Microwave Technology Heats Up

Microwaves provide more than just a quick meal. The transmission of information via microwaves (the type of light, not the appliance) is fundamental to technologies such as Bluetooth communication, mobile phone networks, satellite televisions, radar, and GPS. A team of scientists from Aalto University in Finland recently created a tiny detector that could lead to big advances in microwave technology and have applications at the cutting edge of science.

The electromagnetic spectrum.
Image Credit: NASA’s Imagine the Universe

Microwaves are easy to send wirelessly and transmit across a wide range of frequencies, and they pass through most objects without being affected. These features make microwaves a great option for communication systems. As the technology for sending and receiving microwave signals has improved, so have our communication abilities. However, there is still room to push the current limits on this technology.

Imagine that you are outside on a dark night when you see someone using a lantern to send out an SOS. This call for help works because you—a detector—can see the pulses of light. Now imagine that the person has a keychain flashlight instead of a lantern, so the pulses are much fainter. You can still get the message and call for help as long as you can resolve the light and dark pattern. Now imagine the signal getting fainter and fainter, until the pulses are as faint as they can possibly be—you’d need a much more sensitive detector than your eyes to register the SOS.

Similarly, over time we have developed the technology to send fainter and fainter microwave pulses—all the way down to one photon at a time. Sending messages with individual photons instead of bright pulses means you can do more communicating with less energy. This technology has exciting possibilities in communication and imaging, but also in experiments at the forefront of science that explore how atoms and molecules behave on the quantum level. The trouble is, detecting individual microwave photons efficiently is a challenge.

Scientists are approaching this challenge from multiple angles. One of the most attractive approaches is detecting microwave photons thermally—with heat. Because electromagnetic waves cause the electrons in a conductor to oscillate back and forth, there is a slight increase in the temperature of an object when it absorbs a microwave pulse. If you can detect this temperature change, congratulations! You’ve successfully detected a microwave signal.

It sounds easy enough, but in reality the change in temperature is so small than it often gets lost in the noise when you try to read out the temperature. What you need is a very sensitive thermometer that makes the temperature change easy to see. A team of scientists led by Mikko Möttönen has made significant progress toward achieving this goal.

The detector created by Möttönen’s team consists of a gold nanowire and several tiny pieces of superconducting aluminum. It is smaller than a human blood cell and is chilled to just above absolute zero. When a microwave pulse reaches the detector, it is absorbed by the nanowire and the temperature of the device goes up slightly. This leads to a change in the electrical properties of the device, making it a very sensitive thermometer.

In order to keep the small temperature change from being overwhelmed by noise, the team put a feedback mechanism in place. An external source of energy interacts with the device in such a way that when a microwave pulse is absorbed, the temperature difference is amplified, somewhat similar to the way a guitar amplifier works. The team was able to detect microwave pulses with energy 14 times smaller than previous experiments based on temperature.

Artist’s depiction of a hybrid superconductor-metal microwave detector.
Image Credit: Ella Maru Sudio

The roots of the idea go back seven years to a dinnertime conversation. Möttönen was a postdoc at the Centre for Quantum Computer Technology then, eating with Jukka Pekola, a professor visiting from Aalto University. During conversation an idea sparked and some quick calculations scribbled on a small piece of paper indicated that it should be possible to detect a single microwave photon with a very sensitive thermometer. After starting at Aalto University and securing funding and expertise in the form of PhD student Joonas Govenius and postdocs Rusell Lake and Kuan Yen Tan, the experimental work began.

So far they’ve been able to detect microwave pulses of around 200 photons with 1.1 zeptojoule of energy. This is a tiny amount of energy—the energy required to lift a red blood cell by just 1 nanometer. They are working on optimizing the design, and their paper in Physical Review Letters provides some thoughts on how to get down to the individual photon level, in addition to describing their work.

“I hope that I still have somewhere the little piece of paper that we used to make the back-of-the-envelope calculations,” said Möttönen. But he continues, it’s not enough to have great ideas, you have to have a system that funds the implementation of ideas, as well as motivated, hard-working people.

In addition to other applications, this work is important for cutting-edge experiments exploring quantum-mechanical effects. Experiments in superconducting quantum computers, quantum optics, and quantum thermodynamics require detectors that can measure the energy of quantum systems, which means measuring individual microwave photons. They may sound like science fiction, but these areas could revolutionize our understanding and use of computing, light, and energy.

—Kendra Redmond

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