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How did the universe begin? How did it evolve to what we see now?

There was a time when few people believed such questions could even be posed in scientific terms. Now, as increasingly precise instruments deliver their treasure trove of data, the answers may be within reach.

On Wednesday, October 25, Perimeter Director Emeritus Neil Turok will tackle this intriguing topic in a Perimeter Institute Public Lecture, “Secrets of the Universe: Hiding in Plain Sight?”

Classical simulation techniques are widely used in quantum computation and condensed matter physics. In this talk I will describe algorithms for classically simulating measurement of an n-qubit quantum state in the standard basis, that is, sampling a bit string from the probability distribution determined by the Born rule. Our algorithms reduce the sampling task (known as weak simulation) to computing poly(n) amplitudes of n-qubit states (strong simulation). Two classes of quantum states are considered: output states of polynomial-size quantum circuits and ground states of local Hamiltonians with an inverse polynomial energy gap. We show that our algorithm can significantly accelerate quantum circuit simulations based on tensor network contraction and low-rank stabilizer decompositions. To sample ground state probability distributions we employ the fixed-node Hamiltonian construction, previously used in Quantum Monte Carlo simulations to address the fermionic sign problem. We implement the proposed sampling algorithm numerically and use it to sample from the ground state of Haldane-Shastry Hamiltonian with up to 56 qubits. 

Joint work with Giuseppe Carleo, David Gosset, and Yinchen Liu

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Zoom link https://pitp.zoom.us/j/93297869296?pwd=TVpRdVJmU3lWZjVQM3NNKzBucVVRUT09

I will begin with a broad overview of the main ingredients of causal set theory, and then discuss the partial resolution of a long-standing "puzzle" in the causal set path sum. I will then discuss other causal set curiousities, ending with a brief overview of the challenges in constructing a realistic fully non-perturbative quantum dynamics. --- The presenter will be joining via Zoom for this talk.

The organizers requested an overview of Loop Quantum Gravity (LQG). However, this is an impossible task, especially for a 30 minute talk, given that the subject is over three decades old with tens of thousands of papers. Instead, I will outline some of the key distinguishing features of LQG and discuss an illustrative application. Hopefully these topics will complement those that will be covered by Bianca Dittrich and Lee Smolin, without too much of an overlap.

In the last few years, a remarkable link has been established between the soft theorems and asymptotic symmetries of quantum field theories: soft theorems are Ward identities of the asymptotic symmetry generators. In quantum electrodynamics, Weinberg's soft photon theorem is nothing but the Ward identity of a gauge transformation whose parameter is non-trivial at infinity. Likewise, Low's tree-level subleading soft photon theorem is the Ward identity of a gauge transformation whose parameter diverges linearly at infinity. More recently, it has been shown that Low's theorem receives loop corrections that are logarithmic in soft photon energy. Then, it is natural to ask whether such corrections are associated with some asymptotic symmetry of the S-matrix. There have been proposals for conserved charges whose Ward identities yield the loop-corrected soft theorems, but a clear symmetry interpretation remains elusive. We explore this question in the context of scalar QED, in hopes of shedding light on the connection between asymptotic symmetries and loop-corrected soft theorems.

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Zoom link https://pitp.zoom.us/j/94420835190?pwd=dEpOSHluRzFpVTg3Qm10OS9PTTU3dz09