# The power of CGLMP inequalities

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# Problem

In the setting of Problem 26, consider especially the case $(N,M,K)=(2,2,d)$.

## Problem 27. A

Show that every face of the local polytope $C$, which is not already contained in a face of the no-signalling polytope $P$ is of CGLMP type, i.e., an inequality of the form first written out in [1], but possibly lifted from lower dimensions by fusing together some outcomes.

## Problem 27.B

Numerically, the observables maximally violating the CGLMP inequality on a maximally entangled state are of a very specific form [2], involving measurements in computational basis, transformed by only discrete Fourier transformation and diagonal unitaries [1]. Show that this is necessarily the case. Show also that these measurements realize the highest resistance of violation to noise, and the best discrimination against classical realism in the sense of Kullback-Leibler divergence [3].

# Background

According to the setting $(N,M,K)=(2,2,d)$, the CGLMP inequality features two parties, $X$ and $Y$, with two observables each: $X_1, X_2$ and $Y_1, Y_2$, respectively. Each observable has $d$ possible outcomes. In order to simplify notation, we use the function $m(x) = x \mod d$ where $m(x)\in \{0,1,...,d-1\}$ for integer $x$ and we denote expectation values by $\mathsf{E}$. The inequality can then be written [4]:

$\mathsf{E}(m(X_1-Y_1)) + \mathsf{E}(m(Y_1-X_2)) + \mathsf{E}(m(X_2-Y_2)) + \mathsf{E}(m(Y_2-X_1-1)) \ge d-1.$

This statement also suggests a very elegant proof of the inequality [4]: Note that $(X_1-Y_1) + (Y_1-X_2) + (X_2-Y_2) + (Y_2-X_1-1) = -1.$ Apply the function $m$ to both sides, and use $m(a)+m(b)+m(c)+m(d) \ge m(a+b+c+d)$.

A more detailed discussion of the problem can be found in [5].

# Partial Results

The CGLMP inequality is indeed a facet of the local polytope for the case $(N,M,K) = (2,2,d)$, see [6].

# Literature

1. D. Collins, N. Gisin, N. Linden, S. Massar, and S. Popescu, Phys. Rev. Lett, 88, 040404 (2002)
2. T. Durt, D. Kaszlikowski, and M. Zukowski, Phys. Rev. A, 64, 024101 (2001)
3. W. van Dam, P. Grunwald, and R. Gill, quant-ph/0307125 (2003)
4. R. Gill, private communication
5. A. Acin, R. Gill, and N. Gisin, Phys. Rev. Lett. 95, 210402 (2005)
6. Ll. Masanes, Quant. Inf. Comp. 3, 345 (2002)