Simple Laser Delivers High-Dimensional, Quantum-Like Classical Light

An international team from China and South Africa used a laser to create an arbitrary dimensional light that team members characterized as “quantum like.” Using a simple laser commonly available in university teaching labs, the team showed eight-dimensional, classically entangled light. The demonstration builds on existing properties and principles of light structuring, pushing the limits of the field by charting a course for higher (more than two) dimensions that constrain a qubit quantum state.

The research and a tomography technique that the researchers developed to measure high-dimensional, classically entangled light support future developments in quantum metrology, optical communications, quantum error correction, and more.

Tailoring light typically involves altering its spatial properties, such as its phase, polarization, and/or amplitude. Structuring light, which can involve spatial light modulators, makes it possible to see smaller, more focused images with fewer photons and, accordingly, to store information in light for high-bandwidth communications.

However, the possibility of applying classical light to quantum processes or developing light that harnesses quantum-like properties (in effect, developing this type of tailored light so that it appears “classically entangled”) has so far been beyond the ability to create and control. This stems from the fact that structuring light from laser sources often requires specialized lasers. Further, the commonly considered two-dimensional (pattern and polarization) paradigm only considers classically entangled light in two dimensions.

As it relates to quantum light applications, those two degrees of freedom (pattern and polarization) mimic the two dimensions of the qubit quantum state. Creating higher dimensions requires finding more degrees of freedom in a system that is constrained to just two.

The laser used in the new work contained only a gain crystal and two mirrors. The process followed the quantum mechanical principle of ray-wave duality; the scientists controlled the path and the polarization inside their laser by making a simple adjustment to length to exploit a ray-wave structured laser beam in a tri-partite, eight-dimensional state.

A simple laser comprising just two standard mirrors was used to create higher-dimensional classically entangled light, deviating from the prevailing paradigm of two-dimensional Bell states. The approach combined internal generation, in principle unlimited in what can be created, with external control, allowing user-defined states to be molded. Shown are examples of two-dimensional Bell (left) and high-dimensional states (right), including the famous Greenberger-Horne-Zeilinger (GHZ) states. Courtesy of Yijie Shen et al.

The approach enabled creating any quantum state by marking the wave-like rays that a laser produces and then controlling them externally with a spatial light modulator to mold them into a desirable shape. The researchers’ system allows the laser to itself produce the necessary dimensions before instituting external controls.

As a demonstration, the team produced all of the Greenberger-Horne-Zeilinger (GHZ) states (entangled quantum states that involve three or more subsystems) in vector beams. Measurements necessitated developing a new test and measurement approach; the researchers translated tomography of high-dimensional quantum states into a technique they could gauge. The result, they reported, was a new type of tomography for classically entangled light that reveals its quantum-like correlations beyond two dimensions.

The research was published in Light: Science & Applications (www.doi.org/10.1038/s41377-021-00493-x)