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Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC)

Scattering of light from quantum dipoles in chiral waveguides

Ben Lang, Dara P. S. McCutcheon, Andrew Young, Ruth Oulton

Published Jun 1, 2017• 0 citations
Publisher

Abstract

Summary form only given. There has recently been great interest in chiral waveguides, where the polarisation of the forwards and backwards propagating light fields are oppositely circular at certain positions in space (C-points) [1]. This results in new phenomena, such as the one way emission of light from spin-transitions in quantum dots (QDs) or atoms (simulation, fig. b). The emitted photon's direction is determined by the spin state of the emitter.To date calculations on this topic have either been restricted to certain special cases, or have relied on simplifying “rate forwards/backwards” models [2]. Using a Green's function method we have generalised to the case of any polarisable dipole at a point supporting any polarisation. Several interesting physical effects arise here that could not have been predicted using the previous models. For example in the figure we show how perfect chiral behaviour can be recovered at points of elliptical polarisation by using equally elliptical dipoles stretched along the orthogonal axis. This may be realised in QDs with a vector magnetic field.We further generalise our approach to include the population state of a multi-level system. This allows us to calculate the dynamics of complicated multi-transition “quantum dipoles”, such as a Λ-system, recovering results consistent with experiment [3].

DOI
10.1109/cleoe-eqec.2017.8087593

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Summary form only given. There has recently been great interest in chiral waveguides, where the polarisation of the forwards and backwards propagating light fields are oppositely circular at certain positions in space (C-points) [1]. This results in new phenomena, such as the one way emission of light from spin-transitions in quantum dots (QDs) or atoms (simulation, fig. b). The emitted photon's direction is determined by the spin state of the emitter.To date calculations on this topic have either been restricted to certain special cases, or have relied on simplifying “rate forwards/backwards” models [2]. Using a Green's function method we have generalised to the case of any polarisable dipole at a point supporting any polarisation. Several interesting physical effects arise here that could not have been predicted using the previous models. For example in the figure we show how perfect chiral behaviour can be recovered at points of elliptical polarisation by using equally elliptical dipoles stretched along the orthogonal axis. This may be realised in QDs with a vector magnetic field.We further generalise our approach to include the population state of a multi-level system. This allows us to calculate the dynamics of complicated multi-transition “quantum dipoles”, such as a Λ-system, recovering results consistent with experiment [3].
Unidirectional (chiral) emission of light from a circular dipole emitter into a waveguide is only possible at points of perfect circular polarisation (C points), with elliptical polarisations yielding a lower directional contrast. However, there is no need to restrict engineered systems to circular dipoles and with an appropriate choice of dipole unidirectional emission is possible for any elliptical polarization. Using elliptical dipoles, rather than circular, typically increases the size of the area suitable for chiral interactions (in an exemplary mode by a factor ∼30\sim 30∼30), while simultaneously increasing coupling efficiencies. We propose illustrative schemes to engineer the necessary elliptical transitions in both atomic systems and quantum dots.
We theoretically investigate the optical dipole interaction between a multi-level quantum system and a single-mode optical waveguide of any local polarisation. We investigate several paradoxical seeming situations, for example we find a situation in which there exist two non-orthogonal quantum states, each of which results in a photon flux in the opposite direction to the other. We show how, despite appearances, this does not break the unitary requirements of quantum mechanics. We also find that an isotropic quantum emitter can be either reflective or transmissive to light depending on the waveguide polarisation at the emitter location, indeed in the zero loss limit such a system changes from 100% transmission to 100% reflection due to an infinitesimal polarisation rotation. An example case for a four level system is also considered, which is found to operate as a non-destructive parity measurement of the photon number.
Unidirectional (chiral) emission of light from a circular dipole emitter into a waveguide is only possible at points of perfect circular polarization (C points), with elliptical polarizations yielding a lower directional contrast. However, there is no need to restrict engineered systems to circular dipoles, and with an appropriate choice of dipole unidirectional emission is possible for any elliptical polarization. Using elliptical dipoles, rather than circular, typically increases the size of the area suitable for chiral interactions (in an exemplary mode by a factor ∼30), while simultaneously increasing coupling efficiencies. We propose illustrative schemes to engineer the necessary elliptical transitions in both atomic systems and quantum dots.
We theoretically study the few- and many-body dynamics of photons in chiral waveguides. In particular, we examine pulse propagation through an ensemble of N two-level systems chirally coupled to a waveguide. We show that the system supports correlated multiphoton bound states, which have a well-defined photon number n and propagate through the system with a group delay scaling as 1/n2. This has the interesting consequence that, during propagation, an incident coherent-state pulse breaks up into different bound-state components that can become spatially separated at the output in a sufficiently long system. For sufficiently many photons and sufficiently short systems, we show that linear combinations of n-body bound states recover the well-known phenomenon of mean-field solitons in self-induced transparency. Our work thus covers the entire spectrum from few-photon quantum propagation, to genuine quantum many-body (atom and photon) phenomena, and ultimately the quantum-to-classical transition. Finally, we demonstrate that the bound states can undergo elastic scattering with additional photons. Together, our results demonstrate that photon bound states are truly distinct physical objects emerging from the most elementary light-matter interaction between photons and two-level emitters. Our work opens the door to studying quantum many-body physics and soliton physics with photons in chiral waveguide QED. © 2020 authors.
The ability to engineer chiral light-matter interactions is a valuable resource for realizing quantum networks and non-reciprocal quantum optics. Here, we demonstrate chiral coupling between light and a negatively charged exciton state, i.e., a four-level system, embedded in a standard photonic-crystal waveguide. By controlling the orientation and strength of an external magnetic field, we tune the polarization of the individual optical transition dipoles relative to the fixed elliptically polarized waveguide mode. In a two-sided waveguide, an optimized oblique magnetic field allows a selected transition to decay exclusively into a single propagation direction, enabling near-unity directional emission (0.99−0.02+0.010.99^{+0.01}_{-0.02}0.99−0.02+0.01​), despite the waveguide exhibiting imperfect polarization for chiral coupling. We further observe two tunable chiral branching ratios, the directional analogue of optical cyclicity, for two ΛΛΛ-systems, with one reaching 134−77+∞134_{-77}^{+\infty}134−77+∞​. The magnetic-field-dependent spectroscopy also allows reconstruction of the local guided-mode polarization at the emitter via the Stokes parameters relative to the emitter's dipole moments. Finally, we demonstrate that this local chirality enables coherent control of an electron spin via an optical Raman process mediated by waveguide driving. Building on the simultaneous realization of a large branching ratio, directionality and spin control achieved at the same oblique magnetic-field angle, we propose a protocol with high tolerance to photon loss for generating high-fidelity remote spin-spin entanglement. Our magnetic control opens a reconfigurable and generic route for enhancing on-chip chiral spin-photon coupling in standard nanophotonic interfaces without the need to carefully engineer the polarization of the waveguide.
We theoretically study the few-and many-body dynamics of photons in chiral waveguides.In particular, we examine pulse propagation through an ensemble of N two-level systems chirally coupled to a waveguide.We show that the system supports correlated multiphoton bound states, which have a welldefined photon number n and propagate through the system with a group delay scaling as 1=n 2 .This has the interesting consequence that, during propagation, an incident coherent-state pulse breaks up into different bound-state components that can become spatially separated at the output in a sufficiently long system.For sufficiently many photons and sufficiently short systems, we show that linear combinations of n-body bound states recover the well-known phenomenon of mean-field solitons in self-induced transparency.Our work thus covers the entire spectrum from few-photon quantum propagation, to genuine quantum many-body (atom and photon) phenomena, and ultimately the quantum-to-classical transition.Finally, we demonstrate that the bound states can undergo elastic scattering with additional photons.Together, our results demonstrate that photon bound states are truly distinct physical objects emerging from the most elementary light-matter interaction between photons and two-level emitters.Our work opens the door to studying quantum many-body physics and soliton physics with photons in chiral waveguide QED.