New technique could make quantum networking possible

Caltech engineers have developed an approach for quantum storage that could help pave the way for the development of large-scale optical quantum networks.

The new system relies on nuclear spins – the angular momentum of an atom’s nucleus – collectively oscillating as a spin wave. This collective oscillation effectively chains multiple atoms together to store information.

The work, which is described in an article published on February 16 in the journal Nature, uses a quantum bit (or qubit) made from an ion of ytterbium (Yb), a rare earth element also used in lasers. The team, led by Andrei Faraon (BS ’04), a professor of applied physics and electrical engineering, embedded the ion in a transparent crystal of yttrium orthovanadate (YVO4) and manipulated its quantum states via a combination of optical and microwave fields. The team then used the Yb qubit to monitor the nuclear spin states of several surrounding vanadium atoms in the crystal.

“Based on our previous work, single ytterbium ions were known to be excellent candidates for optical quantum networks, but we needed to bond them with additional atoms. We demonstrate that in this work,” says Faraon, co- corresponding author of Nature paper.

The device was made at Caltech’s Kavli Nanoscience Institute, then tested at very low temperatures in Faraon’s lab.

A new technique for using entangled nuclear spins as quantum memory was inspired by methods used in nuclear magnetic resonance (NMR).

“To store quantum information in nuclear spins, we have developed new techniques similar to those used in NMR machines used in hospitals,” says Joonhee Choi, postdoctoral fellow at Caltech and corresponding co-author of the paper. “The main challenge was to adapt existing techniques to work in the absence of a magnetic field.”

A unique feature of this system is the predetermined placement of vanadium atoms around the ytterbium qubit, as prescribed by the crystal lattice. Each qubit the team measured had an identical memory register, meaning it would store the same information.

“The ability to build technology in a reproducible and reliable way is key to its success,” says graduate student Andrei Ruskuc, first author of the paper. “In the scientific context, this has allowed us to gain unprecedented insight into the microscopic interactions between ytterbium qubits and vanadium atoms in their environment.”

This research is part of a larger effort by Faraon’s lab to lay the foundations for future quantum networks.

Quantum networks would connect quantum computers through a system that operates at a quantum rather than a classical level. In theory, quantum computers will one day be able to perform certain functions faster than classical computers by taking advantage of special properties of quantum mechanics, including superposition, which allows quantum bits to store information in the form a 1 and at 0 simultaneously.

As they can with classical computers, engineers would like to be able to connect multiple quantum computers to share data and work together, creating a “quantum internet”. This would open the door to several applications, including the ability to solve calculations too large for a single quantum computer to handle, as well as establishing unbreakable secure communications using quantum cryptography.

The article is titled “Nuclear spin-wave quantum register for a solid-state qubit”. Co-authors include graduate students Chun-Ju Wu and Jake Rochman (MS ’19). This research was funded by the Institute of Quantum Information and Matter (IQIM), a National Science Foundation Physics Frontiers Center, with support from the Gordon and Betty Moore Foundation, Office of Naval Research, Air Force Office of Scientific Research, Northrop Grumman, General Atomics, and the Weston Havens Foundation.

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Material provided by California Institute of Technology. Original written by Robert Perkins. Note: Content may be edited for style and length.

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