140 research outputs found
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
Renyi entropies as a measure of the complexity of counting problems
Counting problems such as determining how many bit strings satisfy a given
Boolean logic formula are notoriously hard. In many cases, even getting an
approximate count is difficult. Here we propose that entanglement, a common
concept in quantum information theory, may serve as a telltale of the
difficulty of counting exactly or approximately. We quantify entanglement by
using Renyi entropies S(q), which we define by bipartitioning the logic
variables of a generic satisfiability problem. We conjecture that
S(q\rightarrow 0) provides information about the difficulty of counting
solutions exactly, while S(q>0) indicates the possibility of doing an efficient
approximate counting. We test this conjecture by employing a matrix computing
scheme to numerically solve #2SAT problems for a large number of uniformly
distributed instances. We find that all Renyi entropies scale linearly with the
number of variables in the case of the #2SAT problem; this is consistent with
the fact that neither exact nor approximate efficient algorithms are known for
this problem. However, for the negated (disjunctive) form of the problem,
S(q\rightarrow 0) scales linearly while S(q>0) tends to zero when the number of
variables is large. These results are consistent with the existence of fully
polynomial-time randomized approximate algorithms for counting solutions of
disjunctive normal forms and suggests that efficient algorithms for the
conjunctive normal form may not exist.Comment: 13 pages, 4 figure
Quantum Searches in a Hard 2SAT Ensemble
Using a recently constructed ensemble of hard 2SAT realizations, that has a
unique ground-state we calculate for the quantized theory the median gap
correlation length values along the direction of the quantum
adiabatic control parameter . We use quantum annealing (QA) with
transverse field and a linear time schedule in the adiabatic control parameter
. The gap correlation length diverges exponentially in the median with a rate constant , while the run time diverges exponentially with . Simulated
classical annealing (SA) exhibits a run time rate constant that is small and thus finds ground-states exponentially faster
than QA. There are no quantum speedups in ground state searches on constant
energy surfaces that have exponentially large volume. We also determine gap
correlation length distribution functions over the ensemble that at are close to Weibull functions
with i.e., the problems show thin catastrophic tails in
. The inferred success probability distribution functions of the
quantum annealer turn out to be bimodal.Comment: non
High Fidelity Adiabatic Quantum Computation via Dynamical Decoupling
We introduce high-order dynamical decoupling strategies for open system
adiabatic quantum computation. Our numerical results demonstrate that a
judicious choice of high-order dynamical decoupling method, in conjunction with
an encoding which allows computation to proceed alongside decoupling, can
dramatically enhance the fidelity of adiabatic quantum computation in spite of
decoherence.Comment: 5 pages, 4 figure
Picturing counting reductions with the ZH-calculus
Counting the solutions to Boolean formulae defines the problem #SAT, which is
complete for the complexity class #P. We use the ZH-calculus, a universal and
complete graphical language for linear maps which naturally encodes counting
problems in terms of diagrams, to give graphical reductions from #SAT to
several related counting problems. Some of these graphical reductions, like to
#2SAT, are substantially simpler than known reductions via the matrix
permanent. Additionally, our approach allows us to consider the case of
counting solutions modulo an integer on equal footing. Finally, since the
ZH-calculus was originally introduced to reason about quantum computing, we
show that the problem of evaluating ZH-diagrams in the fragment corresponding
to the Clifford+T gateset, is in . Our results show that graphical
calculi represent an intuitive and useful framework for reasoning about
counting problems
Monte Carlo Search for Very Hard KSAT Realizations for Use in Quantum Annealing
Using powerful Multicanonical Ensemble Monte Carlo methods from statistical
physics we explore the realization space of random K satisfiability (KSAT) in
search for computational hard problems, most likely the 'hardest problems'. We
search for realizations with unique satisfying assignments (USA) at ratio of
clause to spin number that is minimal. USA realizations are found
for -values that approach from above with increasing number
of spins . We consider small spin numbers in . The ensemble
mean exhibits very special properties. We find that the density of states of
the first excited state with energy one is consistent with an
exponential divergence in : . The rate
constants for and of KSAT with USA realizations at
are determined numerically to be in the interval at and
at . These approach the unstructured search value
with increasing . Our ensemble of hard problems is expected to provide a
test bed for studies of quantum searches with Hamiltonians that have the form
of general Ising models.Comment: non
On the complexity of trial and error for constraint satisfaction problems
In 2013 Bei, Chen and Zhang introduced a trial and error model of computing, and applied to some constraint satisfaction problems. In this model the input is hidden by an oracle which, for a candidate assignment, reveals some information about a violated constraint if the assignment is not satisfying. In this paper we initiate a systematic study of constraint satisfaction problems in the trial and error model, by adopting a formal framework for CSPs, and defining several types of revealing oracles. Our main contribution is to develop a transfer theorem for each type of the revealing oracle. To any hidden CSP with a specific type of revealing Oracle, the transfer theorem associates another CSP in the normal setting, such that their complexities are polynomial-time equivalent. This in principle transfers the study of a large class of hidden CSPs to the study of normal CSPs. We apply the transfer theorems to get polynomial-time algorithms or hardness results for several families of concrete problems. (C) 2017 Elsevier Inc. All rights reserved
- …