Searching for the perfect quantum bit: New qubit hack could revolutionize quantum computing

Qubit platform one electron on solid neon

New qubit platform: Electrons from a hot light filament (top) land on solid neon (red block), where a single electron (represented as a wavefunction in blue) is trapped and processed by a superconducting quantum circuit (patterned wafer below). Image source: Dafei Jin / Argonne National Laboratory

A new qubit platform could transform quantum information science and technology.

No doubt you are viewing this article on a digital device whose basic unit of information is the bit, either 0 or 1. Scientists around the world are racing to develop a new type of computer based on the use of quantum bits, or qubits.

In a research paper published in the journal on May 4, 2022 temper nature, a team led by the US Department of Energy’s (DOE) Argonne National Laboratory has announced the creation of a new qubit platform that consists of freezing neon gas in a solid at extremely low temperatures, spraying electrons from a light bulb filament onto the solid, and trapping a single electron there. This system has the potential to be developed into ideal building blocks for quantum computers of the future.

“It looks like a perfect qubit may be on the horizon. Thanks to the relative simplicity of the electron platform on neon, it should be easy to manufacture at low cost.” — Dafei Jin, argon scientist at the Center for Nanomaterials

For a useful quantum computer, the quality requirements for qubits are very demanding. While there are different forms of qubits today, none of them are optimal.

What makes a perfect qubit? It has at least three sterling qualities, according to Duffy Jane, Argonne scientist and principal investigator on the project.

It can remain in the 0 and 1 synchronized state (remember the cat!) for a long time. Scientists call this long “cohesion”. Ideally, that time would be about a second, which is a time step that we can perceive on the clock at home in our daily lives.

Second, qubits can be changed from one state to another in a short time. Ideally, that time would be about a billionth of a second (nanosecond), which is a time step of a classic computer clock.

Third, qubits can be easily linked to many other qubits so that they can operate in parallel with each other. Scientists refer to this association as entanglement.

Although known qubits aren’t perfect at the moment, companies like IBM, Intel, Google, Honeywell, and many other startups have picked their favorites. They aggressively pursue technological improvement and commercialization.

“Our ambitious goal is not to compete with those companies, but to discover and build a fundamentally new qubit system that can lead to the perfect platform,” Jin said.

While there are many options for qubit types, the team chose the simplest one – a single electron. Heating a simple filament of light that you might find in a child’s toy can easily release an unlimited supply of electrons.

One of the challenges for any qubit, including the electron, is that it is very sensitive to disturbance from its surroundings. Thus, the team chose to trap an electron on the surface of a high-purity solid neon in a vacuum.

Neon is an inert element that does not interact with other elements. “Because of this inertness, solid neon could be the cleanest solid material possible in a vacuum to host and protect any qubits from disruption,” Jin said.

A key component of the team’s qubit platform is a chip-scale microwave resonator made of a superconductor. (A larger household microwave oven is also a microwave resonator.) Superconductors — metals without electrical resistance — allow electrons and photons to interact together near[{” attribute=””>absolute zero with minimal loss of energy or information.

“The microwave resonator crucially provides a way to read out the state of the qubit,” said Kater Murch, physics professor at the Washington University in St. Louis and a senior co-author of the paper. “It concentrates the interaction between the qubit and microwave signal. This allows us to make measurements telling how well the qubit works.”

“With this platform, we achieved, for the first time ever, strong coupling between a single electron in a near-vacuum environment and a single microwave photon in the resonator,” said Xianjing Zhou, a postdoctoral appointee at Argonne and the first author of the paper. “This opens up the possibility to use microwave photons to control each electron qubit and link many of them in a quantum processor,” Zhou added.

“Our qubits are actually as good as ones that people have been developing for 20 years.” — David Schuster, physics professor at the University of Chicago and a senior co-author of the paper

The team tested the platform in a scientific instrument called a dilution refrigerator, which can reach temperatures as low as a mere 10 millidegrees above absolute zero. This instrument is one of many quantum capabilities in Argonne’s Center for Nanoscale Materials, a DOE Office of Science user facility.

The team performed real-time operations to an electron qubit and characterized its quantum properties. These tests demonstrated that the solid neon provides a robust environment for the electron with very low electric noise to disturb it. Most importantly, the qubit attained coherence times in the quantum state competitive with state-of-the-art qubits.

“Our qubits are actually as good as ones that people have been developing for 20 years,” said David Schuster, physics professor at the University of Chicago and a senior co-author of the paper. “This is only our first series of experiments. Our qubit platform is nowhere near optimized. We will continue improving the coherence times. And because the operation speed of this qubit platform is extremely fast, only several nanoseconds, the promise to scale it up to many entangled qubits is significant.”

There is yet one more advantage to this remarkable qubit platform.“Thanks to the relative simplicity of the electron-on-neon platform, it should lend itself to easy manufacture at low cost,” Jin said. “It would appear an ideal qubit may be on the horizon.”

Reference: “Single electrons on solid neon as a solid-state qubit platform” by Xianjing Zhou, Gerwin Koolstra, Xufeng Zhang, Ge Yang, Xu Han, Brennan Dizdar, Xinhao Li, Ralu Divan, Wei Guo, Kater W. Murch, David I. Schuster and Dafei Jin, 4 May 2022, Nature.
DOI: 10.1038/s41586-022-04539-x

The team published their findings in a Nature article titled “Single electrons on solid neon as a solid-state qubit platform.” In addition to Jin and Zhou, Argonne contributors include Xufeng Zhang, Xu Han, Xinhao Li and Ralu Divan. In addition to David Schuster, the University of Chicago contributors also include Brennan Dizdar. In addition to Kater Murch of Washington University in St. Louis, other researchers include Wei Guo of Florida State University, Gerwin Koolstra of Lawrence Berkeley National Laboratory and Ge Yang of Massachusetts Institute of Technology.

Funding for the Argonne research primarily came from the DOE Office of Basic Energy Sciences, Argonne’s Laboratory Directed Research and Development program and the Julian Schwinger Foundation for Physics Research.