Researchers have pioneered a breakthrough technique that uses lasers to control individual qubits made of barium more robustly than any other method currently known. Reliably controlling qubits is a key step towards realizing future functional quantum computers.
The new method, developed by the University of Waterloo's Institute for Quantum Computing (IQC), uses a small glass waveguide to separate laser beams and focus them four microns apart, about one-fourth the width of a human hair. The precision and degree of focusing each laser beam on the target qubit in parallel is unmatched by previous research.
Dr. K. Rajibul Islam, professor at the IQC and the Department of Physics and Astronomy at the University of Waterloo, said: "Our design limits the amount of crosstalk - the amount of light falling on adjacent ions - to a tiny relative intensity of 0.01%, which is among the best in the quantum world. Unlike previous methods of agile control of individual ions, the fiber-based modulators do not interact with each other."
"This means we can talk to any ion without affecting its neighbors, while still maximizing control of each ion. To our knowledge, in academia and industry, this is the most flexible ion qubit control system with such high precision."
Green lasers are the right energy to manipulate the energy states of barium ions. Source: University of Waterloo
Barium ions: the new favorite in quantum computing
Barium ions are being targeted by scientists as they become increasingly popular in the field of trapped ion quantum computing. Barium ions have convenient energy states that serve as the zero and first orders of qubits and can be manipulated with visible green light, unlike other atom types, which require higher-energy ultraviolet light for the same manipulation. This allows researchers to use commercially available optical technologies not available at ultraviolet wavelengths.
The researchers created a waveguide chip that splits a laser beam into 16 different optical channels. Each channel is then directed to independent fiber-based modulators that provide flexible control of the intensity, frequency and phase of each laser beam independently. The laser beam is then focused to a small distance using a series of telescope-like optical lenses. The researchers confirmed their focus and control by measuring each laser beam using precise camera sensors.
This work is part of a University of Waterloo effort to build a barium ion quantum processor using atomic systems, said Dr. Crystal Senko, Islam's co-principal investigator, IQC and a faculty member in the Department of Physics and Astronomy at the University of Waterloo. "We use ions because they are identical, naturally made qubits, so we don't need to make them. Our task is to find ways to control them."
The innovative waveguide approach demonstrates a simple yet precise control method, raising hope for manipulating ions to encode and process quantum data, as well as for applications in quantum simulations and computing.