Photon-bound states enable “quantum light” manipulation 2023

Researchers saw photons and bound photons interacting differently with a quantum dot. Quantum-enhanced measurement, light-based quantum computing, and metrology could benefit from novel photonic state manipulation.

Photons rarely interact. This provides near-distortion-free information transport at light speed in optical fibres for long-distance communications. Researchers sometimes want light to interact. They intend to develop light states that increase interferometer sensitivity. Photons must interact.

Interacting photons generate bound states, quasiparticles that cause technologically essential physical processes like stimulated emission (lasing). Such states had never been directly observed till today.

Strong photon interactions

In the new study, physicists Sahand Mahmoodian of the University of Sydney in Australia and Natasha Tomm of the University of Basel in Switzerland used a circulator to guide pulses of very dim laser light with few photons into a quantum-dot cavity system. The circulator backscatters and directs the light to a Hanbury Brown-Twiss setup with single photon detectors that record photon impacts.

Mahmoodian says pulses can have zero, one, or two photons, but the likelihood of one is substantially higher than two. Because the two-photon part is smaller, the one-photon part dominates the pulse packet intensity measurement. We overcome this obstacle by measuring the second-order correlation function of light, which allows us to quantify the likelihood of two photons arriving at the detectors within a very little time difference.”

Two delayed photons

The Nature Physics measurement method only records the two-photon pulse since it is insensitive to single photons. The researchers found that the two-photon state was delayed less than the one-photon state. Because their technology creates such strong photon interactions, they could observe the difference between one photon and two. This intense photon–photon interaction creates the two-photon bound state.

“To measure single photons, we measure its time of arrival at just one of our detectors,” says Tomm. We measure photon arrival times at two detectors to determine correlation. The “correlation” between the two detectors is zero if there is just one photon. Two detectors make this measurement insensitive to single-photons. One detector clicks for one photon.”

“Being able to see one photon and two photons interacting differently with a quantum dot (which behaves essentially like a single artificial atom) basically means we are doing nonlinear optics with just two photons,” says Mahmoodian. “By demonstrating that we can identify and manipulate such photon bound states, we have taken a vital first step towards harnessing quantum light for practical use,” he tells Physics World.

In biological microscopy, where high-intensity light can damage delicate samples and the features being observed are particularly small, such quantum states of light can be used to make more sensitive measurements with better resolution using fewer photons, according to the researchers.

“Our platform is flexible,” says Mahmoodian. “It is an almost ideal interface between light and matter at the quantum scale and could be used to generate a variety of quantum light for applications like quantum-enhanced communication, metrology, or computation.”

He claims such a gadget might create specific photon states for “fault-tolerant” quantum computers that are noise-resistant. “We’ll research this.”