Direction switchable single-photon emitter using a Rydberg polariton

This paper demonstrates a direction-switchable single-photon emitter based on Rydberg polaritons that utilizes stimulated Raman transitions to achieve efficient quantum routing across multiple output channels while significantly extending photon lifetime to mitigate motional dephasing.

The Gist

Imagine you have a very special, fragile message written in light—a single photon. In the world of quantum computing, these light particles are like the messengers that carry secret information. The problem is, once you catch a messenger, you usually have to let them go in the exact same direction they came from, or you lose the message.

This paper describes a new way to catch that messenger, change their mind, and send them out in a completely different direction, all while keeping their message perfectly safe.

Here is how the scientists did it, using some creative analogies:

1. The "Freezing" Trick (Storing the Light)

First, the team catches the photon and "freezes" it inside a cloud of super-cooled atoms. They turn the light into a hybrid creature called a Rydberg polariton. Think of this as turning a fast-moving bird into a statue made of the bird and the air around it. The bird (the photon) is now part of the statue (the atoms), so it stops moving and waits.

2. The "Identity Swap" (The Problem)

While the statue is waiting, the atoms inside it are still wiggling around a little bit because they are warm (even if they are very cold). This wiggling is like a crowd of people shifting in a line; if you try to turn the statue back into a bird later, the wiggling makes the bird's feathers ruffled and the message garbled. This is called "motional dephasing."

Usually, to fix this, scientists use a complicated two-step dance to cancel out the wiggling. But in this experiment, the team found a shortcut. They used a specific laser pulse (a "pi pulse") to swap the bird's identity.

• Before: The bird is wearing a red hat (State 1).
• After: The laser swaps the red hat for a blue hat (State 2).

Here is the magic: When the bird gets the blue hat, the way it wiggles changes. The scientists calculated that if they wait just the right amount of time, the wiggling of the blue-hatted bird perfectly cancels out the wiggling of the red-hatted bird. It's like if two people walking in opposite directions on a moving walkway suddenly swap places; their combined movement cancels out, and they end up standing still relative to the ground.

3. The "Traffic Cop" (Changing Direction)

This is the most exciting part. Because the bird now has a blue hat, it can only be released if the "Traffic Cop" (the retrieval laser) stands in a specific spot.

• Old way: To release the bird, the Traffic Cop had to stand exactly where the bird came from.
• New way: Because the bird's "wiggling pattern" changed, the Traffic Cop can now stand in a different spot, and the bird will fly out in a new direction.

The paper shows that by simply moving the laser that acts as the Traffic Cop, they can make the photon fly out in many different directions. It's like having a single mail slot that can be rotated to send a letter to any of 100 different houses, rather than just one.

4. The Result: A Super-Router

The team proved this works in two ways:

1. Direction Switching: They successfully made the photon come out in a different direction than it went in. They showed that by rotating the laser, they could theoretically send the photon to many different "output channels" (like a router on the internet, but for single particles of light).
2. Longer Life: Because they used this "identity swap" trick with just one laser pulse (instead of the usual two), they reduced the noise and kept the photon safe for a long time—over 10 microseconds. In the world of light, that's a very long time (more than 20 times longer than it takes to process the information).

The Bottom Line

The researchers built a "direction switchable" light switch. They caught a single photon, changed its internal state to cancel out the shaking caused by heat, and then used a laser to guide it out in a new direction. This creates a perfect "router" for quantum networks, allowing a single photon to be sent to many different places without losing its message, all while staying safe for a surprisingly long time.

Technical Summary

Technical Summary: Direction Switchable Single-Photon Emitter Using a Rydberg Polariton

Problem Statement
The realization of scalable quantum networks requires quantum routers capable of dynamically reconfiguring single-photon paths and interfacing with quantum memories. While single-photon routing has been demonstrated in various systems (e.g., coupled-resonator waveguides, superconducting qubits), existing solutions often lack the capacity for a large number of output channels compatible with large-scale photonic circuits. Furthermore, Rydberg-mediated single-photon routers have been proposed theoretically (e.g., quantum reflection, chiral routers), but a practical demonstration of a Rydberg-mediated router that simultaneously addresses motional dephasing and offers directional switching has remained unexplored.

Methodology
The authors propose and experimentally demonstrate a protocol based on Rydberg polaritons to achieve a direction-switchable single-photon emitter. The core mechanism involves the manipulation of the stored photon's Rydberg component using a stimulated Raman transition.

1. Storage: A signal photon is stored in a cold Cesium atomic ensemble as a Rydberg polariton $|W_1\rangle$ via two-photon Electromagnetically Induced Transparency (EIT), coupling the ground state $|g\rangle$ to a Rydberg state $|r_1\rangle$ via an intermediate state $|e\rangle$.
2. State Mapping: Instead of using two $\pi$ pulses (as in previous work), the protocol employs a single $\pi$ Raman pulse to convert the Rydberg state $|r_1\rangle$ to a different Rydberg state $|r_2\rangle$ via an intermediate state $|f\rangle$. This converts the polariton to $|W_2\rangle$.
3. Phase Matching and Redirection: The conversion introduces a phase term dependent on the atoms' initial positions and thermal velocities, which typically leads to motional dephasing. The authors derive a specific condition where the wavevector of the retrieval laser ($\vec{k}_{retr}$) is redirected such that the phase mismatch is canceled. Specifically, by satisfying the condition $k - k_r = -k_r \frac{\pi}{2\Omega_r} + t'$, the phase term in the new polariton state $|W_2\rangle$ aligns with the atomic positions at the retrieval time $t_s$.
4. Retrieval: The stored photon is retrieved by applying a retrieval laser ($\lambda_5$) that couples $|r_2\rangle$ back to $|e\rangle$. Crucially, the direction of the emitted photon is determined by the vector relationship between the retrieval laser, the read-out signal, and the polariton wavevector $|W_2\rangle$. To retrieve the signal, these three wavevectors must form a closed triangle. By varying the direction of the retrieval laser, the output direction of the single photon can be switched.

Key Contributions

• $\pi$-Redirection Protocol: The authors introduce a "single $\pi$-pulse" protocol that simultaneously redirects the read-out signal and suppresses motional dephasing. This contrasts with previous methods requiring two $\pi$ pulses, thereby reducing the noise level associated with Raman lasers and enhancing coherence times.
• Direction Switchability: The work demonstrates that the output direction of a stored single photon can be controlled by the direction of the retrieval laser. This enables the creation of a single-photon emitter with multiple output channels.
• Theoretical Router Proposal: Based on the experimental scheme, the authors propose a generic quantum router with $N$ output channels and unity routing efficiency. By rotating the retrieval laser around the wavevectors of the $|W_2\rangle$ polariton, the signal photon can be routed to any of the $N$ channels.

Experimental Results
The authors implemented a proof-of-principle experiment using Case 7 from their theoretical model (where the retrieval laser travels exactly opposite to the coupling laser during loading, $\theta_1 = 0^\circ, \theta_2 = 0^\circ$).

• Directional Verification: Measurements of the second-order correlation function $g^{(2)}(0) = 0.34 \pm 0.08$ confirmed the single-photon nature of the retrieval signal. Crucially, photon retrieval was observed only when the retrieval laser was aligned in the "correct" direction satisfying the wavevector triangle condition. When the laser was set to a "wrong" direction (co-propagating with the coupling laser), no photons were retrieved, verifying the direction-switching mechanism.
• Suppression of Dephasing: The protocol was tested for storage times up to $>10$ µs. The results showed a dramatic increase in retrieval efficiency compared to free decay. The coherence time ($\tau$) was extended from $3.11$ µs (free decay) to $14.86$ µs using the $\pi$-redirection protocol.
• Efficiency Gain: Compared to the authors' previous work using a $\pi$-wait-$\pi$ protocol, the single $\pi$-pulse protocol increased the retrieval efficiency by a factor of two for storage times exceeding $10$ µs, attributed to reduced laser noise.

Significance
The paper claims that this work provides a functional demonstration of a direction-switchable single-photon emitter, a key component for quantum routers. By enabling the redirection of single photons into a rich variety of alternative modes with unity routing efficiency, the scheme addresses the challenge of scaling quantum networks to many output ports. Furthermore, the protocol's ability to extend the photon lifetime to over $10$ µs (more than 20 times the photon processing time) while suppressing motional dephasing makes it suitable for functional quantum devices based on Rydberg polaritons. The authors position this as a step toward practical, large-scale optical quantum networks.