12 research outputs found
A Quantum Version of Sch\"oning's Algorithm Applied to Quantum 2-SAT
We study a quantum algorithm that consists of a simple quantum Markov
process, and we analyze its behavior on restricted versions of Quantum 2-SAT.
We prove that the algorithm solves this decision problem with high probability
for n qubits, L clauses, and promise gap c in time O(n^2 L^2 c^{-2}). If the
Hamiltonian is additionally polynomially gapped, our algorithm efficiently
produces a state that has high overlap with the satisfying subspace. The Markov
process we study is a quantum analogue of Sch\"oning's probabilistic algorithm
for k-SAT
Linear time algorithm for quantum 2SAT
A canonical result about satisfiability theory is that the 2-SAT problem can
be solved in linear time, despite the NP-hardness of the 3-SAT problem. In the
quantum 2-SAT problem, we are given a family of 2-qubit projectors
on a system of qubits, and the task is to decide whether the Hamiltonian
has a 0-eigenvalue, or it is larger than for
some . The problem is not only a natural extension of the
classical 2-SAT problem to the quantum case, but is also equivalent to the
problem of finding the ground state of 2-local frustration-free Hamiltonians of
spin , a well-studied model believed to capture certain key
properties in modern condensed matter physics. While Bravyi has shown that the
quantum 2-SAT problem has a classical polynomial-time algorithm, the running
time of his algorithm is . In this paper we give a classical algorithm
with linear running time in the number of local projectors, therefore achieving
the best possible complexity.Comment: 20 page
On the complexity of probabilistic trials for hidden satisfiability problems
What is the minimum amount of information and time needed to solve 2SAT? When
the instance is known, it can be solved in polynomial time, but is this also
possible without knowing the instance? Bei, Chen and Zhang (STOC '13)
considered a model where the input is accessed by proposing possible
assignments to a special oracle. This oracle, on encountering some constraint
unsatisfied by the proposal, returns only the constraint index. It turns out
that, in this model, even 1SAT cannot be solved in polynomial time unless P=NP.
Hence, we consider a model in which the input is accessed by proposing
probability distributions over assignments to the variables. The oracle then
returns the index of the constraint that is most likely to be violated by this
distribution. We show that the information obtained this way is sufficient to
solve 1SAT in polynomial time, even when the clauses can be repeated. For 2SAT,
as long as there are no repeated clauses, in polynomial time we can even learn
an equivalent formula for the hidden instance and hence also solve it.
Furthermore, we extend these results to the quantum regime. We show that in
this setting 1QSAT can be solved in polynomial time up to constant precision,
and 2QSAT can be learnt in polynomial time up to inverse polynomial precision.Comment: 24 pages, 2 figures. To appear in the 41st International Symposium on
Mathematical Foundations of Computer Scienc
On Efficiently Solvable Cases of Quantum k-SAT
The constraint satisfaction problems k-SAT and Quantum k-SAT (k-QSAT) are canonical NP-complete and QMA_1-complete problems (for k >= 3), respectively, where QMA_1 is a quantum generalization of NP with one-sided error. Whereas k-SAT has been well-studied for special tractable cases, as well as from a parameterized complexity perspective, much less is known in similar settings for k-QSAT. Here, we study the open problem of computing satisfying assignments to k-QSAT instances which have a "matching" or "dimer covering"; this is an NP problem whose decision variant is trivial, but whose search complexity remains open.
Our results fall into three directions, all of which relate to the "matching" setting: (1) We give a polynomial-time classical algorithm for k-QSAT when all qubits occur in at most two clauses. (2) We give a parameterized algorithm for k-QSAT instances from a certain non-trivial class, which allows us to obtain exponential speedups over brute force methods in some cases by reducing the problem to solving for a single root of a single univariate polynomial. (3) We conduct a structural graph theoretic study of 3-QSAT interaction graphs which have a "matching". We remark that the results of (2), in particular, introduce a number of new tools to the study of Quantum SAT, including graph theoretic concepts such as transfer filtrations and blow-ups from algebraic geometry; we hope these prove useful elsewhere
A Quantum Version of Schöning's Algorithm Applied to Quantum 2-SAT
We study a quantum algorithm that consists of a simple quantum Markov process, and we analyze its behavior on restricted versions of Quantum 2-SAT. We prove that the algorithm solves these decision problems with high probability for n qubits, L clauses, and promise gap c in time O(n2L2c-2). If the Hamiltonian is additionally polynomially gapped, our algorithm efficiently produces a state that has high overlap with the satisfying subspace. The Markov process we study is a quantum analogue of Schöning's probabilistic algorithm for k-SAT
Quantum Hamiltonian Complexity
Constraint satisfaction problems are a central pillar of modern computational
complexity theory. This survey provides an introduction to the rapidly growing
field of Quantum Hamiltonian Complexity, which includes the study of quantum
constraint satisfaction problems. Over the past decade and a half, this field
has witnessed fundamental breakthroughs, ranging from the establishment of a
"Quantum Cook-Levin Theorem" to deep insights into the structure of 1D
low-temperature quantum systems via so-called area laws. Our aim here is to
provide a computer science-oriented introduction to the subject in order to
help bridge the language barrier between computer scientists and physicists in
the field. As such, we include the following in this survey: (1) The
motivations and history of the field, (2) a glossary of condensed matter
physics terms explained in computer-science friendly language, (3) overviews of
central ideas from condensed matter physics, such as indistinguishable
particles, mean field theory, tensor networks, and area laws, and (4) brief
expositions of selected computer science-based results in the area. For
example, as part of the latter, we provide a novel information theoretic
presentation of Bravyi's polynomial time algorithm for Quantum 2-SAT.Comment: v4: published version, 127 pages, introduction expanded to include
brief introduction to quantum information, brief list of some recent
developments added, minor changes throughou
Zero and Low Energy Thresholds in Quantum Simulation
Quantum simulation is the process of simulating a quantum mechanical system using either a quantum or a classical computer. Because quantum mechanical systems contain a large number of entangled particles, they are hard to simulate on a classical computer. It is the task of computational complexity theorists to estimate the amount of resources to do the same number of operations on either classical or quantum devices. This report first summarizes the state of the art in the field of quantum computing, and gives an example of a model of quantum computer and examples of quantum algorithms that are currently being researched. Then our own research about k-local quantum Hamiltonians is discussed. We developed programs to determine if a particular kind of k-local Hamiltonian has zero-energy solutions. First, to familiarize ourselves with quantum algorithms, we implemented a recently discovered polynomial-time 2-QSAT algorithm called SolveQ. Then we wrote several versions of brute force 7-variable 3-QSAT solvers and conducted experiments for the threshold of satisfiability. We empirically determined that the thresholds for the four versions, Versions 3, 4, 5, and 6, are 0.741, 1.714, 1.714, and 0.571, respectively. In addition, experiments were conducted involving the 6-qubit Ising model, working on which caused us to realize how inefficient the classical computer really is at simulating quantum mechanical systems. Our conclusion is that quantum simulation is much less feasible than classical simulation on a classical computer
Perturbation Gadgets: Arbitrary Energy Scales from a Single Strong Interaction
In this work we propose a many-body Hamiltonian construction which introduces
only a single separate energy scale of order , for a
small parameter , and for terms in the target Hamiltonian. In its
low-energy subspace, the construction can approximate any normalized target
Hamiltonian with norm ratios
to within relative precision . This comes at the expense
of increasing the locality by at most one, and adding an at most poly-sized
ancilliary system for each coupling; interactions on the ancilliary system are
geometrically local, and can be translationally-invariant.
As an application, we discuss implications for QMA-hardness of the local
Hamiltonian problem, and argue that "almost" translational invariance-defined
as arbitrarily small relative variations of the strength of the local terms-is
as good as non-translational-invariance in many of the constructions used
throughout Hamiltonian complexity theory. We furthermore show that the choice
of geared limit of many-body systems, where e.g. width and height of a lattice
are taken to infinity in a specific relation, can have different
complexity-theoretic implications: even for translationally-invariant models,
changing the geared limit can vary the hardness of finding the ground state
energy with respect to a given promise gap from computationally trivial, to
QMAEXP-, or even BQEXPSPACE-complete
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Quantum Stochastic Processes and Quantum Many-Body Physics
This dissertation investigates the theory of quantum stochastic processes and its applications in quantum many-body physics.
The main goal is to analyse complexity-theoretic aspects of both static and dynamic properties of physical systems modelled by quantum stochastic processes.
The thesis consists of two parts: the first one addresses the computational complexity of certain quantum and classical divisibility questions, whereas the second one addresses the topic of Hamiltonian complexity theory.
In the divisibility part, we discuss the question whether one can efficiently sub-divide a map describing the evolution of a system in a noisy environment, i.e. a CPTP- or stochastic map for quantum and classical processes, respectively, and we prove that taking the nth root of a CPTP or stochastic map is an NP-complete problem.
Furthermore, we show that answering the question whether one can divide up a random variable into a sum of iid random variables , i.e. , is poly-time computable; relaxing the iid condition renders the problem NP-hard.
In the local Hamiltonian part, we study computation embedded into the ground state of a many-body quantum system, going beyond "history state" constructions with a linear clock.
We first develop a series of mathematical techniques which allow us to study the energy spectrum of the resulting Hamiltonian, and extend classical string rewriting to the quantum setting.
This allows us to construct the most physically-realistic QMAEXP-complete instances for the LOCAL HAMILTONIAN problem (i.e. the question of estimating the ground state energy of a quantum many-body system) known to date, both in one- and three dimensions.
Furthermore, we study weighted versions of linear history state constructions, allowing us to obtain tight lower and upper bounds on the promise gap of the LOCAL HAMILTONIAN problem in various cases.
We finally study a classical embedding of a Busy Beaver Turing Machine into a low-dimensional lattice spin model, which allows us to dictate a transition from a purely classical phase to a Toric Code phase at arbitrarily large and potentially even uncomputable system sizes