Physicists Create Quantum Refrigerators

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Physicists Create Quantum Refrigerators

2019-02-18 18:33:01 303 ℃

The new device

confirms that LED can cool other small objects. Photo Source: JOSEPH XU

In recent decades, atomic physicists have used lasers to slow down the "jumping around" atoms in gases and to cool them slightly above absolute zero to study their strange quantum properties. Now, a team of researchers has used similar methods to successfully cool objects. But this time, no laser was used. This technology, which has never been demonstrated in experiments before, may one day be used to cool microelectronics components.

In ordinary laser cooling experiments, physicists emit self-reversing lasers onto rubidium and other gases. They adjust the laser precisely to ensure that if an atom moves toward one of the lasers, it absorbs photons and obtains a slight backward thrust toward the center. Lasers gradually deplete the kinetic energy of atoms, thus cooling the gas to very low temperatures. However, Pramod Reddy, an applied physicist at the University of Michigan, Ann Arbor, wants to try to cool objects without using the special properties of lasers. He and his colleagues started with a small device made of a semiconductor material, the light-emitting diode (LED), which usually appears on a fluorescent screen. LED uses quantum mechanical effect to convert electrical energy into light. Roughly speaking, LED acts as a small "ramp" for electronics. By applying a voltage in the right direction, it pushes the electrons up the ramp and eventually over it, just like a kid on a skateboard. When electrons slide down the ramp and enter a lower energy state, photons are released. For this test, the key is that the LED will not emit light when the voltage is reversed, because electrons cannot cross the ramp in the opposite direction. In fact, the reversal voltage also suppresses the infrared radiation of the device - the broad spectrum of light (including heat) seen when viewing hot objects through night-vision goggles.

This method effectively makes the equipment colder. Meanwhile, Reddy said, this means that the gadget can operate like a miniature refrigerator, but it must be placed close enough to another tiny object. "If you hold a hot object and a cold object, you can get the radiation exchange of heat." Reddy said. To prove that they can use LEDs to cool, scientists placed an object only tens of nanometers (equivalent to the width of hundreds of atoms) away from another heat measuring device called a calorimeter. Because of the quantum tunneling effect, the distance is close enough to increase the photon transfer between two objects. This pore is so small that photons sometimes jump over it. The cooler LED absorbs more photons from the calorimeter than it does back. It relies on capillary action to take heat away from the calorimeter and reduce its temperature by 1/100 degrees Celsius. Reddy and his colleagues reported the results in a recent issue of the Journal Nature. This is a small change, but the size of the LED is also very small, it is equivalent to 6 watts per square meter of energy flux. By contrast, the sun provides about 1,000 watts of energy flux per square metre. Reddy and colleagues believe that they may one day be able to increase the cooling flux to this intensity by reducing the pore size and absorbing the accumulated heat in the LED.

This technology may not replace the traditional refrigeration technology or cool the material below 60 Kelvin. But Shanhui Fan, a theoretical physicist at Stanford University who was not involved in the latest research, believes that it may have potential to be used to cool microelectronic components. In previous work, Fan used computer models to predict that if placed several nanometers away from another object, the LED would have a considerable cooling effect. Now, he says, Reddy and his team have implemented this idea in experiments. (Zonghua)

Relevant Paper Information: DOI:10.1126/science.aax0198