Event Time:
Friday, May 10, 2024 | 9:00 am - 11:00 am
Event Location:
Henn 309 and Zoom, https://ubc.zoom.us/j/69190854282?pwd=amdGR3ovSnhDc0lSaXR6bzNuTkZYQT09
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2024-05-10T09:00:00
2024-05-10T11:00:00
Classical descriptions of quantum computations: Foundations of quantum computation via hidden variable models, quasiprobability representations, and classical simulation algorithms
Event Information:
[abstract] Quasiprobability representations serve as a bridge between classical and quantum descriptions of physical systems. In these representations, nonnegativity allows for a probabilistic interpretation, aligning the description with classical physics.
However, the capacity to model quantum systems hinges on the use of negative quasiprobabilities. Accordingly, negativity is considered a hallmark of genuinely quantum behaviour. This principle has been applied to quantum information processing where negativity in the Wigner function is necessary for a quantum computational advantage. This is demonstrated by an efficient classical simulation algorithm for quantum computations in which all components of the computation remain nonnegative.
However, when constructing quasiprobability representations for quantum computation to which this statement applies, a marked difference arises between the cases of even and odd Hilbert space dimension. We find that Wigner functions with the properties required to describe quantum computation do not exist in any even dimension. We establish that the obstructions to the existence of such Wigner functions are cohomological.
In order to recover the properties required for classical simulation of quantum computation in any dimension, some constraints that traditionally define a Wigner function must be relaxed. We consider several examples of these generalized quasiprobability representations, and we find that when sufficiently general representations are admitted, no negativity is required to represent universal quantum computation.
The result is a hidden variable model that represents all elements of universal quantum computation probabilistically. Since this model can simulate any quantum computation, the simulation must be inefficient in general. However, in certain restricted settings, the simulation is efficient, allowing for a broader class of magic state quantum circuits to be efficiently classically simulated than those covered by the stabilizer formalism and Wigner function methods.
With this hidden variable model, we present a formulation of quantum mechanics that replaces the central notion of state, as a complex vector or density operator, with a bit string. This formulation applies to universal quantum computation, and hence all finite-dimensional quantum mechanics. Thus, we present a surprising response to Wheeler’s "It from Bit" challenge. Alongside coherence, entanglement, and contextuality, this provides a new approach to characterizing quantum advantage.
Event Location:
Henn 309 and Zoom, https://ubc.zoom.us/j/69190854282?pwd=amdGR3ovSnhDc0lSaXR6bzNuTkZYQT09
Event Time:
Monday, May 13, 2024 | 12:30 pm - 2:30 pm
Event Location:
TRIUMF Theory Room, 4004 Wesbrook Mall and zoom; https://ubc.zoom.us/j/68938408525?pwd=MVBBK05ZQWdCK2tJKzNGUXZaazJhdz09 Passcode: 959424
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2024-05-13T12:30:00
2024-05-13T14:30:00
Probing Beyond Standard Model Physics Through Ab Initio Calculations of Exotic Weak Processes in Atomic Nuclei
Event Information:
"Exotic weak decays offer a unique way to probe physics beyond the Standard Model in a low-energy regime using the atomic nucleus as a window to complement the high-energy searches done at particle accelerator facilities. However, in order to extract the relevant physics parameters from experimental observations, inputs from nuclear theory are required.
The hypothetical neutrinoless double beta decay has gathered a lot of interest, as its observation would answer many standing questions in particle physics. First, it would unveil fundamental properties of the most abundant yet most elusive massive particle: the neutrino. A simple observation of this decay would imply the neutrino to be Majorana, meaning that it is its own antiparticle, as well as give insight into its absolute mass. Furthermore, the existence of this decay would explain the matter/antimatter asymmetry of the universe.
In order to extract the neutrino mass and potential couplings to more exotic mechanisms, as well as compare sensitivities of experiments using different isotopes, the nuclear matrix element must be obtained from nuclear theory. Unfortunately, the different models that have historically been used to compute this quantity have shown a large spread with no means of quantifying their respective uncertainties, greatly hindering the experimental precision.
In this thesis, we use recent advances in ab initio methods, which profit from the rapid increase in computational power to calculate nuclear observables directly from the interaction between the nucleons. In particular, we use the ab initio valence-space in medium similarity renormalization group method to compute the matrix element of all relevant candidate isotopes for experimental searches. We further develop a new machine learning emulator that greatly increases the speed of calculations. Using this emulator, we probe the full input parameter space of the calculation to give the first statistical uncertainty on the matrix element.
Our results show smaller values than previous models and are consistent with other ab initio methods. This provides a much tighter constraint than the spread coming from previous models, greatly clarifying the picture for both current and future experimental searches of the decay. “
Event Location:
TRIUMF Theory Room, 4004 Wesbrook Mall and zoom; https://ubc.zoom.us/j/68938408525?pwd=MVBBK05ZQWdCK2tJKzNGUXZaazJhdz09 Passcode: 959424