Universal digital simulator holds promise for quantum computers

Ashley N. Paddock, ashley.paddock@photonics.com

A new digital approach enables universal quantum simulation in a system of trapped ions that can in principle simulate any physical system efficiently, allowing researchers to explore the dynamics of quantum computers.

Previously, quantum simulation was achieved through an analog method: Two years ago, the research group of Christian Roos and Rainer Blatt at the University of Innsbruck re-created the properties of a particle moving close to the speed of light in a quantum system. They encoded the state of the particle into a highly cooled calcium atom and used lasers to manipulate it, simulating the Zitterbewegung (quivering motion) of relativistic particles, which had never before been directly observed in nature.

“Numerical techniques for investigating the dynamics of a quantum system are in general limited to simulating small systems, as the number of parameters needed for describing the system grows exponentially with the system size,” Roos said. “For this reason, it has been proposed to use one quantum system for simulating the physics of another quantum system to overcome this limitation.”

In their current work, the mathematical description of the phenomenon to be investigated is programmed by using a series of laser pulses to perform quantum calculations with atoms. Laser-cooled and electrically trapped calcium atoms are used as carriers of qubits. They demonstrated the method in two experiments at the University of Innsbruck and at the Institute of Quantum Optics and Quantum Information of the Austrian Academy of Sciences, using up to 100 gates and six qubits.

The new scientific results showed that interactions and dynamics not even present in the quantum computer can be simulated, but there is still a need for considerably higher quantities of quantum bits, the physicists say. This means that the researchers would need to control and manipulate up to 40 atoms in the same way that they did in the experiment. The findings appeared in the Sept. 1 issue of Science (doi: 10.1126/science.1208001).

Although a promising application for future quantum computers, Roos said that error sources in the laser-ion interactions used for implementing the quantum dynamics must be further reduced. In addition, the team plans to move from experiments with a few ions to a regime where they operate with ion strings of at least 10 to 20.