February 26, 2024

New techniques for making erbium qubits

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A laser fired at a sheet of titanium dioxide changes the configuration of the crystal it hits – a technique, developed by quantum startup memQ, that allows scientists to design a more efficient multi-qubit device. Credit: memQ

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A laser fired at a sheet of titanium dioxide changes the configuration of the crystal it hits – a technique, developed by quantum startup memQ, that allows scientists to design a more efficient multi-qubit device. Credit: memQ

Qubits are the foundation of quantum technology, and finding or building qubits that are stable and easily manipulated is one of the central goals of quantum technology research. Scientists have discovered that an atom of erbium – a rare earth metal 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 original atoms in the material. Two research groups – one at quantum startup memQ, a corporate partner of the Chicago Quantum Exchange, and one at the US Department of Energy’s Argonne National Laboratory, a member of CQE – have used different host materials for erbium to advance quantum technology, demonstrating the versatility of this type of qubit and highlighting the importance of materials science for quantum computing and quantum communication.

The two projects address challenges that quantum computing researchers have been trying to solve: designing multi-qubit devices and extending the time that qubits can retain information.

“The work that these two efforts have accomplished really highlights how important materials are to quantum technology,” said F. Joseph Heremans, an Argonne staff scientist who was involved in both projects. “The environment in which the qubit resides is as critical as the qubit itself.”

Boot memQ selectively activates erbium qubits, facilitating control of 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 telephone lines; its electrons are also organized in such a way that they are particularly resistant to the kind of environmental changes that can cause a qubit to lose its information.

But the growth process that inserts erbium into the host material spreads atoms throughout the material in a way that scientists can’t precisely control, making it difficult to design multi-qubit devices. In a completely new technique, memQ scientists discovered an alternative solution: “activating” only certain erbium atoms with a laser.

The work is published in the journal Applied Physics Letters.

“We’re not actually putting the erbium in specific locations, the erbium is spread throughout the material,” said Sean Sullivan, CTO and co-founder of memQ, who graduated from Duality, the quantum startup accelerator co-led by the Polsky Center for Entrepreneurship and Innovation at the University of Chicago and the CQE, along with founding partners University of Illinois Urbana-Champaign, Argonne and P33.

“But by using a laser, we can change the crystal structure in a specific area, and that changes the properties of the erbium in that area. So we are selecting which erbium to use as qubits.”

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

In the memQ technique, erbium is spread across a TiO filmtwo this is in a setting. Then, a high-powered laser is focused on the crystal around certain erbium atoms, permanently distorting the TiOtwo in your other configuration only in these locations. Now, the erbium atoms selected by the laser can communicate at the same frequency, completely separate from the others.

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 build something useful,” said Manish Singh, CEO and co-founder of memQ. “With our platform, we can choose which erbium we want to use in the layout we want to use, a capability that has eluded the solid-state community for a long time.”

Argonne scientist achieves long erbium qubit coherence times

A crucial measure of a qubit’s effectiveness is its coherence time: the amount of time it can retain quantum information. This is especially 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 coherence when interacting with something in its environment, like air or heat.

Erbium atoms can retain quantum information using their electrons, which have a property called “spin”. A nucleus, the cluster of protons and neutrons at the center of an atom, also has “spin,” and the spins of electrons and nuclei can influence each other. A common way for an erbium qubit to lose its quantum information is if its electron spin interacts with the nuclear spin of one of the atoms around it.

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

Cerium is the most abundant rare earth element and is used as an oxidizing agent and catalyst in industrial chemistry. Unlike TiOtwowhich has multiple possible structural configurations, CeOtwo it only has one and it is extremely symmetrical. Because of this, erbium qubits in CeOtwo are more stable.

“Two different erbium qubits in ceria will see the same crystalline environment,” Zhang said. “And so it’s very easy to control them simultaneously because they will act very similarly.”

Notably, the new localization technique developed by memQ is not possible with a highly symmetric crystal structure like CeO.two— but Zhang was able to see longer coherence times in the erbium qubits, with even more potential 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 this is very common in quantum,” Zhang said.

More information:
Sean E. Sullivan et al, Near-deterministic localization of Er emitters in TiO thin filmtwo through submicron scale crystalline phase control, Applied Physics Letters (2023). DOI: 10.1063/5.0176610

Jiefei Zhang et al, Optical and spin coherence of Er3+ in epitaxial CeOtwo in silicon, arXiv (2023). DOI: 10.48550/arxiv.2309.16785

Diary information:

Applied Physics Letters

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