The team’s on-chip technology uses orbital angular momentum to encode more information into a single photon

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

Typically, quantum communication systems “write” information about the spin angular momentum of a photon. In this case, the photons either rotate right or left circular or form a quantum superposition of the two known as a two-dimensional qubit. It is also possible to encode information about a photon’s orbital angular momentum – the corkscrew path that light follows as it twists and twists forward, with each photon rotating around the center of the beam . When spin and angular momentum intertwine, it forms a high-dimensional qudit – allowing any theoretically infinite range of values ​​to be encoded and propagated by a single photon.

Qubits and qudits, also called 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 as qubits, which is the basis for turbocharging next-generation quantum communication.

In a cover story for the August 2022 issue of Optical, researchers led by Stefan Strauf, head of Stevens’ NanoPhotonics Lab, show they can create and control individual flying qudits, or “twisted” photons, on demand – a breakthrough that could dramatically expand the capabilities of communication tools quantum. The work builds on the team’s 2018 paper in Nature nanotechnology.

“Normally, spin angular momentum and orbital angular momentum are independent properties of a photon. Our device is the first to demonstrate the 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 of the University of Pennsylvania and Jim Hone of Columbia University.

“What makes this a big deal is that we’ve shown we can do it with single photons rather than conventional light beams, which is the basic requirement for any kind of communication application. quantum,” Ma said.

Encoding information in orbital angular momentum radically increases the information that can be transmitted, Ma explained. Taking advantage of “twisted” photons could increase the bandwidth of quantum communication tools, allowing them to transmit data much faster.

To create meandering photons, Strauf’s team used an atomic-thick film of tungsten diselenide, an upcoming new semiconductor material, to create a quantum emitter capable of emitting single photons.

Next, they coupled the quantum emitter into an internally-reflecting donut-shaped space called a ring resonator. By refining the arrangement of the emitter and gear-like resonator, it is possible to take advantage of the interaction between the photon’s spin and its orbital angular momentum to create individual “twisted” photons on demand. The key to enabling this spin-pulse lock feature lies in the ring resonator’s gear-like pattern, which when carefully engineered into the design, creates the winding vortex beam of light that the device projects at the speed of light.

By embedding these capabilities into a single microchip measuring 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 standardized components in the frame. of a quantum communication system.

Some major challenges remain. While the team’s technology can control which direction a photon spins in a spiral – clockwise or counter-clockwise – there’s still work to be done to control the exact number of orbital angular momentum mode. It is the critical capacity that will allow a theoretically infinite range of different values ​​to be “written” into and later extracted from a single photon. The latest experiments from Strauf’s nanophotonics lab show promising results that this problem can be quickly overcome, according to Ma.

Further work is also needed to create a device capable of creating 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 may require new breakthroughs in materials science to solve, Ma said.

“A lot of challenges lie ahead of us,” he added. “But we have shown the potential for creating more versatile quantum light sources than anything possible before. »

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