A new twist on metasurfaces by researchers from the University of Exeter in the U.K. and the Hong Kong University of Science and Technology generates quantum holograms in which polarization and holographic information are entangled.
The team’s quantum holograms are essentially light patterns with a quantum mechanical property—entanglement—that Einstein famously viewed as “spooky action at a distance.”
“We entangle holograms displaying four letters—H, V, D, A—with the polarization of a separate light beam,” explains Jensen Tsan Hang Li, a professor of computational engineering and metamaterials at the University of Exeter.
It’s intriguing because the correlation is caused by entanglement. “When we measure a photon in one beam to have a certain polarization (horizontal, for example), we see a corresponding destructive interference pattern in the matching letter ‘H’ within the hologram,” says Li. “This demonstrates how quantum information can be encoded and manipulated within spatial light patterns.”
Li and his colleagues’ work builds on previous research with metasurfaces that manipulate quantum properties of light. “Our work was driven by a desire to extend these quantum optical concepts into the spatial domain,” he says. “We became fascinated by the potential connections between traditional holography and quantum entanglement.”
Quantum optics meets advanced nanophotonics
The team’s work combines quantum optics principles with advanced nanophotonics. Metasurfaces, which are specialized surfaces crafted of subwavelength structures that can manipulate light properties with much greater versatility than conventional optics, are at the heart of their approach.
They designed metasurfaces to respond differently to left-handed vs. right-handed polarized light, which create distinct hologram patterns depending on the polarization state.
“Our design required modifying standard computer-generated holography algorithms,” says Li. “The algorithm needs to generate the appropriate phase patterns for our metasurface to produce the desired polarization-dependent holograms.”
How do you generate quantum holograms?
How does the team’s method to generate quantum holograms work? It begins by generating polarization-entangled photon pairs using a nonlinear crystal of beta-barium borate (BBO). When these photon pairs are created, their polarization is perfectly correlated. If one has left-handed polarization its partner will also have left-handed polarization, and the same for right-handed polarization.
“We direct one of these entangled photons toward our metasurface, which then transforms the photon into a specific hologram pattern that directly depends on its polarization state,” says Li.
If the photon has left-handed polarization, a certain hologram pattern appears. With right-handed polarization you’ll see a completely different pattern.
“It’s fascinating that this process creates an entanglement between the polarization of one photon and the hologram pattern of its partner, the quantum hologram, which stores multiple holograms in an entanglement state,” Li says.
Phase relation and timing are everything
A few technical challenges had to be overcome during the team’s experiment. The biggest one was designing the precise phase relation between the two holograms in the entangled state. This phase difference ultimately determines the interference pattern, so the researchers had to carefully control it to successfully erase or maintain the letter pattern.
“We overcame this design problem by using a modified Gerchberg-Saxton algorithm developed by my current postdoc Philip Wai Chun Wong,” says Li. “We were surprised by how well this approach worked, and the resulting letter patterns were remarkably clear and well defined.”
Another challenge was to get their timing system just right. “In our setup, we use a single-photon detector to capture the photon with polarization information, which is in the shorter path,” Li adds. “When this detector registers a photon, it sends a detection signal to our single-photon camera in the other arm to take a picture.”
The trickiest part? Tuning an electronic delay in this signal. They had to match it to the path length difference between the two arms of the detector. If the delay is too short, they’d shoot pictures before the partner photon arrived at the camera. Too long and they’d miss it entirely. Getting this timing calibration right is crucial for observing the quantum effects they were exploring.
“The coolest aspect of our work is that we can use this platform to study very fundamental quantum problems,” says Li. “We visualized the quantum eraser effect, in which inserting a quantum eraser restores quantum interference, and it actually shows up visually as erasing specific holographic content that we designed.”
When they realized this experiment parallels the quantum eraser concept, “it was a genuine ‘a-ha!’ moment,” Li adds. “Our work deepened our understanding of the quantum eraser phenomenon and provided a new way to see these abstract quantum concepts in a more visual form.”
Quantum metasurfaces replace bulky optical components
For quantum optics, the team’s work shows many traditionally bulky optical operations like generating complex holograms can be performed by an ultrathin metasurface instead of conventional optical components. And miniaturization is crucial for practical quantum technologies.
Quantum metasurfaces offer significant advantages because “they’re incredibly compact compared to components in traditional optics, provide remarkable design flexibility to create custom light manipulation, and allow us to integrate multiple optical functions onto a single platform,” says Li. “These benefits make metasurfaces ideal for miniaturizing quantum optical devices, which is essential for transitioning quantum technologies from lab demonstrations to practical applications.”
Anticounterfeiting technology and secure comms ahead
Imagine a credit card or passport with a quantum security feature. “A metasurface would be extremely difficult to copy for two main reasons,” explains Li. “First, manufacturing these metasurfaces requires specialized tools to fabricate the tiny subwavelength structures—it makes physical counterfeiting very challenging. Second, the quantum entanglement created by the metasurface adds another layer of security that’s difficult to replicate without knowledge of quantum optics.”
As for the timeline to practical use, “we’re still very much within the research phase,” he adds. “It will depend on manufacturing processes advances and integration with existing security systems. My best estimate is it’ll be five to 10 years before we might see commercial applications, but the fundamental technology demonstrates a promising new approach to anticounterfeiting.”
The team is now working on using their quantum holograms to secure communication systems. “Our research shows these structured light patterns could significantly improve quantum key distribution,” says Li. “We expect that this approach has a low error rate and can carry more information per measurement than traditional methods. It’s like upgrading from a two-lane road to a four-lane highway—for quantum information. Looking ahead, we plan to scale it up further by modifying our metasurfaces to handle even more complex patterns. The beauty of this approach is that as we increase the complexity we also strengthen the security.”
FURTHER READING
H. Liang, W. C. Wong, T. An, and J. Li, Adv. Photonics, 7, 2, 026006 (Mar. 2025); https://doi.org/10.1117/1.ap.7.2.026006.
Sally Cole Johnson | Editor in Chief
Sally Cole Johnson, Laser Focus World’s editor in chief, is a science and technology journalist who specializes in physics and semiconductors. She wrote for the American Institute of Physics for more than 15 years, complexity for the Santa Fe Institute, and theoretical physics and neuroscience for the Kavli Foundation.