Focus: Entangled Photons Sneak through Hole Unscathed
Future quantum computing devices may include nanoscale waveguides for light, but the quantum information stored in waves passing through such structures could be disturbed as the light interacts with the surrounding material. Starting with the simpler case of light passing through a tiny hole, researchers have now shown how to engineer quantum states of two photons that are protected by fundamental physical principles and pass through unchanged. The work should help in finding ways to keep delicate quantum information from being destroyed in future nanoscale chips.
Researchers building devices to control and process quantum information often use photons to store data. It’s easy to use stable physical properties of light, such as photon polarization (the configuration of the photon’s electric field), to encode digital bits, such as a 0 or 1. However, light often interacts strongly with nanoscale devices, especially those having features smaller than the light’s wavelength. So these devices can modify photonic bits in unexpected ways.
To address this concern, researchers are seeking other properties of light that might be more stable. As Gabriel Molina-Terriza of the Donostia International Physics Center in San Sebastian, Spain, notes, one way to find such properties is by using basic symmetries. A circular hole, for example, has both rotational and mirror symmetry and can serve as a prototype for testing the interactions between light and a nanostructure. Quantum states of photons can be engineered to share symmetries of a circle, and, in principle, even if the interaction with the hole alters the state (changes its wave function), the state’s symmetry should not change. It turns out that for one particular type of symmetry—the so-called antisymmetric symmetry, where the wave function changes sign under rotation and mirror reflection—there is only one possible state. So symmetry protection becomes state protection. The state cannot be altered at all, no matter how complex the interaction might be.
In experiments, Molina-Terriza and colleagues have now demonstrated the protective effect of symmetry. They fashioned a nanoscale aperture, 750 nanometers across, in a metallic sheet and then created several types of entangled photon pairs, each described by a different wave function. While one of these states was the perfectly antisymmetric state, other states were not. The team found that the antisymmetric state was preserved when passing through the hole, while the other states were strongly altered.
“There were experiments showing that the entanglement was not preserved when photons were transmitted through a nanoaperture,” says Molina-Terriza. “We’ve shown that you can protect the entanglement by using the right symmetry in the initial state.”
The achievement, he suggests, provides a new avenue for exploring so-called quantum plasmonics—the behavior of quantum states of light when confined in metallic structures on the scale of atoms and molecules. In this regime, light interacts strongly with plasmons, the wave-like motions of electrons in a metal. Future development, Molina-Terriza suggests, may go in various directions, depending on the purpose being pursued—for example, carrying out logical operations or transporting photons reliably from one part of a device to another.
“This paper is quite exciting,” says solid-state physicist Carsten Rockstuhl of the Karlsruhe Institute of Technology in Germany. Previous research suggested that preserving quantum states of light requires reducing its confinement in a nanostructure (giving it more space), he says. But “here the authors demonstrate a completely different mechanism,” by using symmetry.
The research may also find uses in other areas of quantum technology, says Molina-Terriza. Powerful methods for detecting the presence of single molecules—such as a biomarker protein for cancer—exploit the strong focusing of photons that occurs in a plasmonic nanoaperature. Detection can be made still more accurate by using entangled photons, but this requires a means to ensure the entanglement will be preserved as the photons travel to the biomolecule (in the center of the aperture).
This research is published in Physical Review Letters.
Mark Buchanan is a freelance science writer who splits his time between Wales in the UK and the Normandy region of France.