, you probably measured your weight to the nearest tenth of a pound. That’s probably all you need. But there are times when you want to weigh something a little more accurate, e.g. B. a piece of mail. The scales at the post office weigh an envelope lighter than your bathroom scales. That’s precision, and it’s an important factor in the measurement.

There are cases where extremely accurate measurements are crucial. If you know how to accurately measure location, GPS can help you navigate to the post office. Even more precise measurements will enable a spaceship to land on Mars.

Improved measurements can help us do more and understand more. Quantum systems and entanglement can be used here. They can help us carefully survey an environment and measure it with unprecedented precision.

Decoherence is a big problem for quantum communication. It happens when quantum particles interact with something in their environment – for example the edge of a fiber optic cable – and their wave function collapses.

Decoherence occurs because quantum states are extremely sensitive to their environment. This is a problem for quantum communication, but actually an advantage when it comes to sensing. It is their responses to small changes in the environment that make quantum sensors so accurate, allowing them to achieve levels of accuracy we never dreamed possible before.

A quantum sensor essentially observes how a particle interacts with its environment. There are different types of quantum sensors that can measure all sorts of things – magnetic fields, time, distance, temperature, pressure, rotation, and a host of other observables. By delving deeper into how quantum sensors work, we can get a glimpse of their power and how they can affect our lives.

Look deep into the ground

In the original Jurassic Park, paleontologists used undefined, fictional technology to create an image of dinosaur bones hidden underground. The scene is a bit ridiculous, but it helps us understand the implications of a tool that allows us to see underground without digging. Such technology might not help us find surprisingly intact dinosaur skeletons, but it could help us locate a variety of other things – abandoned mine shafts, pipes or cables, aquifers, and any number of underground irregularities. Knowing where things are underground before you start digging could help companies save millions of dollars on everything from subways to skyscrapers.

How can atoms help? Just like the sun and the earth, things around us have a gravitational pull – albeit a much smaller one. Dense matter like a vein of granite would have a greater gravitational pull than an empty subway tunnel. The difference might be tiny when measured from above, but a sufficiently precise sensor could detect it.

Using atoms as quantum sensors, a group at the University of Birmingham illustrated how precise such sensors can be. They placed two atoms in a gravitational field and gave one a little “kick” upwards. This atom fell back under gravity. Since particles can act like waves, the two atoms get in each other’s way and create an interference pattern. Two crests of atomic waves can align and cause constructive interference. Alternatively, a peak may align with a valley, causing destructive interference. A tiny gravitational difference would change the interference pattern of the atoms and allow tiny measurements in the gravitational field.

Not only does this allow us to know what is beneath our feet, but it can also help us predict when volcanoes will erupt. Magma filling an empty chamber beneath a volcano alters local gravity. Sensors spread across a volcano could potentially detect when a chamber is filling and hopefully warn of an eruption.

There is no time like quantum time

Atomic clocks are another example of quantum sensors that can produce extreme precision. These clocks are based on the quantum nature of atoms. First of all, all the electrons in an atom have some energy. Imagine the electron orbiting the nucleus at a certain distance. The electron can only orbit in discrete states separated by highly specific energy levels. To move from one energy level to another, the electron can either absorb a photon of a precise frequency to move up or emit a photon to move down. An atomic clock works when an electron changes its energy state around the atom.

Currently, United States Standard Time is determined by a cesium atomic clock at the National Institute of Standards and Technology. This clock is so accurate that in 100 million years it will not gain or lose a second. To measure time with such accuracy, the watch uses a laser beam to shower cesium atoms with extremely precise frequencies of light and eject their electrons to higher levels. Only the precise calibration of the light frequency of the laser makes it possible to save time. (Remember that frequency is the inverse of time.)

We can do it even better if our atoms don’t work alone, but are entangled with each other. In 2020, a team at MIT fabricated an atomic clock from entangled atoms. The accuracy of this clock is truly mind-boggling: it loses just 100 milliseconds over the age of the universe.

From very small to very large

Quantum sensors can allow our telescopes and microscopes to show us more.

When we think of exploring the universe, we usually picture a telescope that collects photons – whether they’re optical, infrared, or radio. But we can also explore the universe with gravitational waves.

When two black holes merge or a supernova explodes, the fabric of space and time stretches and compresses like ripples on a pond. We can detect these waves with an interferometer that accurately compares the distance for two perpendicular directions. To measure this, the instrument sends a beam of light along each axis. The rays bounce off mirrors, return to the source, and recombine, creating an interference pattern. When a wave of a gravitational wave passes the interferometer in one direction, it could be slightly stretched while in the other direction it would be compressed, changing the interference pattern. This difference is small, but would indicate the passage of a gravitational wave.

Again, entangled photons can offer an advantage. The measuring ability of the interferometer is limited by the different arrival times of the photons in the light beam. Put simply, some of the photons arrive at the detector earlier than others. By combining entangled photons and a technique called “photon squeezing” with Heisenberg’s uncertainty principle, we can reduce the scatter in the arrival times of these photons at the expense of another observable. Using this method, interferometers such as LIGO and Virgo can detect vibrations 100,000 times smaller than an atomic nucleus.

Squeezing light can also help improve the sensitivity of microscopes. For a microscope to work, light must illuminate the object. When this light bounces off the sample and returns to the microscope, the randomness of the photon arrival time introduces noise. Normally, this so-called shot noise can be reduced by increasing the brightness. But at some point, the light intensity actually damages the sample, especially if it’s some kind of biological tissue. A team from the University of Queensland showed that using entangled photons and squeezing them together increased the sensitivity of the microscope without frying the sample.

Measuring is about understanding our environment on a deeper level. Whether temperature, electric field, pressure or time, such measurements are about more than numbers. It’s about understanding what these numbers mean and how to use small changes. Quantum sensors can be used in MRIs and in navigating without GPS systems. They can help self-driving cars better sense their surroundings, and scientists can predict volcanic eruptions. Quantum entanglement may remain mysterious, but it also has a very practical side.