Lifetimes and quantum efficiencies of quantum dots deterministically positioned in photonic-crystal waveguides

Xiao-Liu Chu, Tommaso Pregnolato, Rüdiger Schott, Andreas D. Wieck, Arne Ludwig, Nir Rotenberg, Peter Lodahl

Interfacing single emitters and photonic nanostructures enables modifying their emission properties, such as enhancing individual decay rates or controlling the emission direction. To achieve full control, the single emitter must be positioned in the nanostructures deterministically. Here, we use spectroscopy to gain spectral and spatial information about individual quantum dots in order to position each emitter in a pre-determined location in a unit cell of a photonic-crystal waveguide. Depending on the spatial and spectral positioning within the structured nanophotonic mode, we observe that the quantum dot emission can either be suppressed or enhanced. These results demonstrate the capacity of photonic-crystal waveguides to control the emission of single photons and that the ability to position quantum dots will be crucial to the creation of complex multi-emitter quantum photonic circuits.

Experimental reconstruction of the few-photon nonlinear scattering matrix from a single quantum dot in a nanophotonic waveguide

Hanna Le Jeannic, Tomás Ramos, Signe F. Simonsen, Tommaso Pregnolato, Zhe Liu, Rüdiger Schott, Andreas D. Wieck, Arne Ludwig, Nir Rotenberg, Juan José García-Ripoll, Peter Lodahl

Coherent photon-emitter interfaces offer a way to mediate efficient nonlinear photon-photon interactions, much needed for quantum information processing. Here we experimentally study the case of a two-level emitter, a quantum dot, coupled to a single optical mode in a nanophotonic waveguide. We carry out few-photon transport experiments and record the statistics of the light to reconstruct the scattering matrix elements of 1- and 2-photon components. This provides direct insight to the complex nonlinear photon interaction that contains rich many-body physics.

Roadmap on Neuromorphic Photonics

Daniel Brunner, Bhavin J. Shastri, Mohammed A. Al Qadasi, H. Ballani, Sylvain Barbay, Stefano Biasi, Peter Bienstman, Simon Bilodeau, Wim Bogaerts, Fabian Böhm, G. Brennan, Sonia Buckley, Xinlun Cai, Marcello Calvanese Strinati, B. Canakci, Benoit Charbonnier, Mario Chemnitz, Yitong Chen, Stanley Cheung, Jeff Chiles, Suyeon Choi, Demetrios N. Christodoulides, Lukas Chrostowski, J. Chu, J. H. Clegg, D. Cletheroe, Claudio Conti, Qionghai Dai, Luigi Di Lauro, Nikolaos Panteleimon Diamantopoulos, Niyazi Ulas Dinc, Jacob Ewaniuk, Shanhui Fan, Lu Fang, Riccardo Franchi, Pedro Freire, Silvia Gentilini, Sylvain Gigan, Gian Luca Giorgi, C. Gkantsidis, J. Gladrow, Elena Goi, M. Goldmann, A. Grabulosa, Min Gu, Xianxin Guo, Matěj Hejda, F. Horst, Jih Liang Hsieh, Jianqi Hu, Juejun Hu, Chaoran Huang, Antonio Hurtado, Lina Jaurigue, K. P. Kalinin, Morteza Kamalian Kopae, D. J. Kelly, Mercedeh Khajavikhan, H. Kremer, Jeremie Laydevant, Joshua C. Lederman, Jongheon Lee, Daan Lenstra, Gordon H. Y. Li, Mo Li, Yuhang Li, Xing Lin, Zhongjin Lin, Mieszko Lis, Kathy Lüdge, Alessio Lugnan, Alessandro Lupo, A. I. Lvovsky, Egor Manuylovich, Alireza Marandi, Federico Marchesin, Serge Massar, Adam N. McCaughan, Peter L. McMahon, Miltiadis Moralis Pegios, Roberto Morandotti, Christophe Moser, David J. Moss, Avilash Mukherjee, Mahdi Nikdast, B. J. Offrein, Ilker Oguz, Bakhrom Oripov, G. O'Shea, Aydogan Ozcan, F. Parmigiani, Sudeep Pasricha, Fabio Pavanello, Lorenzo Pavesi, Nicola Peserico, L. Pickup, Davide Pierangeli, Nikos Pleros, Xavier Porte, Bryce A. Primavera, Paul Prucnal, Demetri Psaltis, Lukas Puts, Fei Qiao, B. Rahmani, Fabrice Raineri, Carlos A. Ríos Ocampo, Joshua Robertson, Bruno Romeira, Charles Roques Carmes, Nir Rotenberg, A. Rowstron, Steffen Schoenhardt, Russell L . T. Schwartz, Jeffrey M. Shainline, Sudip Shekhar, Anas Skalli, Mandar M. Sohoni, Volker J. Sorger, Miguel C. Soriano, James Spall, Ripalta Stabile, Birgit Stiller, Satoshi Sunada, Anastasios Tefas, Bassem Tossoun, Apostolos Tsakyridis, Sergei K. Turitsyn, Guy Van der Sande, Thomas Van Vaerenbergh, Daniele Veraldi, Guy Verschaffelt, E. A. Vlieg, Hao Wang, Tianyu Wang, Gordon Wetzstein, Logan G. Wright, Changming Wu, Chu Wu, Jiamin Wu, Fei Xia, Xingyuan Xu, Hangbo Yang, Weiming Yao, Mustafa Yildirim, S. J. Ben Yoo, Nathan Youngblood, Roberta Zambrini, Haiou Zhang, Weipeng Zhang

Quantum optics with near lifetime-limited quantum-dot transitions in a nanophotonic waveguide

Henri Thyrrestrup, Gabija Kiršanskė, Hanna Le Jeannic, Tommaso Pregnolato, Liang Zhai, Laust Raahauge, Leonardo Midolo, Nir Rotenberg, Alisa Javadi, Rüdiger Schott, Andreas D. Wieck, Arne Ludwig, Matthias C. Löbl, Immo Söllner, Richard J. Warburton, Peter Lodahl

Establishing a highly efficient photon-emitter interface where the intrinsic linewidth broadening is limited solely by spontaneous emission is a key step in quantum optics. It opens a pathway to coherent light-matter interaction for, e.g., the generation of highly indistinguishable photons, few-photon optical nonlinearities, and photon-emitter quantum gates. However, residual broadening mechanisms are ubiquitous and need to be combated. For solid-state emitters charge and nuclear spin noise is of importance and the influence of photonic nanostructures on the broadening has not been clarified. We present near lifetime-limited linewidths for quantum dots embedded in nanophotonic waveguides through a resonant transmission experiment. It is found that the scattering of single photons from the quantum dot can be obtained with an extinction of $66 \pm 4 \%$, which is limited by the coupling of the quantum dot to the nanostructure rather than the linewidth broadening. This is obtained by embedding the quantum dot in an electrically-contacted nanophotonic membrane. A clear pathway to obtaining even larger single-photon extinction is laid out, i.e., the approach enables a fully deterministic and coherent photon-emitter interface in the solid state that is operated at optical frequencies.

Optimizing the chiral Purcell factor for unidirectional single photon emitters in topological photonic crystal waveguides using inverse design

Eric Nussbaum, Nir Rotenberg, Stephen Hughes

We present an inverse design approach to significantly improve the figures-of-merit for chiral photon elements and quantum emitters in topological photonic crystal slab waveguides. Beginning with a topological waveguide mode with a group index of approximately 10 and a maximum forwards or backwards Purcell factor at a chiral point of less than 0.5, we perform optimizations of the directional Purcell factor. We use a fully three dimensional guided-mode expansion method to efficiently calculate waveguide band dispersion properties and modes, while automatic differentiation is employed to calculate the gradient of objective functions. We present two example improved designs: (i) a topological mode with an accessible group index of approximately 30 and a maximum unidirectional Purcell factor at a chiral point greater then 4.5 representing a nearly 10-fold improvement to the Purcell factor, and (ii) a slow light mode, well away from the Brillouin zone edge with a group index greater then 350 and a maximum unidirectional Purcell factor at a chiral point greater than 45.

Chiral quantum optics in broken-symmetry and topological photonic crystal waveguides

Nils Hauff, Stephen Hughes, Hanna Le Jeannic, Peter Lodahl, Nir Rotenberg

On-chip chiral quantum light-matter interfaces, which support directional interactions, provide a promising platform for efficient spin-photon coupling, non-reciprocal photonic elements, and quantum logic architectures. We present full-wave three-dimensional calculations to quantify the performance of conventional and topological photonic crystal waveguides as chiral emitter-photon interfaces. Specifically, the ability of these structures to support and enhance directional interactions while suppressing subsequent backscattering losses is quantified. Broken symmetry waveguides, such as the non-topological glide-plane waveguide and topological bearded interface waveguide are found to act as efficient chiral interfaces, with the topological waveguide modes allowing for operation at significantly higher Purcell enhancement factors. Finally, although all structures suffer from backscattering losses due to fabrication imperfections, these are found to be smaller at high enhancement factors for the topological waveguide. These reduced losses occur because the optical mode is pushed away from the air-dielectric interfaces where scattering occurs, and not because of any topological protection. These results are important to the understanding of light-matter interactions in topological photonic crystals and to the design of efficient, on-chip chiral quantum devices.

Coherent nonlinear optics of quantum emitters in nanophotonic waveguides

Pierre Türschmann, Hanna Le Jeannic, Signe F. Simonsen, Harald R. Haakh, Stephan Götzinger, Vahid Sandoghdar, Peter Lodahl, Nir Rotenberg

Coherent quantum optics, where the interaction of a photon with an emitter does not scramble phase coherence, lies at the heart of many quantum optical effects and emerging technologies. Solid-state emitters coupled to nanophotonic waveguides are a promising platform for quantum devices, as this combination is scalable. Yet, reaching full coherence in these systems is challenging due to the dynamics of the solid-state environment of the emitters. Here, we review progress towards coherent light-matter interactions with solid-state quantum emitters coupled to nanophotonic waveguides. We first lay down the theoretical foundation for coherent and nonlinear light-matter interactions of a two-level system in a quasi-one-dimensional system, and then benchmark experimental realizations. We then discuss higher-order nonlinearities that arise due to the addition of photons of different frequencies, more complex energy-level schemes of the emitters, and the coupling of multiple emitters via a shared photonic mode. Throughout, we highlight protocols for applications and novel effects that are based on these coherent interactions, the steps taken towards their realization, and the challenges that remain to be overcome.

An integrated whispering-gallery-mode resonator for solid-state coherent quantum photonics

Arianne Brooks, Xiao-Liu Chu, Zhe Liu, Rudiger Schott, Arne Ludwig, Andreas D. Wieck, Leonardo Midolo, Peter Lodahl, Nir Rotenberg

Tailored photonic cavities allow enhancing light-matter interaction ultimately to create a fully coherent quantum interface. Here, we report on an integrated microdisk cavity containing self-assembled quantum dots to coherently route photons between different access waveguides. We measure a Purcell factor of $F_{exp}=6.9\pm0.9$ for a cavity quality factor of about 10,000, allowing us to observe clear signatures of coherent scattering of photons by the quantum dots. We show how this integrated system can coherently re-route photons between the drop and bus ports, and how this routing is controlled by detuning the quantum dot and resonator, or through the strength of the excitation beam, where a critical photon number less than one photon per lifetime is required. We discuss the strengths and limitations of this approach, focusing on how the coherent scattering and single-photon nonlinearity can be used to increase the efficiency of quantum devices such as routers or Bell-state analyzers.

Realistic quantum photonic neural networks

Jacob Ewaniuk, Jacques Carolan, Bhavin J. Shastri, Nir Rotenberg

Quantum photonic neural networks are variational photonic circuits that can be trained to implement high-fidelity quantum operations. However, work-to-date has assumed idealized components, including a perfect $π$ Kerr nonlinearity. Here, we investigate the limitations of realistic quantum photonic neural networks that suffer from fabrication imperfections leading to photon loss and imperfect routing, and weak nonlinearities, showing that they can learn to overcome most of these errors. Using the example of a Bell-state analyzer, we demonstrate that there is an optimal network size, which balances imperfections versus the ability to compensate for lacking nonlinearities. With a sub-optimal $π/10$ effective Kerr nonlinearity, we show that a network fabricated with current state-of-the-art processes can achieve an unconditional fidelity of 0.891, that increases to 0.999999 if it is possible to precondition success on the detection of a photon in each logical photonic qubit. Our results provide a guide to the construction of viable, brain-inspired quantum photonic devices for emerging quantum technologies.

Deterministic positioning of nanophotonic waveguides around single self-assembled quantum dots

Tommaso Pregnolato, Xiao-Liu Chu, Tim Schröder, Rüdiger Schott, Andreas D. Wieck, Arne Ludwig, Peter Lodahl, Nir Rotenberg

The capability to embed self-assembled quantum dots (QDs) at predefined positions in nanophotonic structures is key to the development of complex quantum photonic architectures. Here, we demonstrate that QDs can be deterministically positioned in nanophotonic waveguides by pre-locating QDs relative to a global reference frame using micro-photoluminescence ($μ$PL) spectroscopy. After nanofabrication, $μ$PL images reveal misalignments between the central axis of the waveguide and the embedded QD of only $(9\pm46$) nm and $(1\pm33$) nm, for QDs embedded in undoped and doped membranes, respectively. A priori knowledge of the QD positions allows us to study the spectral changes introduced by nanofabrication. We record average spectral shifts ranging from 0.1 to 1.1 nm, indicating that the fabrication-induced shifts can generally be compensated by electrical or thermal tuning of the QDs. Finally, we quantify the effects of the nanofabrication on the polarizability, the permanent dipole moment and the emission frequency at vanishing electric field of different QD charge states, finding that these changes are constant down to QD-surface separations of only 70 nm. Consequently, our approach deterministically integrates QDs into nanophotonic waveguides whose light-fields contain nanoscale structure and whose group index varies at the nanometer level.

Small slot waveguide rings for on-chip quantum optical circuits

Nir Rotenberg, Pierre Tuerschmann, Harald Haakh, Diego Martin-Cano, Stephan Goetzinger, Vahid Sandoghdar

Nanophotonic interfaces between single emitters and light promise to enable new quantum optical technologies. Here, we use a combination of finite element simulations and analytic quantum theory to investigate the interaction of various quantum emitters with slot-waveguide rings. We predict that for rings with radii as small as 1.44 $μ$m (Q = 27,900), near-unity emitter-waveguide coupling efficiencies and emission enhancements on the order of 1300 can be achieved. By tuning the ring geometry or introducing losses, we show that realistic emitter-ring systems can be made to be either weakly or strongly coupled, so that we can observe Rabi oscillations in the decay dynamics even for micron-sized rings. Moreover, we demonstrate that slot waveguide rings can be used to directionally couple emission, again with near-unity efficiency. Our results pave the way for integrated solid-state quantum circuits involving various emitters.

Hybrid Quantum-Classical Photonic Neural Networks

Tristan Austin, Simon Bilodeau, Andrew Hayman, Nir Rotenberg, Bhavin Shastri

Neuromorphic (brain-inspired) photonics leverages photonic chips to accelerate artificial intelligence, offering high-speed and energy efficient solutions in RF communication, tensor processing, and data classification. However, the limited physical size of integrated photonic hardware constrains network complexity and computational capacity. In light of recent advances in photonic quantum technology, it is natural to utilize quantum exponential speedup to scale photonic neural network capabilities. Here we show a combination of classical network layers with trainable continuous variable quantum circuits yields hybrid networks with improved trainability and accuracy. On a classification task, hybrid networks achieve the same performance when benchmarked against fully classical networks that are twice the size. When the bit precision of the optimized networks is reduced through added noise, the hybrid networks still achieve greater accuracy when evaluated at state of the art bit precision. These hybrid quantum classical networks demonstrate a unique route to improve computational capacity of integrated photonic neural networks without increasing the physical network size.

Approaching transform-limited linewidths in telecom-wavelength transitions of ungated quantum dots

Andrew N. Wakileh, Dan Dalacu, Philip J. Poole, Boris Lamontagne, Simona Moisa, Robin L. Williams, Nir Rotenberg

Highly coherent quantum emitters operating in the telecommunication C-band (1530 - 1565nm), where ultra-low-loss fibers and photonic circuits are available, are crucial to the development of scalable quantum technologies. In this work, we report on a modified Stranski-Krastanov growth scheme using chemical beam epitaxy to enable the generation of high-quality InAs/InP quantum dots, characterized by near-transform-limited linewidths ($Γ_{\mathrm{TL}}$). We demonstrate the growth of highly-symmetric quantum dots with aspect ratios >0.8 and densities ranging from 2 to 22$\,μ$m$^{-2}$. Optical characterization of these sources reveal fine-structure splittings down to $25\pm4\,μ$eV and a single-photon purity of $g^{(2)}(0) = 0.012\pm\mathrm{0.007}$, confirming the quality of these dots. Further, using an etalon to measure the linewidth, in combination with rigorous modelling, we find an upper-bound to the mean, low-power linewidths of only $12.1\pm 6.7\,Γ_\mathrm{TL}$ and, in the best case, $2.8\pm 1.8\,Γ_\mathrm{TL}$. These results represent a significant step in the development of telecom-wavelength quantum light sources which are essential for complex quantum networks and devices.

Quantum photonic neural networks in time

Ivanna M. Boras Vazquez, Jacob Ewaniuk, Nir Rotenberg

We introduce the architecture and timing algorithm to realize a time-bin-encoded quantum photonic neural network (QPNN): a reconfigurable nonlinear photonic circuit inspired by the brain and trained to process quantum information. Unlike the typical spatially-encoded QPNN, time-encoded networks require the same number of photonic elements (e.g. phase shifters or switches) regardless of their size or depth. Here, we present a model of such a network and show how to include imperfections such as losses, routing errors and most notably distinguishable photons. As an example, we train the QPNN to realize a controlled-NOT gate, based on a hypothetical ideal Kerr nonlinearity. We then extend our model to a realistic two-photon nonlinearity due to scattering from a single, semiconductor quantum dot coupled to a photonic waveguide. We show that, using this realistic nonlinearity, the QPNN can be trained to act as a Bell-state analyzer which operates with a fidelity of 0.96 and at a rate only limited by losses. We further show that time gating can raise this fidelity to over 0.99, while still maintaining an efficiency exceeding 0.9. Overall, this work lays a framework for the first QPNN encoded in time, and provides a clear path to the scaling of these networks.

Sub-radiant states for imperfect quantum emitters coupled by a nanophotonic waveguide

Xiao-Liu Chu, Vasiliki Angelopoulou, Peter Lodahl, Nir Rotenberg

Coherent interactions between quantum emitters in tailored photonic structures is a fundamental building block for future quantum technologies, but remains challenging to observe in complex solid-state environments, where the role of decoherence must be considered. Here, we investigate the optical interaction between two quantum emitters mediated by one-dimensional waveguides in a realistic solid-state environment, focusing on the creation, population and detection of a sub-radiant state, in the presence of dephasing. We show that as dephasing increases, the signatures of sub-radiance quickly vanish in intensity measurements yet remain pronounced in photon correlation measurements, particularly when the two emitters are pumped separately so as to populate the sub-radiant state efficiently. The applied Green's tensor approach is used to model a photonic crystal waveguide, including the dependence on the spatial position of the integrated emitter. The work lays out a route to the experimental realization of sub-radiant states in nanophotonic waveguides containing solid-state emitters.

Independent electrical control of two quantum dots coupled through a photonic-crystal waveguide

Xiao-Liu Chu, Camille Papon, Nikolai Bart, Andreas D. Wieck, Arne Ludwig, Leonardo Midolo, Nir Rotenberg, Peter Lodahl

Efficient light-matter interaction at the single-photon level is of fundamental importance in emerging photonic quantum technology. A fundamental challenge is addressing multiple quantum emitters at once, as intrinsic inhomogeneities of solid-state platforms require individual tuning of each emitter. We present the realization of two semiconductor quantum dot emitters that are efficiently coupled to a photonic-crystal waveguide and individually controllable by applying a local electric Stark field. We present resonant transmission and fluorescence spectra in order to probe the coupling of the two emitters to the waveguide. We exploit the single-photon stream from one quantum dot to perform spectroscopy on the second quantum dot positioned 16$μ$m away in the waveguide. Furthermore, power-dependent resonant transmission measurements reveals signatures of coherent coupling between the emitters. Our work provides a scalable route to realizing multi-emitter collective coupling, which has inherently been missing for solid-state deterministic photon emitters.

Dynamical photon-photon interaction mediated by a quantum emitter

Hanna Le Jeannic, Alexey Tiranov, Jacques Carolan, Tomás Ramos, Ying Wang, Martin H. Appel, Sven Scholz, Andreas D. Wieck, Arne Ludwig, Nir Rotenberg, Leonardo Midolo, Juan José García-Ripoll, Anders S. Sørensen, Peter Lodahl

Single photons constitute a main platform in quantum science and technology: they carry quantum information over extended distances in the future quantum internet and can be manipulated in advanced photonic circuits enabling scalable photonic quantum computing. The main challenge in quantum photonics is how to generate advanced entangled resource states and efficient light-matter interfaces. Here we utilize the efficient and coherent coupling of a single quantum emitter to a nanophotonic waveguide for realizing quantum nonlinear interaction between single-photon wavepackets. This inherently multimode quantum system constitutes a new research frontier in quantum optics. We demonstrate control of a photon with another photon and experimentally unravel the dynamical response of two-photon interactions mediated by a quantum emitter, and show that the induced quantum correlations are controlled by the pulse duration. The work will open new avenues for tailoring complex photonic quantum resource states.

Large-Scale Tree-Type Photonic Cluster State Generation with Recurrent Quantum Photonic Neural Networks

Jacob Ewaniuk, Bhavin J. Shastri, Nir Rotenberg

Large, multi-dimensional clusters of entangled photons are among the most powerful resources for emerging quantum technologies, as they are predicted to enable global quantum networks or universal quantum computation. Here, we propose an entirely new architecture and protocol for their generation based on recurrent quantum photonic neural networks (QPNNs) and focusing on tree-type cluster states. Unlike other approaches, QPNN-based generators are not limited by the the coherence of quantum emitters or by probabilistic multi-photon operations, enabling arbitrary scaling only limited by loss (which, unavoidably, also affects all other methods). We show that a single QPNN can learn to perform all of the many different operations needed to create a cluster state, from photon routing to entanglement generation, all with near-perfect fidelity and at loss-limited rates, even when it is created from imperfect photonic components. Although these losses ultimately place a limit on the size of the cluster states, we show that state-of-the-art photonics should already allow for clusters of 60 photons, which can grow into the 100s with modest improvements to losses. Finally, we present an analysis of a one-way quantum repeater based on these states, determining the requisite platform quality for a global quantum network and highlighting the potential of the QPNN to play a vital role in high-impact quantum technologies.

Chiral emission into nanophotonic resonators

Diego Martín-Cano, Harald R. Haakh, Nir Rotenberg

Chiral emission, where the handedness of a transition dipole determines the direction in which a photon is emitted, has recently been observed from atoms and quantum dots coupled to nanophotonic waveguides. Here, we consider the case of chiral light-matter interactions in resonant nanophotonic structures, deriving closed-form expressions for the fundamental quantum electrodynamic quantities that describe these interactions. We show how parameters such as the position dependent, directional Purcell factors and mode volume can be calculated using computationally efficient two dimensional eigenmode simulations. As an example, we calculate these quantities for a prototypical ring resonator with a geometric footprint of only 4.5~$μ$m$^2$, showing that perfect directionality with a simultaneous Purcell enhancement upwards of 400 are possible. The ability to determine these fundamental properties of nanophotonic chiral interfaces is crucial if they are to form elements of quantum circuits and networks.

On-demand single photon emission in the telecom C-band from nanowire-based quantum dots

Andrew N. Wakileh, Lingxi Yu, Doğa Dokuz, Sofiane Haffouz, Xiaohua Wu, Jean Lapointe, David B. Northeast, Robin L. Williams, Nir Rotenberg, Philip J. Poole, Dan Dalacu

Single photon sources operating on-demand at telecom wavelengths are required in fiber-based quantum secure communication technologies. In this work we demonstrate single photon emission from position-controlled nanowire quantum dots emitting at λ > 1530 nm. Using above-band pulsed excitation, we obtain single photon purities of g(2)(0) = 0.062. These results represent an important step towards the scalable manufacture of high efficiency, high rate single photon emitters in the telecom C-band.