Quantum error correction is needed for future quantum computers. Classical error correcting codes are not suitable for this due to the nature of quantum mechanics. Therefore, new codes need to be developed. A promising candidate is the toric code, a surface code, because of its locality and its high error correcting capability and thresholds (the error probability below which increasing the size of the code decreases the failure rate). This thesis provides an introduction to quantum error correction and the stabilizer formalism, which is used to introduce the toric code. Several decoders are looked at, including the Minimum Weight Perfect Matching (MWPM) decoder and a recently developed decoder, the Union-Find (UF) decoder. The UF decoder is useful because of its almost-linear time complexity, while only reducing the threshold by a marginal amount compared to the MWPM decoder. In this thesis, the time complexity of the weighted growth version of the UF decoder is analysed. The toric code and the decoders have been implemented and simulated, and the thresholds have been determined. For the MWPM decoder, the threshold was determined to be 11.5±0.2%, which is not in agreement with the threshold in literature of 10.3%, and should not be possible according to the optimal threshold of about 11%. This probably is a result of the low grid sizes (up to a gridsize of 11) of the code used due to the time needed to simulate the MWPM decoder. For the UF decoder, the threshold found was 9.7 ± 0.9%, which is in agreement with the threshold of 9.9% found by N. Delfosse and N. Nickerson. The time complexity of the UF decoder has also been determined, and is indeed almost-linear as expected by the analysis.

In this thesis we shall consider a generalization on Pólya Processes as have been described by Chung et al. [7]. Given finitely many bins, containing an initial configuration of balls, additional balls arrive one at a time. For each new ball, a new bin is created with probability 푝, or with probability 1 − 푝 this new ball shall be placed in an existing bin such that the probability of this ball ending in a specific bin, is proportional to 푓(푚) where 푚 is the number of balls currently in that bin and 푓 is some feedback function. We shall show that for 푝 = 0, which will be defined as Finite Pólya Processes, the behaviour of the process can be classified into one of three mutual exclusive regimes: Monopolistic Regime, Eventual Leadership Regime or Almost-Balanced Regime. This behaviour solely depends on the convergence of the following sums: Ση≥1 푓(푛)–1 and Ση≥1 푓(푛)-2. We shall explore the limiting distribution of fractions of balls in bins when 푓(푥) = 푥, which is a known result for classical multi-coloured Pólya Urn problems.Using a similar method, we find a limiting distribution for Finite Pólya Processes with general positive linear feedback functions, which has not previously been researched. We then consider the case where 푝 > 0, which are defined as Infinite Pólya Processes and restrict the feedback function to be of the form 푓(푥) = 푥γ where 훾 ∈ ℝ. We shall show that if 훾 > 1, almost surely one bin will dominate or a new bin will be created. We shall show that for 훾 = 1 a preferential attachment scheme arises. We consider 훾 < 1 under the assumptions that some limits exist and show that the fraction of bins having 푚 balls shrinks exponentially as a function of 푚. Finally, we reflect on our results and discuss interesting future research subjects.

In the divisible sandpile model, we consider a collection of i.i.d. Gaussian heights on a finite graph. It was shown by Levine et al. (2015) that the odometer function in this case equals a discrete, bi-Laplacian field. Subsequently, Cipriani et al. (2016) proved that the scaling limit of the odometer is a continuum bi-Laplacian field, this time on the unit torus. In this thesis, we will determine the scaling limit of a divisible sandpile with an initial configuration of correlated Gaussians, where the covariance is given by a stationary covariance function K(x-y). We show that after appropriate scaling, the odometer still converges to a bi-Laplacian field on the unit torus.

In this thesis, the Random Energy Model (REM) is applied to the highly complex problem of random heteropolymer folding. The relevance of the REM lies in the fact that it is an analytically solvable model, which makes it possible to learn more about the (thermodynamical) behaviour of folding proteins.
Firstly, a rigorous proof of the existence of a critical point in the REM with non-zero mean is presented. The mathematical properties of the REM with non-zero mean are used to derive the thermodynamical properties of this special case of the REM, such as the free energy.
In the second part of this thesis, applications of the model to folding polymers are investigated and a simple simulation of protein folding using the REM is suggested. It turns out that this simulation is barely useful, so a more realistic version of the REM for polymer folding is suggested.

A variation on the random Kuramoto model was analyzed, a mean-field model which describes the behavior of coupled oscillators in a random environment. The analytic results are based on the behavior of the system in the infinite volume limit. In the analysis critical values were found for the parameters in the model which can be used to determine when the oscillators synchronize. Numerical simulations were used to verify the analytic results.

[ not the actual abstract] In this thesis, the sandpile models distributions are analysed on the finite and infinite bethe lattice. To do so, the recursive property of the lattice is used.