‘Twisted’ photons can charge the next generation of q

image: The image shows a quantum emitter capable of emitting single photons combined with a shape vector resonator. By adjusting the arrangement of the emitter and the resonator to the shape of a gear, it is possible to take advantage of the interaction between a photon’s spin and its orbital angular momentum to create individual “twisted” photons on demand.
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Credit: Stevens Institute of Technology

Computers and quantum communication devices work by encoding information into single or entangled photons, allowing quantum data to be safely transmitted and processed exponentially faster than is possible with conventional electronics. Now, quantum researchers at Stevens Institute of Technology have demonstrated a way to encode dramatically more information into a single photon, opening the door to faster and more powerful quantum communication tools.

Quantum communication systems typically “write” information on the photon’s spin angular momentum. In this case, the photons either spin left or right, or form a quantum superposition of the two known as two-dimensional qubits. It is also possible to encode information on the photon’s orbital angular momentum – the spiral path that light follows as it moves and torques forward, with each photon rotating around the beam’s center. When the spin and angular momentum are entangled, it forms a high-dimensional standard – allowing any theoretically unlimited range of values ​​to be encoded and propagated by a single photon.

Qubits and qudits, also known as flying qubits and flying qudits, are used to spread information stored in photons from one point to another. The main difference is that qubits can carry much more information over the same distance compared to qubits, providing the basis for charging the next generation of quantum communications.

in Cover story in the August 2022 issue of Visualled by researchers Stephen Struve, head of the NanoPhotonics Laboratory at Stevens, explains that they can create and control individual flying parts, or “twisted” photons, on demand — a breakthrough that could greatly expand the capabilities of quantum communication tools. Work is based on the 2018 team paper in Nature’s nanotechnology.

“Usually the spin angular momentum and orbital angular momentum are independent properties of the photon. Our device is the first to show simultaneous control of both properties through the controlled coupling between the two,” explained Yishen Ma, a graduate student in the Struve Laboratory of NanoPhotonics, who led Research in collaboration with Liang Feng at the University of Pennsylvania, and Jim Sharpen at Columbia University.

“What makes it important is that we’ve shown that we can do this with single photons instead of traditional beams of light, which is a basic requirement for any kind of quantum communication application,” Ma said.

Ma explained that encoding information in orbital angular momentum drastically increases the information that can be transmitted. Taking advantage of “twisted” photons could increase the bandwidth of quantum communication tools, enabling them to transmit data more quickly.

To create the twisted photons, Struve’s team used an atom-thick film of tungsten diselenide, a new semiconductor material, to create a quantum emitter capable of emitting single photons.

Next, they wired the quantum emitter into an internally reflective, doughnut-shaped space called a circular resonator. By adjusting the arrangement of the emitter and the resonator to the shape of a gear, it is possible to take advantage of the interaction between a photon’s spin and its orbital angular momentum to create individual “twisted” photons on demand. This rotating momentum lock function enable switch in the decoration adopts the gear shape of the annular resonator, which when carefully engineered into the design creates a twisted vortex beam of light that is emitted by the device at the speed of light.

By incorporating these capabilities into a single chip just 20 microns in diameter — about a quarter of the width of a human hair — the team created a twisted photon emitter capable of interacting with other standard components as part of a quantum communications system.

There are still some major challenges. While the team’s technique can control the direction in which the photon rotates – clockwise or counterclockwise – more work is needed to control the exact orbital angular momentum mode number. It is this critical ability that will enable “writing” in a theoretically unlimited range of different values ​​and subsequently extracting them from a single photon. The latest experiments at Struve’s Nanophotonics Laboratory have shown promising results that this problem can soon be overcome, according to Ma.

More work is also needed to create a device that can create twisted photons with strictly consistent quantum properties, that is, indistinguishable photons – a prerequisite for enabling the quantum internet. Such challenges affect everyone working in quantum photonics, Ma said, and may require new breakthroughs in materials science to solve them.

“There are many challenges ahead,” he added. “But we’ve shown that quantum light sources can be created more diverse than anything previously possible.”

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