Realization of waveguide many-body quantum optics

This paper demonstrates the realization of many-body quantum optics in a waveguide by coherently coupling multiple solid-state artificial atoms to a nanophotonic structure, thereby enabling the deterministic generation and control of higher-order photon correlations, such as genuine three-photon correlations, through scalable light-matter interactions.

The Gist

Imagine light not as a smooth, continuous beam like a garden hose, but as a stream of individual, tiny marbles called photons. In the world of quantum physics, these marbles usually don't talk to each other; they just pass right by. But what if you could make them interact, bounce off one another, or even dance in a synchronized group? That is the goal of quantum optics.

This paper describes a breakthrough where researchers successfully made groups of light particles interact with each other by using a special "dance floor" made of light and matter.

The Setup: A One-Lane Highway for Light

Think of a nanophotonic waveguide as a microscopic, one-lane highway built inside a piece of glass. On the sides of this highway, the researchers placed tiny, artificial atoms called quantum dots.

Usually, if you throw a marble (photon) down a highway, it just rolls straight through. But these quantum dots act like bouncers at a club. They can grab a marble, hold it for a split second, and then let it go. If you have just one bouncer, they can only handle one marble at a time. If two marbles arrive together, the bouncer gets overwhelmed, and the interaction is simple.

The Magic Trick: Teamwork Makes the Dream Work

The researchers' big innovation was to get multiple bouncers (quantum dots) to work together on the same highway.

1. The Single Bouncer (One Atom): When they used just one quantum dot, it acted like a standard gatekeeper. It could reflect one marble or let one pass, but it couldn't create complex group behaviors.
2. The Team of Bouncers (Two Atoms): When they tuned two quantum dots to work as a team, something magical happened. The two dots formed a "collective" unit.
   • The Analogy: Imagine a two-person team trying to catch marbles. They can easily catch two marbles together. But if a third marble comes along, the team is already full. They can't catch it. Instead, the team gets excited by the presence of that third marble and suddenly spits out a burst of marbles in a specific direction.
   • The Result: The researchers observed that when they sent light at these two dots, the system naturally filtered out single and double marbles. Instead, it created a rare, synchronized burst of three marbles arriving at the exact same time. This is what they call "genuine three-photon correlations."

The "Superradiant Burst"

The paper describes this event as a "superradiant burst."

• Imagine a crowded room: If one person claps, it's just a clap. If two people clap in perfect sync, it's louder. But if a whole group of people, who are all holding hands, suddenly clap together because they are all excited by a third person entering the room, it creates a massive, synchronized thunderclap.
• In the experiment, the two quantum dots absorbed two photons (marbles) and became "full." When a third photon arrived, it triggered the whole team to release a burst of three photons together in the forward direction, while sending the "leftover" single photons backward.

Scaling Up: Adding More Dancers

The researchers didn't stop at two. They showed they could add a third quantum dot to the mix.

• Just as adding a second bouncer changed the rules for two marbles, adding a third bouncer changed the rules for three marbles.
• They demonstrated that by adding more emitters (bouncers), they could control the light on a "marble-by-marble" basis. If you have m emitters, the system naturally prefers to create groups of m+1 photons.

Why This Matters (According to the Paper)

The paper claims this is the beginning of "many-body quantum optics."

• Before: Scientists could mostly control light one particle at a time or with simple pairs.
• Now: They have a scalable way to control groups of particles. They can engineer light to have specific, complex correlations (like a specific rhythm of three marbles hitting a wall at once) that don't happen in nature.

Summary

In simple terms, the researchers built a microscopic highway where they placed teams of artificial atoms. By making these atoms work together, they turned the highway into a machine that takes in a stream of light and spits out highly synchronized, complex groups of light particles. They proved that by adding more atoms to the team, they can control larger and larger groups of light particles, opening the door to creating new types of quantum states that were previously impossible to make.

Technical Summary

Technical Summary: Realization of Waveguide Many-Body Quantum Optics

Problem and Motivation
Controlling light photon-by-photon is central to quantum optics, relying fundamentally on the mediation of photon interactions through their coupling to atoms. While proof-of-concept photon-photon nonlinear interactions (such as photon blockade and two-photon transport) have been realized in atomic and solid-state systems, extending this paradigm to radiatively couple multiple individual atoms in a deterministic and scalable manner remains a significant challenge. Accessing the realm of many-body quantum dynamics requires strong, coherent multi-photon interactions for a well-defined number of particles. The authors aim to realize a setting where the interaction between multiple emitters and photons is controlled at the level of individual particles, thereby opening the arena of many-body quantum optics.

Methodology
The researchers utilized highly efficient light-matter waveguide interactions with a controllable number of solid-state artificial atoms (quantum dots, QDs) embedded in a photonic crystal waveguide (PCW).

• Device Architecture: The device features a two-sided PCW with a shallow trench etched into the center, allowing for independent electrical control of QDs on either side via the Stark effect. A third emitter can be brought into resonance using Zeeman tuning with an external magnetic field.
• Coupling Regime: The emitters are coupled via the waveguide mode, creating long-range dipole-dipole interactions (up to $r = 26\lambda$) that effectively transform the typical $1/r^3$ interaction into an infinite-range interaction in a one-dimensional geometry. The coupling phase between the emitters was tuned to $\phi = 0.8\pi$, placing the system in an intermediate regime between dispersive and dissipative coupling.
• Experimental Protocol: The team scattered weak coherent light pulses ($\langle n \rangle \leq 0.1$) off the emitters. They compared two configurations: a reference case with a single resonant QD (two-level system) and a many-body case with two or three coupled resonant QDs (multi-level system).
• Measurement: The scattered fields were split into three detection channels to measure temporal correlations up to the third order ($g^{(3)}$). The data was analyzed using Jacobi coordinates to separate center-of-mass motion from relative temporal coordinates. The authors distinguished between "connected" (genuine many-body) and "disconnected" (lower-order) correlation components by utilizing measurements from single pulses, partially correlated pulses, and uncorrelated pulses.

Key Contributions and Results

1. Observation of Genuine Three-Photon Correlations: By coupling two QDs, the authors demonstrated the generation of genuine three-photon correlations in the forward-propagating field. While a single QD primarily exhibits two-photon correlations, the coupled pair suppresses lower-order components and enhances the three-photon component. Specifically, the zero-delay value of the connected three-photon correlation function, $g_c^{(3)}(0,0)$, increased from $0.83(5)$ for a single QD to $4.3(2)$ for the coupled pair.
2. Mechanism of Superradiant Burst: The enhanced correlations originate from a specific interaction pathway: the first two photons excite the collectively coupled emitters to a doubly excited state, and a third photon stimulates a burst-like emission (superradiance) from this populated state. This results in a temporal burst of strongly correlated photons.
3. Directional Filtering: The system exhibits distinct directional behavior. In the backward direction, the coupled emitters reflect two-photon components (due to the resonant two-photon process) but suppress three-photon reflections (as two emitters cannot simultaneously reflect three photons). Conversely, the forward direction shows enhanced three-photon correlations while suppressing lower-order components.
4. Scalability: The study successfully scaled the system to three resonant quantum emitters. Transmission measurements showed that the suppression of transmission (enhanced reflection) scales with the number of emitters $m$, with deeper transmission dips observed for $m=2$ and $m=3$ compared to $m=1$.
5. Theoretical Validation: Experimental results were in strong agreement with theoretical simulations, which predicted that the highest value of connected correlations $g_c^{(n)}$ in the forward field is achieved when $n = m+1$.

Significance and Claims
The paper claims to demonstrate the "onset of many-body quantum optics in waveguide quantum electrodynamics." The primary significance lies in the ability to engineer non-equilibrium many-body systems one emitter and one photon at a time. The authors state that these advancements enable:

• The creation of many-body entangled states.
• The exploration of novel quantum phase transitions.
• The realization of new photonic quantum simulators.

The work establishes a scalable route to realizing quantum nonlinear optics where the number of quantum emitters directly controls the order of photon correlations, moving beyond sequential generation of complex states to the direct formation of strongly correlated states through simultaneous multi-photon interactions. The authors note that while the current results are modest in terms of absolute correlation values (limited by spectral diffusion and detector jitter), the underlying principle of controlling correlations via the number of coupled emitters is robust and scalable.