310 research outputs found
SurfBraid: A concept tool for preparing and resource estimating quantum circuits protected by the surface code
The first generations of quantum computers will execute fault-tolerant
quantum circuits, and it is very likely that such circuits will use surface
quantum error correcting codes. To the best of our knowledge, no complete
design automation tool for such circuits is currently available. This is to a
large extent because such circuits have three dimensional layouts (e.g. two
dimensional hardware and time axis as a third dimension) and their optimisation
is still ongoing research. This work introduces SurfBraid, a tool for the
automatic design of surface code protected quantum circuits -- it includes a
complete workflow that compiles an arbitrary quantum circuit into an
intermediary Clifford+T equivalent representation which is further synthesised
and optimised to surface code protected structures (for the moment, braided
defects). SurfBraid is arguably the first flexible (modular structure,
extensible through user provided scripts) and interactive (automatically
updating the results based on user interaction, browser based) tool for such
circuits. One of the prototype's methodological novelty is its capability to
automatically estimate the resources necessary for executing large
fault-tolerant circuits. A prototype implementation and the corresponding
source code are available at https://alexandrupaler.github.io/quantjs/
Circular CNOT Circuits: Definition, Analysis and Application to Fault-Tolerant Quantum Circuits
The work proposes an extension of the quantum circuit formalism where qubits
(wires) are circular instead of linear. The left-to-right interpretation of a
quantum circuit is replaced by a circular representation which allows to select
the starting point and the direction in which gates are executed. The
representation supports all the circuits obtained after computing cyclic
permutations of an initial quantum gate list. Two circuits, where one has a
gate list which is a cyclic permutation of the other, will implement different
functions. The main question appears in the context of scalable quantum
computing, where multiple subcircuits are used for the construction of a larger
fault-tolerant one: can the same circular representation be used by multiple
subcircuits? The circular circuits are sufficient for constructing
computationally universal, fault-tolerant circuits formed entirely of qubit
initialisation, CNOT gates and qubit measurements. The main result of modelling
circular CNOT circuits is that a derived Boolean representation allows to
define a set of equations for and stabiliser transformations. Through a
well defined set of steps it is possible to reduce the initial equations to a
set of stabiliser transformations given a series of cuts through the circular
circuit.Comment: 14 pages, 8 figures, accepted at 8th Conference on Reversible
Computation, 201
An Efficient Methodology for Mapping Quantum Circuits to the IBM QX Architectures
In the past years, quantum computers more and more have evolved from an
academic idea to an upcoming reality. IBM's project IBM Q can be seen as
evidence of this progress. Launched in March 2017 with the goal to provide
access to quantum computers for a broad audience, this allowed users to conduct
quantum experiments on a 5-qubit and, since June 2017, also on a 16-qubit
quantum computer (called IBM QX2 and IBM QX3, respectively). Revised versions
of these 5-qubit and 16-qubit quantum computers (named IBM QX4 and IBM QX5,
respectively) are available since September 2017. In order to use these, the
desired quantum functionality (e.g. provided in terms of a quantum circuit) has
to be properly mapped so that the underlying physical constraints are satisfied
- a complex task. This demands solutions to automatically and efficiently
conduct this mapping process. In this paper, we propose a methodology which
addresses this problem, i.e. maps the given quantum functionality to a
realization which satisfies all constraints given by the architecture and, at
the same time, keeps the overhead in terms of additionally required quantum
gates minimal. The proposed methodology is generic, can easily be configured
for similar future architectures, and is fully integrated into IBM's SDK.
Experimental evaluations show that the proposed approach clearly outperforms
IBM's own mapping solution. In fact, for many quantum circuits, the proposed
approach determines a mapping to the IBM architecture within minutes, while
IBM's solution suffers from long runtimes and runs into a timeout of 1 hour in
several cases. As an additional benefit, the proposed approach yields mapped
circuits with smaller costs (i.e. fewer additional gates are required). All
implementations of the proposed methodology is publicly available at
http://iic.jku.at/eda/research/ibm_qx_mapping
An introduction to Fault-tolerant Quantum Computing
In this paper we provide a basic introduction of the core ideas and theories
surrounding fault-tolerant quantum computation. These concepts underly the
theoretical framework of large-scale quantum computation and communications and
are the driving force for many recent experimental efforts to construct small
to medium sized arrays of controllable quantum bits. We examine the basic
principals of redundant quantum encoding, required to protect quantum bits from
errors generated from both imprecise control and environmental interactions and
then examine the principals of fault-tolerance from largely a classical
framework. As quantum fault-tolerance essentially is avoiding the
uncontrollable cascade of errors caused by the interaction of quantum-bits,
these concepts can be directly mapped to quantum information.Comment: Intro to fault-tolerant quantum computing from the perspective of the
classical community, 7 page
NISQ circuit compilation is the travelling salesman problem on a torus
Noisy, intermediate-scale quantum (NISQ) computers are expected to execute
quantum circuits of up to a few hundred qubits. The circuits have to conform to
NISQ architectural constraints regarding qubit allocation and the execution of
multi-qubit gates. Quantum circuit compilation (QCC) takes a nonconforming
circuit and outputs a compatible circuit. Can classical optimisation methods be
used for QCC? Compilation is a known combinatorial problem shown to be solvable
by two types of operations: 1) qubit allocation, and 2) gate scheduling. We
show informally that the two operations form a discrete ring. The search
landscape of QCC is a two dimensional discrete torus where vertices represent
configurations of how circuit qubits are allocated to NISQ registers. Torus
edges are weighted by the cost of scheduling circuit gates. The novelty of our
approach uses the fact that a circuit's gate list is circular: compilation can
start from any gate as long as all the gates will be processed, and the
compiled circuit has the correct gate order. By showing that QCC can be solved
as a travelling salesman problem, we bridge a theoretical and practical gap
between classical circuit design automation and the emerging field of quantum
circuit optimisation.Comment: rewritten. added torus. showing similarity with ts
Retrospective Causal Inference with Machine Learning Ensembles: An Application to Anti-Recidivism Policies in Colombia
We present new methods to estimate causal effects retrospectively from micro
data with the assistance of a machine learning ensemble. This approach
overcomes two important limitations in conventional methods like regression
modeling or matching: (i) ambiguity about the pertinent retrospective
counterfactuals and (ii) potential misspecification, overfitting, and otherwise
bias-prone or inefficient use of a large identifying covariate set in the
estimation of causal effects. Our method targets the analysis toward a well
defined ``retrospective intervention effect'' (RIE) based on hypothetical
population interventions and applies a machine learning ensemble that allows
data to guide us, in a controlled fashion, on how to use a large identifying
covariate set. We illustrate with an analysis of policy options for reducing
ex-combatant recidivism in Colombia
Synthesis of Arbitrary Quantum Circuits to Topological Assembly: Systematic, Online and Compact
It is challenging to transform an arbitrary quantum circuit into a form
protected by surface code quantum error correcting codes (a variant of
topological quantum error correction), especially if the goal is to minimise
overhead. One of the issues is the efficient placement of magic state
distillation sub circuits, so-called distillation boxes, in the space-time
volume that abstracts the computation's required resources. This work presents
a general, systematic, online method for the synthesis of such circuits.
Distillation box placement is controlled by so-called schedulers. The work
introduces a greedy scheduler generating compact box placements. The
implemented software, whose source code is available online, is used to
illustrate and discuss synthesis examples. Synthesis and optimisation
improvements are proposed
Online Scheduled Execution of Quantum Circuits Protected by Surface Codes
Quantum circuits are the preferred formalism for expressing quantum
information processing tasks. Quantum circuit design automation methods mostly
use a waterfall approach and consider that high level circuit descriptions are
hardware agnostic. This assumption has lead to a static circuit perspective:
the number of quantum bits and quantum gates is determined before circuit
execution and everything is considered reliable with zero probability of
failure. Many different schemes for achieving reliable fault-tolerant quantum
computation exist, with different schemes suitable for different architectures.
A number of large experimental groups are developing architectures well suited
to being protected by surface quantum error correcting codes. Such circuits
could include unreliable logical elements, such as state distillation, whose
failure can be determined only after their actual execution. Therefore,
practical logical circuits, as envisaged by many groups, are likely to have a
dynamic structure. This requires an online scheduling of their execution: one
knows for sure what needs to be executed only after previous elements have
finished executing. This work shows that scheduling shares similarities with
place and route methods. The work also introduces the first online schedulers
of quantum circuits protected by surface codes. The work also highlights
scheduling efficiency by comparing the new methods with state of the art static
scheduling of surface code protected fault-tolerant circuits.Comment: accepted in QI
Reliable quantum circuits have defects
State of the art quantum computing architectures are founded on the decision
to use scalable but faulty quantum hardware in conjunction with an efficient
error correcting code capable of tolerating high error rates. The promised
effect of this decision is that the first large-scale practical quantum
computer is within reach. Coming to grips with the strategy and the challenges
of preparing reliable executions of an arbitrary quantum computation is not
difficult. Moreover, the article explains why defects are good.Comment: preprint of the paper from XRD
Synthesis of Arbitrary Quantum Circuits to Topological Assembly
Given a quantum algorithm, it is highly nontrivial to devise an efficient
sequence of physical gates implementing the algorithm on real hardware and
incorporating topological quantum error correction. In this paper, we present a
first step towards this goal, focusing on generating correct and simple
arrangements of topological structures that correspond to a given quantum
circuit and largely neglecting their efficiency. We detail the many challenges
that will need to be tackled in the pursuit of efficiency. The software source
code can be consulted at https://github.com/alexandrupaler/tqec.Comment: 24 pages, 28 figure
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