Nanoscale systems represent a frontier in contemporary physics, where classical physics intersects with quantum mechanics, resulting in unique physical properties and phenomena. These properties not only deepen our understanding of fundamental physical principles but also enable the development of novel technological applications. Precise control over the state of systems at the nanoscale is essential for understanding and leveraging their full potential. The main objective of this thesis is to propose novel methods for the control of classical and quantum nanoscale systems, focusing both on theoretical understanding and practical implementation. In the first part of the thesis, we address the tayloring of pulse propagation inside waveguides, with applications for classical information processing and quantum technologies. The confinement of light fields to nanoscale sizes allows for exotic properties such as frequency dependent group velocity, discretization of the frequency spectrum and unconventional pulse propagation. This confinement also leads to an enhanced interaction with quantum emitters and results in interesting properties of light-matter interaction. In Chapter 2, we introduce a family of chirped pulses that dynamically self-compress during their evolution in a generic medium with a quadratic dispersion relation. We propose to use these pulses to remotely addresss individual quantum emitters embedded in the medium. In Chapter 3, we present an experimental demonstration of the proposal introduced in Chapter 2, performed in collaboration with the group of Prof. Kirchmair at IQOQI and the University of Innsbruck. We show self-compression of pulses inside a rectangular microwave waveguide and spatial addressing of two superconducting qubits separated by a sub-wavelength distance. In Chapter 4, we turn our attention to spin-wave pulses, namely excitations of the magnetization that propagate in a magnetic material. We provide a method based on inverse design to generate spin-wave pulses in magnonic waveguides with a desired spatial-temporal profile. In the second part of the thesis, we focus on the quantum control of the motional state of a levitated nanoparticle. This system presents a promising experimental platform for the exploration of quantum mechanics with mesoscopic objects, as they could allow for the observation of macroscopic quantum superposition states and matter-wave interferometry, and have applications as ultraprecise sensors. In Chapter 5, we introduce an optimization method to determine optimal static potentials for the generation of largely delocalized and non-Gaussian quantum states of levitated nanoparticles under decoherence.
Nanoscale systems represent a frontier in contemporary physics, where classical physics intersects with quantum mechanics, resulting in unique physical properties and phenomena. These properties not only deepen our understanding of fundamental physical principles but also enable the development of novel technological applications. Precise control over the state of systems at the nanoscale is essential for understanding and leveraging their full potential. The main objective of this thesis is to propose novel methods for the control of classical and quantum nanoscale systems, focusing both on theoretical understanding and practical implementation. In the first part of the thesis, we address the tayloring of pulse propagation inside waveguides, with applications for classical information processing and quantum technologies. The confinement of light fields to nanoscale sizes allows for exotic properties such as frequency dependent group velocity, discretization of the frequency spectrum and unconventional pulse propagation. This confinement also leads to an enhanced interaction with quantum emitters and results in interesting properties of light-matter interaction. In Chapter 2, we introduce a family of chirped pulses that dynamically self-compress during their evolution in a generic medium with a quadratic dispersion relation. We propose to use these pulses to remotely addresss individual quantum emitters embedded in the medium. In Chapter 3, we present an experimental demonstration of the proposal introduced in Chapter 2, performed in collaboration with the group of Prof. Kirchmair at IQOQI and the University of Innsbruck. We show self-compression of pulses inside a rectangular microwave waveguide and spatial addressing of two superconducting qubits separated by a sub-wavelength distance. In Chapter 4, we turn our attention to spin-wave pulses, namely excitations of the magnetization that propagate in a magnetic material. We provide a method based on inverse design to generate spin-wave pulses in magnonic waveguides with a desired spatial-temporal profile. In the second part of the thesis, we focus on the quantum control of the motional state of a levitated nanoparticle. This system presents a promising experimental platform for the exploration of quantum mechanics with mesoscopic objects, as they could allow for the observation of macroscopic quantum superposition states and matter-wave interferometry, and have applications as ultraprecise sensors. In Chapter 5, we introduce an optimization method to determine optimal static potentials for the generation of largely delocalized and non-Gaussian quantum states of levitated nanoparticles under decoherence.
Abstract Preparing and observing quantum states of nanoscale particles is a challenging task with great relevance for quantum technologies and tests of fundamental physics. In contrast to atomic systems with discrete transitions, nanoparticles exhibit a practically continuous absorption spectrum and thus their quantum dynamics cannot be easily manipulated. Here, we demonstrate that charged nanoscale dielectrics can be artificially endowed with a discrete level structure by coherently interfacing their rotational and translational motion with a superconducting qubit. We propose a pulsed scheme for the generation and read-out of motional quantum superpositions and entanglement between several levitated nanoparticles, providing an all-electric platform for networked hybrid quantum devices.
Realizing quantum superpositions of massive objects provides a pathway to explore the quantum-to-classical transition. Levitated nanoparticles appear as a promising platform for this purpose due to their large mass and unbound motion. This thesis follows the proposal that particle evolution in a double-well potential expands the particle’s motional state within an inverted harmonic region, while the quartic potential generates the non-Gaussian states needed to certify quantum delocalization. As this protocol has been recently developed, we perform experiments in the classical regime as an important milestone to identify the technical challenges and the decoherence sources. While the nanoparticle’s motional state grows linearly under free evolution, inverted harmonic potentials offer exponentially accelerated state expansion. In this thesis, we experimentally demonstrate the expansion of the center-of-mass state of a 125 nm silica nanoparticle from an initial position uncertainty of 45.6 pm to 43.4 nm within 260 μs. This expansion represents a 952-fold increase, reaching a state size comparable to the particle’s radius, and is achieved by employing an electrostatic inverted potential to disable decoherence from photon recoil. The incoherent dynamics allow us to estimate the remaining decoherence sources in the dark. To accomplish dark dynamics, we developed two segmented linear Paul trap devices providing 2D confinement while generating DC potentials in the third axis of motion: a planar micro-fabricated chip and a PCB based wheel trap. Simulations of tunable double-well potentials of tens to hundreds of micrometers based on the device layouts are performed alongside a study of the resulting classical non-Gaussian states. This work marks a key advancement in the effort to generate macroscopic quantum superpositions at unprecedented mass and length scales.
Waveguide quantum electrodynamics (waveguide QED) describes the interaction between an electromagnetic field confined to a one-dimensional waveguide with atom-like quantum emitters close by. The characteristics of these kinds of systems are the possibility for strong and even ultrastrong interactions between the photonic and atom-like systems, and practically infinite range interactions between the emitters. These properties are valuable for a wide range of quantum optical applications, like quantum communication, quantum networks and quantum metrology. In this thesis we focus on two challenges in quantum optics, namely the generation and scattering of multiphoton states. In the first part of this thesis we demonstrate how waveguide QED systems can be exploited for the generation of multiphoton states, in particular of single-mode Fock states and superpositions thereof as well as multi-mode photonic states with metrological applications. The basic setup for this goal is an ensemble of quantum emitters coupled to a waveguide. In the so-called atomic mirror configuration symmetric Dicke states (or a superposition thereof) decay superradiantly to the ground state and emit the desired multiphoton state, which can be efficiently collected at the ends of the waveguide. We propose various protocols for the preparation of these Dicke states in waveguide QED systems and characterize the emitted photonic state. It turns out that in the low excitation regime, that is, if the number of photons is much lower than the ensemble size, the photonic state is emitted into a single mode. This single-mode structure is fundamental to current proposals for applications in quantum metrology with optical interferometers. Outside the low excitation regime, a multi-mode photonic state is generated, for which the metrological capabilities were unknown. We were able to show that these states still lead to quantum-enhanced optical interferometry. In the second part of this thesis we demonstrate how the scattering of multiphoton states on a single quantum emitter coupled to the waveguide can be used for testing the limits of the light-matter interaction strength. In particular, we investigate systems in the so-called ultrastrong coupling regime, where the coupling strength between the photonic and atom-like system is of the order of the emitter's transition energy. In this regime, many methods and approximations used in quantum optics, especially the Rotating Wave Approximation, break down and one needs to develop new analytical and numerical methods to study these systems. One approach is the so-called polaron Transformation. We show how this transformation can be applied for predicting the scattering amplitude of few photons scattering on the emitter. The comparison of these results with numerical simulations using matrix product states shows a good agreement for moderate coupling strengths. Both parts together show that waveguide QED systems are promising candidates for the generation of multiphoton states as well as for the investigation of fundamental limits probed through the scattering of multiphoton states. The latter also finds application in the implementation of photon-photon nonlinearities induced by the interaction with the quantum emitters. Apart from these and many other theoretical predictions, waveguide QED systems are also undergoing rapid progress in experiments. Therefore, we expect these kinds of systems to bring forth several advances in the field of quantum optics.
Every massive particle behaves like a wave, according to quantum physics. Yet, this characteristic wave nature has only been observed in double-slit experiments with microscopic systems, such as atoms and molecules. The key aspect is that the wavefunction describing the motion of these systems extends coherently over a distance comparable to the slit separation, much larger than the size of the system itself. Preparing these states of more massive and complex objects remains an outstanding challenge. While the motion of solid-state oscillators can now be controlled at the level of single quanta, their coherence length remains comparable to the zero-point motion, limited to subatomic distances. Here, we prepare a delocalized state of a levitating solid-state nanosphere with coherence length exceeding the zero-point motion. We first cool its motion to the ground state. Then, by modulating the stiffness of the confinement potential, we achieve more than a threefold increment of the initial coherence length with minimal added noise. Optical levitation gives us the necessary control over the confinement that other mechanical platforms lack. Our work is a stepping stone towards the generation of delocalization scales comparable to the object size, a crucial regime for macroscopic quantum experiments, and towards quantum-enhanced force sensing with levitated particles.
Quantum light-matter interactions, described by the theory of quantum electrodynamics (QED), have motivated numerous scientific discoveries and technological developments over the past few decades. Considerable effort has been made to develop different QED platforms, e.g., free space, waveguides, cavities, and photonic crystals, for engineering quantum light-matter interactions. This thesis explores a new frontier, deep microscopic QED, which leverages the deep microscopic response of materials to reveal new topological, quantum, and thermal photonic phenomena with applications to quantum sensing and information processing. In the first part of this thesis, we investigate unique topological photonic effects arising in the deep microscopic QED regime. Previous efforts focused on topological photonic phases and edge states arising from artificial spatial structures of materials (e.g., photonic crystals or metamaterials). One fundamental question is whether topological photonic states can also arise from the intrinsic response of many-body systems and do not rely on materials' macroscopic structures. Here, we first discuss a new topological electromagnetic phase - ‘optical N-phase’ arising in two-dimensional (2D) viscous hydrodynamic electrons in graphene, which originates from the microscopic response of 2D materials and does not rely on materials' macroscopic structures. We then introduce the associated edge states, optical N-plasmons, which are topologically protected ultra-subwavelength edge magneto-plasmons. We further show how these topological plasmons can be harnessed to design chiral radio-frequency nonreciprocal circuit components, topological hydrodynamic circulators, that interface quantum and classical hardware.In the second part of this thesis, we focus on collective atom-photon interactions in the deep microscopic QED regime. In atom interactions with photonic environments, there exist two non-unitary processes -- pure dephasing and spontaneous emission. Despite considerable interest in collective effects in spontaneous emission since the 1950s, one long-standing problem is whether collective effects in pure dephasing can arise in atom-photon interactions. This question is particularly important in the quantum information era since pure dephasing is the main obstacle in current quantum information technologies. Here, we first introduce a new collective quantum phenomenon, ‘nanophotonic superdephasing,’ occurring in atom/spin ensembles in nanoscale photonic environments. This nanophotonic superdephasing phenomenon originates from low-frequency photonic environment correlations, and leads to collectively accelerated disentanglement without photon emission, in stark contrast to conventional collective spontaneous emission/superradiance effects. We further reveal the effects of nanophotonic environment mediated correlated dynamics on quantum computing and sensing. We demonstrate that correlations of nanoscale EM environments in spin qubit quantum computing devices can induce correlated errors and limit quantum gate fidelity. Meanwhile, based on nanophotonic collective dephasing, we also propose a multiqubit quantum sensing protocol that exploits spatially correlated noise near materials to enable order parameter symmetry characterization of materials beyond single-qubit methods.In the final part, we explore thermal photonic phenomena in the deep microscopic regime. Conventional techniques in engineering near-field radiative heat transfer typically rely on phonons, plasmons, and electromagnetic modes in structured materials. Here, we reveal a new approach to enhance near-field thermal radiation: leveraging intrinsic quantum coherence in superconducting materials. We reveal the quantum fingerprint of superconductivity in thermal photonics, ‘superconducting coherence peak’ in near-field thermal radiation, which originates from quantum coherence of Bogoliubov quasiparticles, with potential applications in thermal engineering in superconducting circuits.Overall, the thesis establishes a deep microscopic QED platform for new quantum, topological, and thermal photonic phenomena in many-body systems, with applications in quantum sensors and devices.
Waveguide quantum electrodynamics—the study of atomic systems interacting with propagating electromagnetic fields—is a powerful platform for understanding the complex interplay between light and matter. Qubit control is an indispensable tool in this field, and most experiments thus far have used narrowband control pulses to interact with qubits at specific frequencies. Dynamics can change significantly with fast, broadband pulses, since waveguide properties such as dispersion affect the pulse evolution and its impact on the qubit. Here we use dispersion to achieve spatial addressing of superconducting qubits separated by a subwavelength distance within a microwave waveguide. This approach relies on a self-focusing effect to create a position-dependent interaction between a chirped pulse and an individual qubit. This method emphasizes the importance of dispersion in the design and analysis of quantum experiments, and offers avenues for the rapid control of quantum states.