Quantum computing is a rapidly-emerging technology that harnesses the laws of quantum mechanics to solve problems too complex for classical computers. When scientists and engineers encounter difficult problems, they turn to supercomputers. These are very large classical computers, often with thousands of classical CPU and GPU cores. However, even supercomputers struggle to solve certain kinds of problems. If a supercomputer gets stumped, that’s probably because the big classical machine was asked to solve a problem with a high degree of complexity. When classical computers fail, it’s often due to complexity. Complex problems are problems with lots of variables interacting in complicated ways. Modeling the behavior of individual atoms in a molecule is a complex problem, because of all the different electrons interacting with one another. Sorting out the ideal routes for a few hundred tankers in a global shipping network is complex too.
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 known 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. Qubits and qudits, also known as 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.
In a new study published in journal Optica, researchers led by Stefan Strauf, head of the NanoPhotonics Lab at Stevens Institute of Technology, show that they can create and control individual flying qudits, or “twisty” photons, on demand a breakthrough that could dramatically expand the capabilities of quantum communication tools. Dr. Liang Feng at the University of Pennsylvania, and Dr. Jim Hone at Columbia University were co-authors and contributed to the research.
Encoding information into orbital angular momentum radically increases the information that can be transmitted. Leveraging “twisty” photons could boost the bandwidth of quantum communication tools, enabling them to transmit data far more quickly.
To create twisty photons, the authors 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 an internally reflective donut-shaped space called 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 about a quarter of the width of a human hair 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.
Future research will focus on creating 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. it would be interesting to create hybrid devices embedding plasmonic gap mode cavities with deterministically coupled QEs directly into the mode maximum of these patterned SiN ring resonators that provide spin–orbit locking, which allows for efficient on-chip OAM state creation and manipulation. In conclusion the new study has shown the potential for creating quantum light sources that are more versatile than anything that was previously possible.
Yichen Ma, Haoqi Zhao, Na Liu, Zihe Gao, Seyed Sepehr Mohajerani, Licheng Xiao, James Hone, Liang Feng, and Stefan Strauf. On-chip spin-orbit locking of quantum emitters in 2D materials for chiral emission. https://opg.optica.org/optica/fulltext.cfm?uri=optica-9-8-953&id=492743