A Parallel Repetition Theorem for Entangled Projection Games

Thomas VidickSimons Institute, Berkeley

Joint work with Irit Dinur (Weizmann) and David Steurer (Cornell)

Direct Product Theorems• Consider task ; requires resources to achieve success

• What is the optimal way to solve independent instances ?

• DPT: solving instances of with resources must have success

• Question arises in diverse settings: – Circuits; power of restricted models of computation;

– Information theory; communication complexity;

– Multiplayer games: hardness amplification, cryptography

? –resource solver

Answer!

?

?

–resource solver

!

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Instance of

Success

independent instances of

Multiplayer games

• Study originates in foundations of QM: EPR’35, Bell’64 → “Bell inequalities”;CHSH’69, Mermin’90s → nonlocal games

• Renewed interest over past years:randomness certification [Col’09,…,MS’14],

device-independent key distribution [BHK’05,…,VV’13],

testing quantum systems [MY’98,…,RUV’13]

• Independent line of work in classical complexity theory: PCP theorem, interactive proofs and hardness of approximation

• Referee interacts with playersMakes choice of inputs (questions); observes outputs (answers)

• Referee decides to accept/reject

:= max. prob. accept optimize over all strategies = state + measurements Accept! Reject!!

:= max. prob. accept optimize over all classical strategies

:= max. prob. accept optimize over all quantum strategies

Parallel repetition of multiplayer games• a two-player game. : copies of in parallel

– Select independent pairs of questions– Send all questions at once, receive all answers– Accept if and only if all instances would accept

• Can we relate to ?– (play each game independently)

– [Feige,Watrous] Simple game such that

• Parallel repetition question: does ? At what rate?

• Introduced in classical setting as means to “amplify soundness”;long sequence of results [Ver’94,…,Raz’98,Hol’09] yields almost-optimal bounds

• Quantum? XOR games [CSUU’07], unique games [KRT’08]: based on tight SDP [KV’12]: general games, but only polynomial decay

Check! Check! Check!

Results: parallel repetition of projection games• Projection game: for every pair of questions,

any answer from B determines unique validanswer from A

• Your favorite two-player game is a projection game!

• Exists universal transformation such that projection game and Could have

Main result: for every projection game ,

• Matches optimal bound in classical case, up to value of constant ([Hol’09]: )

𝑢𝑣

𝑎1𝑎2

𝑏1

𝑏5

XOR games

Uniquegames

Magic square

3SATgame

Projectiongames

Hidden matchinggame

Proof outline

1. Introduce a relaxation such that:

(i) is a tight relaxation:

(ii) is perfectly multiplicative:

2. Conclude: if ,

Rounding argument based on quantum correlated sampling lemmaUses projection game

𝑣𝑎𝑙 +¿

∗ (𝐺) ¿

𝜔∗(𝐺)

: semidefinite relaxation + positivity constraint on vector coefficients

→ shares multiplicativity properties of semidefinite relaxations

The relaxation • Three steps:

1. Relax condition on measurements (“”)

2. SDP-like relaxation to vector-valued matrices

3. Additional positivity constraint on coordinates

𝜔∗ (𝐺 )=¿ 𝐴𝑢𝑎 ,𝐵𝑣

𝑏 ,∨𝜓 ⟩Σ𝑢 ,𝑣 ,𝑎 ,𝑏𝑔𝑢 ,𝑣𝑎,𝑏⟨ 𝜓|𝐴𝑢

𝑎⊗𝐵𝑣𝑏|𝜓 ⟩

Game coefficients

𝜔∗ (𝐺 )=¿ 𝐴𝑢𝑎 ,𝐵𝑣

𝑏‖Σ𝑢 ,𝑣 ,𝑎 ,𝑏𝑔𝑢 ,𝑣𝑎 ,𝑏 𝐴𝑢

𝑎⊗𝐵𝑣𝑏‖∞

Σ𝑎 𝐴𝑢𝑎=Id‖Σ𝑎𝐴𝑢𝑎‖∞≤1

𝜔𝑆𝐷𝑃∗ (𝐺 )=¿ �⃗�𝑢

𝑎 , �⃗�𝑣𝑏‖Σ𝑢 ,𝑣 ,𝑎 ,𝑏𝑔𝑢 ,𝑣

𝑎 ,𝑏 �⃗�𝑢𝑎⊗�⃗�𝑣

𝑏‖∞‖Σ𝑎 �⃗�𝑢

𝑎⊗ �⃗�𝑢𝑎‖∞1 /2≤1

𝑎1

𝑎9¿

𝐴Ω𝑢𝑎

𝐴1𝑢𝑎

𝜔∗ (𝐺 )≤¿ 𝐴𝑢𝑎 ,𝐵𝑣

𝑏‖Σ𝑢 ,𝑣 ,𝑎 ,𝑏𝑔𝑢 ,𝑣𝑎 ,𝑏𝐴𝑢

𝑎⊗𝐵𝑣𝑏‖∞

‖Σ𝑎𝐴𝑢𝑎⊗𝐴𝑢

𝑎‖∞1 /2≤1

Variables are vector-valued matrices:

𝐴𝜔𝑢𝑎 ≥0 ∀𝜔 ,𝑢 ,𝑎

𝑣𝑎 𝑙+¿∗ (𝐺 )=¿ �⃗�𝑢

𝑎,𝐵 𝑣𝑏‖Σ𝑢,𝑣 ,𝑎 ,𝑏𝑔𝑢,𝑣

𝑎 ,𝑏 �⃗�𝑢𝑎⊗𝐵 𝑣

𝑏‖∞¿E𝜔∈Ω 𝐴𝜔𝑢

𝑎 ⊗𝐵𝜔𝑣𝑏

�⃗�𝑢𝑎=¿(i)

(ii)

A quantum correlated sampling lemma

• Consider the following distributed task:

– A,B share of their choice

– Receive classical descriptions of , in s. t.

– Goal: using local operations alone, create s.t.

• : quantum state embezzlement [vDH’09]

→ Use embezzling state

• We give robust procedure; achieves Main challenge is dealing with near-degeneracies in spectrum

• Generalizes classical correlated sampling lemma from [Hol’09]

Procedure in [vDH’09] is not robust: |Ψ ⟩= 1

√2|0 ⟩|1 ⟩+ 1

√2|1 ⟩|0 ⟩

¿ Γ ⟩→|Ψ ⟩⊗∨ 𝑗𝑢𝑛𝑘⟩

¿ Γ ⟩→|Ψ ⟩⊗∨ 𝑗𝑢𝑛𝑘⟩

LO

Summary• First exponential parallel repetition theorem for entangled games

– Applies to projection games

– Proof introduces tight, multiplicative relaxation inspired from semidefinite relaxation

– Rounding argument based on quantum correlated sampling lemma

• Extend to general games? (See next talk for different approach)

• Low soundness case? Direct product results? (Exist in classical case!)

• General paradigm: relaxation optimizes over “partial strategies”→ Useful in other contexts? Parallel repetition for QMA(2)? Channels?

Questions

Thank you!