Light’s Entangled State Steers Energy Flow in Novel Material

Precise control over biexciton eigenstate distribution is achieved using entangled photons in monolayer WSe 2. Minseok A. Jang and Hongki Yoo, at the Centre for Quantum Materials (KIST), and the Department of Physics at KAIST, reveal that the Bell-state phase of entangled photon pairs governs the resulting biexciton states generated through entangled two-photon absorption. A selection rule, unattainable with classical light sources, partitions excitation according to the Bell-state phase, selectively driving either bright or exchange-dark eigenstates. Achieving a phase-scan visibility exceeding 0.97 at 4 K with broadband photon sources, this work signifies a key advance in manipulating quantum states of matter with high-fidelity entangled light.

Entangled photon generation and biphoton phase control in tungsten diselenide

Generating entangled photons with defined polarisation underpinned this work, utilising a technique called spontaneous parametric down-conversion (SPDC). SPDC creates pairs of entangled photons from a single laser pulse, intrinsically linking them and sharing correlated properties like polarisation. The process employed a frequency-nondegenerate ladder scheme, meaning the two photons produced differed in energy, allowing specific interaction with the material. This carefully designed process ensured the relative amplitude between the material’s two valley pathways, analogous to alternative routes for energy transfer, was dictated by the biphoton phase, a key element in controlling the experiment.

Two-photon absorption was investigated using a monolayer of tungsten diselenide (WSe2) at 4 Kelvin. The material excited two independent valley pathways, and , without a shared intermediate state, leveraging its unique coupling between photon polarisation and valley degrees of freedom. Broadband photons, with approximately 100 femtosecond pulses, were generated using spontaneous parametric down-conversion (SPDC), enabling control over biexciton eigenstate distribution via the biphoton phase.

High-visibility phase-scan control of biexciton eigenstates via entangled photon absorption

A phase-scan visibility exceeding 0.97, surpassing the previous limit of 0.8, demonstrates unprecedented control over entangled two-photon absorption in monolayer WSe2. This threshold represents a qualitative leap, enabling clear identification of entanglement signatures previously obscured by classical two-photon absorption processes. The ability to distinguish genuine entanglement is vital for advancing quantum technologies reliant on precise light-matter interactions. This work establishes a selection rule, unattainable with conventional light sources, that selectively drives either bright or exchange-dark biexciton eigenstates based on the Bell-state phase of the entangled photons.

This precise manipulation of biexciton eigenstates allows for tailored material responses unavailable with classical excitation methods, opening possibilities for deterministic quantum light sources and novel optoelectronic devices. Biexcitons, bound states of two excitons, act as a key stepping stone to more complex quantum phenomena, and scientists confirmed that the polarization of entangled photons directly influences which biexciton eigenstates are created within monolayer WSe2. Specifically, a symmetric Bell state, with a phase of 0, preferentially excites bright biexciton eigenstates, while an antisymmetric state, at a phase of π, selectively drives the exchange-dark eigenstate. This control is achieved because the biphoton phase dictates the relative contribution of two distinct valley pathways, K and K’, within the material, effectively acting as a switch and coherently imprinting the polarization structure of quantum light onto the valley degree of freedom.

Entangled photons precisely steer energy-carrying excitons in atomically thin material

Controlling the flow of energy at the nanoscale promises breakthroughs in areas ranging from more efficient solar cells to ultra-fast computing. This research offers a new level of precision in directing that energy, utilising the unique properties of entangled photons to manipulate excitons, bound pairs of electrons and holes, within a single layer of tungsten diselenide. Maintaining this delicate control, however, presents a significant hurdle, as the experiment was conducted at the extremely low temperature of 4 Kelvin, raising questions about its viability in practical, room-temperature devices.

Despite the need for cryogenic cooling, the significance of this work remains substantial. Demonstrating precise exciton manipulation via entangled photons establishes a fundamental principle for future nanoscale devices. Excitons, created when electrons absorb light, act as energy carriers, and controlling their behaviour is crucial for advancements in areas like solar energy conversion and computing. This technique directs these key energy carriers using the unique characteristics of paired photons.

This research establishes a direct link between the phase of entangled photons and the resulting quantum states within a material, specifically monolayer tungsten diselenide. Utilising entangled two-photon absorption, scientists demonstrated precise control over biexciton eigenstates, which are fundamental energy configurations. A symmetric Bell state preferentially creates bright eigenstates, while an antisymmetric state favours an exchange-dark eigenstate, a distinction impossible with traditional light sources. This control, evidenced by a phase-scan visibility exceeding 0.97, signifies a new method for manipulating quantum systems, moving beyond simply creating excitons to directing their specific properties.

The research demonstrated that the phase of entangled photons controls the distribution of biexciton eigenstates within a single layer of tungsten diselenide. This precise control over energy carriers is achieved through entangled two-photon absorption, allowing scientists to selectively drive different quantum states. Specifically, symmetric and antisymmetric Bell states produce distinct eigenstate distributions unattainable with classical light sources. The authors report a phase-scan visibility exceeding 0.97 at 4 Kelvin, establishing a fundamental principle for manipulating quantum systems at the nanoscale.