Tag Archives: Andrea Morello

A jellybean solution to a problem with quantum computing chips

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A May 11, 2023 news item on phys.org heralds this new development, Note: A link has been removed,

The silicon microchips of future quantum computers will be packed with millions, if not billions of qubits—the basic units of quantum information—to solve the greatest problems facing humanity. And with millions of qubits needing millions of wires in the microchip circuitry, it was always going to get cramped in there.

But now engineers at UNSW [University of New South Wales] Sydney have made an important step toward solving a long-standing problem about giving their qubits more breathing space—and it all revolves around jellybeans.

Not the kind we rely on for a sugar hit to get us past the 3pm slump. But jellybean quantum dots –elongated areas between qubit pairs that create more space for wiring without interrupting the way the paired qubits interact with each other.

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A May 10, 2023 University of New South Wales (UNSW) press release (also published on EurekAlert), which originated the news item, delves further into the ‘jellbean solution’, Note: A link has been removed,

As lead author Associate Professor Arne Laucht explains, the jellybean quantum dot is not a new concept in quantum computing, and has been discussed as a solution to some of the many pathways towards building the world’s first working quantum computer.

“It has been shown in different material systems such as gallium arsenide. But it has not been shown in silicon before,” he says.

Silicon is arguably one of the most important materials in quantum computing, A/Prof. Laucht says, as the infrastructure to produce future quantum computing chips is already available, given we use silicon chips in classical computers. Another benefit is that you can fit so many qubits (in the form of electrons) on the one chip.

“But because the qubits need to be so close together to share information with one another, placing wires between each pair was always going to be a challenge.”

In a study published today in Advanced Materials, the UNSW team of engineers describe how they showed in the lab that jellybean quantum dots were possible in silicon. This now opens the way for qubits to be spaced apart to ensure that the wires necessary to connect and control the qubits can be fit in between.

How it works

In a normal quantum dot using spin qubits, single electrons are pulled from a pool of electrons in silicon to sit under a ‘quantum gate’ – where the spin of each electron represents the computational state. For example, spin up may represent a 0 and spin down could represent a 1. Each qubit can then be controlled by an oscillating magnetic field of microwave frequency.

But to implement a quantum algorithm, we also need two-qubit gates, where the control of one qubit is conditional on the state of the other. For this to work, both quantum dots need to be placed very closely, just a few 10s of nanometres apart so their spins can interact with one another. (To put this in perspective, a single human hair is about 100,000 nanometres thick.)

But moving them further apart to create more real estate for wiring has always been the challenge facing scientists and engineers. The problem was as the paired qubits move apart, they would then stop interacting.

The jellybean solution represents a way of having both: nicely spaced qubits that continue to influence one another. To make the jellybean, the engineers found a way to create a chain of electrons by trapping more electrons in between the qubits. This acts as the quantum version of a string phone so that the two paired qubit electrons at each end of the jellybean can continue to talk to another. Only the electrons at each end are involved in any computations, while the electrons in the jellybean dot are there to keep them interacting while spread apart.

The lead author of the paper, former PhD student Zeheng Wang says the number of extra electrons pulled into the jellybean quantum dot is key to how they arrange themselves.

“We showed in the paper that if you only load a few electrons in that puddle of electrons that you have underneath, they break into smaller puddles. So it’s not one continuous jellybean quantum dot, it’s a smaller one here, and a bigger one in the middle and a smaller one there. We’re talking of a total of three to maybe ten electrons.

“It’s only when you go to larger numbers of electrons, say 15 or 20 electrons, that the jellybean becomes more continuous and homogeneous. And that’s where you have your well-defined spin and quantum states that you can use to couple qubits to another.”

Post-jellybean quantum world

A/Prof. Laucht stresses that there is still much work to be done. The team’s efforts for this paper focused on proving the jellybean quantum dot is possible. The next step is to insert working qubits at each end of the jellybean quantum dot and make them talk to another.

“It is great to see this work realised. It boosts our confidence that jellybean couplers can be utilised in silicon quantum computers, and we are excited to try implementing them with qubits next.”

Whoever wrote the press release seems to have had a lot of fun with the jellybeans. Thank you.

Here’s a link to and a citation for the paper,

Jellybean Quantum Dots in Silicon for Qubit Coupling and On-Chip Quantum Chemistry by Zeheng Wang, MengKe Feng, Santiago Serrano, William Gilbert, Ross C. C. Leon, Tuomo Tanttu, Philip Mai, Dylan Liang, Jonathan Y. Huang, Yue Su, Wee Han Lim, Fay E. Hudson,  Christopher C. Escott, Andrea Morello, Chih Hwan Yang, Andrew S. Dzurak, Andre Saraiva, Arne Laucht. Advanced Materials Volume 35, Issue 19 May 11, 2023 2208557 DOI: https://doi.org/10.1002/adma.202208557 First published online: 20 February 2023

This paper is open access.

Modernizing ‘Maxwell’s demon’ for a quantum computing feat

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Maxwell is James Clerk Maxwell, a Scottish mathematician and scientist, considered a genius for his work on electromagnetism. His ‘demon’ is a thought experiment that has influenced research for over 150 years as this November 29, 2022 news item on ScienceDaily makes clear,

A team of quantum engineers at UNSW [University of New South Wales] Sydney has developed a method to reset a quantum computer — that is, to prepare a quantum bit in the ‘0’ state — with very high confidence, as needed for reliable quantum computations. The method is surprisingly simple: it is related to the old concept of ‘Maxwell’s demon’, an omniscient being that can separate a gas into hot and cold by watching the speed of the individual molecules.

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A November 30, 2022 UNSW press release (also on EurekAlert but published on November 29, 2022), which originated the news item, modernizes the demon,

“Here we used a much more modern ‘demon’ – a fast digital voltmeter – to watch the temperature of an electron drawn at random from a warm pool of electrons. In doing so, we made it much colder than the pool it came from, and this corresponds to a high certainty of it being in the ‘0’ computational state,” says Professor Andrea Morello of UNSW, who led the team.

“Quantum computers are only useful if they can reach the final result with very low probability of errors. And one can have near-perfect quantum operations, but if the calculation started from the wrong code, the final result will be wrong too. Our digital ‘Maxwell’s demon’ gives us a 20x improvement in how accurately we can set the start of the computation.”

The research was published in Physical Review X, a journal published by the American Physical Society.

Watching an electron to make it colder

Prof. Morello’s team has pioneered the use of electron spins in silicon to encode and manipulate quantum information, and demonstrated record-high fidelity – that is, very low probability of errors – in performing quantum operations. The last remaining hurdle for efficient quantum computations with electrons was the fidelity of preparing the electron in a known state as the starting point of the calculation.

“The normal way to prepare the quantum state of an electron is go to extremely low temperatures, close to absolute zero, and hope that the electrons all relax to the low-energy ‘0’ state,” explains Dr Mark Johnson, the lead experimental author on the paper. “Unfortunately, even using the most powerful refrigerators, we still had a 20 per cent chance of preparing the electron in the ‘1’ state by mistake. That was not acceptable, we had to do better than that.”

Dr Johnson, a UNSW graduate in Electrical Engineering, decided to use a very fast digital measurement instrument to ‘watch’ the state of the electron, and use real-time decision-making processor within the instrument to decide whether to keep that electron and use it for further computations. The effect of this process was to reduce the probability of error from 20 per cent to 1 per cent.

A new spin on an old idea

“When we started writing up our results and thought about how best to explain them, we realized that what we had done was a modern twist on the old idea of the ‘Maxwell’s demon’,” Prof. Morello says.

The concept of ‘Maxwell’s demon’ dates back to 1867, when James Clerk Maxwell imagined a creature with the capacity to know the velocity of each individual molecule in a gas. He would take a box full of gas, with a dividing wall in the middle, and a door that can be opened and closed quickly. With his knowledge of each molecule’s speed, the demon can open the door to let the slow (cold) molecules pile up on one side, and the fast (hot) ones on the other.

“The demon was a thought experiment, to debate the possibility of violating the second law of thermodynamics, but of course no such demon ever existed,” Prof. Morello says.

“Now, using fast digital electronics, we have in some sense created one. We tasked him with the job of watching just one electron, and making sure it’s as cold as it can be. Here, ‘cold’ translates directly in it being in the ‘0’ state of the quantum computer we want to build and operate.”

The implications of this result are very important for the viability of quantum computers. Such a machine can be built with the ability to tolerate some errors, but only if they are sufficiently rare. The typical threshold for error tolerance is around 1 per cent. This applies to all errors, including preparation, operation, and readout of the final result.

This electronic version of a ‘Maxwell’s demon’ allowed the UNSW team to reduce the preparation errors twenty-fold, from 20 per cent to 1 per cent.

“Just by using a modern electronic instrument, with no additional complexity in the quantum hardware layer, we’ve been able to prepare our electron quantum bits within good enough accuracy to permit a reliable subsequent computation,” Dr Johnson says.

“This is an important result for the future of quantum computing. And it’s quite peculiar that it also represents the embodiment of an idea from 150 years ago!”

Hat’s off to whoever prepared the opening sequences for this informative and entertaining video from UNSW,

Here’s a link to and a citation for the paper,

Beating the Thermal Limit of Qubit Initialization with a Bayesian Maxwell’s Demon by Mark A. I. Johnson, Mateusz T. Mądzik, Fay E. Hudson, Kohei M. Itoh, Alexander M. Jakob, David N. Jamieson, Andrew Dzurak, and Andrea Morello. Phys. Rev. X 12, 041008 Vol. 12, Iss. 4: October – December 2022 Published 25 October 2022

This paper is open access.

For years, James Clerk Maxwell’s role as a poet has fascinated me. Yes, a physicist who wrote poetry about physics and other matters as noted in my April 24, 2019 (The poetry of physics from Canada’s Perimeter Institute) where you’ll find poems by various physicists including the aforementioned Maxwell, as well as, a link to the original Perimeter Institute for Theoretical Physics (PI) posting featuring the excerpted poems even more physics poems.