Quantum computers and communi­cation devices work by encoding information into individual or entangled photons, enabling data to be quantum securely transmitted and manipulated expo­nentially faster than is possible with conven­tional elec­tronics. Now, quantum researchers at Stevens Institute of Techno­logy have demonstrated a method for encoding vastly more information into a single photon, opening the door to even faster and more powerful quantum communi­cation tools.

Typically, quantum communi­cation systems write information onto a photon’s spin angular momentum. In this case, photons carry out either a right or left circular rotation, or form a quantum superposition of the two as a two-dimen­sional qubit. It’s also possible to encode infor­mation onto a photon’s orbital angular momentum – the corkscrew path that light follows as it twists and torques forward, with each photon circling around the center of the beam. When the spin and angular momentum interlock, it forms a high-dimen­sional qudit – enabling any of a theoretically infinite range of values to be encoded into and propagated by a single photon.

These flying qubits and flying qudits are used to propagate information stored in photons from one point to another. The main difference is that qudits can carry much more information over the same distance than qubits, providing the foundation for turbo­charging next generation quantum communication. The researchers led by Stefan Strauf, head of the Nano­Photonics Lab at Stevens, show that they can create and control individual flying qudits, or twisty photons, on demand. That could dramatically expand the capa­bilities of quantum communi­cation tools. “Normally the spin angular momentum and the orbital angular momentum are independent properties of a photon. Our device is the first to demonstrate simul­taneous control of both properties via the controlled coupling between the two,” explained Yichen Ma, a graduate student in Strauf’s Nano­Photonics Lab, who led the research in collaboration with Liang Feng at the University of Penn­sylvania, and Jim Hone at Columbia University.

“What makes it a big deal is that we’ve shown we can do this with single photons rather than classical light beams, which is the basic requirement for any kind of quantum communi­cation appli­cation,” Ma said. Encoding information into orbital angular momentum radically increases the information that can be transmitted, Ma explained. Leveraging twisty photons could boost the bandwidth of quantum communi­cation tools, enabling them to transmit data far more quickly. To create twisty photons, Strauf’s team used an atom-thick film of tungsten diselenide, an upcoming novel semi­conductor material, to create a quantum emitter capable of emitting single photons.

Next, they coupled the quantum emitter in a ring resonator. By fine-tuning the arrange­ment of the emitter and the gear-shaped resonator, it’s possible to leverage the inter­action between the photon’s spin and its orbital angular momentum to create indi­vidual twisty photons on demand. The key to enabling this spin-momentum-locking func­tionality relies in the gear-shaped patterning of the ring resonator, that when carefully engineered in the design, creates the twisty vortex beam of light that the device shoots out at the speed of light. By integrating those capabilities into a single microchip measuring just 20 microns across, the team has created a twisty-photon emitter capable of interacting with other stan­dardized components as part of a quantum communi­cations system.

Some key challenges remain. While the team’s technology can control the direction in which a photon spirals – clockwise or anti­clockwise – more work is needed to control the exact orbital angular momentum mode number. That’s the critical capability that will enable a theoretically infinite range of different values to be written into and later extracted from a single photon. Latest experiments in Strauf’s Nano­photonics Lab show promising results that this problem can be soon overcome, according to Ma.

Further work is also needed to create a device that can create twisted photons with rigorously consistent quantum properties, i.e., indis­tinguishable photons – a key requirement to enable the quantum internet. Such challenges affect everyone working in quantum photonics and could require new break­throughs in material science to solve, Ma said. “Plenty of challenges lie ahead,” he added. “But we’ve shown the potential for creating quantum light sources that are more versa­tile than anything that was previously possible.” (Source: Stevens)

Reference: Y. Ma et al.: On-chip spin-orbit locking of quantum emitters in 2D materials for chiral emission, Optica 9, 953 (2022); DOI: 10.1364/OPTICA.463481

Link: Center for Quantum Science and Engineering, Stevens Institute of Technology, Hoboken, USA