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Nature Communications

Giant optical polarisation rotations induced by a single quantum dot spin

Elham Mehdi, Manuel Gundín, C. Millet, Niccolò Somaschi, Aristide Lemaître, Isabelle Sagnes, Luc Le Gratiet, Dario Fioretto, Nadia Belabas, O. Krebs, P. Senellart, L. Lanco

Published Jan 18, 2024• 13 citations
Publisher

Abstract

In the framework of optical quantum computing and communications, a major objective consists in building receiving nodes implementing conditional operations on incoming photons, using 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-dependent photon state, in a deterministic way. Here we use an electrically-contacted pillar-based cavity, embedding a single InGaAs quantum dot, to demonstrate giant polarisation rotations induced on reflected photons by a single electron spin. A complete tomography approach is introduced to extrapolate the output polarisation Stokes vector, conditioned by a specific spin state, in presence of spin and charge fluctuations. We experimentally approach polarisation states conditionally rotated by [Formula: see text], π, and [Formula: see text] in the Poincaré sphere with extrapolated fidelities of (97 ± 1) %, (84 ± 7) %, and (90 ± 8) %, respectively. We find that an enhanced light-matter coupling, together with limited cavity birefringence and reduced spectral fluctuations, allow targeting most conditional rotations in the Poincaré sphere, with a control both in longitude and latitude. Such polarisation control may prove crucial to adapt spin-photon interfaces to various configurations and protocols for quantum information.

DOI
10.1038/s41467-023-44651-8

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In the framework of optical quantum computing and communications, a major objective consists in building receiving nodes implementing conditional operations on incoming photons, using 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-dependent photon state, in a deterministic way. Here we use an electrically-contacted pillar-based cavity, embedding a single InGaAs quantum dot, to demonstrate giant polarisation rotations induced on reflected photons by a single electron spin. A complete tomography approach is introduced to extrapolate the output polarisation Stokes vector, conditioned by a specific spin state, in presence of spin and charge fluctuations. We experimentally approach polarisation states conditionally rotated by [Formula: see text], π, and [Formula: see text] in the Poincaré sphere with extrapolated fidelities of (97 ± 1) %, (84 ± 7) %, and (90 ± 8) %, respectively. We find that an enhanced light-matter coupling, together with limited cavity birefringence and reduced spectral fluctuations, allow targeting most conditional rotations in the Poincaré sphere, with a control both in longitude and latitude. Such polarisation control may prove crucial to adapt spin-photon interfaces to various configurations and protocols for quantum information.
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.
We introduce a tomography approach to describe the optical response of a cavity quantum electrodynamics device, beyond the semiclassical image of polarization rotation, by analyzing the polarization density matrix of the reflected photons in the Poincaré sphere. Applying this approach to an electrically-controlled quantum dot-cavity device, we show that a single resonantly-excited quantum dot induces a large optical polarization rotation by 20∘20^\circ20∘ in latitude and longitude in the Poincaré sphere, with a polarization purity remaining above 84%84\%84%. The quantum dot resonance fluorescence is shown to contribute to the polarization rotation via its coherent part, whereas its incoherent part contributes to degrading the polarization purity.
Coherent light-matter interfaces controllably modifying the state of a photon upon interaction with a stationary qubit are a key resource for implementing deterministic entangling gates for optical quantum technologies. This requires a one-to-one mapping between the state of the scattered photon and that of the embedded qubit. Here, we present an experimental signature of such a bijection by leveraging the spin-induced Kerr rotation present in a low-noise charged quantum dot-micropillar cavity device. Through time-resolved polarization measurements, we project the electron spin to one of its eigenstates with 95±2%95\pm2\%95±2% fidelity with a single reflected photon detection, and follow the subsequent spin relaxation through the detection of a second reflected photon. We demonstrate that, after a transient regime governed by the trion radiative lifetime, two orthogonal polarization states can be produced, each associated to a given spin eigenstate. While the current results are limited by a timescale competition between electron spin relaxation and trion radiative lifetime, they could be improved using hole spins displaying increased relaxation times. Our work paves the way towards deterministic logic gates exploiting this one-to-one mapping between a spin and the polarization of a scattered photon.
We propose a quantum nondemolition method---a giant optical Faraday rotation near the resonant regime to measure a single-electron spin in a quantum dot inside a microcavity where a negatively charged exciton strongly couples to the cavity mode. Left-circularly and right-circularly polarized lights reflected from the cavity obtain different phase shifts due to cavity quantum electrodynamics and the optical spin selection rule. This yields giant and tunable Faraday rotation that can be easily detected experimentally. Based on this spin-detection technique, a deterministic photon-spin entangling gate and a scalable scheme to create remote spin entanglement via a single photon are proposed.
Polarization-encoded spin-photon interfaces constitute promising candidates for the development of stationary nodes used as photon receivers, for quantum communication and distributed quantum computing. Here we introduce a time-resolved tomography approach which allows observing the dynamics of an electron spin, in a semiconductor quantum dot, mapped onto the dynamics of the polarization state of reflected photons. Through a single tomography experiment, we infer all the relevant spin dynamics timescales, including precession, decoherence and relaxation times. We also demonstrate and quantify the measurement back-action induced, on the embedded spin qubit, by the detection of a single reflected photon. We show that the induced population and coherence of the spin state can be tuned by the chosen polarization basis of the measurement. The control of the photon-induced back-action on the embedded spin qubit constitutes a crucial requirement for the use of spin-photon interfaces as quantum receivers.
Quantum dots (QDs) can be incorporated into solid state photonic devices such as cavities or waveguides that enhance the light-matter interaction. A near unit efficiency light-matter interaction is essential for deterministic, scalable quantum information devices [1]. In this limit, a single photon input into the device will undergo a large rotation of the polarization of the light field due to the strong interaction with the QD. In the past preliminary results have indicated that a low quality-factor (Q~290) pillar microcavity possesses a high ß-factor and that the instantaneous interactions should be deterministic and with high fidelity [2].