U-M Emulates Impossible “Unipolar” Laser Pulses to Develop Quantum Information

The semiconductor nanosheets in the water-cooled copper mount turn an infrared laser pulse into an effectively unipolar terahertz pulse. // Courtesy of hristian Meineke/Huber Lab/ U-R
The semiconductor nanosheets in the water-cooled copper mount turn an infrared laser pulse into an effectively unipolar terahertz pulse. // Courtesy of hristian Meineke/Huber Lab/ U-R

Researchers from the University of Michigan in Ann Arbor, with aid from the University of Regensburg, have emulated impossible “unipolar” laser pulses for the development of quantum information.

Quantum materials emit light restricted to a positive pulse, rather than a positive-negative oscillation. In most digital circuits, these pulses can be either positive or negative. A laser pulse that evades the light symmetry could manipulate quantum information, bringing into sight room temperature quantum computing and conventional computing.

Quantum computing, using quantum bits or qubits, has the potential to increase production of solutions that explore several variables at the same time, including drug discovery, weather prediction, and encryption for cybersecurity.

Conventional computer bits encode either a 1 or 0, but qubits can encode both at the same time, enabling quantum computers to work through multiple scenarios simultaneously. A caveat is that the mixed states don’t last long, so the information processing must be faster than electronic circuits can gather.

“The optimum would be a completely directional, unipolar ‘wave,’ so there would be only the central peak, no oscillations. That would be the dream,” says Mackillo Kira, U-M professor of electrical engineering and computer science and leader of the theory aspects of the study in Light: Science & Applications. “But the reality is that light fields that propagate have to oscillate, so we try to make the oscillations as small as we can.”

Different ways of computing are possible if the charge carriers used to encode quantum information could be moved around — including a room temperature approach. In between infrared and microwave radiation sits terahertz light that oscillates fast, however, the shape of the wave is an issue.

The electromagnetic waves are made to produce oscillations that are both positive and negative, which equal zero. The positive cycle may move charge carriers, such as electrons, though the negative cycle pulls the charges back to the start, thus, to control the quantum information, asymmetric light is needed.

Since waves that are strictly positive or strictly negative are impossible, the international team found a solution by creating an effectively unipolar wave with a sharp, high-amplitude positive peak flanked by two long, low-amplitude negative peaks. This makes the negative peaks too small for much effect, while the positive peak is powerful enough to move charge carriers.

The researchers did this by engineering nanosheets of a gallium arsenide semiconductor to design the terahertz emission through the motion of electrons and holes, which are spaces left behind when electrons move in semiconductors. The nanosheets, each as fine as one one-thousandth of a hair, were made in the lab of Dominique Bougeard, professor of physics at the University of Regensburg in Germany.

The group of Rupert Huber, professor of physics at the University of Regensburg, stacked the semiconductor nanosheets in front of a laser and when the pulse hit the nanosheet, it generated electrons.

The design of the nanosheet made for separation from the holes, so the electrons shot forward, then pulled back from the holes. As the electrons rejoined the holes, they released the energy from the laser pulse as a strong positive terahertz half-cycle, preceded and followed by a weak, long negative half-cycle.

“The resulting terahertz emission is stunningly unipolar, with the single positive half-cycle peaking about four times higher than the two negative ones,” says Huber. “We have been working for many years on light pulses with fewer and fewer oscillation cycles. The possibility of generating terahertz pulses so short that they effectively comprise less than a single half-oscillation cycle was beyond our bold dreams.”

Ultimately, the team intends to use these pulses to manipulate electrons in room temperature quantum materials, exploring mechanisms for quantum information processing. The pulses could also be used for ultrafast processing of conventional information.

“Now that we know the key factor of unipolar pulses, we may be able to shape terahertz pulses to be even more asymmetric and tailored for controlling semiconductor qubits,” says Qiannan Wen, a doctoral candidate in applied physics at U-M and a co-first author of the study, along with Christian Meineke and Michael Prager, doctoral candidates in physics at the University of Regensburg.

Collaborators at Justus Liebig University Giessen and Helmut Schmidt University, both in Germany, contributed to the experiment and the characterization of the nanosheets.

This research was supported by the German Research Foundation (DFG), W.M. Keck Foundation, and the National Science Foundation.