Big ideas on making quantum computers smaller

A quantum computer isn’t like your laptop. It needs sub-zero temperatures, superconducting circuits and extremely sensitive electronics to make it work. On top of that, the infrastructure required to deliver a massive amount of computing power also requires a massive amount of space. It varies depending on the device, but at the University of Calgary, Dr. Shabir Barzanjeh, PhD, has one that measures about nine cubic metres. He's also got ideas on how to make it smaller.

Killam Fellowship recipient Barzanjeh and an international team of colleagues from Malta, the United States, the Netherlands and Germany explained those ideas in a review article recently published in Nature Physics titled “Nonreciprocity in Quantum Technologies.” The article surveys existing research to show where the field currently stands and explains the basic principles needed for others to build their own smaller quantum computer devices.

Innovating to make components smaller and more reliable could help reduce the size, cost and complexity of future quantum systems. Barzanjeh says that’s important for quantum technologies to move beyond specialized laboratories and become practical tools for medicine, communications and scientific research. All this hinges on the complex topic of nonreciprocity.

“It's really difficult for our brain and our classical intuition to understand those concepts, but they exist, so that's what we're working on,” says Barzanjeh, an associate professor in the Department of Physics and Astronomy in the Faculty of Science.

The door swings both ways

According to Barzanjeh, this is the basic idea. Most systems carry information in a two-way fashion. For example, light comes in through a window while you can also see out. A wire can carry a signal from one end to the other and back again. This is reciprocity. When it comes to quantum technology, this is a challenge because the signals are extraordinarily weak. The path carrying information away from a quantum device also offers a way for unwanted interference to travel back to the source. This degrades measurements and disturbs the fragile quantum states stored in quantum bits (also known as qubits). Engineers tackle this challenge by using an isolator or a circulator that forces signals to travel in one direction. This is nonreciprocity. 

Conventional versions of nonreciprocal systems use magnetic materials. They work well, but they tend to be bulky, introduce signal loss and produce stray magnetic fields that can interfere with superconducting quantum circuits. This poses what Barzanjeh calls an increasingly serious engineering problem. 

“Discussing with people in different communities, we could see gaps of the basics and foundation of nonreciprocal systems,” says Barzanjeh. Writing a review article offered an opportunity to bridge those gaps, bring the community together and move the technology forward. 

Sharing the information

Barzanjeh’s research offers a description of what you need to build a nonreciprocal system. He likens it to a cookbook. Instead of each lab inventing its own recipe, the article lays out the ingredients and methods side by side, so others can choose the approach that suits their own platform. 

Work over the last few years has seen developments that allow for the creation of nonreciprocal quantum systems without the use of magnets. These devices use carefully timed microwave or laser drives. The drives ensure the signals moving one direction do what they need to do while anything travelling backward cancels itself out preventing interference. Imagine a traffic roundabout, where cars can only circulate in one direction. Physicists call the quantum version of this device a circulator. Because these one-way streets can be built from ordinary chip components and no magnets, they take up far less space than their magnetic counterparts. 

“We showed it's possible to make the chip extremely small,” says Barzanjeh. “Two orders of magnitude smaller than the existing nonreciprocal systems — and we did not use any magnetic field.” Earlier experiments by Barzanjeh and colleagues demonstrated a magnet-free circulator directly on a chip, with a footprint that measured millimeters compared to the centimetre-scale magnetic components used today. 

“That would be an amazing sort of step towards building more compact quantum computers,” says Barzanjeh. “We can put tons of those nonreciprocal systems inside one chip and you can use them for quantum computing applications.”

The same idea could also improve technologies that detect extremely weak signals. In the future, this could support more sensitive medical imaging, navigation systems and scientific instruments, where protecting tiny signals from interference is essential.

There are still important challenges. These devices need to become more reliable, work across a wider range of signals and operate for long periods without disturbing the fragile quantum information they are designed to protect. Barzanjeh says solving these problems will be an important step toward making quantum technologies practical outside research laboratories.

Seeing the results

After spending two years working on the paper, Barzanjeh is excited to finally see it published, adding that preliminary drafts of the paper have already been cited 20 times by other researchers.

“It becomes a resource for the community to build on and to shape their ideas about how to build these components,” says Barzanjeh. “That’s the main goal.”