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Optical physics

Optical physics

Near-unipolar laser pulses could control qubits

01 Jul 2022 Isabelle Dumé
unipolar light wave
Effectively unipolar light wave. (Courtesy: Christian Meineke, Huber Lab, University of Regensburg)

Physicists have created a light wave that is effectively unipolar, meaning it behaves as though it is solely a positive field pulse rather than the usual positive–negative oscillation found in electromagnetic waves. The positive pulse has a sharp peak and high amplitude and is powerful enough to switch or move electronic states, meaning that it could be used to manipulate quantum information and perhaps accelerate conventional computing as well.

Electromagnetic waves, and in particular light pulses, can be used to switch, characterize, and control electronic quantum states with incredible accuracy, explain team leaders Mackillo Kira and Rupert Huber of the University of Michigan in the US and the University of Regensburg in Germany. However, the shape of such pulses is fundamentally restricted to a combination of positive and negative oscillations that sum to zero. As a result, the positive cycle may move charge carriers (electrons or holes), but then the negative cycle pulls them back to square one.

Positive peak is strong enough to switch or move electronic states

An ideal quantum-electronic switch pulse would be so highly asymmetrical as to be completely unidirectional – in other words, it would contain only a positive (or negative) half-cycle of field oscillation. Under these conditions, such a pulse could flip a quantum state, such as a quantum bit, in minimum time (a half cycle) and with maximum efficiency (no back-and-forth oscillations).

This is fundamentally impossible for freely-propagating waves, but Kira, Huber and colleagues found a way to create the “next best thing” in the form of a quasi-unipolar wave consisting of a very short, high-amplitude positive peak sandwiched between two long, low-amplitude negative peaks. “The positive peak is strong enough to switch or move electronic states,” Kira and Huber explain, “while the negative peaks are too small to have much of an effect.”

In their work, the researchers started with a newly developed stack of nanofilms made of different semiconductor materials, such as indium gallium arsenide (InGaAs) that was grown epitaxially on gallium arsenide antimonide (GaAsSb). Each of the nanofilms is only a few atoms thick, and at the interface between them, ultrashort laser pulses can excite electrons mainly in the InGaAs film. The holes left behind by the excited electrons remain in the GaAsSb film, creating a charge separation.

Effective half-cycle light pulses

“We then made use of our quantum-theoretical breakthrough in exploiting the electrostatic attraction between the oppositely charged electrons and holes to pull them back together in a precisely controlled way,” Kira tells Physics World. “The fast charging and slower charge oscillations combined emitted the unipolar wave that we tailored as effective half-cycle light pulses in the far-infrared and terahertz part of the electromagnetic spectrum.”

Huber describes the resulting terahertz emission as “stunningly unipolar”, with the single positive half-cycle peaking about four times higher than the two negative peaks. While researchers have been working for a long time on producing 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, he adds, “beyond our bold dreams”.

Kira and Huber say that these unipolar terahertz fields could be a powerful tool for controlling novel quantum materials on time scales that are comparable to microscopic electronic motion. The researchers suggest that the fields could also serve as superior, well-defined “clockworks” for next-generation ultrafast electronics. Finally, the new emitters are, they claim, “perfectly adapted” to operate in combination with industry-grade high-power solid-state lasers and could thus form “an extremely scalable platform for applications in both fundamental science and industry”.

The researchers, who report their work in Light: Science & Applications, say they have begun to use these pulses to explore new platforms for quantum information processing. “Other applications include coupling these pulses into a scanning tunnelling microscope, which allows us to speed up atomic-resolution microscopy to few-femtosecond time scales (1 fs = 10-15 s), and thus capture the real-space and -time motion of electrons in actual ultraslow-motion microscopic videos,” they explain.

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