Physicists create quantum refrigerator

Physicists create quantum refrigerator
For decades, atomic physicists have used lasers to slow down the speed of atoms “jumping around" in the gas and cooled them to slightly above absolute zero to study their weird quantum properties. Today, a research team uses similar methods to successfully cool objects. Only this time, the laser was not used. This technology, which has never been demonstrated in experiments before, may one day be used to cool microelectronic components.

Physicists create quantum refrigerator

In the ordinary laser cooling test, physicists emit laser light from the opposite direction onto gas such as rubidium. They adjusted the laser precisely to ensure that if the atoms moved towards one of the laser beams, they would absorb the photons and gain a slight back pressure toward the center. The laser will gradually deplete the kinetic energy of the atoms, thereby cooling the gas to a very low temperature.

However, Pramod Reddy, an applied physicist at the University of Michigan, Ann Arbor, wanted to try to cool objects without using the special properties of the laser. He and his colleagues started with a small device made of light-emitting diodes (LEDs), a semiconductor material usually found in fluorescent screens. LEDs use quantum mechanical effects to convert electrical energy into light. Roughly speaking, the LED acts as a small "ramp" of electronics. Applying voltage in the right direction, it will push the electron up the "ramp" and eventually over it, like a skateboarder. 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 the electron cannot cross the "ramp" in the opposite direction. In fact, the reverse voltage also suppresses the device ’s infrared radiation—the broad spectrum of light (including heat) seen when viewing hot objects through night vision goggles.

This method effectively makes the equipment cooler. At the same time, Reddy said that this means that the above-mentioned small device can operate like a mini 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 heat radiation exchange." Reddy said. To confirm that they can use LED cooling, scientists placed an object only a few tens of nanometers (equivalent to a few hundred atoms in width) from another calorimeter measuring device called a calorimeter. Due to the existence of quantum tunneling, this distance is close enough to increase the photon transfer between two objects. This pore is so small that photons can sometimes jump over it.

The colder LED absorbs more photons from the calorimeter than it returns, relying on capillary action to take the heat away from the calorimeter and reduce its temperature by 1/100 ° C. Reddy and colleagues reported this result in the recently published Nature magazine. This is a small change, but the size of the LED is also very small, which is equivalent to 6 watts per square meter of energy flux. In contrast, the sun provides about 1,000 watts of energy flux per square meter. 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 heat accumulated in the LED.

This technology may not replace traditional refrigeration technology, or cool the material below about 60 Kelvin. But Shanhui Fan, a theoretical physicist at Stanford University who did not participate in the latest research, believes that it may have the potential to be used to cool microelectronic components. In his previous work, Fan used a computer model to predict that if placed a few nanometers away from another object, the LED could produce a considerable cooling effect. Today, he said, Reddy and his team have implemented this idea in experiments.

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