Adiabatic quantum computation

Adiabatic quantum computation (AQC) is a form of quantum computing which relies on the adiabatic theorem to do calculations and is closely related to quantum annealing. Description First, a (potentially complicated) Hamiltonian is found whose ground state describes the solution to the problem of interest. Next, a system with a simple Hamiltonian is prepared and initialized to the ground state. Finally, the simple Hamiltonian is adiabatically evolved to the desired complicated Hamiltonian. By the adiabatic theorem, the system remains in the ground state, so at the end the state of the system describes the solution to the problem. Adiabatic quantum computing has been shown to be polynomially equivalent to conventional quantum computing in the circuit model. The time complexity for an adiabatic algorithm is the time taken to complete the adiabatic evolution which is dependent on the gap in the energy eigenvalues (spectral gap) of the Hamiltonian. Specifically, if the system

Novel materials and algorithms for quantum technologies

Francesco Libbi

The enormous advancements in the ability to detect and manipulate single quantum states have lead to the emerging field of quantum technologies. Among these, quantum computation is the most far-reaching and challenging, aiming to solve problems that the classic computers could never address because of the exponential scaling, while quantum sensing exploits the ability to address single quantum states to realize ultra-sensitive and precise detectors. Defect centers in semiconductors play a primary role in these fields. The possibility to store information in the spin of their ground state, manipulate it through microwaves, and read it optically allows to use them as qubits. In addition, the very sharp dependence of their properties on temperature, strain and magnetic fields makes them very promising quantum sensors. In this Thesis we aim at contributing to the progress of quantum technologies both at the hardware and software level. From a hardware point of view, we study a key property of defect centers in semiconductors, the phonon-assisted luminescence, which can be measured to perform the readout of the information stored in a quantum bit, or to detect temperature variations. We predict the luminescence and study the exciton-phonon couplings within a rigorous many-body perturbation theory framework,an analysis that has never been performed for defect centers.In particular, we study the optical emission of the negatively-charged boron vacancy in 2D hexagonal boron nitride, which currently stands out among defect centers in 2D materials thanks to its promise for applications in quantum information and quantum sensing. We show that phonons are responsible for the observed luminescence, which otherwise would be dark due to symmetry. We also show that the symmetry breaking induced by the static Jahn-Teller effect is not able to describe the presence of the experimentally observed peak at 1.5 eV.The knowledge of the coupling between electrons and phonons is fundamental for the accurate prediction of all the features of the photoluminescence spectrum. However, the large number of atoms in a defect supercell hinders the possibility use density functional perturbation theory to study this coupling. In this work we present a finite-differences technique to calculate the electron-phonon matrix elements, which exploits the symmetries of the defect in such a way to use the very same set of displacement needed for the calculation of phonons. The computation of electron-phonon coupling thus becomes a simple post-processing of the finite-differences phonons calculation. On the quantum software side, we propose an improved quantum algorithm to calculate the Green's function through real-time propagation, and use it to compute the retarded Green's function for the 2-, 3- and 4-site Hubbard models. This novel protocol significantly reduces the number of controlled operations when compared to those previously suggested in literature. Such reduction is quite remarkable when considering the 2-site Hubbard model, for which we show that it is possible to obtain the exact time propagation of the

\ket{N\pm 1}

states by exponentiating one single Pauli component of the Hamiltonian, allowing us to perform the calculations on an actual superconducting quantum processor.

EPFL2022

In-depth Cryogenic Characterization of 22 nm FDSOI Technology for Quantum Computation

Edoardo Charbon, Christian Enz, Hung-Chi Han, Farzan Jazaeri

In this paper, the influence of temperature and back-gate bias is experimentally investigated on 22 nm FDSOI CMOS process. Cryogenic DC characterization was carried out under various back-gate voltages, V back , from 2.95 K back to 300 K. An abrupt drop-off in drain current due to intersubband scattering is experienced in the transfer characteristic with a certain V back . Moreover, resonant and source-to-drain tunneling transports are observed in devices with minimal channel length at cryogenic temperatures. The threshold voltage, V T , and free carrier mobility, µ eff , and their dependence on the back-gate voltage over a wide range of temperatures are extracted and discussed in detail. This work aims at investigating the impact of back gate potential on V T and carrier transport at cryogenic temperatures, further paving the way towards up-scaling of quantum computers.

IEEE2021

Novel materials and algorithms for quantum technologies

Francesco Libbi

\ket{N\pm 1}

states by exponentiating one single Pauli component of the Hamiltonian, allowing us to perform the calculations on an actual superconducting quantum processor.

EPFL2022

Novel materials and algorithms for quantum technologies

Quantum information and quantum computation

In-depth Cryogenic Characterization of 22 nm FDSOI Technology for Quantum Computation

Quantum information and quantum computation

Novel materials and algorithms for quantum technologies

In-depth Cryogenic Characterization of 22 nm FDSOI Technology for Quantum Computation