An international group of physicists has found a novel method to organize ions into two stable layers, which may lead to exciting new designs for quantum computers and other atom-based technologies.
Various quantum devices, ranging from quantum sensors to quantum computers, utilize trapped ions or charged atoms, held in place by electric and magnetic fields, for processing information.
Nevertheless, existing trapped-ion systems face significant obstacles. Most studies are confined to one-dimensional chains or two-dimensional arrangements of ions, which limit the scalability and capabilities of quantum devices. Researchers have long envisioned the possibility of assembling these ions into three-dimensional configurations, but this has proven to be quite challenging due to the difficulties in maintaining stability and control over the ions in more intricate arrangements.
To tackle these challenges, a collaborative effort between physicists from India, Austria, and the USA—including JILA and NIST Fellow Ana Maria Rey, along with NIST scientists Allison Carter and John Bollinger—was established. They proposed that altering the electric fields used to trap the ions could enable the creation of stable, multilayer structures, paving the way for exciting advancements in future quantum technologies. Their research was published in Physical Review X.
“The ability to trap large groups of ions in two or more spatially distinct layers under fully controllable conditions opens up thrilling possibilities for investigating new regimes and phenomena that are not easily achievable in purely 2D crystals, such as topological chiral modes, teleportation, and precise measurements of spatially varying fields—all vital for quantum information science,” Rey stated.
Utilizing Penning traps
Among the many platforms considered for quantum computing, trapped ions stand out due to their high level of control and the capability to carry out precise quantum operations. These ions can be adjusted using laser or microwave pulses that alter their quantum states, enabling them to be “encoded” with specific information. These encoded ions are referred to as quantum bits or “qubits.”
During this process, ions also interact through the Coulomb force, which can be used to entangle them. This interaction decreases the overall noise within the system and improves its measurements.
“Previous studies have indicated that ion crystals can form 3D spherical shapes, but our aim was to find a method to create stacked arrays of 2D layers,” explained Samarth Hawaldar, the lead author from the Indian Institute of Science. “We began investigating how to achieve such arrangements in a type of ion trap known as a Penning trap, recognized for its capability to hold large quantities of ions, often ranging from hundreds to thousands.”
In a Penning trap, ions can naturally group together into crystalline structures due to the balance between repulsive Coulomb interactions and the confinement potential, which is the combined electric and magnetic force that maintains the ions in a specified region.
“Confinement is accomplished through electromagnetic forces created by a series of electrodes and by making the ions rotate within a strong magnetic field,” Carter explained.
Penning traps are particularly valuable to physicists because they accommodate a large number of ions, making them ideal for researching more complex three-dimensional configurations. These traps have been successfully used to arrange ions into a single two-dimensional layer or more rounded three-dimensional shapes. The rounded structures arise because the confining electric field in these traps typically increases linearly with distance from the center, akin to an ideal spring, which guides ions into these simpler formations.
However, the researchers sought to adjust the trap’s electric field to be more sophisticated and dependent on distance from the trap’s center. This minor modification allowed them to encourage the ions to form a new type of structure—a bilayer crystal consisting of two flat layers of ions, one above the other.
The team performed extensive numerical simulations to support their new method, demonstrating that this bilayer structure could be stabilized under specific conditions and even hinting at the potential to extend this technique to build crystals with more than two layers.
“We’re thrilled to attempt forming bilayer crystals in the lab with our current Penning trap setup,” remarked John Bollinger, a co-author and experimental physicist. “In the long run, I believe this concept will inspire a redesign of our traps’ electrode structures.”
A new horizon for ion trapping
Transitioning ion trapping from 2D to 3D has profound implications for the future of quantum devices, including sensors and quantum computers.
“Bilayer crystals provide several new capabilities for processing quantum information that are not straightforward with 1D chains or 2D planes,” stated Dr. Athreya Shankar, a postdoctoral researcher at the Indian Institute of Science.
“For example, generating quantum entanglement between large subsystems located at a distance, such as the two layers in this system, is a highly sought-after capability across various quantum hardware platforms.
The research team is eager to conduct experimental tests of their findings using their Penning traps. If successful, these efforts could lead to novel quantum hardware designs that optimize the use of 3D space, thereby enhancing the scalability and reliability of quantum technologies.
In addition to hardware advancements, bilayers could introduce new possibilities for quantum simulations and sensing.
“For instance, the normal modes of ions in a bilayer can interact with both vertical and radial dimensions, favoring clockwise over counterclockwise circulation or vice versa,” Rey added. “This could simulate complex behaviors seen in electrons under strong magnetic fields, but in completely controllable environments. Plus, the presence of more ions can enhance the signal-to-noise ratio in measurements, enabling more accurate assessments of quantities like time, electric fields, or accelerations, which are crucial for discovering new physics.”
This collaboration between researchers from India, Austria, and the USA is essential as quantum technology continues to advance. Innovations like these are pivotal in unlocking the full potential of quantum computing, sensing, and beyond.
This study received support from the U.S. Department of Energy’s Office of Science, The National Quantum Initiative (NQI) Science Research Centers, and the Quantum Systems Accelerator (QSA).
