1,888 research outputs found
A note on quantum algorithms and the minimal degree of epsilon-error polynomials for symmetric functions
The degrees of polynomials representing or approximating Boolean functions
are a prominent tool in various branches of complexity theory. Sherstov
recently characterized the minimal degree deg_{\eps}(f) among all polynomials
(over the reals) that approximate a symmetric function f:{0,1}^n-->{0,1} up to
worst-case error \eps: deg_{\eps}(f) = ~\Theta(deg_{1/3}(f) +
\sqrt{n\log(1/\eps)}). In this note we show how a tighter version (without the
log-factors hidden in the ~\Theta-notation), can be derived quite easily using
the close connection between polynomials and quantum algorithms.Comment: 7 pages LaTeX. 2nd version: corrected a few small inaccuracie
Error-Correcting Data Structures
We study data structures in the presence of adversarial noise. We want to
encode a given object in a succinct data structure that enables us to
efficiently answer specific queries about the object, even if the data
structure has been corrupted by a constant fraction of errors. This new model
is the common generalization of (static) data structures and locally decodable
error-correcting codes. The main issue is the tradeoff between the space used
by the data structure and the time (number of probes) needed to answer a query
about the encoded object. We prove a number of upper and lower bounds on
various natural error-correcting data structure problems. In particular, we
show that the optimal length of error-correcting data structures for the
Membership problem (where we want to store subsets of size s from a universe of
size n) is closely related to the optimal length of locally decodable codes for
s-bit strings.Comment: 15 pages LaTeX; an abridged version will appear in the Proceedings of
the STACS 2009 conferenc
A Survey of Quantum Learning Theory
This paper surveys quantum learning theory: the theoretical aspects of
machine learning using quantum computers. We describe the main results known
for three models of learning: exact learning from membership queries, and
Probably Approximately Correct (PAC) and agnostic learning from classical or
quantum examples.Comment: 26 pages LaTeX. v2: many small changes to improve the presentation.
This version will appear as Complexity Theory Column in SIGACT News in June
2017. v3: fixed a small ambiguity in the definition of gamma(C) and updated a
referenc
Average-Case Quantum Query Complexity
We compare classical and quantum query complexities of total Boolean
functions. It is known that for worst-case complexity, the gap between quantum
and classical can be at most polynomial. We show that for average-case
complexity under the uniform distribution, quantum algorithms can be
exponentially faster than classical algorithms. Under non-uniform distributions
the gap can even be super-exponential. We also prove some general bounds for
average-case complexity and show that the average-case quantum complexity of
MAJORITY under the uniform distribution is nearly quadratically better than the
classical complexity.Comment: 14 pages, LaTeX. Some parts rewritten. This version to appear in the
Journal of Physics
Optimizing the Number of Gates in Quantum Search
In its usual form, Grover's quantum search algorithm uses
queries and other elementary gates to find a solution in
an -bit database. Grover in 2002 showed how to reduce the number of other
gates to for the special case where the database has a
unique solution, without significantly increasing the number of queries. We
show how to reduce this further to gates for any
constant , and sufficiently large . This means that, on average, the
gates between two queries barely touch more than a constant number of the qubits on which the algorithm acts. For a very large that is a power of
2, we can choose such that the algorithm uses essentially the minimal
number of queries, and only
other gates.Comment: 11 pages LaTeX. Version 2: small improvements in the proof
Communication Complexity Lower Bounds by Polynomials
The quantum version of communication complexity allows the two communicating
parties to exchange qubits and/or to make use of prior entanglement (shared
EPR-pairs). Some lower bound techniques are available for qubit communication
complexity, but except for the inner product function, no bounds are known for
the model with unlimited prior entanglement. We show that the log-rank lower
bound extends to the strongest model (qubit communication + unlimited prior
entanglement). By relating the rank of the communication matrix to properties
of polynomials, we are able to derive some strong bounds for exact protocols.
In particular, we prove both the "log-rank conjecture" and the polynomial
equivalence of quantum and classical communication complexity for various
classes of functions. We also derive some weaker bounds for bounded-error
quantum protocols.Comment: 16 pages LaTeX, no figures. 2nd version: rewritten and some results
adde
Locally Decodable Quantum Codes
We study a quantum analogue of locally decodable error-correcting codes. A
q-query locally decodable quantum code encodes n classical bits in an m-qubit
state, in such a way that each of the encoded bits can be recovered with high
probability by a measurement on at most q qubits of the quantum code, even if a
constant fraction of its qubits have been corrupted adversarially. We show that
such a quantum code can be transformed into a classical q-query locally
decodable code of the same length that can be decoded well on average (albeit
with smaller success probability and noise-tolerance). This shows, roughly
speaking, that q-query quantum codes are not significantly better than q-query
classical codes, at least for constant or small q.Comment: 15 pages, LaTe
Rational approximations and quantum algorithms with postselection
We study the close connection between rational functions that approximate a
given Boolean function, and quantum algorithms that compute the same function
using postselection. We show that the minimal degree of the former equals (up
to a factor of 2) the minimal query complexity of the latter. We give optimal
(up to constant factors) quantum algorithms with postselection for the Majority
function, slightly improving upon an earlier algorithm of Aaronson. Finally we
show how Newman's classic theorem about low-degree rational approximation of
the absolute-value function follows from these algorithms.Comment: v2: 12 pages LaTeX, to appear in Quantum Information and Computation.
Compared to version 1, the writing has been improved but the results are
unchange
Quantum Zero-Error Algorithms Cannot be Composed
We exhibit two black-box problems, both of which have an efficient quantum
algorithm with zero-error, yet whose composition does not have an efficient
quantum algorithm with zero-error. This shows that quantum zero-error
algorithms cannot be composed. In oracle terms, we give a relativized world
where ZQP^{ZQP}\=ZQP, while classically we always have ZPP^{ZPP}=ZPP.Comment: 7 pages LaTeX. 2nd version slightly rewritte
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