188 research outputs found
Codes and Protocols for Distilling , controlled-, and Toffoli Gates
We present several different codes and protocols to distill ,
controlled-, and Toffoli (or ) gates. One construction is based on
codes that generalize the triorthogonal codes, allowing any of these gates to
be induced at the logical level by transversal . We present a randomized
construction of generalized triorthogonal codes obtaining an asymptotic
distillation efficiency . We also present a Reed-Muller
based construction of these codes which obtains a worse but performs
well at small sizes. Additionally, we present protocols based on checking the
stabilizers of magic states at the logical level by transversal gates
applied to codes; these protocols generalize the protocols of 1703.07847.
Several examples, including a Reed-Muller code for -to-Toffoli distillation,
punctured Reed-Muller codes for -gate distillation, and some of the check
based protocols, require a lower ratio of input gates to output gates than
other known protocols at the given order of error correction for the given code
size. In particular, we find a T-gate to Toffoli gate code with
distance as well as triorthogonal codes with parameters
with very low prefactors in front of
the leading order error terms in those codes.Comment: 28 pages. (v2) fixed a part of the proof on random triorthogonal
codes, added comments on Clifford circuits for Reed-Muller states (v3) minor
chang
Quantization of Hall Conductance For Interacting Electrons on a Torus
We consider interacting, charged spins on a torus described by a gapped
Hamiltonian with a unique groundstate and conserved local charge. Using
quasi-adiabatic evolution of the groundstate around a flux-torus, we prove,
without any averaging assumption, that the Hall conductance of the groundstate
is quantized in integer multiples of e^2/h, up to exponentially small
corrections in the linear size of the system. In addition, we discuss
extensions to the fractional quantization case under an additional topological
order assumption on the degenerate groundstate subspace.Comment: 28 pages, 4 figures, This paper significantly simplifies the proof
and tightens the bounds previously shown in arXiv:0911.4706 by the same
authors. Updated to reflect published versio
Diffusion Processes on Power-Law Small-World Networks
We consider diffusion processes on power-law small-world networks in
different dimensions. In one dimension, we find a rich phase diagram, with
different transient and recurrent phases, including a critical line with
continuously varying exponents. The results were obtained using self-consistent
perturbation theory and can also be understood in terms of a scaling theory,
which provides a general framework for understanding processes on small-world
networks with different distributions of long-range links.Comment: 4 pages, 3 figures, added references, modified Fig. 2 with added data
(PRL, in press
Quantum Codes from High-Dimensional Manifolds
We construct toric codes on various high-dimensional manifolds. Assuming a conjecture in geometry we find families of
quantum CSS stabilizer codes on N qubits with logarithmic weight stabilizers and distance N^{1-epsilon} for any epsilon>0.
The conjecture is that there is a constant C>0 such that for any n-dimensional torus {mathbb T}^n={mathbb R}^n/Lambda, where Lambda is a lattice, the least volume unoriented n/2-dimensional cycle (using the Euclidean metric) representing nontrivial homology has volume at least C^n times the volume of the least volume n/2-dimensional hyperplane representing nontrivial homology; in fact, it would suffice to have this result for Lambda an integral lattice with the cycle restricted to faces of a cubulation by unit hypercubes.
The main technical result is an estimate of Rankin invariants for certain random lattices, showing that in a certain sense they are optimal.
Additionally, we construct codes with square-root distance, logarithmic weight stabilizers, and inverse polylogarithmic soundness factor (considered as quantum locally testable codes.
We also provide an short, alternative proof that the shortest vector in the exterior power of a lattice may be non-split
Finite Size Scaling of Mutual Information: A Scalable Simulation
We develop a quantum Monte Carlo procedure to compute the Renyi mutual
information of an interacting quantum many-body system at non-zero temperature.
Performing simulations on a spin-1/2 XXZ model, we observe that for a subregion
of fixed size embedded in a system of size L, the mutual information converges
at large L to a limiting function which displays non-monotonic temperature
behavior corresponding to the onset of correlations. For a region of size L/2
embedded in a system of size L, the mutual information divided by L converges
to a limiting function of temperature, with apparently nontrivial corrections
near critical points.Comment: 4 pages, 4 figure
Magic State Distillation with Low Space Overhead and Optimal Asymptotic Input Count
We present an infinite family of protocols to distill magic states for
-gates that has a low space overhead and uses an asymptotic number of input
magic states to achieve a given target error that is conjectured to be optimal.
The space overhead, defined as the ratio between the physical qubits to the
number of output magic states, is asymptotically constant, while both the
number of input magic states used per output state and the -gate depth of
the circuit scale linearly in the logarithm of the target error (up to
). Unlike other distillation protocols, this protocol
achieves this performance without concatenation and the input magic states are
injected at various steps in the circuit rather than all at the start of the
circuit. The protocol can be modified to distill magic states for other gates
at the third level of the Clifford hierarchy, with the same asymptotic
performance. The protocol relies on the construction of weakly self-dual CSS
codes with many logical qubits and large distance, allowing us to implement
control-SWAPs on multiple qubits. We call this code the "inner code". The
control-SWAPs are then used to measure properties of the magic state and detect
errors, using another code that we call the "outer code". Alternatively, we use
weakly-self dual CSS codes which implement controlled Hadamards for the inner
code, reducing circuit depth. We present several specific small examples of
this protocol.Comment: 39 pages, (v2) renamed "odd" and "even" weakly self-dual CSS codes of
(v1) to "normal" and "hyperbolic" codes, respectively. (v3) published in
Quantu
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