Researchers develop a superconducting quantum refrigerator

Imagine a refrigerator so cold that it could turn atoms into quantum states, which would give them unique properties defying the rules of classical physics.

In an article published in Applied physical examinationAndrew Jordan, a professor of physics at the University of Rochester, and his graduate student Sreenath Manikandan, in collaboration with their colleague Francesco Giazotto of NEST Istituto Nanoscienze-CNR and Scuola Normale Superiore in Italy, have come up with an idea for this guy refrigerator. would cool atoms to near-zero temperatures (about minus 459 degrees Fahrenheit). Scientists could use the refrigerator, based on the quantum property of superconductivity, to facilitate and improve the performance of quantum sensors or circuits for high-speed quantum computers.

What is superconductivity?

Conductivity is the degree of conductivity of a material. When a material has a high conductivity, it is easily traversed by an electric current. Metals, for example, are good conductors, while wood, or shielding wrapped around wires, are insulators. However, if the metal wires are good conductors, they still encounter resistance due to friction.

In an ideal scenario, a material would conduct electricity without encountering resistance; that is, that it would carry a current indefinitely without losing any energy. This is precisely what happens with a superconductor.

"When you cool a system to extreme temperatures, the electrons enter a quantum state where they behave more like a collective fluid that flows without resistance," says Manikandan. "This is done by electrons in a pair of superconductors, called pairs of copper, at very low temperatures."

The researchers believe that all metals can become superconductors if they are sufficiently cooled, but each metal has a different "critical temperature" at which its resistance disappears.

"When you reach that magical temperature – and it's not a gradual thing, it's a steep thing -, the resistance just falls as a stone to zero and there's a phase transition going on product, "says Jordan. "As far as I know, a practical superconducting refrigerator has not been manufactured yet."

Similarities with a traditional refrigerator

The superconducting quantum refrigerator uses the principles of superconductivity to function and generate an ultra-cold environment. The cold environment is then conducive to the generation of the quantum effects needed to improve quantum technologies. The superconducting quantum refrigerator would create an environment in which researchers could transform materials into a superconducting state, similar to transforming a material into gas, liquid or solid.

Although superconducting quantum refrigerators are not used in a person's kitchen, the principles of operation are quite similar to those of traditional refrigerators, says Jordan. "What your kitchen refrigerator has in common with our superconducting refrigerators, is that it uses a phase transition to achieve cooling power."

If you walk into your kitchen and stay next to your refrigerator, you'll notice that it's cold inside, but warm at the back. A conventional refrigerator does not work by making its contents cold, but by removing the heat. To do this, it passes a fluid, the refrigerant, between hot and cold tanks, and goes from liquid to gas.

"Refrigerators do not create cold from scratch," says Jordan. "There is a principle of energy conservation.Heat is a kind of energy, so the refrigerator takes the heat from one region of space and transmits it to another region. . "

In a conventional refrigerator, the refrigerant in the liquid state passes through a regulator. When the liquid is relaxed, its pressure and temperature drop when it goes into the gaseous state. The now cold refrigerant passes through an evaporator coil located inside the refrigerator, thus absorbing the heat from the refrigerator's contents. It is then recompressed by a compressor powered by electricity, further increasing its temperature and pressure and passing it from a gas to a hot liquid. The hot condensed liquid, warmer than the outside environment, flows through the condenser coils to the outside of the refrigerator, emitting heat into the environment. The liquid then enters the regulator and the cycle is repeated.

The superconducting refrigerator is similar to a conventional refrigerator in that it moves material between hot and cold tanks. However, instead of a refrigerant that changes from a liquid state to a gas, the electrons of a metal go from the paired superconducting state to an unpaired normal state.

"We are doing exactly the same thing as a traditional refrigerator, but with a superconductor," says Manikandan.

The workings of a superconducting quantum refrigerator

In the superconducting quantum refrigerator, researchers place a stack of metals in a cold and cryogenic dilution refrigerator:

• The bottom layer of the stack is a superconducting niobium sheet, which acts as a hot reservoir, similar to the environment located on the outside of a traditional refrigerator.
• The intermediate layer is the superconducting tantalum, which is the working substance, similar to the refrigerant of a traditional refrigerator
• The top layer is copper, which is the cold tank, similar to the inside of a traditional refrigerator

When researchers slowly apply a current of electricity to the niobium, they generate a magnetic field that enters the middle tantalum layer, causing the decoupling of its superconducting electrons, their transition to their normal state and their cooling. The now-cold tantalum layer absorbs the heat of the now warmer copper layer. The researchers then slowly extinguish the magnetic field, causing the tantalum electrons to couple and return to a superconducting state. Tantalum then becomes hotter than the niobium layer. Excess heat is then transferred to niobium. The cycle is repeated maintaining a low temperature in the upper layer of copper.

This is similar to refrigerant in a traditional refrigerator, going from cold cycles where it is extended to a gas and hot where it is compressed into a fluid. But since the working substance in the superconducting quantum refrigerator is a superconductor, "it is rather the pairs of barrels that dissociate and cool down when you apply a magnetic field slowly at very low temperatures, taking as a reference the current advanced refrigerator. baseline and cooling it even more, "says Manikandan.

While you use your kitchen's fridge to store milk and vegetables, what can a researcher put into a superconducting quantum refrigerator?

"You use a kitchen refrigerator to cool your food," says Jordan. "But it's a super fridge, super cold." Instead of storing food, the superconducting quantum refrigerator could be used to store objects such as qubits, the basic units of quantum computers, by placing them above the metal stack. Researchers could also use the refrigerator to cool down quantum sensors, which measure light very efficiently and are useful for studying stars and other galaxies and could be used to develop deeper tissue imaging more efficient in medical devices. ; MRI.

"It's really a bit of a surprise to think about how it works, it's basically about taking energy and turning it into transformative heat."