KINGSTON, R.I. – May 22, 2023 – A research team including a University of Rhode Island physicist have shown a new way to strengthen the interactions between particles in collective quantum states. The technique could be useful in making powerful quantum sensors or more robust quantum computing systems.
In a paper published recently in the journal Physical Review A, the researchers demonstrated two “quantum squeezing” techniques that enhanced quantum coherence in a system consisting of 100 beryllium ions (atoms with an electric charge) trapped in an electromagnetic field. The work was led by researchers from National Institute of Standards and Technology in Boulder, Colorado, with Wenchao Ge, an assistant professor at URI, serving as the project’s theorist.
“The idea here is to strengthen the interaction between quantum particles so that external noise doesn’t affect the system as much,” Ge said. “By doing that, we can make more sensitive quantum sensors or faster quantum logic gates, which are essential for quantum computers.”
Quantum devices harness the strange rules that apply to matter at the scales of individual particles and atoms. Quantum particles don’t exist in single states. Instead, they exist in a range of possible states called a superposition, which is the basis of what’s known as a quantum bit or qubit. In classical computers, a bit is a single unit of information represented by either a zero or one. But qubits exist in a superposition of zero, one, or something in between. That enables new possibilities for how information can be processed and calculations can be performed.
However, making a useful quantum device requires lots of particles to be in correlated quantum states. One way of doing that is known as a Penning trap, in which ions are suspended in an electromagnetic field. Scientists have demonstrated that the technique can be used to create quantum systems with 100 or more ions. By shining lasers to the ions in a carefully designed way, the internal states of the ions can be correlated by coupling them to the collective motion of the group. But keeping the quantum states of all those ions in a coherent collective state remains a challenge.
A major reason coherent quantum states in trapped ions are hard to maintain has to do with the lasers that are used to create the correlation. Tiny random fluctuations in the laser light perturbs the system slightly, leading to the loss of coherence. But through a technique known as quantum squeezing, scientists can minimize those perturbations. The technique involves modulating the electric field in which the particles are trapped to increase coherent interactions.
In this new study, the research team tested two techniques for squeezing the quantum states of 100 beryllium ions in a Penning trap. One technique used pulsing modulation of the trapping potential to squeeze the particles’ external motion, which makes the entire system more sensitive to small displacements. That could make for quantum sensors that can make precise measurements of electromagnetic fields, chemical compositions, or even dark matter—the invisible and as yet undetected stuff thought to account for the vast majority of matter in the universe.
The second technique used a continuous modulation technique to squeeze the ions’ motion. That technique could be useful in quantum computers and quantum simulation by enabling quantum systems to hold onto their information for a longer period.
“It enables us to perform many more computations before external noise degrades the system,” Ge said. “If it takes the system a millisecond to lose its information, but our system operates at microsecond speeds, we could do lots of calculations before the information is gone.”
In addition to showing that the techniques could indeed increase coherence in the quantum system, it also revealed limitations, some of which were related to the frequency fluctuation of the motional mode.
Despite those limitations, Ge said, the research points to a promising way of making larger, more coherent quantum systems.