John P. Philbin, Joseph Kelly, Lintao Peng, Igor Coropceanu, Abhijit Hazarika, Dmitri V. Talapin, Eran Rabani, Xuedan Ma, and Prineha Narang. 4/13/2021. “Room temperature single-photon superfluorescence from a single epitaxial cuboid nano-heterostructure.” arXiv. Publisher's VersionAbstract
Single-photon superradiance can emerge when a collection of identical emitters are spatially separated by distances much less than the wavelength of the light they emit, and is characterized by the formation of a superradiant state that spontaneously emits light with a rate that scales linearly with the number of emitters. This collective phenomena has only been demonstrated in a few nanomaterial systems, all requiring temperatures below 10K. Here, we rationally design a single colloidal nanomaterial that hosts multiple (nearly) identical emitters that are impervious to the fluctuations which typically inhibit room temperature superradiance in other systems such as molecular aggregates. Specifically, by combining molecular dynamics, atomistic electronic structure calculations, and model Hamiltonian methods, we show that the faces of a heterostructure nanocuboid mimic individual quasi-2D nanoplatelets and can serve as the robust emitters required to realize superradiant phenomena at room temperature. Leveraging layer-by-layer colloidal growth techniques to synthesize a nanocuboid, we demonstrate single-photon superfluorescence via single-particle time-resolved photoluminescence measurements at room temperature. This robust observation of both superradiant and subradiant states in single nanocuboids opens the door to ultrafast single-photon emitters and provides an avenue to entangled multi-photon states via superradiant cascades.
Dominik M. Juraschek and Prineha Narang. 3/11/2021. “Highly confined phonon polaritons in monolayers of oxide perovskites.” arXiv. Publisher's VersionAbstract
Two-dimensional (2D) materials are able to strongly confine light hybridized with collective excitations of atoms, enabling electric-field enhancements and novel spectroscopic applications. Recently, freestanding monolayers of oxide perovskites have been synthesized, which possess highly infrared-active phonon modes and a complex interplay of competing interactions. In this study, we evaluate central figures of merit for phonon polaritons in the tetragonal phases of the 2D perovskites SrTiO3, KTaO3, and LiNbO3, using density functional theory calculations. Specifically, we compute the 2D phonon-polariton dispersions, the propagation-quality, confinement, and deceleration factors, and we show that they are comparable to those found in the prototypical 2D dielectric hexagonal boron nitride. Our results suggest that monolayers of oxide perovskites are promising candidates for polaritonic platforms in the terahertz spectral range that enable possibilites to control complex phases of matter through strongly enhanced electromagnetic fields.
Derek S. Wang, Tomáš Neuman, and Prineha Narang. 3/10/2021. “Spin emitters beyond the Point Dipole Approximation in Nanomagnonic Cavities.” The Journal of Physical Chemistry C. Publisher's VersionAbstract
Control over transition rates between spin states of emitters is crucial in a wide variety of fields ranging from quantum information science to the nanochemistry of free radicals. We present an approach to drive both electric and magnetic dipole-forbidden transitions of a spin emitter by placing it in a nanomagnonic cavity, requiring a description of both the spin emitter beyond the point dipole approximation and the vacuum magnetic fields of the nanomagnonic cavity with a large spatial gradient over the volume of the spin emitter. We specifically study the silicon vacancy (SiV) defect in diamond, whose Zeeman-split ground states comprise a logical qubit for solid-state quantum information processing, coupled to a magnetic nanoparticle serving as a model nanomagnonic cavity capable of concentrating microwave magnetic fields into deeply subwavelength volumes. Through first-principles modeling of the SiV spin orbitals, we calculate the spin transition densities of magnetic dipole-allowed and -forbidden transitions and calculate their coupling rates to various multipolar modes of the nanomagnonic cavity. We envision using such a framework for manipulation of quantum spin states.
Derek S. Wang, Tomáš Neuman, Johannes Flick, and Prineha Narang. 3/9/2021. “Light-matter interaction of a molecule in a dissipative cavity from first principles.” The Journal of Chemical Physics, 154, Pp. 104109. Publisher's VersionAbstract
Cavity-mediated light-matter coupling can dramatically alter opto-electronic and physico-chemical properties of a molecule. Ab initio theoretical predictions of these systems need to combine non-perturbative, many-body electronic structure theory-based methods with cavity quantum electrodynamics and theories of open quantum systems. Here we generalize quantum-electrodynamical density functional theory to account for dissipative dynamics and describe coupled cavity-molecule interactions in the weak-to-strong-coupling regimes. Specifically, to establish this generalized technique, we study excited-state dynamics and spectral responses of benzene and toluene under weak-to-strong light-matter coupling. By tuning the coupling we achieve cavity-mediated energy transfer between electronic excited states. This generalized ab initio quantum-electrodynamical density functional theory treatment can be naturally extended to describe cavity-mediated interactions in arbitrary electromagnetic environments, accessing correlated light-matter observables and thereby closing the gap between electronic structure theory and quantum optics.
Nicholas R. Poniatowski, Jonathan B. Curtis, Amir Yacoby, and Prineha Narang. 3/9/2021. “Spectroscopic signatures of time-reversal symmetry breaking superconductivity.” arXiv. Publisher's VersionAbstract
The collective mode spectrum of a symmetry-breaking state, such as a superconductor, provides crucial insight into the nature of the order parameter. In this context, we present a microscopic weak-coupling theory for the collective modes of a generic multi-component time-reversal symmetry breaking superconductor, and show that fluctuations in the relative amplitude and phase of the two order parameter components are well-defined underdamped collective modes, even in the presence of nodal quasiparticles. We then demonstrate that these "generalized clapping modes" can be detected using a number of experimental techniques including ac electronic compressibility measurements, ultrafast THz spectroscopy, and microwave power absorption. Finally, we discuss the implications of our work as a new form of "collective mode spectroscopy" that drastically expands the number of experimental probes capable of detecting time-reversal symmetry breaking in unconventional superconductors such as Sr2RuO4, UTe2, and moiré heterostructures.
Dominik M. Juraschek, Derek S. Wang, and Prineha Narang. 3/4/2021. “Sum-frequency excitation of coherent magnons.” Physical Review B, 103, 9, Pp. 094407. Publisher's VersionAbstract
Coherent excitation of magnons is conventionally achieved through Raman scattering processes, in which the difference-frequency components of the driving field are resonant with the magnon energy. Here, we describe mechanisms by which the sum-frequency components of the driving field can be used to coherently excite magnons through two-particle absorption processes. We use the Landau-Lifshitz-Gilbert formalism to compare the spin-precession amplitudes that different types of impulsive stimulated and ionic Raman scattering processes and their sum-frequency counterparts induce in an antiferromagnetic model system. We show that sum-frequency mechanisms enabled by linearly polarized driving fields yield excitation efficiencies comparable or larger than established Raman techniques, while elliptical polarizations produce only weak and circularly polarizations no sum-frequency components at all. The mechanisms presented here complete the map for dynamical spin control by the means of Raman-type processes.
Kate Reidy, Georgios Varnavides, Joachim Dahl Thomsen, Abinash Kumar, Thang Pham, Arthur M. Blackburn, Polina Anikeeva, Prineha Narang, James M. LeBeau, and Frances M. Ross. 2/26/2021. “Direct imaging and electronic structure modulation of moiré superlattices at the 2D/3D interface.” Nature Communications, 12, Pp. 1290. Publisher's VersionAbstract
The atomic structure at the interface between a two-dimensional (2D) and a three-dimensional (3D) material influences properties such as contact resistance, photo-response, and high-frequency performance. Moiré engineering has yet to be explored for tailoring this 2D/3D interface, despite its success in enabling correlated physics at 2D/2D twisted van der Waals interfaces. Using epitaxially aligned MoS2 /Au{111} as a model system, we apply a geometric convolution technique and four-dimensional scanning transmission electron microscopy (4D STEM) to show that the 3D nature of the Au structure generates two coexisting moiré periods (18 Angstroms and 32 Angstroms) at the 2D/3D interface that are otherwise hidden in conventional electron microscopy imaging. We show, via ab initio electronic structure calculations, that charge density is modulated with the longer of these moiré periods, illustrating the potential for (opto-)electronic modulation via moiré engineering at the 2D/3D interface.
Kade Head-Marsden, Stefan Krastanov, David A. Mazziotti, and Prineha Narang. 2/25/2021. “Capturing non-Markovian dynamics on near-term quantum computers.” Physical Review Research, 3, 1, Pp. 013182. Publisher's VersionAbstract
With the rapid progress in quantum hardware, there has been an increased interest in new quantum algorithms to describe complex many-body systems searching for the still-elusive goal of “useful quantum advantage.” Surprisingly, quantum algorithms for the treatment of open quantum systems (OQSs) have remained underexplored, in part due to the inherent challenges of mapping non-unitary evolution into the framework of unitary gates. Evolving an open system unitarily necessitates dilation into a new effective system to incorporate critical environmental degrees of freedom. In this context, we present and validate a new quantum algorithm to treat non-Markovian dynamics in OQSs built on the ensemble of Lindblad's trajectories approach, invoking the Sz.-Nagy dilation theorem. Here we demonstrate our algorithm on the Jaynes-Cummings model in the strong-coupling and detuned regimes, relevant in quantum optics and driven quantum system studies. This algorithm, a key step towards generalized modeling of non-Markovian dynamics on a noisy-quantum device, captures a broad class of dynamics and opens up a new direction in OQS problems.
Conor Delaney, Kaushik P. Seshadreesan, Ian MacCormack, Alexey Galda, Saikat Guha, and Prineha Narang. 2/25/2021. “Demonstration of quantum advantage by a joint detection receiver for optical communications using quantum belief propagation on a trapped-ion device.” arXiv. Publisher's VersionAbstract
Demonstrations of quantum advantage have largely focused on computational speedups and on quantum simulation of many-body physics, limited by fidelity and capability of current devices. Discriminating laser-pulse-modulated classical-communication codewords at the minimum allowable probability of error using universal-quantum processing presents a promising parallel direction, one that is of both fundamental importance in quantum state discrimination, as well as of technological relevance in deep-space laser communications. Here we present an experimental realization of a quantum joint detection receiver for binary phase shift keying modulated codewords of a 3-bit linear tree code using a recently-proposed quantum algorithm: belief propagation with quantum messages. The receiver, translated to a quantum circuit, was experimentally implemented on a trapped-ion device -- the recently released Honeywell LT-1.0 system using 171Yb+ ions, which possesses all-to-all connectivity and mid-circuit measurement capabilities that are essential to this demonstration. We conclusively realize a previously postulated but hitherto not-demonstrated joint quantum detection scheme, and provide an experimental framework that surpasses the quantum limit on the minimum average decoding error probability associated with pulse-by-pulse detection in the low mean photon number limit. The full joint-detection scheme bridges across photonic and trapped-ion based quantum information science, mapping the photonic coherent states of the modulation alphabet onto inner product-preserving states of single-ion qubits. Looking ahead, our work opens new avenues in hybrid realizations of quantum-enhanced receivers with applications in astronomy and emerging space-based platforms
Derek S. Wang, Christopher J. Ciccarino, Johannes Flick, and Prineha Narang. 2/18/2021. “Hybridized Defects in Solid-State Materials as Artificial Molecules.” ACS Nano. Publisher's VersionAbstract
Two-dimensional materials can be crafted with structural precision approaching the atomic scale, enabling quantum defects-by-design. These defects are frequently described as artificial atoms and are emerging optically-addressable spin qubits. However, interactions and coupling of such artificial atoms with each other, in the presence of the lattice, is remarkably underexplored. Here we present the formation of artificial molecules in solids, introducing a new degree of freedom in control of quantum optoelectronic materials. Specifically, in monolayer hexagonal boron nitride as our model system, we observe configuration- and distance-dependent dissociation curves and hybridization of defect orbitals within the bandgap into bonding and antibonding orbitals, with splitting energies ranging from ∼ 10 meV to nearly 1 eV. We calculate the energetics of cis and trans out-of-plane defect pairs CHB-CHB against an in-plane defect pair CB-CB and find that in-plane defect pair interacts more strongly than out-of-plane pairs. We demonstrate an application of this chemical degree of freedom by varying the distance between CB and VN of CBVN and observe changes in the predicted peak absorption wavelength from the visible to the near-infrared spectral band. We envision leveraging this chemical degree of freedom of defect complexes to precisely control and tune defect properties towards engineering robust quantum memories and quantum emitters for quantum information science.
John P. Philbin, Amikam Levy, Prineha Narang, and Wenjie Dou. 2/15/2021. “Spin-Dependent Transport Through a Colloidal Quantum Dot: The Role of Exchange Interactions.” arXiv. Publisher's VersionAbstract
The study of charge and spin transport through semiconductor quantum dots is experiencing a renaissance due to recent advances in nano-fabrication and the realization of quantum dots as candidates for quantum computing. In this work, we combine atomistic electronic structure calculations with quantum master equation methods to study the transport of electrons and holes through strongly confined quantum dots coupled to two leads with a voltage bias. We find that a competition between the energy spacing between the two lowest quasiparticle energy levels and the strength of the exchange interaction determines the spin states of the lowest two quasiparticle energy levels. Specifically, the low density of electron states results in a spin singlet being the lowest energy two-electron state whereas, in contrast, the high density of states and significant exchange interaction results in a spin triplet being the lowest energy two-hole state. The exchange interaction is also responsible for spin blockades in transport properties, which could persist up to temperatures as high as 77K for strongly confined colloidal quantum dots from our calculations. Lastly, we relate these findings to the preparation and manipulation of singlet and triplet spin qubit states in quantum dots using voltage biases.
Gavin B. Osterhoudt, Vincent M. Plisson, Yaxian Wang, Christina A. C. Garcia, Johannes Gooth, Claudia Felser, Prineha Narang, and Kenneth S. Burch. 1/27/2021. “Evidence for dominant phonon-electron scattering in Weyl semimetal WP2.” Physical Review X, 11, 1, Pp. 011017. Publisher's VersionAbstract
Topological semimetals have revealed a wide array of novel transport phenomena, including electron hydrodynamics, quantum field theoretic anomalies, and extreme magnetoresistances and mobilities. However, the scattering mechanisms central to these behaviors remain largely unexplored. Here we reveal signatures of significant phonon-electron scattering in the type-II Weyl semimetal WP2 via temperature-dependent Raman spectroscopy. Over a large temperature range, we find that the decay rates of the lowest energy Amodes are dominated by phonon-electron rather than phonon-phonon scattering. In conjunction with first-principles calculations, a combined analysis of the momentum, energy, and symmetry-allowed decay paths indicates this results from intraband scattering of the electrons. The excellent agreement with theory further suggests that such results could be true for the acoustic modes. We thus provide evidence for the importance of phonons in the transport properties of topological semimetals and identify specific properties that may contribute to such behavior in other materials.
Zixuan Hu, Kade Head-Marsden, David A. Mazziotti, Prineha Narang, and Sabre Kais. 1/13/2021. “A general quantum algorithm for open quantum dynamics demonstrated with the Fenna-Matthews-Olson complex.” arXiv. Publisher's VersionAbstract
Using quantum algorithms to simulate complex physical processes and correlations in quantum matter has been a major direction of quantum computing research, towards the promise of a quantum advantage over classical approaches. In this work we develop a generalized quantum algorithm to simulate any dynamical process represented by either the operator sum representation or the Lindblad master equation. We then demonstrate the quantum algorithm by simulating the dynamics of the Fenna-Matthews-Olson (FMO) complex on the IBM QASM quantum simulator. This work represents a first demonstration of a quantum algorithm for open quantum dynamics with a moderately sophisticated dynamical process involving a realistic biological structure. We discuss the complexity of the quantum algorithm relative to the classical method for the same purpose, presenting a decisive query complexity advantage of the quantum approach based on the unique property of quantum measurement. An accurate yet tractable quantum algorithm for the description of complex open quantum systems (like the FMO complex) has a myriad of significant applications from catalytic chemistry and correlated materials physics to descriptions of hybrid quantum systems.
Stefan Krastanov, Mikkel Heuck, Jeffrey H. Shapiro, Prineha Narang, Dirk R. Englund, and Kurt Jacobs. 1/8/2021. “Room-temperature photonic logical qubits via second-order nonlinearities.” Nature Communications, 12, 191. Publisher's VersionAbstract
Recent progress in nonlinear optical materials and microresonators has brought quantum computing with bulk optical nonlinearities into the realm of possibility. This platform is of great interest, not only because photonics is an obvious choice for quantum networks, but also as a promising route to quantum information processing at room temperature. We propose an approach for reprogrammable room-temperature photonic quantum logic that significantly simplifies the realization of various quantum circuits, and in particular, of error correction. The key element is the programmable photonic multi-mode resonator that implements reprogrammable bosonic quantum logic gates, while using only the bulk χ(2) nonlinear susceptibility. We theoretically demonstrate that just two of these elements suffice for a complete, compact error-correction circuit on a bosonic code, without the need for measurement or feed-forward control. Encoding and logical operations on the code are also easily achieved with these reprogrammable quantum photonic processors. An extrapolation of current progress in nonlinear optical materials and photonic circuits indicates that such circuitry should be achievable within the next decade.
Ian MacCormack, Conor Delaney, Alexey Galda, Nidhi Aggarwal, and Prineha Narang. 12/28/2020. “Branching Quantum Convolutional Neural Networks.” arXiv. Publisher's VersionAbstract
Neural network-based algorithms have garnered considerable attention in condensed matter physics for their ability to learn complex patterns from very high dimensional data sets towards classifying complex long-range patterns of entanglement and correlations in many-body quantum systems. Small-scale quantum computers are already showing potential gains in learning tasks on large quantum and very large classical data sets. A particularly interesting class of algorithms, the quantum convolutional neural networks (QCNN) could learn features of a quantum data set by performing a binary classification task on a nontrivial phase of quantum matter. Inspired by this promise, we present a generalization of QCNN, the branching quantum convolutional neural network, or bQCNN, with substantially higher expressibility. A key feature of bQCNN is that it leverages mid-circuit (intermediate) measurement results, realizable on current trapped-ion systems, obtained in pooling layers to determine which sets of parameters will be used in the subsequent convolutional layers of the circuit. This results in a branching structure, which allows for a greater number of trainable variational parameters in a given circuit depth. This is of particular use on current-day NISQ devices, where circuit depth is limited by gate noise. We present an overview of the ansatz structure and scaling, and provide evidence of its enhanced expressibility compared to QCNN. Using artificially-constructed large data sets of training states as a proof-of-concept we demonstrate the existence of training tasks in which bQCNN far outperforms an ordinary QCNN. Finally, we present future directions where the classical branching structure and increased density of trainable parameters in bQCNN would be particularly valuable.
Christina A. C. Garcia, Dennis M. Nenno, Georgios Varnavides, and Prineha Narang. 12/16/2020. “Anisotropic phonon-mediated electronic transport in chiral Weyl semimetals.” arXiv. Publisher's VersionAbstract
Discovery and observations of exotic, quantized optical and electrical responses have sparked renewed interest in nonmagnetic chiral crystals. Within this class of materials, six group V transition metal ditetrelides, that is, XY2 (X = V, Nb, Ta and Y = Si, Ge), host composite Weyl nodes on high-symmetry lines, with Kramers-Weyl fermions at time-reversal invariant momenta. In addition, at least two of these materials, NbGe2 and NbSi2, exhibit superconducting transitions at low temperatures. The interplay of strong electron-phonon interaction and complex Fermi surface topology present an opportunity to study both superconductivity and hydrodynamic electron transport in these systems. Towards this broader question, we present an ab initio theoretical study of the electronic transport and electron-phonon scattering in this family of materials, with a particular focus on NbGe2 vs. NbSi2, and the other group V ditetrelides. We shed light on the microscopic origin of NbGe2's large and anisotropic room temperature resistivity and contextualize its strong electron-phonon scattering with a presentation of other relevant scattering lifetimes, both momentum-relaxing and momentum-conserving. Our work explores the intriguing possibility of observing hydrodynamic electron transport in these chiral Weyl semimetals.
Kade Head-Marsden, Johannes Flick, Christopher J. Ciccarino, and Prineha Narang. 12/16/2020. “Quantum Information and Algorithms for Correlated Quantum Matter.” Chemical Reviews. Publisher's VersionAbstract
Discoveries in quantum materials, which are characterized by the strongly quantum-mechanical nature of electrons and atoms, have revealed exotic properties that arise from correlations. It is the promise of quantum materials for quantum information science superimposed with the potential of new computational quantum algorithms to discover new quantum materials that inspires this Review. We anticipate that quantum materials to be discovered and developed in the next years will transform the areas of quantum information processing including communication, storage, and computing. Simultaneously, efforts toward developing new quantum algorithmic approaches for quantum simulation and advanced calculation methods for many-body quantum systems enable major advances toward functional quantum materials and their deployment. The advent of quantum computing brings new possibilities for eliminating the exponential complexity that has stymied simulation of correlated quantum systems on high-performance classical computers. Here, we review new algorithms and computational approaches to predict and understand the behavior of correlated quantum matter. The strongly interdisciplinary nature of the topics covered necessitates a common language to integrate ideas from these fields. We aim to provide this common language while weaving together fields across electronic structure theory, quantum electrodynamics, algorithm design, and open quantum systems. Our Review is timely in presenting the state-of-the-art in the field toward algorithms with nonexponential complexity for correlated quantum matter with applications in grand-challenge problems. Looking to the future, at the intersection of quantum information science and algorithms for correlated quantum matter, we envision seminal advances in predicting many-body quantum states and describing excitonic quantum matter and large-scale entangled states, a better understanding of high-temperature superconductivity, and quantifying open quantum system dynamics.
Maarten R. van Delft, Yaxian Wang, Carsten Putzke, Jacopo Oswald, Georgios Varnavides, Christina A. C. Garcia, Chunyu Guo, Heinz Schmid, Vicky Süss, Horst Borrmann, Jonas Diaz, Yan Sun, Claudia Felser, Bernd Gotsmann, Prineha Narang, and Philip J.W. Moll. 12/15/2020. “Sondheimer oscillations as a probe of non-ohmic flow in type-II Weyl semimetal WP2.” arXiv. Publisher's VersionAbstract
As conductors in electronic applications shrink, microscopic conduction processes lead to strong deviations from Ohm's law. Depending on the length scales of momentum conserving (lMC) and relaxing (lMR) electron scattering, and the device size (d), current flows may shift from ohmic to ballistic to hydrodynamic regimes and more exotic mixtures thereof. So far, an in situ, in-operando methodology to obtain these parameters self-consistently within a micro/nanodevice, and thereby identify its conduction regime, is critically lacking. In this context, we exploit Sondheimer oscillations, semi-classical magnetoresistance oscillations due to helical electronic motion, as a method to obtain lMR in micro-devices even when lMR≫d. This gives information on the bulk lMR complementary to quantum oscillations, which are sensitive to all scattering processes. We extract lMR from the Sondheimer amplitude in the topological semi-metal WP2, at elevated temperatures up to T∼50 K, in a range most relevant for hydrodynamic transport phenomena. Our data on micrometer-sized devices are in excellent agreement with experimental reports of the large bulk lMR and thus confirm that WP2 can be microfabricated without degradation. Indeed, the measured scattering rates match well with those of theoretically predicted electron-phonon scattering, thus supporting the notion of strong momentum exchange between electrons and phonons in WP2 at these temperatures. These results conclusively establish Sondheimer oscillations as a quantitative probe of lMR in micro-devices in studying non-ohmic electron flow.
Emiliano Cortés, Lucas V. Besteiro, Alessandro Alabastri, Andrea Baldi, Giulia Tagliabue, Angela Demetriadou, and Prineha Narang. 12/14/2020. “Challenges in Plasmonic Catalysis.” ACS Nano. Publisher's VersionAbstract
The use of nanoplasmonics to control light and heat close to the thermodynamic limit enables exciting opportunities in the field of plasmonic catalysis. The decay of plasmonic excitations creates highly nonequilibrium distributions of hot carriers that can initiate or catalyze reactions through both thermal and nonthermal pathways. In this Perspective, we present the current understanding in the field of plasmonic catalysis, capturing vibrant debates in the literature, and discuss future avenues of exploration to overcome critical bottlenecks. Our Perspective spans first-principles theory and computation of correlated and far-from-equilibrium light–matter interactions, synthesis of new nanoplasmonic hybrids, and new steady-state and ultrafast spectroscopic probes of interactions in plasmonic catalysis, recognizing the key contributions of each discipline in realizing the promise of plasmonic catalysis. We conclude with our vision for fundamental and technological advances in the field of plasmon-driven chemical reactions in the coming years.
Tomáš Neuman, Derek S. Wang, and Prineha Narang. 12/9/2020. “Nanomagnonic Cavities for Strong Spin-Magnon Coupling and Magnon-Mediated Spin-Spin Interactions.” Physical Review Letters, 125, 24, Pp. 247702. Publisher's VersionAbstract
We present a theoretical approach to use ferro- or ferrimagnetic nanoparticles as microwave nanomagnonic cavities to concentrate microwave magnetic fields into deeply subwavelength volumes ∼10−13 mm3. We show that the field in such nanocavities can efficiently couple to isolated spin emitters (spin qubits) positioned close to the nanoparticle surface reaching the single magnon-spin strong-coupling regime and mediate efficient long-range quantum state transfer between isolated spin emitters. Nanomagnonic cavities thus pave the way towards magnon-based quantum networks and magnon-mediated quantum gates.