Quantum shift shows itself in coupled light and matter

April 16, 2018, Rice University
A simplified schematic shows the basic idea behind a Rice University experiment to detect a Bloch-Siegert shift in strongly coupled light and matter. In this illustration, a light field rotating in the opposite direction to an orbiting electron still interacts with the electron in a cavity, in this case the empty space between two mirrors. The influence of resonance on the counter-rotating element defines the shift. Credit: Xinwei Li/Kono Lab at Rice University

A team led by Rice University scientists used a unique combination of techniques to observe, for the first time, a condensed matter phenomenon about which others have only speculated. The research could aid in the development of quantum computers.

The researchers, led by Rice physicist Junichiro Kono and graduate student Xinwei Li, observed and measured what's known as a Bloch-Siegert shift in strongly coupled light and .

Results of the complicated combination of modeling and experimentation are the subject of a paper in Nature Photonics. The technique could lead to a greater understanding of theoretical predictions in quantum phase transitions because the experimental parameters used in the Rice experiments are highly adjustable, according to Kono. Ultimately, he said, it may help in the development of robust quantum bits for advanced computing.

The Bloch-Siegert shift, a theory born in the 1940s, is a quantum interaction in which counter-rotating fields are able to interact. But such interactions have been difficult to detect.

The theory suggested to Kono and Li that it might be possible to detect such a shift when a light rotating in one direction strongly couples with a matter-bound electron field rotating in the opposite direction. These interactions have proven difficult to create without the unique tools assembled by the Rice-led team.

Researchers at Rice University, including graduate student Xinwei Li, have observed and measured a Bloch-Siegert shift in strongly coupled light and matter in a vacuum. The project could aid in the development of quantum computers. Credit: Jeff Fitlow/Rice University
"Light and matter should not resonate with each other when they are rotating in opposite directions," Kono said. "However, in our case, we proved they can still strongly couple, or interact, even though they are not resonating with each other."

Kono and his colleagues created the resonance frequency shift in a two-level electron system induced by coupling with an electromagnetic field inside a cavity even when the electrons and field are rotating in opposite directions - a truly surprising effect that occurs only in a regime where light and matter are mixed together to an extreme degree.

In this case, the levels are those of two-dimensional electrons in solid gallium arsenide in a strong perpendicular magnetic field. They hybridize with the "vacuum" electromagnetic field in the cavity to form quasiparticles known as polaritons. This vacuum-matter hybridization had been expected to lead to a finite frequency shift, a vacuum Bloch-Siegert shift, in optical spectra for circularly polarized light counter-rotating with the electrons. The Rice team can now measure it.

"In condensed matter physics, we often look for new ground states (lowest-energy states). For that purpose, light-matter coupling is usually considered an enemy because light drives matter to an excited (higher-energy) state," Kono said. "Here we have a unique system that is predicted to go into a new ground state because of strong light-matter coupling. Our technique will help us know when the strength of light-matter coupling exceeds a certain threshold."

The research builds upon a strong vacuum field-matter coupling in a high-quality-factor cavity the lab first created and reported in 2016. The results at the time only hinted at the presence of a Bloch-Siegert shift. "Experimentally, we just demonstrated the new regime," Li said. "But here, we have a very deep understanding of the physics involved."

Kono and Li credited physicist Motoaki Bamba of Osaka University for providing a theoretical basis for the discovery and Katsumasa Yoshioka of Yokohama National University and a former visiting scholar at Rice for providing a device to produce circularly polarized light in the terahertz range of the electromagnetic spectrum.

The lab used the light to probe the shift in an ultra-high quality, two-dimensional electron gas supplied by Purdue University physicist Michael Manfra and set in a gallium arsenide quantum well (to contain the particles) under the influence of a strong magnetic field and low temperature. A terahertz spectroscope measured activity in the system.

"Linearly polarized light means an alternating current electric field that is always oscillating in one direction," Kono said. "In , the electric field is rotating." That allowed the researchers to distinguish between left- and right-rotating electrons in their vacuum-bound in a , and from that, measure the shift.

"In this work, both theoretically and experimentally, we demonstrated that even though the electron is rotating this way and the light is rotating (the other) way, they still strongly interact with each other, which leads to a finite frequency shift known as the Bloch-Siegert shift," Kono said.

Observing the shift is a direct indication that ultra-strong light-matter coupling invalidated the rotating wave approximation, he said. "That approximation is behind almost all light-matter interaction phenomenon, including lasers, nuclear magnetic resonance and quantum computing," Kono said. "In any resonant light-matter interaction, people are satisfied with this approximation, because the coupling is usually weak. But if the coupling between and matter is strong, it doesn't work. That's clear evidence that we are in the ultra-strong coupling regime."

Explore further: Light and matter merge in quantum coupling

More information: Xinwei Li et al, Vacuum Bloch–Siegert shift in Landau polaritons with ultra-high cooperativity, Nature Photonics (2018). DOI: 10.1038/s41566-018-0153-0

Related Stories

Light and matter merge in quantum coupling

August 22, 2016

Where light and matter intersect, the world illuminates. Where light and matter interact so strongly that they become one, they illuminate a world of new physics, according to Rice University scientists.

Plasmons triggered in nanotube quantum wells

March 16, 2018

A novel quantum effect observed in a carbon nanotube film could lead to the development of unique lasers and other optoelectronic devices, according to scientists at Rice University and Tokyo Metropolitan University.

Bridging the gap between the quantum and classical worlds

August 2, 2016

In the quantum world, physicists study the tiny particles that make up our classical world - neutrons, electrons, photons - either one at a time or in small numbers because the behaviour of the particles is completely different ...

Polarization has strong impact on electrons, study shows

April 10, 2018

The movement of thousands of electrons underlies electronics. Yet, ubiquitous as electrons are, the particulars of their behavior continue to stump physicists. One phenomenon has proven especially puzzling: how electrons ...

Recommended for you

Magic number colloidal clusters

December 14, 2018

Complexity in nature often results from self-assembly, and is considered particularly robust. Compact clusters of elemental particles can be shown to be of practical relevance, and are found in atomic nuclei, nanoparticles ...

Tangled magnetic fields power cosmic particle accelerators

December 13, 2018

Magnetic field lines tangled like spaghetti in a bowl might be behind the most powerful particle accelerators in the universe. That's the result of a new computational study by researchers from the Department of Energy's ...

1 comment

Adjust slider to filter visible comments by rank

Display comments: newest first

milnik
not rated yet Apr 17, 2018
Bloch Siegert shift and this condensate-based experiment coincides with my finding, which I did not publish, which explains many phenomena resulting from the interaction of that condensate and substance that fills the infinite universe, which is AETHER.
In this way, the celestial bodies and a large number of subatomic particles move. This Bloch-Siegert shift, described by two spins of the same size and opposite directions, is derived from the kinetic energy of the radial velocity of motion. It will be known one day if the science has interest in it.

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.