Quantum Distributed Complexity of Set Disjointness on a Line
- URL: http://arxiv.org/abs/2002.11795v5
- Date: Tue, 8 Mar 2022 22:07:19 GMT
- Title: Quantum Distributed Complexity of Set Disjointness on a Line
- Authors: Frederic Magniez and Ashwin Nayak
- Abstract summary: Set Disjointness on a Line is a variant of the Set Disjointness problem in a distributed computing scenario.
We prove an unconditional lower bound of $widetildeOmegabig(sqrt[3]n d2+sqrtn, big)$ rounds for Set Disjointness on a Line with $d + 1$ processors.
- Score: 3.2590610391507444
- License: http://arxiv.org/licenses/nonexclusive-distrib/1.0/
- Abstract: Set Disjointness on a Line is a variant of the Set Disjointness problem in a
distributed computing scenario with $d+1$ processors arranged on a path of
length $d$. It was introduced by Le Gall and Magniez (PODC 2018) for proving
lower bounds on the quantum distributed complexity of computing the diameter of
an arbitrary network in the CONGEST model. However, they were only able to
provide a lower bound when the local memory used by the processors on the
intermediate vertices of the path consists of O$( \log n)$ qubits for $n$-bit
inputs. We prove an unconditional lower bound of
$\widetilde{\Omega}\big(\sqrt[3]{n d^2}+\sqrt{n} \, \big)$ rounds for Set
Disjointness on a Line with $d + 1$ processors. The result gives us a new lower
bound of $\widetilde{\Omega} \big( \sqrt[3]{n\delta^2}+\sqrt{n} \, \big)$ on
the number of rounds required for computing the diameter $\delta$ of any
$n$-node network with quantum messages of size O$(\log n)$ in the CONGEST
model.
We draw a connection between the distributed computing scenario above and a
new model of query complexity. The information-theoretic technique we use for
deriving the round lower bound for Set Disjointness on a Line also applies to
the number of rounds in this query model. We provide an algorithm for Set
Disjointness in this query model with round complexity that matches the round
lower bound stated above, up to a polylogarithmic factor. This presents a
barrier for obtaining a better round lower bound for Set Disjointness on the
Line. At the same time, it hints at the possibility of better communication
protocols for the problem.
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