Fudge - a sweet solution to quantum circuit congestion

The silicon microchips of future quantum computers will be packed with millions, if not billions, of quantum bits (the basic unit of quantum information) to solve the biggest problems facing humanity. Since millions of quantum bits require millions of wires in the microchip circuit, the chip will always get crowded there.

Now, engineers at the University of New South Wales in Sydney have taken an important step toward solving a long-standing problem: giving their quantum bits more room to breathe - all around "(jellybean) fudge ".

It's not the kind of candy we eat to get through the 3 p.m. slump; it's "jellybean quantum dots": extended regions between pairs of quantum bits that scientists have created more space for wiring without interfering with the way pairs of quantum bits interact with each other.

As lead author Associate Professor Arne Laucht explains, fondant quantum dots are not a new concept in quantum computing, but a solution that has been discussed many times.

"It has been shown in different material systems, such as gallium arsenide. But it hasn't been shown in silicon before." He said.

Silicon is arguably one of the most important materials in quantum computing, A/Prof. Laucht said, adding, "Given that we use silicon chips in classical computers, the infrastructure to produce future quantum computing chips is already in place. Another benefit is that you can hold many quantum bits (in the form of electrons) on a single chip."

"However, because quantum bits need to be close together to share information with each other, placing wires between each pair of quantum bits is always a challenge."

In a study published in February in Advanced Materials, a team of UNSW engineers described how they showed in the lab that fudge-like quantum dots are possible in silicon. This opens up new paths for spacing quantum bits to ensure that the wires needed to connect and control them can be placed in the middle.

The research results were published in Advanced Materials on February 20, 2023, as Jellybean Quantum Dots in Silicon for Qubit Coupling and On-Chip Quantum Chemistry.

In ordinary quantum dots using spin quantum bits, individual electrons are pulled from a pool of electrons in silicon under a "quantum gate". Each quantum bit can then be controlled by an oscillating magnetic field at microwave frequencies.

But in order to implement quantum algorithms, we also need double quantum bit gates, where the control of one quantum bit is conditional on the state of the other quantum bit. To do this, the two quantum dots need to be placed very closely together (only a few tens of nanometers apart; a human hair is about 100,000 nanometers thick) so that their spins can interact with each other.

But separating them further apart to create more wiring space - that's always been a challenge for scientists and engineers. The problem is that as pairs of quantum bits move, they will stop interacting with each other.

Device structure and jellybean quantum dot transport measurements.

The fudge solution this time represents a way to do both: well-spaced quantum bits will continue to interact with each other. To make fudge, engineers found a way to create a chain of electrons by capturing more electrons between quantum bits - it's like a quantum version of a string phone, so the two paired quantum bit electrons at each end of the fudge can continue to talk to the other. Only the electrons at each end are involved in any computation, and the electrons in the fudge dots are there to keep them interacting as they spread out.

The number of extra electrons pulled into the fudge dots is key to how they arrange themselves, said Zeheng Wang, the paper's lead author and a former doctoral student.

"We showed in the paper that if you only load a few electrons in the puddle below you, they break up into smaller puddles. So it's not a continuous fudge quantum dot; it's a smaller one here, a larger one in the middle and a smaller one there. We're talking about a total of three to maybe ten electrons."

"It's only when you get to a larger number of electrons (say 15 or 20 electrons) that the fudge becomes more continuous and homogeneous. That's where there's a clear spin and quantum state that we can use to couple quantum bits to another."

Experimentally adjusting the distance between artificial atoms.

Magnetic spectrum analysis on a fondant quantum dot. This shows that the fondant quantum dot formed in the device has a well-defined spin state.

Finally, the team says that this result is the first demonstration of a fondant structure in a SiMOS architecture and will help to understand the physics of fondant couplers in silicon: as we are moving towards using fondant as a medium between quantum bits for exchange coupling.

Professor Laucht emphasizes that there is still much work to be done. The team's efforts for this paper have focused on demonstrating that fondant-like quantum dots are possible. The next step is to insert working quantum bits at the ends of the fondant quantum dots and make them "talk" to another quantum bit.

"It's great to see this work come to fruition. It boosts our confidence that fondant couplers can be exploited in silicon quantum computers, and we are excited to try to implement them with quantum bits next."

Reference links:

[1] https://onlinelibrary.wiley.com/doi/10.1002/adma.202208557

[2] https://thequantuminsider.com/2023/05/15/mmmm-jellybeans-and-chips-scientists-say-jellybeans-could-solve-overcrowding-in- quantum-computer-chips/

[3]https://www.gizmodo.com.au/2023/05/quantum-jellybean-australia/

[4]https://newsroom.unsw.edu.au/news/science-tech/jellybeans-%E2%80%93-sweet-solution-overcrowded-circuitry-quantum-computer-chips