108 research outputs found
Single query learning from abelian and non-abelian Hamming distance oracles
We study the problem of identifying an n-bit string using a single quantum
query to an oracle that computes the Hamming distance between the query and
hidden strings. The standard action of the oracle on a response register of
dimension r is by powers of the cycle (1...r), all of which, of course,
commute. We introduce a new model for the action of an oracle--by general
permutations in S_r--and explore how the success probability depends on r and
on the map from Hamming distances to permutations. In particular, we prove that
when r = 2, for even n the success probability is 1 with the right choice of
the map, while for odd n the success probability cannot be 1 for any choice.
Furthermore, for small odd n and r = 3, we demonstrate numerically that the
image of the optimal map generates a non-abelian group of permutations.Comment: 14 page
Quantum algorithms for highly non-linear Boolean functions
Attempts to separate the power of classical and quantum models of computation
have a long history. The ultimate goal is to find exponential separations for
computational problems. However, such separations do not come a dime a dozen:
while there were some early successes in the form of hidden subgroup problems
for abelian groups--which generalize Shor's factoring algorithm perhaps most
faithfully--only for a handful of non-abelian groups efficient quantum
algorithms were found. Recently, problems have gotten increased attention that
seek to identify hidden sub-structures of other combinatorial and algebraic
objects besides groups. In this paper we provide new examples for exponential
separations by considering hidden shift problems that are defined for several
classes of highly non-linear Boolean functions. These so-called bent functions
arise in cryptography, where their property of having perfectly flat Fourier
spectra on the Boolean hypercube gives them resilience against certain types of
attack. We present new quantum algorithms that solve the hidden shift problems
for several well-known classes of bent functions in polynomial time and with a
constant number of queries, while the classical query complexity is shown to be
exponential. Our approach uses a technique that exploits the duality between
bent functions and their Fourier transforms.Comment: 15 pages, 1 figure, to appear in Proceedings of the 21st Annual
ACM-SIAM Symposium on Discrete Algorithms (SODA'10). This updated version of
the paper contains a new exponential separation between classical and quantum
query complexit
On the uselessness of quantum queries
Given a prior probability distribution over a set of possible oracle
functions, we define a number of queries to be useless for determining some
property of the function if the probability that the function has the property
is unchanged after the oracle responds to the queries. A familiar example is
the parity of a uniformly random Boolean-valued function over ,
for which classical queries are useless. We prove that if classical
queries are useless for some oracle problem, then quantum queries are also
useless. For such problems, which include classical threshold secret sharing
schemes, our result also gives a new way to obtain a lower bound on the quantum
query complexity, even in cases where neither the function nor the property to
be determined is Boolean
Learning and Testing Variable Partitions
Let be a multivariate function from a product set to an
Abelian group . A -partition of with cost is a partition of
the set of variables into non-empty subsets such that is -close to
for some with
respect to a given error metric. We study algorithms for agnostically learning
partitions and testing -partitionability over various groups and error
metrics given query access to . In particular we show that
Given a function that has a -partition of cost , a partition
of cost can be learned in time
for any .
In contrast, for and learning a partition of cost is NP-hard.
When is real-valued and the error metric is the 2-norm, a
2-partition of cost can be learned in time
.
When is -valued and the error metric is Hamming
weight, -partitionability is testable with one-sided error and
non-adaptive queries. We also show that even
two-sided testers require queries when .
This work was motivated by reinforcement learning control tasks in which the
set of control variables can be partitioned. The partitioning reduces the task
into multiple lower-dimensional ones that are relatively easier to learn. Our
second algorithm empirically increases the scores attained over previous
heuristic partitioning methods applied in this context.Comment: Innovations in Theoretical Computer Science (ITCS) 202
Full Quantum Equivalence of Group Action DLog and CDH, and More
Cryptographic group actions are a relaxation of standard cryptographic groups that have less structure. This lack of structure allows them to be plausibly quantum resistant despite Shor\u27s algorithm, while still having a number of applications. The most famous example of group actions are built from isogenies on elliptic curves.
Our main result is that CDH for abelian group actions is quantumly *equivalent* to discrete log. Galbraith et al. (Mathematical Cryptology) previously showed *perfectly* solving CDH to be equivalent to discrete log quantumly; our result works for any non-negligible advantage. We also explore several other questions about group action and isogeny protocols
Structure in Communication Complexity and Constant-Cost Complexity Classes
Several theorems and conjectures in communication complexity state or
speculate that the complexity of a matrix in a given communication model is
controlled by a related analytic or algebraic matrix parameter, e.g., rank,
sign-rank, discrepancy, etc. The forward direction is typically easy as the
structural implications of small complexity often imply a bound on some matrix
parameter. The challenge lies in establishing the reverse direction, which
requires understanding the structure of Boolean matrices for which a given
matrix parameter is small or large. We will discuss several research directions
that align with this overarching theme.Comment: This is a column to be published in the complexity theory column of
SIGACT New
A quantum view on convex optimization
In this dissertation we consider quantum algorithms for convex optimization. We start by considering a black-box setting of convex optimization. In this setting we show that quantum computers require exponentially fewer queries to a membership oracle for a convex set in order to implement a separation oracle for that set. We do so by proving that Jordan's quantum gradient algorithm can also be applied to find sub-gradients of convex Lipschitz functions, even though these functions might not even be differentiable. As a corollary we get a quadraticly faster algorithm for convex optimization using membership queries. As a second set of results we give sub-linear time quantum algorithms for semidefinite optimization by speeding up the iterations of the Arora-Kale algorithm. For the problem of finding approximate Nash equilibria for zero-sum games we then give specific algorithms that improve the error-dependence and only depend on the sparsity of the game, not it's size. These last results yield improved algorithms for linear programming as a corollary. We also show several lower bounds in these settings, matching the upper bounds in most or all parameters
Generalized Learning Problems and Applications to Non-commutative Cryptography
Abstract. We propose a generalization of the learning parity with noise (LPN) and learning with errors (LWE) problems to an abstract class of group-theoretic learning problems that we term learning homomorphisms with noise (LHN). This class of problems contains LPN and LWE as spe-cial cases, but is much more general. It allows, for example, instantiations based on non-abelian groups, resulting in a new avenue for the applica-tion of combinatorial group theory to the development of cryptographic primitives. We then study a particular instantiation using relatively free groups and construct a symmetric cryptosystem based upon it
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