B. Royall

Chirality of nanophotonic waveguide with embedded quantum emitter for unidirectional spin transfer

Scalable quantum technologies may be achieved by faithful conversion between matter qubits and photonic qubits in integrated circuit geometries. Within this context, quantum dots possess well-defined spin states (matter qubits), which couple efficiently to photons. By embedding them in nanophotonic waveguides, they provide a promising platform for quantum technology implementations. In this paper, we demonstrate that the naturally occurring electromagnetic field chirality that arises in nanobeam waveguides leads to unidirectional photon emission from quantum dot spin states, with resultant in-plane transfer of matter-qubit information. The chiral behaviour occurs despite the non-chiral geometry and material of the waveguides. Using dot registration techniques, we achieve a quantum emitter deterministically positioned at a chiral point and realize spin-path conversion by design. We further show that the chiral phenomena are much more tolerant to dot position than in standard photonic crystal waveguides, exhibit spin-path readout up to 95±5% and have potential to serve as the basis of spin-logic and network implementations.

Waveguide Coupled Resonance Fluorescence from On-Chip Quantum Emitter

Resonantly driven quantum emitters offer a very promising route to obtain highly coherent sources of single photons required for applications in quantum information processing (QIP). Realizing this for on-chip scalable devices would be important for scientific advances and practical applications in the field of integrated quantum optics. Here we report on-chip quantum dot (QD) resonance fluorescence (RF) efficiently coupled into a single-mode waveguide, a key component of a photonic integrated circuit, with a negligible resonant laser background and show that the QD coherence is enhanced by more than a factor of 4 compared to off-resonant excitation. Single-photon behavior is confirmed under resonant excitation, and fast fluctuating charge dynamics are revealed in autocorrelation g((2)) measurements. The potential for triggered operation is verified in pulsed RF. These results pave the way to a novel class of integrated quantum-optical devices for on-chip quantum information processing with embedded resonantly driven quantum emitters.

Ultra-low-power hybrid light-matter solitons.

New functionalities in nonlinear optics will require systems with giant optical nonlinearity as well as compatibility with photonic circuit fabrication techniques. Here we introduce a platform based on strong light–matter coupling between waveguide photons and quantum-well excitons. On a sub-millimetre length scale we generate picosecond bright temporal solitons at a pulse energy of only 0.5 pJ. From this we deduce a nonlinear refractive index three orders of magnitude larger than in any other ultrafast system. We study both temporal and spatio-temporal nonlinear effects and observe dark–bright spatio-temporal polariton solitons. Theoretical modelling of soliton formation in the strongly coupled system confirms the experimental observations. These results show the promise of our system as a high speed, low power, integrated platform for physics and devices based on strong interactions between photons.

Exciton polaritons in semiconductor waveguides

We report the observation of strong coupling between quantum well excitons and a guided mode of a semiconductor planar waveguide by observation of anticrossing in the dispersion. Strong spatial confinement of the optical mode allows a splitting between the two polariton modes of 5–6 meV for a single quantum well. Polaritons on resonance are shown to propagate with a characteristic decay length of 280 μm and a group velocity of 26 μm ps−1. This is a promising first step towards developing an alternative to microcavities for the study of rapidly propagating polaritons, which is particularly well suited to prospective on-chip polaritonic circuit applications.

Linearly polarized emission from an embedded quantum dot using nanowire morphology control.

GaAs nanowires with elongated cross sections are formed using a catalyst-free growth technique. This is achieved by patterning elongated nanoscale openings within a silicon dioxide growth mask on a (111)B GaAs substrate. It is observed that MOVPE-grown vertical nanowires with cross section elongated in the [21̅1̅] and [1̅12] directions remain faithful to the geometry of the openings. An InGaAs quantum dot with weak radial confinement is realized within each nanowire by briefly introducing indium into the reactor during nanowire growth. Photoluminescence emission from an embedded nanowire quantum dot is strongly linearly polarized (typically >90%) with the polarization direction coincident with the axis of elongation. Linearly polarized PL emission is a result of embedding the quantum dot in an anisotropic nanowire structure that supports a single strongly confined, linearly polarized optical mode. This research provides a route to the bottom-up growth of linearly polarized single photon sources of interest for quantum information applications.

Waveguide-coupled photonic crystal cavity for quantum dot spin readout

We present a waveguide-coupled photonic crystal H1 cavity structure in which the orthogonal dipole modes couple to spatially separated photonic crystal waveguides. Coupling of each cavity mode to its respective waveguide with equal efficiency is achieved by adjusting the position and orientation of the waveguides. The behavior of the optimized device is experimentally verified for where the cavity mode splitting is larger and smaller than the cavity mode linewidth. In both cases, coupled Q-factors up to 1600 and contrast ratios up to 10 are achieved. This design may allow for spin state readout of a self-assembled quantum dot positioned at the cavity center or function as an ultra-fast optical switch operating at the single photon level.

Dark Solitons in High Velocity Waveguide Polariton Fluids

P. M. Walker, ∗ L. Tinkler, B. Royall, D. V. Skryabin, 3 I. Farrer, D. A. Ritchie, M. S. Skolnick, and D. N. Krizhanovskii Department of Physics and Astronomy, University of Sheffield, S3 7RH Sheffield, UK Department of Physics, University of Bath, BA2 7AY Bath, UK ITMO University, Kronverksky Avenue 49, St. Petersburg 197101, Russia Department of Electronic and Electrical Engineering, University of Sheffield, S3 7HQ Sheffield, UK Cavendish Laboratory, University of Cambridge, CB3 0HE Cambridge, UKWe study exciton-polariton nonlinear optical fluids in the high momentum waveguide regime for the first time. We demonstrate the formation of dark solitons with the expected dependence of width on fluid density for both main classes of soliton-forming fluid defects. The results are well described by numerical modeling of the fluid propagation. We deduce a continuous wave nonlinearity more than ten times that on picosecond time scales, arising due to interaction with the exciton reservoir.

Single-photon electroluminescence for on-chip quantum networks

An electrically driven single-photon source has been monolithically integrated with nano-photonic circuitry. Electroluminescent emission from a single InAs/GaAs quantum dot (QD) is channelled through a suspended nanobeam waveguide. The emission line has a linewidth of below 6 μeV, demonstrating the ability to have a high coherence, electrically driven, waveguide coupled QD source. The single-photon nature of the emission is verified by g(2)(τ) correlation measurements. Moreover, in a cross-correlation experiment, with emission collected from the two ends of the waveguide, the emission and propagation of single photons from the same QD is confirmed. This work provides the basis for the development of electrically driven on-chip single-photon sources, which can be readily coupled to waveguide filters, directional couplers, phase shifters, and other elements of quantum photonic networks.

Backward Cherenkov radiation emitted by polariton solitons in a microcavity wire

Exciton-polaritons in semiconductor microcavities form a highly nonlinear platform to study a variety of effects interfacing optical, condensed matter, quantum and statistical physics. We show that the complex polariton patterns generated by picosecond pulses in microcavity wire waveguides can be understood as the Cherenkov radiation emitted by bright polariton solitons, which is enabled by the unique microcavity polariton dispersion, which has momentum intervals with positive and negative group velocities. Unlike in optical fibres and semiconductor waveguides, we observe that the microcavity wire Cherenkov radiation is predominantly emitted with negative group velocity and therefore propagates backwards relative to the propagation direction of the emitting soliton. We have developed a theory of the microcavity wire polariton solitons and of their Cherenkov radiation and conducted a series of experiments, where we have measured polariton-soliton pulse compression, pulse breaking and emission of the backward Cherenkov radiation.Cherenkov radiation is generated from fast electrons and its optical analogue plays an important role in many nonlinear optical effects. Here, Skryabin et al. demonstrate backward-propagating photonic Cherenkov radiation generated from solitons in a semiconductor exciton-polariton system.

Electrical control of nonlinear quantum optics in a nano-photonic waveguide

Quantum photonics is a rapidly developing platform for future quantum network applications. Waveguide-based architectures, in which embedded quantum emitters act as both nonlinear elements to mediate photon–photon interactions and as highly coherent single-photon sources, offer a highly promising route to realize such networks. A key requirement for the scale-up of the waveguide architecture is local control and tunability of individual quantum emitters. Here, we demonstrate electrical control, tuning, and switching of the nonlinear photon–photon interaction arising due to a quantum dot embedded in a single-mode nano-photonic waveguide. A power-dependent waveguide transmission extinction as large as 40±2% is observed on resonance. Photon statistics measurements show clear, voltage-controlled bunching of the transmitted light and antibunching of the reflected light, demonstrating the single-photon, quantum character of the nonlinearity. Importantly, the same architecture is also shown to act as a source of highly coherent, electrically tunable single photons. Overall, the platform presented addresses the essential requirements for the implementation of photonic gates for scalable nano-photonic-based quantum information processing.