The study of quantum correlations is important for fundamental reasons as well as for quantum communication and information processing tasks. On the one hand, it is of tremendous interest to derive the correlations produced by measurements on separated composite quantum systems from within the set of all correlations obeying the no-signaling principle of relativity, by means of information-theoretic principles [1][2][3][4][5]. On the other hand, cryptographic protocols based on quantum non-local correlations have been proposed for the generation of secure keys [6] and the amplification and expansion of randomness [7,8] against general no-signaling adversaries. In both these research programs, a fundamental question arises : can any measurements on quantum states realize the correlations present in pure extremal no-signaling boxes? Here, we answer this question in full generality showing that no non-trivial (not local realistic) extremal boxes of general no-signaling theories can be realized in quantum theory. We then explore some important consequences of this fact.It is well-known that quantum mechanics allows for non-local correlations between spatially separated systems, i.e., correlations that cannot be explained by any local hidden variable theory as shown by the violation of Bell inequalities. The correlations described by quantum theory form a convex set which is sandwiched between the sets of classical correlations and general nosignaling correlations. By how much and why the quantum set is inside the no-signaling set has been the subject of intense research [1][2][3][4][5]9].The classical and no-signaling sets are known to form convex polytopes, the quantum set while being convex is not a polytope in general. Each point in the set describes a box of correlations, that is a set of conditional probability distributions for outputs given inputs. The extremal boxes (vertices) of the no-signaling polytope are the pure states of the no-signaling theory and their purity implies that they are completely uncorrelated with the environment. Consequently, access to an extremal non-local box would be of great advantage in cryptographic tasks, providing intrinsic certified randomness and key secure against any eavesdropper even when the latter is limited only by the no-signaling principle. Extremal boxes are also of crucial importance in the program of deriving the set of quantum correlations from purely information-theoretic principles, being essential test-beds for checking the validity of such principles [1][2][3][4][5]. Furthermore, the extremal boxes are the ones that maximally violate Bell inequalities in no-signaling theories; for some Bell inequalities there exists a single unique box that gives maximal violation. For instance, for the celebrated CHSH inequality [10], there exists a unique extremal box known as the Popescu-Rohrlich (PR) box [11] which maximally violates the inequality. From these considerations, one arrives at the fundamental question: does quantum mechanics allow for the realization of extre...