63 research outputs found
Classical simulations of Abelian-group normalizer circuits with intermediate measurements
Quantum normalizer circuits were recently introduced as generalizations of
Clifford circuits [arXiv:1201.4867]: a normalizer circuit over a finite Abelian
group is composed of the quantum Fourier transform (QFT) over G, together
with gates which compute quadratic functions and automorphisms. In
[arXiv:1201.4867] it was shown that every normalizer circuit can be simulated
efficiently classically. This result provides a nontrivial example of a family
of quantum circuits that cannot yield exponential speed-ups in spite of usage
of the QFT, the latter being a central quantum algorithmic primitive. Here we
extend the aforementioned result in several ways. Most importantly, we show
that normalizer circuits supplemented with intermediate measurements can also
be simulated efficiently classically, even when the computation proceeds
adaptively. This yields a generalization of the Gottesman-Knill theorem (valid
for n-qubit Clifford operations [quant-ph/9705052, quant-ph/9807006] to quantum
circuits described by arbitrary finite Abelian groups. Moreover, our
simulations are twofold: we present efficient classical algorithms to sample
the measurement probability distribution of any adaptive-normalizer
computation, as well as to compute the amplitudes of the state vector in every
step of it. Finally we develop a generalization of the stabilizer formalism
[quant-ph/9705052, quant-ph/9807006] relative to arbitrary finite Abelian
groups: for example we characterize how to update stabilizers under generalized
Pauli measurements and provide a normal form of the amplitudes of generalized
stabilizer states using quadratic functions and subgroup cosets.Comment: 26 pages+appendices. Title has changed in this second version. To
appear in Quantum Information and Computation, Vol.14 No.3&4, 201
Contextuality as a resource for models of quantum computation on qubits
A central question in quantum computation is to identify the resources that
are responsible for quantum speed-up. Quantum contextuality has been recently
shown to be a resource for quantum computation with magic states for odd-prime
dimensional qudits and two-dimensional systems with real wavefunctions. The
phenomenon of state-independent contextuality poses a priori an obstruction to
characterizing the case of regular qubits, the fundamental building block of
quantum computation. Here, we establish contextuality of magic states as a
necessary resource for a large class of quantum computation schemes on qubits.
We illustrate our result with a concrete scheme related to measurement-based
quantum computation.Comment: Published version. We have revised the title, introduction and
discussion, as well as slightly simplified the setting in this versio
Normalizer Circuits and Quantum Computation
(Abridged abstract.) In this thesis we introduce new models of quantum
computation to study the emergence of quantum speed-up in quantum computer
algorithms.
Our first contribution is a formalism of restricted quantum operations, named
normalizer circuit formalism, based on algebraic extensions of the qubit
Clifford gates (CNOT, Hadamard and -phase gates): a normalizer circuit
consists of quantum Fourier transforms (QFTs), automorphism gates and quadratic
phase gates associated to a set , which is either an abelian group or
abelian hypergroup. Though Clifford circuits are efficiently classically
simulable, we show that normalizer circuit models encompass Shor's celebrated
factoring algorithm and the quantum algorithms for abelian Hidden Subgroup
Problems. We develop classical-simulation techniques to characterize under
which scenarios normalizer circuits provide quantum speed-ups. Finally, we
devise new quantum algorithms for finding hidden hyperstructures. The results
offer new insights into the source of quantum speed-ups for several algebraic
problems.
Our second contribution is an algebraic (group- and hypergroup-theoretic)
framework for describing quantum many-body states and classically simulating
quantum circuits. Our framework extends Gottesman's Pauli Stabilizer Formalism
(PSF), wherein quantum states are written as joint eigenspaces of stabilizer
groups of commuting Pauli operators: while the PSF is valid for qubit/qudit
systems, our formalism can be applied to discrete- and continuous-variable
systems, hybrid settings, and anyonic systems. These results enlarge the known
families of quantum processes that can be efficiently classically simulated.
This thesis also establishes a precise connection between Shor's quantum
algorithm and the stabilizer formalism, revealing a common mathematical
structure in several quantum speed-ups and error-correcting codes.Comment: PhD thesis, Technical University of Munich (2016). Please cite
original papers if possible. Appendix E contains unpublished work on Gaussian
unitaries. If you spot typos/omissions please email me at JLastNames at
posteo dot net. Source: http://bit.ly/2gMdHn3. Related video talk:
https://www.perimeterinstitute.ca/videos/toy-theory-quantum-speed-ups-based-stabilizer-formalism
Posted on my birthda
Classical simulation complexity of extended Clifford circuits
Clifford gates are a winsome class of quantum operations combining
mathematical elegance with physical significance. The Gottesman-Knill theorem
asserts that Clifford computations can be classically efficiently simulated but
this is true only in a suitably restricted setting. Here we consider Clifford
computations with a variety of additional ingredients: (a) strong vs. weak
simulation, (b) inputs being computational basis states vs. general product
states, (c) adaptive vs. non-adaptive choices of gates for circuits involving
intermediate measurements, (d) single line outputs vs. multi-line outputs. We
consider the classical simulation complexity of all combinations of these
ingredients and show that many are not classically efficiently simulatable
(subject to common complexity assumptions such as P not equal to NP). Our
results reveal a surprising proximity of classical to quantum computing power
viz. a class of classically simulatable quantum circuits which yields universal
quantum computation if extended by a purely classical additional ingredient
that does not extend the class of quantum processes occurring.Comment: 17 pages, 1 figur
Generalized Cluster States Based on Finite Groups
We define generalized cluster states based on finite group algebras in
analogy to the generalization of the toric code to the Kitaev quantum double
models. We do this by showing a general correspondence between systems with CSS
structure and finite group algebras, and applying this to the cluster states to
derive their generalization. We then investigate properties of these states
including their PEPS representations, global symmetries, and relationship to
the Kitaev quantum double models. We also discuss possible applications of
these states.Comment: 23 pages, 4 figure
- …