# Symmetry, Phases of Matter, and Resources in Quantum Computing

Symmetry, Phases of Matter, and Resources in Quantum Computing

## Error correction with the color code

Friday Nov 29, 2019
Speaker(s):

The color code is a topological quantum code with many valuable fault-tolerant logical gates. Its two-dimensional version may soon be realized with currently available superconducting hardware despite constrained qubit connectivity. In the talk, I will focus on how to perform error correction with the color code in d ≥ 2 dimensions. I will describe an efficient color code decoder, the Restriction Decoder, which uses as a subroutine any toric code decoder.

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## How Wilson lines in AdS redundantly compute CFT correlation functions

Friday Nov 29, 2019
Speaker(s):

In the AdS/CFT correspondence, global symmetries of the CFT are realized as local symmetries of AdS; this feature underlies the error-correcting property of AdS. I will explain how this allows AdS3 to realize multiple redundant computations of any CFT2 correlation function in the form of networks of Wilson lines. The main motivation is to rigorously define the CFT at a cutoff and study it as a model of computational complexity; in that regard we will find agreement with the holographic "Complexity = Volume" proposal. But the framework might be useful more generally.

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## Entanglement and extended conformal field theory (or how to get a tensor network from a CFT path integral)

Friday Nov 29, 2019
Speaker(s):

In a continuum field theory the Hilbert space does not factorize into local tensor products. How then can we define entanglement and the basic protocols of quantum information theory? In this talk we will show how the factorization problem can be solved in a class of 2D conformal field theories by directly appealing to the fusion rules. The solution suggests a tensor network description of a CFT path integral using the OPE data.

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## The representation theory of the Clifford group, with applications to resource theories

Thursday Nov 28, 2019
Speaker(s):

I will report on an ongoing project to work out and exploit an analogue of Schur-Weyl duality for the Clifford group. Schur-Weyl establishes a one-one correspondence between irreps of the unitary group and those of the symmetric group. A similar program can be carried out for Cliffords.
The permutations are then replaced by certain discrete orthogonal maps.

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## Two-dimensional AKLT states as ground states of gapped Hamiltonians and resource for universal quantum computation

Thursday Nov 28, 2019
Speaker(s):

Affleck, Kennedy, Lieb, and Tasaki (AKLT) constructed one-dimensional and two-dimensional spin models invariant under spin rotation. These are recognized as paradigmatic examples of symmetry-protected topological phases, including the spin-1 AKLT chain with a provable nonzero spectral gap that strongly supports Haldane’s conjecture on the spectral gap of integer chains.

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## No-go theorems for quantum resource purification

Thursday Nov 28, 2019
Speaker(s):

The manipulation of quantum "resources" such as entanglement and coherence lies at the heart of quantum advantages and technologies. In practice, a particularly important kind of manipulation is to "purify" the quantum resources, since they are inevitably contaminated by noises and thus often lost their power or become unreliable for direct usage. Here we derive fundamental limitations on how effectively generic noisy resources can be purified enforced by the laws of quantum mechanics, which universally apply to any reasonable kind of quantum resource.

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## Stabilizer codes for prime power qudits

Thursday Nov 28, 2019
Speaker(s):

There is a standard generalization of stabilizer codes to work with qudits which have prime dimension, and a slightly less standard generalization for qudits whose dimension is a prime power. However, for prime power dimensions, the usual generalization effectively treats the qudit as multiple prime-dimensional qudits instead of one larger object. There is a finite field GF(q) with size equal to any prime power, and it makes sense to label the qudit basis states with elements of the finite field, but the usual stabilizer codes do not make use of the structure of the finite field.

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## Self-correction from symmetry

Thursday Nov 28, 2019
Speaker(s):

A self-correcting quantum memory can store and protect quantum information for a time that increases without bound in the system size, without the need for active error correction. Unfortunately, the landscape of Hamiltonians based on stabilizer (subspace) codes is heavily constrained by numerous no-go results and it is not known if they can exist in three dimensions or less. In this talk, we will discuss the role of symmetry in self-correcting memories.

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## Towards local testability for quantum coding

Wednesday Nov 27, 2019
Speaker(s):

We introduce the hemicubic codes, a family of quantum codes obtained by associating qubits with the

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## Fine-grained quantum supremacy and stabilizer rank

Wednesday Nov 27, 2019
Speaker(s):

It is known that several sub-universal quantum computing models cannot be classically simulated unless the polynomial-time
hierarchy collapses. However, these results exclude only polynomial-time classical simulations. In this talk, based on fine-grained
complexity conjectures, I show more fine-grained" quantum supremacy results that prohibit certain exponential-time classical simulations.
I also show the stabilizer rank conjecture under fine-grained complexity conjectures.

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## LECTURES ON-DEMAND

### Gabriela González, Louisiana State University

Speaker: Gabriela Gonzalez