Unlocking the Third Dimension in Quantum Computing

Researchers have devised a new technique to trap ions in 3D structures using modified electric fields in Penning traps, creating stable bilayer crystals.

The discovery paves the way for more complex quantum devices and could be revolutionary Quantum computing and realization through more efficient use of space.

Quantum device challenges

Many quantum devices, from quantum sensors to quantum computers, process information using ions, or charged atoms trapped in electric and magnetic fields, as a hardware platform.

However, current trapped-ion systems face important challenges. Most experiments have been limited to one-dimensional chains or two-dimensional planes of ions, which limit the scalability and functionality of quantum devices. Scientists have long dreamed of stacking these ions into three-dimensional structures, but this has proved difficult because the ions are difficult to stably and well control when arranged in more complex ways.

Breakthrough in ion trapping technology

To address these challenges, an international collaboration of physicists from India, Austria, and the United States—Jila and NIST fellow Ana Maria Ray, along with NIST scientists Allison Carter and John Bollinger—proposed that the electric fields that trap ions could be modulated. Stable, multilayer structures open up exciting new possibilities for future quantum technologies. The researchers published their findings Physical examination X.

„The ability to trap large ensembles of ions in two or more spatially separated layers under fully controllable conditions opens up exciting opportunities to explore new regimes and phenomena, such as topological chiral modes, teleportation, and precision measurements. These are fields well-suited to quantum information science,” says Ray.

Enhancing Quantum Computing with Penning Traps

Among the various platforms being explored for quantum computing, entangled ions have emerged as a leading candidate due to their high degree of controllability and ability to perform precise quantum operations. These ions can be manipulated with laser or microwave pulses, which change their quantum states and allow them to be „encoded” with specific information. These encoded ions are often called quantum bits or „qubits”.

During this process, the ions experience Coulomb forces or interactions with other ions that physicists can use to trap them, reducing the overall noise of the system and improving its measurements.

„Previous work has shown that ion crystals can form 3D spherical structures, but what we’re looking for is a way to realize a stacked array of 2D layers,” said Samarth Havaldar, first author of the paper and a researcher at the Indian Institute of Science. Illustrated in a Recent writing About the paper. „We began investigating ways to realize such structures in a specific type of ion trap called a Penning trap, because these traps are good at storing large numbers of ions, typically several hundreds to thousands.”

In a Penning trap, ions can be forced to self-assemble into crystal structures created by the competition between the coordinated electric and magnetic forces—repulsive Coulomb interactions and confinement potential—that hold the ions securely in place.

„The confinement is achieved through electromagnetic forces generated by the stack of electrodes and by spinning the ions in a powerful magnetic field,” explains Carter.

For physicists, Penning traps are particularly useful because they can store large numbers of ions, making them an excellent choice for probing very complex, three-dimensional structures. Penning traps are used to arrange ions into a single, two-dimensional layer or more rounded, three-dimensional shapes. The rounded, three-dimensional shape occurs because the confining electric field in these traps typically increases linearly with distance from the center of the trap, like an ideal spring, naturally guiding the ions into these simple, circular shapes.

However, researchers including Prakriti Shahi of the Indian Institute of Technology, Bombay, have attempted to vary the trap’s electric field more finely and depending on the distance from the center of the trap. This subtle change allowed the ions to bind together to form a new type of structure—a bilayer crystal, where two flat layers of ions are stacked on top of each other.

The team conducted extensive numerical simulations to validate their new approach, showing that under certain conditions this bilayer configuration can be stabilized and suggesting the potential to extend the method to form crystals beyond two layers.

„We are excited to try to make bilayer crystals in the lab with our current Penning trap set-up,” says John Bollinger, experimental physicist and co-author of the publication. „In the long term, I think this idea will inspire a redesign of the detailed electrode architecture of our traps.”

A new frontier for ion trapping

Changing ion trapping from 2D to 3D has significant implications for the future of quantum devices such as sensors or quantum computers.

„Bilayer crystals open up many new possibilities for direct quantum information processing through 1D chains or 2D planes,” said Dr. Athreya Shankar, a postdoctoral researcher at the Indian Institute of Science. Latest Report About the study. „For example, creating quantum entanglement between large subsystems separated by a distance, such as the two layers in this system, is a sought-after capability in all quantum hardware.”

The team is eager to test these findings experimentally in their penning traps. If successful, this could lead to new quantum hardware architectures that make more efficient use of 3D space, thus increasing the scalability and robustness of quantum technologies.

In addition to hardware opportunities, bilayers open up new quantum simulations and sensing possibilities.

„For example, the normal modes of ions in a bilayer can combine vertical and radial degrees of freedom, rotating clockwise or counterclockwise,” explains Ray. „It can be used to mimic the rich behaviors that electrons experience in strong magnetic fields, but under fully controllable settings. Also, having more ions can improve the signal-to-noise in the measurement, enabling accurate estimation of quantities such as time, electric fields or acceleration, which are critical to discovering new physics.” .

This partnership between researchers in India, Austria and the US is important as the field of quantum technology continues to develop. Discoveries like these will be essential to realizing the full potential of quantum computing, sensing and beyond.

Reference: Samarth Hawaldar, Prakriti Shahi, Alison L. Carter, Anna Maria Rae, John J. Bollinger and Atreya Shankar, “Bilayer Crystals of Trapped Ions for Quantum Information Processing”, 16 August 2024. Physical examination X.
DOI: 10.1103/PhysRevX.14.031030

This work was supported by the US Department of Energy’s Office of Science, the National Quantum Initiative (NQI) Science Research Centers, and the Quantum Systems Accelerator (QSA).