twisted light quantum electrons control

Twisted Light Spins Electrons: A Breakthrough in Quantum Interaction Control

The Profound Power of Light

Enjoying a sunny day at the beach might make light seem gentle and serene, but its power is profound. A beam of light not only transmits energy but also carries momentum. This includes linear momentum, the force that challenges braking a moving train, and orbital angular momentum, fundamental to Earth's solar revolution.

The Experiment: Advancing Quantum Science

The Role of Orbital Angular Momentum in Light

On November 26, 2024, Nature Photonics published a paper detailing a novel experiment in quantum science. Researchers showcased how a beam of light could impart orbital angular momentum to electrons in graphene, advancing the understanding of light-matter quantum interactions.

The Importance of Light-Matter Interaction

Why Manipulating Light-Matter Interaction Matters

Precise manipulation of light-matter interactions is critical for advancing technologies such as quantum computing and quantum sensing. Researchers have particularly focused on encouraging electrons to respond to the unique and exotic patterns of light beams.

The Nature of Orbital Angular Momentum in Light

What is Orbital Angular Momentum?

A beam of light carrying orbital angular momentum rotates around its axis as it moves forward. Viewed directly, it features a dark center—a vortex shaped by its helical nature.

Theoretical Background and Challenges

"The interaction between light carrying orbital angular momentum and matter has been theorized since the 1990s," notes Deric Session, a postdoctoral researcher at JQI and the University of Maryland (UMD), who led the study. "But only a handful of experiments have demonstrated this phenomenon."

The Scale Mismatch Problem

The challenge arises from a mismatch in scale. For electrons to interact with a light beam carrying momentum, they must detect its changing characteristics as it propagates. Unfortunately, the spatial extent of these changes typically exceeds the dimensions of the material being studied, making electron targeting particularly arduous.

Addressing the Challenge: Expanding Electron Space

The Limitations of Traditional Approaches

For example, atoms and their orbiting electrons—fundamental to quantum physics research and prime candidates for precise control—are approximately 1,000 times smaller than the light beams used to interact with them.

The Problem with Photons and Atom Interaction

Light propagates as oscillating waves of electric and magnetic fields, and the distance it covers before repeating itself is known as the wavelength.

Beyond being a fundamental characteristic, the wavelength of light also dictates the energy carried by its individual particles, known as photons. Only photons with specific energy levels can interact with atoms, and these photons typically have wavelengths much larger than the atoms they engage with.

Although atoms as a whole can absorb energy and momentum from photons, the wavelength of these photons is too large for the atom's internal components—the nucleus and electrons—to detect any significant difference. This poses a challenge for transferring orbital angular momentum directly to the electrons.

Potential Solutions and Innovations

One potential solution to this challenges is to reduce the wavelength of light. However, doing so increases the energy carried by each photon, making atoms impractical as reliable targets. In their latest experiment, researchers, including JQI Fellows Mohammad Hafezi and Nathan Schine, JQI Co-Director Jay Sau, and JQI Adjunct Fellow Glenn Solomon, took a different approach: rather than shortening the light's wavelength, they expanded the electrons to occupy a larger space.

The Role of Graphene in the Ex periment

Why Graphene Was Chosen

Electrons that are bound to an atom's nucleus can only move within a limited a range before being freed from the atom, making them unsuitable for experimental use. In contrast, conductive materials allow electrons greater freedom to travel over longer distance while still being kept under control. To explore how to expand the electrons' spatial distribution, the researchers chose graphene—a highly efficient electrical conductor.

Experimental Setup and Findings

When a sample of graphene is cooled to 4 degrees above absolute zero and exposed to a powerful magnetic field, the electrons that typically move freely become ensnared in loop-like structures called cyclotron orbits. As the field strength increases, these orbits tighten further, packing more and more electrons until no additional ones can fit. Though compact, these orbits remain much larger than electron orbitals in atoms, providing an ideal setup for them to interact with light that carries orbital angular momentum.

Methodology and Results

Setting Up the Experiment

The experiment involved a graphene sample equipped with electrodes. One electrode was situated in the center, while another was arranged in a ring surrounding the edges of the sample.

Theoretical Predictions and Observations

Theoretical studies from 2021, led by Bin Cao, a former graduate student at JQI and UMD, and three other authors of the present paper, indicated that electrons circulating in this type of sample might gain angular momentum in distinct increments from incoming light, which would enlarge their orbits and ultimately result in their absorption by the electrodes.

Key Experimental Findings

"The principle is to modify the size fo the cyclotron orbits by transferring orbital angular momentum to or from the electrons, which in turn moves them throughout the sample and creates a current," says Session.

In their latest paper, the research team reported observing a stable current that persisted across a broad range of experimental conditions. They directed light with orbital angular momentum rotating clockwise at their graphene sample and detected the current moving in a single direction.

The research team then illuminated the sample with light carrying counterclockwise orbital angular momentum and noted that the direction of the current flipped. When they reversed the applied magnetic field, they observed the same directional change in the current—an anticipated finding since reversing the magnetic field direction also reverses the electron flow in cyclotron orbits. Changing the voltage across the inner and outer electrodes still resulted in the same difference between currents generated by clockwise and counterclockwise vortex light.

The researchers also exposed the sample to circularly polarized light, which possesses intrinsic angular momentum, and observed that it generated only a negligible current. The data consistently showed that a current was only detected when the light carried orbital angular momentum, with its direction directly related to whether the light's momentum was clockwise or counterclockwise.

Overcoming Technical Challenges

Sample Fabrication and Alignment Difficulties

This outcome was the result of years of dedicated effort, which included initial challenges with sample fabrication and difficulties in gathering sufficient reliable data for the experiment.

"I spent more than a year trying to fabricate graphene samples with this precise geometry," says Session. Ultimately, Session and the research team connected with a group they had previously worked with, headed by Roman Sordan, a physicist from the Polytechnic University of Milan with extensive experience in graphene sample fabrication.

According to Session, "They delivered and created the samples that we ultimately used."

Beam Alignment and Data Collection

Once they achieved samples that worked as expected, they still encountered difficulties aligning their twisted light with the sample to observe the current.

"The signal we were analyzing wasn't entirely consistent," says Mahmoud Jalali Mehrabad, a postdoctoral researcher at JQI and UMD and co-author of the paper. "Then one day, with Deric, we decided to do a spatial sweep. We essentially mapped the sample with exceptional precision. Once we identified the optimal  position for the beam, everything fell into place."

Within a week, they  had amassed all the data needed and were able to isolate the signals that demonstrated the current's relationship with the beam's orbital angular momentum.

Implications and Future Prospects

Significance of the Findings

Mehrabad states that, in addition to offering a new method for light-based control of matter, this technique could enable unprecedented electron measurements in quantum materials. Specially engineered light beams, when combined with interference techniques, could serve as a microscope to visualize the electron's spatial extent, providing a direct assessment of their quantum properties.

Potential for Future Quantum Research

"Measuring these spatial degrees of freedom in free electrons is vital for evaluating their coherence properties in a controlled way and for manipulating them," Mehrabad says. "It's not just detection; it's about control. That's what makes this achievement so significant."

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