A new peer-reviewed study by physicists at the University of Bonn in Germany and the Technion - Israel Institute of Technology examines which factors determine the speed at which calculations can be performed by a quantum computer. The study draws on previous research by Soviet physicists Leonid Mandelstam and Igor Tamm.
Quantum computers, unlike their conventional counterparts, use quantum mechanics to process information, which enables them to solve a wider range of problems - but still, there are limits.
Conventional computers use binary code composed of sequences of 1s and 0s known as "bits" to store information, whereas quantum computers use quantum bits. Also known as "qubits," these units of information "resemble a wave rather than a series of discrete values," according to the Technion.
Information is linked together in conventional computers by building blocks known as "gates," and when gates are combined, simple calculations may be performed. In quantum computers, information processing occurs in a similar manner where "gates change the wave function."
Dr. Andrea Alberti of the Institute of Applied Physics at the University of Bonn and one of the study's authors explained that "They require a minimum amount of time to transform the wave function and the information this contains." Mandelstam and Tamm theoretically deduced this required minimum time in their research, and this new study investigates the limit they determined.
To investigate that limit, this study's authors initially observed the motion of cesium atoms as they rolled "like marbles" down a light bowl, though this method proved to have variables that hindered the researchers' ability to identify information changes. "We therefore devised a different method to detect the deviation from the initial state," Alberti explained.
Another strategy was then attempted. Gal Ness, a doctoral student at the Technion and the lead author of the study explained that the team "used fast light pulses to create a so-called quantum superposition of two states of the atom. Figuratively speaking, the atom behaves as if it had two different colors at the same time." Each copy of the atom "takes a different position in the light bowl: One is high up on the edge and 'rolls' down from there. The other, conversely, is already at the bottom of the bowl. This twin does not move - after all, it cannot roll up the walls and so does not change its wave function."
The atom clones were then compared at regular intervals using a technique called "quantum interference" to determine exactly when a significant change of the matter wave occurred.
The height above the bottom of the light bowl was varied at the start of the experiment to control the atom's "position energy." Technion Prof. Yoav Sagi explained: "We were able to demonstrate that the minimum time for the matter wave to change depends on this energy uncertainty. The greater the uncertainty, the shorter the Mandelstam-Tamm time."
While these results aligned with the predictions of the two Russian researchers, another effect that was discovered did not. When the physicists increased the energy uncertainty "until it exceeded the average energy of the atom, then the minimum time did not decrease further - contrary to what the Mandelstam-Tamm limit would actually suggest." The new findings proved that there is a speed limit imposed by the atom's average energy.
The study was funded by the Reinhard Frank Foundation in collaboration with the German Technion Society, the German Research Foundation (DFG), the Helen Diller Quantum Center at the Technion, and the German Academic Exchange Service (DAAD).