## On experimentally relevant quantum speedups

This is an editorial on the article Achieving quantum supremacy with sparse and noisy commuting quantum computations by Michael J. Bremner, Ashley Montanaro and Dan J. Shepherd, published in Quantum …

## The surface code with a twist

Theodore J. Yoder and Isaac H. Kim

Quantum 1, 2 (2017). https://doi.org/10.22331/q-2017-04-25-2

The surface code is one of the most successful approaches to topological quantum error-correction. It boasts the smallest known syndrome extraction circuits and correspondingly largest thresholds. Defect-based logical encodings of a new variety called twists…

## Better local hidden variable models for two-qubit Werner states and an upper bound on the Grothendieck constant $K_G(3)$

Flavien Hirsch, Marco Túlio Quintino, Tamás Vértesi, Miguel Navascués, and Nicolas Brunner

Quantum 1, 3 (2017). https://doi.org/10.22331/q-2017-04-25-3

We consider the problem of reproducing the correlations obtained by arbitrary local projective measurements on the two-qubit Werner state $\rho = v |\psi_- \rangle \langle\psi_- | + (1- v ) \frac{1}{4}$ via a local hidden…

## Limits on the storage of quantum information in a volume of space

Steven T. Flammia, Jeongwan Haah, Michael J. Kastoryano, and Isaac H. Kim

Quantum 1, 4 (2017). https://doi.org/10.22331/q-2017-04-25-4

We study the fundamental limits on the reliable storage of quantum information in lattices of qubits by deriving tradeoff bounds for approximate quantum error correcting codes. We introduce a notion of local approximate correctability and…

## QInfer: Statistical inference software for quantum applications

Christopher Granade, Christopher Ferrie, Ian Hincks, Steven Casagrande, Thomas Alexander, Jonathan Gross, Michal Kononenko, and Yuval Sanders

Quantum 1, 5 (2017). https://doi.org/10.22331/q-2017-04-25-5

Characterizing quantum systems through experimental data is critical to applications as diverse as metrology and quantum computing. Analyzing this experimental data in a robust and reproducible manner is made challenging, however, by the lack of…

## Extremal eigenvalues of local Hamiltonians

Aram W. Harrow and Ashley Montanaro

Quantum 1, 6 (2017). https://doi.org/10.22331/q-2017-04-25-6

We apply classical algorithms for approximately solving constraint satisfaction problems to find bounds on extremal eigenvalues of local Hamiltonians. We consider spin Hamiltonians for which we have an upper bound on the number of terms…

## Proposal for witnessing non-classical light with the human eye

A. Dodel, A. Mayinda, E. Oudot, A. Martin, P. Sekatski, J.-D. Bancal, and N. Sangouard

Quantum 1, 7 (2017). https://doi.org/10.22331/q-2017-04-25-7

We give a complete proposal showing how to detect the non-classical nature of photonic states with naked eyes as detectors. The enabling technology is a sub-Poissonian photonic state that is obtained from single photons, displacement…

## Achieving quantum supremacy with sparse and noisy commuting quantum computations

Michael J. Bremner, Ashley Montanaro, and Dan J. Shepherd

Quantum 1, 8 (2017). https://doi.org/10.22331/q-2017-04-25-8

The class of commuting quantum circuits known as IQP (instantaneous quantum polynomial-time) has been shown to be hard to simulate classically, assuming certain complexity-theoretic conjectures. Here we study the power of IQP circuits in the…

## Finite-density phase diagram of a $(1+1)-d$ non-abelian lattice gauge theory with tensor networks

Pietro Silvi, Enrique Rico, Marcello Dalmonte, Ferdinand Tschirsich, and Simone Montangero

Quantum 1, 9 (2017). https://doi.org/10.22331/q-2017-04-25-9

We investigate the finite-density phase diagram of a non-abelian SU(2) lattice gauge theory in (1+1)-dimensions using tensor network methods. We numerically characterise the phase diagram as a function of the matter filling and of the…

## A purification postulate for quantum mechanics with indefinite causal order

Mateus Araújo, Adrien Feix, Miguel Navascués, and Časlav Brukner

Quantum 1, 10 (2017). https://doi.org/10.22331/q-2017-04-26-10

To study which are the most general causal structures which are compatible with local quantum mechanics, Oreshkov et al. introduced the notion of a process: a resource shared between some parties that allows for quantum…