Littérature scientifique sur le sujet « Molecular electronic qubits »

We propose a scenario to realize quantum computers utilizing heteronuclear diatomic rovibrational states as qubits. We focused on rovibrational qubits created by simple transform limited infrared laser pulse instead of using chirped pulse. Numerical calculations show that single qubit gate operation in the electronic ground state of LiH molecule can be obtained. We also discuss the effect of temperature on the initially rotational states, and a suitable experiment condition is indicated.

Silicon-based quantum-computer architectures have attracted attention because of their promise for scalability and their potential for synergetically utilizing the available resources associated with the existing Si technology infrastructure. Electronic and nuclear spins of shallow donors (e.g. phosphorus) in Si are ideal candidates for qubits in such proposals due to the relatively long spin coherence times. For these spin qubits, donor electron charge manipulation by external gates is a key ingredient for control and read-out of single-qubit operations, while shallow donor exchange gates are frequently invoked to perform two-qubit operations. More recently, charge qubits based on tunnel coupling in P+2 substitutional molecular ions in Si have also been proposed. We discuss the feasibility of the building blocks involved in shallow donor quantum computation in silicon, taking into account the peculiarities of silicon electronic structure, in particular the six degenerate states at the conduction band edge. We show that quantum interference among these states does not significantly affect operations involving a single donor, but leads to fast oscillations in electron exchange coupling and on tunnel-coupling strength when the donor pair relative position is changed on a lattice-parameter scale. These studies illustrate the considerable potential as well as the tremendous challenges posed by donor spin and charge as candidates for qubits in silicon.

Quantum dot-based quantum computation employs extensively the exchange interaction between nearby electronic spins in order to manipulate and couple different qubits. The exchange interaction, however, couples the qubit states to charge noise, which reduces the fidelity of the quantum gates that employ it. The effect of charge noise can be mitigated by working at noise sweetspots in which the sensitivity to charge variations is reduced. In this work we study the response to charge noise of a double quantum dot based qubit in the presence of ac gates, with arbitrary driving amplitudes, applied either to the dot levels or to the tunneling barrier. Tuning with an ac driving allows to manipulate the sign and strength of the exchange interaction as well as its coupling to environmental electric noise. Moreover, we show the possibility of inducing a second-order sweetspot in the resonant spin-triplet qubit in which the dephasing time is significantly increased.

The trinuclear copper(ii) complex [Cu₃(saltag)(py)₆]ClO₄ (H₅saltag = tris(2-hydroxybenzylidene)triaminoguanidine) was synthesized and characterized by experimental as well as theoretical methods.

Abstract Preservation of an entangled state in a quantum system is one of the major goals in quantum technological applications. However, entanglement can be quickly lost into dissipation when the effective interaction among the qubits becomes smaller compared to the noise-injection from the environment. Thus, a medium that can sustain the entanglement of distantly spaced qubits is essential for practical implementations. This work introduces the fabrication of a rolled-up zero-index waveguide which can serve as a unique reservoir for the long-range qubit–qubit entanglement. We also present the numerical evaluation of the concurrence (entanglement measure) via Ansys Lumerical FDTD simulations using the parameters determined experimentally. The calculations demonstrate the feasibility and supremacy of the experimental method. We develop and fabricate this novel structure using cost-effective self-rolling techniques. The results of this study redefine the range of light-matter interactions and show the potential of the rolled-up zero-index waveguides for various classical and quantum applications such as quantum communication, quantum information processing, and superradiance.

Classical simulation of Noisy Intermediate Scale Quantum computers is a crucial task for testing the expected performance of real hardware. The standard approach, based on solving Schrödinger and Lindblad equations, is demanding when scaling the number of qubits in terms of both execution time and memory. In this article, attempts in defining compact models for the simulation of quantum hardware are proposed, ensuring results close to those obtained with standard formalism. Molecular Nuclear Magnetic Resonance quantum hardware is the target technology, where three non-ideality phenomena—common to other quantum technologies—are taken into account: decoherence, off-resonance qubit evolution, and undesired qubit-qubit residual interaction. A model for each non-ideality phenomenon is embedded into a MATLAB simulation infrastructure of noisy quantum computers. The accuracy of the models is tested on a benchmark of quantum circuits, in the expected operating ranges of quantum hardware. The corresponding outcomes are compared with those obtained via numeric integration of the Schrödinger equation and the Qiskit’s QASMSimulator. The achieved results give evidence that this work is a step forward towards the definition of compact models able to provide fast results close to those obtained with the traditional physical simulation strategies, thus paving the way for their integration into a classical simulator of quantum computers.

Conventional computing faces a huge technical challenge as traditional transistors will soon reach their size limitations. This will halt progress in reaching faster processing speeds and to overcome this problem, require an entirely new approach. Quantum computing (QC) is a natural solution offering a route to miniaturisation by, for example, storing information in electron or nuclear spin states, whilst harnessing the power of quantum physics to perform certain calculations exponentially faster than its classical counterpart. However, QCs face many difficulties, such as, protecting the quantum-bit (qubit) from the environment and its irreversible loss through the process of decoherence. Hybrid systems provide a route to harnessing the benefits of multiple degrees of freedom through the coherent transfer of quantum information between them. In this thesis I show coherent qubit transfer between electron and nuclear spin states in a ¹⁵N@C₆₀ molecular system (comprising a nitrogen atom encapsulated in a carbon cage) and a solid state system, using phosphorous donors in silicon (Si:P). The propagation uses a series of resonant mi- crowave and radiofrequency pulses and is shown with a two-way fidelity of around 90% for an arbitrary qubit state. The transfer allows quantum information to be held in the nuclear spin for up to 3 orders of magnitude longer than in the electron spin, producing a ¹⁵N@C₆₀ and Si:P ‘quantum memory’ of up to 130 ms and 1.75 s, respectively. I show electron and nuclear spin relaxation (T₁), in both systems, is dominated by a two-phonon process resonant with an excited state, with a constant electron/nuclear T₁ ratio. The thesis further investigates the decoherence and relaxation properties of metal atoms encapsulated in a carbon cage, termed metallofullerenes, discovering that exceptionally long electron spin decoherence times are possible, such that these can be considered a viable QC candidate.

In this thesis, large organic molecules (DM15N, DM18N, Cu(dbm)2) were deposited. These molecules are cannot be deposited by thermal sublimation due the fact that they decompose at lower temperature than they sublime. The employed molecules to single molecular magnets, which can be potentially used as quantum bites (qubit). The new method of deposition atomic layer injection made by Bihur Crystal company was introduced and tested. The method uses liquid solution with molecules which is driven by argon gas through pulse valve to the sample placed in ultra-high vacuum chamber. During the deposition, droplets of solution are formed on the sample surface. The solvent can be removed by light annealing or by keeping the sample in the vacuum for couple of days. The molecules were investigated by x-ray photoelectron spectroscopy and by scanning electron microscopy to determine fragmentation of the molecules, to study topography of the resultant surface and homogeneity of the deposited layer. We found conditions at which the intact molecules are deposited on the sample surfaces and form molecular nano- and micro- crystals.

The development of quantum mechanics led to the birth of the quantum computing, a new frontier of the computer science which exploits quantum phenomena such as the quantum states superposition and the entanglement to perform computation. The basic element of a quantum computer is the Qubit, a two-level quantum system that can be addressed whether in 0 or 1 , but also in a coherent superposition of these two states. Among the great variety of potential Qubit systems, molecular electron spins show interesting features as the great tunability of the electronic and magnetic properties as well as the possibility to interact with other Qubits by adopting an ad-hoc synthetic strategy. Despite these positive features, they suffer from relatively short coherence time, Tm, namely the time during which the quantum information stored inside the Qubit is maintained. The aim of this PhD work is the optimization of the molecular Qubits performances through the research of the key “elements” to enhance the spin-lattice relaxation time, T1, that sets the upper limit of Tm. The study has been focused on Vanadium(IV) coordination compounds whose magnetic properties are due to the single unpaired electron, resulting in a S= 1/2 spin value. This element shows a weak spin-orbit coupling and, consequently, a small spin-phonon coupling, that makes it particularly suitable for the purpose. Two classes of complexes have been taken into account: vanadyl-based molecules, which show the VO2+ metal centre in a square pyramidal coordination geometry, and V(IV)-based ones in which the metal centre is octahedrally coordinated. Several ligands have been used in order to test different structural effects on the spin relaxation. In this study we utilized different experimental techniques to investigate different aspects of the issue. We jointed standard magnetic techniques, as alternate-current (AC) susceptometry, continuous-wave (CW) and pulse electron paramagnetic resonance (EPR) spectroscopy, and TeraHertz time-domain spectroscopy (THz-TDS) for the characterization of the phonons of the complexes in the crystal phase. Moreover, a deeper investigation of the spin-lattice relaxation mechanisms required a more exotic technique, that is time-resolved THz-pump EPR-probe (TR-THz-EPR) measurements, which exploits the free electron laser radiation as THz source at the Novosibirsk Free Electron Laser (NovoFEL) facility. The whole work has been supported by theoretical calculations that has been fundamental in the rationalization of several experimental results. The manuscript is organized in several parts and each of them includes different chapters. Part I provides a general introduction about Qubits and, in particular, those based on molecular electronic spins. In part II, many theoretical concepts that I consider to be essential in the comprehension of this work are discussed, such as the theory of the spin relaxation and the spin-phonon interaction, with a short mention to the spin-orbit coupling. In part III, the techniques based on custom setups, as THz-TDS setups and the NovoFEL equipment, as well as the samples preparation, are examined. Part IV, which represents the main one, is devoted to the results obtained during the PhD. It is divided in two chapters, according to the two main topics: the former contains the outcomes obtained by the combination of magnetic techniques and THz-TD spectroscopy, arranged according to the publications; while the latter includes the investigation of THz-induced effects on the spin dynamics, performed at NovoFEL. In part V, that is the last one, a summary of this work helps in getting an overall view of it.

This work highlights the different possibilities given by a rational chemical approach to obtain molecular-based quantum bits and quantum logic gates as an appealing and alternative platform for implementing quantum computation. This work resumes the three years of the author's work on this subject, mainly presenting and focusing on experimental results obtained for some new potential hydrogen-free molecular qubits, multi-qubit structures, and state-of-the-art EPR-based (Electron Paramagnetic Resonance) experiments on an archetypical vanadyl-based qubit.