In new quantum computer design, qubits use magnets to communicate selectively

Researchers have begun using magnets to entangle qubits, the building blocks of quantum computers. This simple technique could unlock complex abilities.

When you press a button to open a garage door, it doesn’t open all the garage doors in the neighborhood. This is because the opener and the door communicate using a specific microwave frequency, a frequency that no other door nearby uses.

Researchers at the U.S. Department of Energy’s (DOE) Argonne National Laboratory, the University of Chicago, the University of Iowa, and Tohoku University in Japan have begun developing devices that could use the same principles, i.e. sending signals via magnets rather than through the air. to connect individual qubits on a chip, as shown in a new paper published in the Proceedings of the National Academy of Sciences.

“This is a proof of concept, at room temperature, of a scalable and robust quantum technology that uses conventional materials,” said David Awschalom, Liew Family Professor of Molecular Engineering and Physics at the Pritzker School of Molecular Engineering from the University of Chicago; the director of the Chicago Quantum Exchange; the director of Q-NEXT, a DOE national quantum information science research center housed at Argonne; and the principal investigator of the project. “The beauty of this experiment lies in its simplicity and its use of well-established technology to design and ultimately entangle quantum devices.

Connecting qubits via quantum entanglement is necessary to build a quantum computer, but it can often be tricky. With nitrogen vacancy (NV) centers – defects in diamond that can be used as qubits – the challenge is that to communicate with each other, they have to be very, very close to each other. Normal quantum interaction between NV centers has a maximum range of only a few nanometers, or a thousandth of the width of a hair, and when NV centers are so close together, they cannot be designed in a Useful setup.

“You have to be able to get your hands on things to connect wires and make a device,” said Michael Flatté, a professor of physics and astronomy at the University of Iowa who contributed to the work. Flatté is also the chief scientist at quantum technology company QuantCAD LLC, a partner company of the Chicago Quantum Exchange. “And nanometers are just too close for that.”

This is where magnets come in.

Two years ago, Flatté and his collaborators published a theoretical paper proposing using a magnetic material to establish a quantum connection between NV centers so that they could be entangled while being further apart from each other. The normal interaction between two NV centers involves microwaves. In this proposed device, the magnet receives the microwave from the NV center and transmits it via “magnon” to the NV on the other side.

In a magnet, the spins of all the electrons inside point in the same direction, like stalks of grain pointing upwards. A magnon is a slight disturbance of the waves through these rotations, like a wave the wind would make across a field of grain. Magnons can go much further than nanometers, even a thousand times farther, in fact, up to several micrometers.

“The micrometer scale is quite interesting because it is the typical scale of many integrated electronic devices, such as silicon transistors in a computer chip,” Flatté said. “So if you were to make objects of this size, you could get a reasonable number of them on a chip.”

Connecting the central NV qubits with magnets also allows selective interaction: if two qubits in the quantum computer spoke at a slightly different frequency, they could entangle without disturbing or being affected by the other qubits, even if there had other qubits between them. This capability is extremely important for the type of complex work that scientists want quantum computers to do.

This experiment carried out by Awschalom and his collaborators verified that the NV center could “talk” to the magnetic material, transmitting its microwaves like a magnon. What’s more, the numbers match almost perfectly what was predicted in the theoretical paper from two years ago.

“This work represents a good synergy between experiment and theory,” said Masaya Fukami, first author of the paper. Fukami was a postdoctoral fellow at UChicago’s Pritzker School of Molecular Engineering during the experiment and now works at the quantum computing company PsiQuantum. “I was really impressed with how well the model predicted the experiment. It gives me a lot of confidence in this system.”

Now that they have established that the NV center can communicate with the magnet, the next step is to place another NV center on the other side and see if the magnet can establish a quantum connection between the two.

“This is the first integration modality with magnets,” Flatté said. “I think this is a very powerful approach that could in principle also be applied to other solid-state qubit systems.”