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arXiv:2404.16025 · Advanced Quantum Technologies

Birefringent Spin‐Photon Interface Generates Polarization Entanglement

N. V. Leppenen, Dmitry S. Smirnov

Published Aug 26, 2024• 2 citations
arXivHTMLPDF

Abstract

Abstract A spin‐photon interface based on the luminescence of a singly charged quantum dot in a micropillar cavity allows for the creation of photonic entangled states. Current devices suffer from cavity birefringence, which limits the generation of spin‐photon entanglement. In this study, we conduct a theoretical analysis of the light absorption and emission by the interface with an anisotropic cavity and derive the maximal excitation and spin‐photon entanglement conditions. It is shown that the concurrence of the spin‐photon state equal to one and complete quantum dot population inversion can be reached for a micropillar cavity with any degree of birefringence by tuning the quantum dot resonance strictly between the cavity modes. This sweet spot is also valid for generating a multiphoton cluster state, as demonstrated by calculating the three‐tangle and fidelity with the maximally entangled state.

DOI
10.1002/qute.202400193

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Laser & Photonics Review•Nov 14, 2025•0 citations
Abstract A spin‐photon interface based on the luminescence of a singly charged quantum dot in a micropillar cavity allows for the creation of photonic entangled states. Current devices suffer from cavity birefringence, which limits the generation of spin‐photon entanglement. In this study, we conduct a theoretical analysis of the light absorption and emission by the interface with an anisotropic cavity and derive the maximal excitation and spin‐photon entanglement conditions. It is shown that the concurrence of the spin‐photon state equal to one and complete quantum dot population inversion can be reached for a micropillar cavity with any degree of birefringence by tuning the quantum dot resonance strictly between the cavity modes. This sweet spot is also valid for generating a multiphoton cluster state, as demonstrated by calculating the three‐tangle and fidelity with the maximally entangled state.
We present a deterministic and scalable scheme to generate photon polarization entanglement via a single electron spin confined in a charged quantum dot inside a microcavity. This scheme is based on giant circular birefringence and giant Faraday rotation induced by a single electron spin. Two independent photons are sequentially sent to the cavity and get entangled after measuring the spin state. We show that this scheme can be extended to generate multiphoton polarization entanglement including Greenberger-Horne-Zeilinger states and cluster states in a deterministic way.
In the framework of optical quantum computing and communications, a major objective consists in building receiving nodes that implement conditional operations on incoming photons, using the interaction with a single stationary qubit. In particular, the quest for scalable nodes motivated the development of cavity-enhanced spin-photon interfaces with solid-state emitters. An important challenge remains, however, to produce a stable, controllable, spin-dependant photon state, in a deterministic way. Here we use a pillar-based high-Q cavity, embedding a singly-charged semiconductor quantum dot, to demonstrate the control of giant polarisation rotations induced by a single electron spin. A complete tomography approach is used to deduce the output polarisation Stokes vector, conditioned by a single spin state. We experimentally demonstrate rotation amplitudes such as ±π2\pm \fracπ{2}±2π​ and πππ in the Poincaré sphere, as required for applications based on spin-polarisation mapping and spin-mediated photon-photon gates. In agreement with our modeling, we observe that the environmental noise does not limit the amplitude of the spin-induced rotation, yet slightly degrades the polarisation purity of the output states. We find that the polarisation state of the reflected photons can be manipulated in most of the Poincaré sphere, through controlled spin-induced rotations, thanks to moderate cavity birefringence and limited noise. This control allows the operation of spin-photon interfaces in various configurations, including at zero or low magnetic fields, which ensures compatibility with key protocols for photonic cluster state generation.
Entangling a single spin to the polarization of a single incoming photon, generated by an external source, would open new paradigms in quantum optics such as delayed-photon entanglement, deterministic logic gates or fault-tolerant quantum computing. These perspectives rely on the possibility that a single spin induces a macroscopic rotation of a photon polarization. Such polarization rotations induced by single spins were recently observed, yet limited to a few 10(-3) degrees due to poor spin-photon coupling. Here we report the enhancement by three orders of magnitude of the spin-photon interaction, using a cavity quantum electrodynamics device. A single hole spin in a semiconductor quantum dot is deterministically coupled to a micropillar cavity. The cavity-enhanced coupling between the incoming photons and the solid-state spin results in a polarization rotation by ± 6° when the spin is optically initialized in the up or down state. These results open the way towards a spin-based quantum network.
Graph states of mutually entangled photons are key resources for quantum computation and communication and can be generated by leveraging the entanglement between a single resident spin and emitted photons from a charged semiconductor quantum dot (QD). This approach is intrinsically limited by the decoherence of the spin. We study how to mitigate this decoherence with dynamical decoupling of an electron spin in the weak transverse magnetic field regime using spin echo and Carr-Purcell-Meiboom-Gill (CPMG) techniques. Application of these techniques allows us to extend the coherence time of a spin by more than two orders of magnitude, extracting a T2CPMGT_2^{CPMG}T2CPMG​ of 298±53298\pm53298±53 ns. We further demonstrate that this technique is compatible with the generation of a spin-photon-photon entangled state at a high rate enabled by a micropillar cavity, with a 20% improvement in simulated state fidelity when using dynamical decoupling. These results pave the way for generating larger and more complex entangled states with QDs.
ABSTRACT Bidirectional coherent control between photonic and matter qubits in solid‐state quantum networks demand a tailored photon‐emitter interface that can operate at the few‐photon level. Cavity‐coupled quantum dots with high input coupling efficiency provides a viable path toward such interface. However, reflection from the bare cavity can contaminate, or even overwhelm, the desired output photons that have interacted with the emitter. To overcome this problem, we implement a low‐reflectivity ( 1%) quantum dot‐micropillar cavity system with 97% input coupling efficiency. We achieve an optical nonlinearity threshold of 5.6 incident photons per pulse while maintaining high antibunching fidelity ( 0.2), measured at a pulse duration of twice the quantum dot exciton lifetime. Furthermore, we observe quantum dot transition inversion at 9 photons when the pulse duration is reduced to roughly match the quantum dot lifetime. This highly efficient photon‐emitter interface paves the way for key quantum photonic resources and functionalities.