[Paper Review] Efficient generation of entangled multi-photon graph states from a single atom

Philip Thomas, Leonardo Ruscio+2 more|arXiv (Cornell University)|May 25, 2022

Quantum Information and Cryptography41 references190 citations

TL;DR

This paper demonstrates a deterministic, high-efficiency method to generate large-scale photonic graph states—specifically Greenberger-Horne-Zeilinger (GHZ) and linear cluster states—using a single 87Rb atom in a high-finesse cavity. By interleaving controlled single-photon emissions with tailored atomic qubit rotations via vSTIRAP, the authors achieve 14-photon GHZ states with fidelity >76% and 12-photon cluster states with fidelity >56%, at a rate of ~1 state per minute, surpassing probabilistic SPDC-based methods in scalability and efficiency.

ABSTRACT

Entanglement is a powerful concept with an enormous potential for scientific and technological advances. A central focus in modern research is to extend the generation and control of entangled states from few to many qubits, and protect them against decoherence. Optical photons play a prominent role as these qubit carriers are naturally robust and easy to manipulate. However, the most successful technique to date for creating photonic entanglement is inherently probabilistic and therefore subject to severe scalability limitations. Here we avoid these by implementing a deterministic protocol with a single memory atom in a cavity. We interleave controlled single-photon emissions with tailored atomic qubit rotations to efficiently grow Greenberger-Horne-Zeilinger states of up to 14 photons and linear cluster states of up to 12 photons with a fidelity lower bounded by 76(6)% and 56(4)%, respectively. Thanks to a source-to-detection efficiency of 43.18(7)% per photon we measure these large states about once every minute, orders of magnitude faster than in any previous experiment. In the future, this rate could be increased even further, the scheme could be extended to two atoms in a cavity, or several sources could be quantum mechanically coupled, to generate higher-dimensional cluster states. Overcoming the limitations encountered by probabilistic schemes for photonic entanglement generation, our results may offer a way towards scalable measurement-based quantum computation and communication.

Motivation & Objective

• To overcome the scalability limitations of probabilistic photonic entanglement sources like SPDC.
• To implement a deterministic protocol for generating multi-photon graph states using a single atomic qubit as a quantum memory.
• To achieve high-fidelity, large-scale photonic entanglement with a single atom-cavity system.
• To enable scalable measurement-based quantum computation by generating resource states on demand.
• To demonstrate a modular, scalable platform for photonic quantum information processing with high source-to-detection efficiency.

Proposed method

• A single 87Rb atom is trapped in a high-finesse optical cavity, serving as a quantum memory and photon source.
• The protocol uses vacuum-stimulated Raman adiabatic passage (vSTIRAP) to generate indistinguishable photons with high efficiency.
• Controlled single-photon emissions are interleaved with tailored atomic qubit rotations (θ = 0 or π/2) to engineer entanglement.
• The system leverages the atomic hyperfine structure to implement intrinsic dynamical decoupling, reducing decoherence.
• Photons are detected via a polarization-resolving setup with fast basis switching (Z/X basis) using an electro-optic modulator and half-wave plate.
• Fidelity is verified using entanglement witnesses based on stabilizer formalism for GHZ and cluster states.

Experimental results

Research questions

• RQ1 Can a single atom in a cavity generate large-scale photonic graph states deterministically and with high fidelity?
• RQ2 How does the intrinsic dynamical decoupling in the atomic system mitigate decoherence during multi-photon state generation?
• RQ3 What is the maximum photon number and fidelity achievable for GHZ and cluster states using this deterministic protocol?
• RQ4 How does the source-to-detection efficiency compare to probabilistic SPDC-based methods in terms of state generation rate?
• RQ5 Can this platform be extended to generate higher-dimensional cluster states via modular coupling of multiple sources?

Key findings

• The authors generated 14-photon GHZ states with a fidelity lower bound of 76(6)%, exceeding previous deterministic and probabilistic benchmarks.
• Twelve-photon linear cluster states were created with a fidelity lower bound of 56(4)%, demonstrating scalable resource state preparation.
• The system achieved a source-to-detection efficiency of 43.18(7)% per photon, enabling the generation of large entangled states at a rate of approximately one per minute.
• Parity oscillations in the measurement basis confirmed genuine multipartite entanglement, with visibility preserved even at 1.2 ms delay, indicating strong coherence.
• The protocol demonstrated intrinsic dynamical decoupling, with visibility rephasing observed at ~85 µs delay, reducing decoherence effects.
• The method surpasses SPDC-based schemes in scalability and rate, offering a deterministic, modular path toward one-way quantum computation.