Quantum Opto-Mechanical Systems for Quantum Technologies

In this thesis we theoretically explore the perspectives of control of the quantum dynamics of experimentally relevant systems for quantum technological applications. In Part II we analyze a quantum memory for single photons consisting of a single atom confined in an optical cavity and driven by a laser. We optimize the absorption of the single photon into an atomic excitation by suitably tailoring the laser pulse. In Part III we analyze a monolithic diamond structure which embeds a nitrogen-vacancy center. We identify the parameters regime and the processes that lead to radiative cooling of a mechanical mode of the diamond structure by laser-driving the NV-center. In Part IV we investigate spontaneous spin-spatial pattern formation in an ensemble of laser driven thermal atoms confined in an optical cavity and explore the perspectives of using this system as quantum simulator of quantum magnetism

Low-lying Adiabatic Electronic States of NO: a QMC Study

Using Diffusion Monte Carlo in the fixed node approximation (FN-DMC) we have computed the electronic energy curves for some low lying doublet and quartet adiabatic states of NO. By starting with compact trial wave functions, we obtained accurate results in a wide range of internuclear distances. We have also been able to compute some spectroscopic properties and to interpret UV and visible absorption and emission spectra of NO in the gas phase by computing the Franck-Condon factors in the Born- Oppenheimer approximation. The comparison with available theoretical and experimental data is good.(doi: 10.5562/cca2302

Super-Adiabatic Transfer in Three-Level Systems

La ricerca di metodi con cui controllare il trasferimento di popolazione tra specifici stati quantistici è un tema molto importante in fisica atomica e molecolare, ed è un requisito fondamentale in molte applicazioni della fisica moderna come per esempio l’informazione quantistica. In questa tesi analizziamo il sistema a tre livelli nelle configurazioni lambda (E1, E3 < E2) e ladder (E1 < E2 < E3). Scopo della tesi è la determinazione di un metodo robusto che permetta il trasferimento totale di popolazione dallo stato iniziale |1 allo stato finale |3> , i quali hanno stessa parità e quindi non sono accoppiati direttamente tramite radiazione laser. Per effettuare il trasferimento si usa quindi lo stato intermedio |2> . Tuttavia questo stato può essere instabile (cioè può avere vita media piccola rispetto ai tempi del processo di trasferimento). Quindi se parte della popolazione passa da questo stato si hanno delle perdite e non si raggiunge il trasferimento completo. Lo STIRAP (STImulated Raman Adiabatic Passage) è un metodo che sfrutta la coerenza atomica creata dai campi elettromagnetici per effettuare un trasferimento “quasi” completo. Si utilizzano due impulsi laser: un impulso di pompa che accoppia lo stato iniziale |1> con lo stato intermedio |2> e un impulso di Stokes che accoppia lo stato intermedio con lo stato finale |3>. L’ingrediente principale di questo metodo è la sequenza di impulsi contro-intuitiva: prima si accende l’impulso di Stokes, che accoppia i due stati vuoti, poi, con una certa sovrapposizione con l’impulso precedente, si accende l’impulso di pompa. L’impulso di Stokes che accoppia due stati vuoti crea una sovrapposizione coerente tra di essi, l’impulso di pompa, poi, fa interagire lo stato inizialmente popolato |1> con questa sovrapposizione coerente di stati |2> e |3> . Se sono soddisfatte le condizioni di adiabaticità, si forma un dark state (con proiezione nulla sullo stato |2> ) e la popolazione passa direttamente dallo stato |1> allo stato |3>. Quello che succede è che, se le frequenze di Rabi sono abbastanza grandi o se il tempo di interazione è abbastanza lungo, l’approssimazione adiabatica vale e il sistema segue adiabaticamente il dark state dallo stato iniziale allo stato finale. In questa tesi sviluppiamo i protocolli “super-adiabatici” dello STIRAP: aggiungendo delle interazioni ausiliarie a quelle dello STIRAP è possibile ottenere una evoluzione “perfettamente adiabatica”, cioè il sistema segue esattamente il dark state senza bisogno di condizioni particolari sulle frequenze di Rabi e sul tempo di interazione. In questo modo la popolazione nello stato intermedio è identicamente nulla per tutta la durata del processo e si ottiene il trasferimento completo di popolazione. Le interazioni ausiliarie, oltre a modificare la forma e la fase degli impulsi di pompa e di Stokes, introducono anche un’interazione tra lo stato iniziale e lo stato finale essenziale per il funzionamento super-adiabatico. Abbiamo studiato come questa interazione possa essere realizzata attraverso campi magnetici ausiliari o con transizioni a due fotoni. Analizzando le interazioni ausiliarie con metodi analitici e simulazioni numeriche si trova che il protocollo super-adiabatico dello STIRAP è strettamente collegato al ben noto impulso π della risonanza magnetica, che non è un metodo robusto. Studiando il protocollo super-adiabatico abbiamo trovato che gli impulsi dello STIRAP possono essere usati per rendere l’impulso π più robusto. Inoltre, sia per lo STIRAP che per il suo protocollo super-adiabatico, abbiamo studiato il tempo necessario per ottenere un’ efficienza di trasferimento del 99% in funzione della forma degli impulsi e dei loro parametri. Per controllare sperimentalmente i risultati ottenuti con la teoria, abbiamo usato degli atomi di Rubidio in una trappola magneto-ottica a temperature dell’ordine di 100 μK. Alcuni risultati preliminari sono stati ottenuti applicando lo STIRAP nella configurazione ladder fra il livello fondamentale ed uno stato di Rydberg

Territorialisation dynamics for Italian farms adhering to Alternative Food Networks

The demographic processes of the last decades have led to variations in urban and peri-urban territorial configurations, questioning the patterns of traditional productive localisation. They have begun to outline new perspectives related to proximity to trading and commerce sites as well as to the services offered by the city. Business strategies, such as multifunctionality and diversification, have begun to consider these new possibilities that, at a larger scale, have triggered the process of territorialisation. The study analyses the influence of proximity to the city on the strategies of farms diversifying income through short food chains, with the aim of identifying the prevailing behaviours adopted in three different concentric areas at the urban centre of gravity: peri-urban, belt and rural. The study involves a dataset constituted by 217 farms, where each farm has been associated with a set of explanatory variables that outline some structural, social and economic characteristics. The sample has been segmented through a hierarchical cluster analysis, which allowed us to identify 5 groups of farms, after having reduced the number of variables through PCA (Principal Component Analysis). The results show that short food chains and, more generally, AFNs, are based on strategies alternative to those of traditional chains, and which involve a different economic dimension of the same chains and the construction of a different place-based agro-food system, also envisaging a re-localisation of space near the final market

Adiabatic quantum trajectories in engineered reservoirs

We analyze the efficiency of protocols for adiabatic quantum state transfer assisted by an engineered reservoir. The target dynamics is a quantum trajectory in the Hilbert space and is the fixed point of a time-dependent master equation. We specialize to quantum state transfer in a qubit and determine the optimal schedule for a class of time-dependent Lindblad equations. The speed limit on state transfer is extracted from a physical model of a qubit coupled to a reservoir, from which the Lindblad equation is derived in the Born-Markov limit. Our analysis shows that the resulting efficiency is comparable to the efficiency of the optimal unitary dynamics. Numerical studies indicate that reservoir-engineered protocols could outperform unitary protocols outside the regime of the Born-Markov master equation, namely, when correlations between the qubit and reservoir become relevant. Our study contributes to the theory of shortcuts to adiabaticity for open quantum systems and to the toolbox of protocols of the NISQ era.Comment: 14+7 pages, 7 figure

Integrated conversion and photodetection of virtual photons in an ultrastrongly coupled superconducting quantum circuit

The ground-state of an artificial atom ultrastrongly coupled to quantized modes is entangled thus it contains an arbitrary number of virtual photons. The problem of their detection has been raised since the very birth of the field but despite the theoretical efforts still awaits experimental demonstration. Recently experimental problems have been addressed in detail showing that they can be overcome by combining an unconventional design of the artificial atom with advanced coherent control. In this work we study a simple scheme of control-integrated continuous measurement which makes remarkably favourable the tradeoff between measurement efficiency and backaction showing that the unambiguous detection of virtual photons can be achieved within state-of-the art quantum technologies.Comment: 10 pages, 2 figure

Optimal storage of a single photon by a single intra-cavity atom

We theoretically analyse the efficiency of a quantum memory for single photons. The photons propagate along a transmission line and impinge on one of the mirrors of a high-finesse cavity. The quantum memory is constituted by a single atom within the optical resonator. Photon storage is realised by the controlled transfer of the photonic excitation into a metastable state of the atom and occurs via a Raman transition with a suitably tailored laser pulse, which drives the atom. Our study is supported by numerical simulations, in which we include the modes of the transmission line and we use the experimental parameters of existing experimental setups. It reproduces the results derived using input-output theory in the corresponding regime and can be extended to compute dynamics where the input-output formalism cannot be straightforwardly applied. Our analysis determines the maximal storage efficiency, namely, the maximal probability to store the photon in a stable atomic excitation, in the presence of spontaneous decay and cavity parasitic losses. It further delivers the form of the laser pulse that achieves the maximal efficiency by partially compensating parasitic losses. We numerically assess the conditions under which storage based on adiabatic dynamics is preferable to non-adiabatic pulses. Moreover, we systematically determine the shortest photon pulse that can be efficiently stored as a function of the system parameters.Comment: 13 pages, 8 figure

Fast storage of photons in cavity-assisted quantum memories

Ideal photonic quantum memories can store arbitrary pulses of light with unit efficiency. This requires operating in the adiabatic regime, where pulses have a duration much longer than the bandwidth of the memory. In the non-adiabatic regime of short pulses, memories are therefore imperfect, and information is always lost. We theoretically investigate the bandwidth limitations for setups based on individual atoms, or ensembles thereof, confined inside optical cavities. We identify an effective strategy for optimizing the efficiencies of the storage and retrieval process regardless of the duration of the pulses. Our protocol is derived almost completely analytically and attains efficiencies better than or comparable to those obtained by numerical optimization. Furthermore, our results provide an improved understanding of the performance of quantum memories in several regimes. When considering pulses defined on an infinite time interval, the shapes can be divided into two categories, depending on their asymptotic behaviours. If the intensity of the pulse increases with time slower than or as an exponential function, then the storage efficiency is only limited by the pulse width. For pulses defined on a finite interval, on the other hand, the efficiency is determined by the shape at the beginning of the storage or, correspondingly, at the end of the retrieval process.Comment: 21 pages, 8 figures, all kinds of feedback welcom