Researchers from the Faculty of Physics at Vilnius University have developed a theoretical model that allows atoms to be “pre-programmed” by light alone to reshape laser beams that carry both a twist and a polarisation. The study by master’s student Dharma Prasetya Permana, alongside Dr Mažena Mackoit-Sinkevičienė, Dr Julius Ruseckas, and Dr Hamid Reza Hamedi from the Institute of Theoretical Physics and Astronomy, opens a magnet-free route to controlling structured light for quantum technologies. The research was recently published in the prestigious journal “Physical Review A”.
Dr Mažena Mackoit-Sinkevičienė, Dharma Prasetya Permana and Dr Hamid Reza Hamedi. Photo from the personal archive.
Twisted light
The team’s research focuses on optical vortices – special light beams whose structure twists as they propagate. “Unlike whirlpools in water, where the matter itself spins, optical vortices are twists in the wave structure of light. As the beam travels, its wavefronts form a helical, spiral-like structure,” explains D. P. Permana.
At the centre of such a beam, the light intensity drops to zero, leaving a small dark core. The size of this region depends on a quantity called the topological charge, which counts how many full twists the wavefront makes around the beam’s axis.
“A topological charge of zero means there is no twist. When the charge increases to one, a small dark core appears at the centre. As the charge grows, the structure becomes more pronounced. You can think of it like the thread of a screw – the higher the charge, the tighter the twist,” explains D. P. Permana.
Dharma Prasetya Permana. Photo by Nail Garejev.
In theory, this number has no upper limit; it can take any integer value, positive or negative. That makes optical vortices particularly attractive for encoding information in many distinct states. In practice, it’s possible to create up to ten thousand distinct states.
“Researchers have already begun using these light vortices to build advanced quantum communication channels,” adds Dr H. R. Hamedi. “Instead of a standard quantum bit, or qubit, which holds only two states, light vortices allow information to be encoded in higher-dimensional quantum states, called qudits, dramatically increasing how much data a single photon can carry.”
Spin and swirl in light
Light travels as a wave, and there are two distinct ways it can be manipulated: polarisation and vortices. Normally, light from a bulb or the Sun is chaotic, with waves vibrating wildly in every direction at once.
“Polarisation is how the wave vibrates. Imagine tying a rope to a wall and shaking it. Shake your hand up and down, and the wave vibrates vertically; shake it side to side, and it vibrates horizontally. This direction of vibration is what physicists call the wave's polarisation. Polarised sunglasses act like a fence with vertical slats: they let through vertically vibrating light and block the horizontal glare bouncing off roads or water. Speaking of a vortex, it describes the overall shape of the beam. Instead of travelling as a flat wall of waves, the light is twisted into a spiral staircase as it moves forward,” says Dr M. Mackoit-Sinkevičienė.
Dr Mažena Mackoit-Sinkevičienė. Photo by Vilnius University.
When scientists combine these two concepts, they get a vector vortex: a beam with a structured pattern and vibration. “Imagine a spiral staircase where the direction of the steps changes as you climb. At the bottom, the steps face north – that is, vertical polarisation. As you walk up the spiral, the steps gradually turn to face east – horizontal polarisation. In a vector vortex, the light beam is twisting like a whirlpool, and the direction in which the light waves vibrate changes depending on where you are inside that whirlpool, “explains Dr M. Mackoit-Sinkevičienė.
“Programmable” atoms
To manipulate vector vortices and harness them for advanced information processing, the VU researchers studied how these beams interact with an atomic gas. They chose a medium of three-energy-level atoms, a standard model in quantum optics.
“We developed a theoretical model showing how these atoms can be ‘pre-programmed’ to modify the shape of optical vector vortices. When such light passes through the prepared atomic medium, the atoms respond in a highly structured way. They effectively inherit the spatial pattern of the light, forming regions where they absorb strongly and regions where they become almost transparent to the light,” says Dr H. R. Hamedi.
This creates a feedback mechanism between light and matter: the light shapes the atomic response, and the atomic response reshapes the light. The beam is transformed as it propagates. Instead of a simple ring-shaped intensity profile with a dark centre, it evolves into a petal-like pattern, where light concentrates in several bright lobes arranged around the centre. At the same time, the beam's polarisation structure evolves, showing that the medium actively controls both its spatial shape and its spin.
“This demonstrates that by preparing the atomic medium in advance, we can control not only how complex light structures evolve in space, but also how their polarisation changes as they pass through matter,” says Dr J. Ruseckas.
Applications in quantum technologies
The study positions pre-programmed atoms as a powerful tool for manipulating light, with implications for quantum computing and high-density data transfer.
“Until now, controlling structured light in this way required strong external magnetic fields. Generating those fields takes complex, bulky and often expensive equipment, which limits how easily these systems can be integrated into compact technologies,” says D. P. Permana.
“Our method is entirely optical. By using light itself to ‘program’ the atoms, we eliminate the need for magnetic fields. This offers a much more flexible, scalable, and elegant way to control light-matter interactions. Ultimately, it paves the way for faster quantum processors, highly secure quantum communication networks, and incredibly precise optical sensors,” says Dr H. R. Hamedi.