New techniques for making qubits from erbium

Qubits are the cornerstone of quantum technology, and finding or building stable, easy-to-manipulate qubits is one of the central goals of quantum technology research. Scientists have discovered that an atom of erbium – a rare earth sometimes used in lasers or to color glass – can be a very effective qubit.

To make erbium qubits, erbium atoms are placed in “host materials,” where the erbium atoms replace some of the material’s original atoms. Two research groups – one at quantum startup memQ, a partner company of the Chicago Quantum Exchange, and the other at the U.S. Department of Energy’s Argonne National Laboratory, a EQC member – used different materials hosts for erbium to advance quantum technology, demonstrating the versatility of this technology. kind of qubit and highlighting the importance of materials science for quantum computing and quantum communication.

Both projects address the challenges quantum computing researchers are trying to solve: designing multi-qubit devices and extending the length of time qubits can hold information.

“The work done by these two efforts really highlights the importance of materials for quantum technology,” said F. Joseph Heremans, a scientist at Argonne who was involved in both projects. “The environment in which the qubit resides is just as critical as the qubit itself.”

Startup memQ selectively activates erbium qubits, making it easier to control multi-qubit devices

Erbium is popular as a qubit because it can efficiently transmit quantum information over the same type of optical fiber that channels the Internet and phone lines; its electrons are also arranged in such a way that it is particularly resistant to the kind of environmental changes that can cause a qubit to lose information.

But the growth process that inserts the erbium into the host material disperses the atoms throughout the material in a way that scientists cannot precisely control, making it difficult to design multi-qubit devices. Using a completely new technique, memQ scientists discovered a workaround: “activating” only certain erbium atoms with a laser.

The work is published in the journal Applied physics letters.

“We don’t actually place the erbium in specific places, the erbium is dispersed throughout the material,” said Sean Sullivan, CTO and co-founder of memQ, a graduate of Duality, the quantum startup accelerator co -led by the Polsky Center for Entrepreneurship and Innovation at the University of Chicago and CQE with founding partners University of Illinois Urbana-Champaign, Argonne and P33.

“But using a laser, we can change the crystal structure in a particular area, which changes the properties of the erbium in that area. So we select the erbium to use as qubits.”

The technique relies on the properties of the host material, titanium dioxide (TiO₂). Due to its symmetry, a TiO crystal lattice₂ has two possible configurations. An erbium atom inserted into the lattice will communicate at a different frequency depending on the configuration of the TiO₂ he moved.

In the memQ technique, erbium is dispersed in a TiO film₂ it’s in a configuration. Next, a high-power laser is focused on the crystal around certain erbium atoms, permanently distorting the TiO.₂ in its other configuration at these locations only. Now the erbium atoms selected by the laser can all communicate at the same frequency, completely separated from each other.

The new procedure represents a significant advance in this area of ​​quantum technology, known as solid-state technology.

“You can’t use qubits in 100 random locations to create something useful,” said Manish Singh, CEO and co-founder of memQ. “With our platform, we can choose the erbium we want to use in the configuration we want to use, a capability that has long eluded the semiconductor community.”

Argonne scientist achieves long coherence times of erbium qubits

A crucial measure of a qubit’s efficiency is its coherence time: the length of time it can retain quantum information. This is particularly important for qubits intended for use as quantum memory, the quantum equivalent of classical computer memory. But coherence is very fragile: a qubit can lose its coherence by interacting with something in its environment, such as air or heat.

Erbium atoms can retain quantum information through their electrons, which have a property called “spin.” A nucleus, the cluster of protons and neutrons at the center of an atom, also has a “spin,” and the spins of electrons and nuclei can influence each other. An erbium qubit often loses its quantum information when its electronic spin interacts with the nuclear spin of one of the atoms surrounding it.

For this reason, Argonne researcher Jiefei Zhang sought a host material for erbium that had the lowest possible nuclear spin, but that could also be made with more traditional silicon technologies. She found it with a different oxide, this time of a rare earth element: cerium dioxide, also known as cerium oxide (CeO₂).

Cerium is the most abundant rare earth element and is used as an oxidizing agent and catalyst in industrial chemistry. Unlike TiO₂which has several possible structural configurations, CeO₂ has only one and is extremely symmetrical. For this reason, the erbium qubits in CeO₂ are more stable.

“Two different erbium qubits in cerium oxide will see the same crystal environment,” Zhang said. “So it’s very easy to control them simultaneously because they will act very similar.”

Notably, the new localization technique developed by memQ is not possible with a highly symmetrical crystal structure like CeO.₂– but Zhang was able to observe longer coherence times from erbium qubits, with the potential for even longer as they continue to develop the experiment. The work can be found on the preprint server arXiv.

“There are definitely pros and cons to each material, and that’s very common in the quantum realm,” Zhang said.