28 research outputs found
On Fortification of Projection Games
A recent result of Moshkovitz \cite{Moshkovitz14} presented an ingenious
method to provide a completely elementary proof of the Parallel Repetition
Theorem for certain projection games via a construction called fortification.
However, the construction used in \cite{Moshkovitz14} to fortify arbitrary
label cover instances using an arbitrary extractor is insufficient to prove
parallel repetition. In this paper, we provide a fix by using a stronger graph
that we call fortifiers. Fortifiers are graphs that have both and
guarantees on induced distributions from large subsets. We then show
that an expander with sufficient spectral gap, or a bi-regular extractor with
stronger parameters (the latter is also the construction used in an independent
update \cite{Moshkovitz15} of \cite{Moshkovitz14} with an alternate argument),
is a good fortifier. We also show that using a fortifier (in particular
guarantees) is necessary for obtaining the robustness required for
fortification.Comment: 19 page
A parallel repetition theorem for all entangled games
The behavior of games repeated in parallel, when played with quantumly
entangled players, has received much attention in recent years. Quantum
analogues of Raz's classical parallel repetition theorem have been proved for
many special classes of games. However, for general entangled games no parallel
repetition theorem was known. We prove that the entangled value of a two-player
game repeated times in parallel is at most for a
constant depending on , provided that the entangled value of is
less than 1. In particular, this gives the first proof that the entangled value
of a parallel repeated game must converge to 0 for all games whose entangled
value is less than 1. Central to our proof is a combination of both classical
and quantum correlated sampling.Comment: To appear in the 43rd International Colloquium on Automata,
Languages, and Programming (ICALP
Parallel Repetition for the GHZ Game: Exponential Decay
We show that the value of the -fold repeated GHZ game is at most
, improving upon the polynomial bound established by Holmgren
and Raz. Our result is established via a reduction to approximate subgroup type
questions from additive combinatorics
Quantum hedging in two-round prover-verifier interactions
We consider the problem of a particular kind of quantum correlation that
arises in some two-party games. In these games, one player is presented with a
question they must answer, yielding an outcome of either 'win' or 'lose'.
Molina and Watrous (arXiv:1104.1140) studied such a game that exhibited a
perfect form of hedging, where the risk of losing a first game can completely
offset the corresponding risk for a second game. This is a non-classical
quantum phenomenon, and establishes the impossibility of performing strong
error-reduction for quantum interactive proof systems by parallel repetition,
unlike for classical interactive proof systems. We take a step in this article
towards a better understanding of the hedging phenomenon by giving a complete
characterization of when perfect hedging is possible for a natural
generalization of the game in arXiv:1104.1140. Exploring in a different
direction the subject of quantum hedging, and motivated by implementation
concerns regarding loss-tolerance, we also consider a variation of the protocol
where the player who receives the question can choose to restart the game
rather than return an answer. We show that in this setting there is no possible
hedging for any game played with state spaces corresponding to
finite-dimensional complex Euclidean spaces.Comment: 34 pages, 1 figure. Added work on connections with other result
A No-Go Theorem for Derandomized Parallel Repetition: Beyond Feige-Kilian
In this work we show a barrier towards proving a randomness-efficient
parallel repetition, a promising avenue for achieving many tight
inapproximability results. Feige and Kilian (STOC'95) proved an impossibility
result for randomness-efficient parallel repetition for two prover games with
small degree, i.e., when each prover has only few possibilities for the
question of the other prover. In recent years, there have been indications that
randomness-efficient parallel repetition (also called derandomized parallel
repetition) might be possible for games with large degree, circumventing the
impossibility result of Feige and Kilian. In particular, Dinur and Meir
(CCC'11) construct games with large degree whose repetition can be derandomized
using a theorem of Impagliazzo, Kabanets and Wigderson (SICOMP'12). However,
obtaining derandomized parallel repetition theorems that would yield optimal
inapproximability results has remained elusive.
This paper presents an explanation for the current impasse in progress, by
proving a limitation on derandomized parallel repetition. We formalize two
properties which we call "fortification-friendliness" and "yields robust
embeddings." We show that any proof of derandomized parallel repetition
achieving almost-linear blow-up cannot both (a) be fortification-friendly and
(b) yield robust embeddings. Unlike Feige and Kilian, we do not require the
small degree assumption.
Given that virtually all existing proofs of parallel repetition, including
the derandomized parallel repetition result of Dinur and Meir, share these two
properties, our no-go theorem highlights a major barrier to achieving
almost-linear derandomized parallel repetition
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Information Value of Two-Prover Games
We introduce a generalization of the standard framework for studying the difficulty of two-prover games. Specifically, we study the model where Alice and Bob are allowed to communicate (with information constraints) - in contrast to the usual two-prover game where they are not allowed to communicate after receiving their respective input. We study the trade-off between the information cost of the protocol and the achieved value of the game after the protocol.
In particular, we show the connection of this trade-off and the amortized behavior of the game (i.e. repeated value of the game).
We show that if one can win the game with at least (1 - epsilon)-probability by communicating at most epsilon bits of information,
then one can win n copies with probability at least 2^{-O(epsilon n)}. This gives an intuitive explanation why Raz\u27s counter-example to strong parallel repetition [Raz2008] (the odd cycle game) is a counter-example to strong parallel repetition - one can win the odd-cycle game on a cycle of length by communicating O(m^{-2})-bits where m is the number of vertices.
Conversely, for projection games, we show that if one can win n copies with probability larger than (1-epsilon)^n,
then one can win one copy with at least (1 - O(epsilon))-probability by communicating O(epsilon) bits of information.
By showing the equivalence between information value and amortized value, we give an alternative direction for further works in studying amortized behavior of the two-prover games.
The main technical tool is the "Chi-Squared Lemma" which bounds the information cost of the protocol in terms of Chi-Squared distance,
instead of usual divergence. This avoids the square loss from using Pinsker\u27s Inequality
Information Value of Two-Prover Games
We introduce a generalization of the standard framework for studying the difficulty of two-prover games. Specifically, we study the model where Alice and Bob are allowed to communicate (with information constraints) - in contrast to the usual two-prover game where they are not allowed to communicate after receiving their respective input. We study the trade-off between the information cost of the protocol and the achieved value of the game after the protocol.
In particular, we show the connection of this trade-off and the amortized behavior of the game (i.e. repeated value of the game).
We show that if one can win the game with at least (1 - epsilon)-probability by communicating at most epsilon bits of information,
then one can win n copies with probability at least 2^{-O(epsilon n)}. This gives an intuitive explanation why Raz\u27s counter-example to strong parallel repetition [Raz2008] (the odd cycle game) is a counter-example to strong parallel repetition - one can win the odd-cycle game on a cycle of length by communicating O(m^{-2})-bits where m is the number of vertices.
Conversely, for projection games, we show that if one can win n copies with probability larger than (1-epsilon)^n,
then one can win one copy with at least (1 - O(epsilon))-probability by communicating O(epsilon) bits of information.
By showing the equivalence between information value and amortized value, we give an alternative direction for further works in studying amortized behavior of the two-prover games.
The main technical tool is the "Chi-Squared Lemma" which bounds the information cost of the protocol in terms of Chi-Squared distance,
instead of usual divergence. This avoids the square loss from using Pinsker\u27s Inequality
Multiplayer Parallel Repetition for Expanding Games
We investigate the value of parallel repetition of one-round games with any number of players k>=2. It has been an open question whether an analogue of Raz\u27s Parallel Repetition Theorem holds for games with more than two players, i.e., whether the value of the repeated game decays exponentially with the number of repetitions. Verbitsky has shown, via a reduction to the density Hales-Jewett theorem, that the value of the repeated game must approach zero, as the number of repetitions increases. However, the rate of decay obtained in this way is extremely slow, and it is an open question whether the true rate is exponential as is the case for all two-player games.
Exponential decay bounds are known for several special cases of multi-player games, e.g., free games and anchored games. In this work, we identify a certain expansion property of the base game and show all games with this property satisfy an exponential decay parallel repetition bound. Free games and anchored games satisfy this expansion property, and thus our parallel repetition theorem reproduces all earlier exponential-decay bounds for multiplayer games. More generally, our parallel repetition bound applies to all multiplayer games that are *connected* in a certain sense.
We also describe a very simple game, called the GHZ game, that does not satisfy this connectivity property, and for which we do not know an exponential decay bound. We suspect that progress on bounding the value of this the parallel repetition of the GHZ game will lead to further progress on the general question
Parallel Repetition of Free Entangled Games: Simplification and Improvements
In a two-player game, two cooperating but non communicating players, Alice
and Bob, receive inputs taken from a probability distribution. Each of them
produces an output and they win the game if they satisfy some predicate on
their inputs/outputs. The entangled value of a game is the
maximum probability that Alice and Bob can win the game if they are allowed to
share an entangled state prior to receiving their inputs.
The -fold parallel repetition of consists of instances of
where Alice and Bob receive all the inputs at the same time and must
produce all the outputs at the same time. They win if they win each
instance of . Recently, there has been a series of works showing parallel
repetition with exponential decay for projection games [DSV13], games on the
uniform distribution [CS14] and for free games, i.e. games on a product
distribution [JPY13].
This article is meant to be a follow up of [CS14], where we improve and
simplify several parts of our previous paper. Our main result is that for any
free game with value , we have where is the size of
the output set of the game. This result improves on both the results in [JPY13]
and [CS14]. The framework we use can also be extended to free projection games.
We show that for a free projection game with value
, we have .Comment: 17 pages, this paper is a follow up and supersedes our previous paper
'Parallel Repetition of Entangled Games with Exponential Decay via the
Superposed Information Cost' [CS14, arXiv:1310.7787] v2: updated GS
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