Laura Mančinska

In complexity theory, gap-preserving reductions play a crucial role in studying hardness of approximation and in analyzing the relative complexity of multiprover interactive proof systems. In the quantum setting, multiprover interactive proof systems with entangled provers correspond to gapped promise problems for nonlocal games, and the recent result MIP*=RE shows that these are in general undecidable. However, the relative complexity of problems within MIP* is still not well-understood, as establishing gap-preserving reductions in the quantum setting presents new challenges. In this paper, we introduce a framework to study such reductions and use it to establish MIP*-completeness of the gapped promise problem for the natural class of independent set games. In such a game, the goal is to determine whether a given graph contains an independent set of a specified size. We construct families of independent set games with constant question size for which the gapped promise problem is undecidable. In contrast, the same problem is decidable in polynomial time in the classical setting. To carry out our reduction, we establish a new stability theorem, which could be of independent interest, allowing us to perturb families of almost PVMs to genuine PVMs.

Random numbers are used in a wide range of sciences. In many applications, generating unpredictable private random numbers is indispensable. Device-independent quantum random number generation is a framework that makes use of the intrinsic randomness of quantum processes to generate numbers that are fundamentally unpredictable according to our current understanding of physics. While device-independent quantum random number generation is an exceptional theoretical feat, the difficulty of controlling quantum systems makes it challenging to carry out in practice. It is therefore desirable to harness the full power of the quantum degrees of freedom (the dimension) that one can control. It is known that no more than 2log(d) bits of private device-independent randomness can be extracted from a quantum system of local dimension d. In this paper we demonstrate that this bound can be achieved for all dimensions d by providing a family of explicit protocols. In order to obtain our result, we develop new certification techniques that can be of wider interest in device-independent applications for scenarios in which complete certification ('self-testing') is impossible or impractical. With our C*-algebra representation tools, we are able to device-independently certify non-projective measurements for the purpose of randomness generation. Our protocols use a class of measurements we call "balanced informationally complete" (BIC) POVMs, which we anticipate to be useful in scenarios where normally symmetric informationally complete (SIC) POVMs are useful. Moreover, we explicitly construct BIC-POVMs in every dimension, circumventing the problem with SIC-POVMs which are only conjectured to exist in every dimension.

We show that every real-valued projective measurement can be self-tested from correlations. To achieve this, we develop the theory of post-hoc self-testing, which extends existing self-tested strategies to incorporate new measurements. A sufficient and computationally feasible condition for a projective measurement to be post-hoc self-tested by a given strategy is proven. Recent work by Mančinska et al. [arXiv:2103.01729] showed that a strategy containing d+1 two-output projective measurements and the maximally entangled state with the local dimension d is self-tested. Applying the post-hoc self-testing technique to this work results in an extended strategy that can incorporate any real-valued projective measurement. We further study the general theory of iterative post-hoc self-testing whenever the state in the initial strategy is maximally entangled and characterize the iteratively post-hoc self-tested measurements in terms of a Jordan algebra generated by the initial strategy.

Self-testing results allow us to characterize measurements and states in a quantum device from classical output probabilities in a nonlocal game. However, there is some variation in how the individual self- testing statements are stated and proven, including whether they apply robustly or assume purity of the state or protectiveness of the measurements. We investigate these differences, and as one of our main results show that if the reference strategy of a given game has full Schmidt rank, the notions of self-testing strategies using either mixed or pure states coincide. Additionally, we show by example that not all types of self-testing statements are equivalent: We both provide an example of a nonlocal game which is an exact, but not a robust, self-test, and an example of another game which does not self-test any state.

Abstract Over 50 years ago, Lovasz proved that two graphs are isomorphic if and only if they admit the same number of homomorphisms from any graph. Other equivalence relations on graphs, such as cospectrality or fractional isomorphism, can be characterized by equality of homomorphism counts from an appropriately chosen class of graphs. Dvorak [J. Graph Theory 2010] showed that taking this class to be the graphs of treewidth at most $k$ yields a tractable relaxation of graph isomorphism known as $k$-dimensional Weisfeiler-Leman equivalence. Together with a famous result of Cai, Furer, and Immerman [FOCS 1989], this shows that homomorphism counts from graphs of bounded treewidth do not determine a graph up to isomorphism. Dell, Grohe, and Rattan [ICALP 2018] raised the questions of whether homomorphism counts from planar graphs determine a graph up to isomorphism, and what is the complexity of the resulting relation. We answer the former in the negative by showing that the resulting relation is equivalent to the so-called quantum isomorphism [Mancinska et al, ICALP 2017]. Using this equivalence, we further resolve the latter question, showing that testing whether two graphs have the same number of homomorphisms from any planar graph is, surprisingly, an undecidable problem, and moreover is complete for the class coRE (the complement of recursively enumerable problems). Quantum isomorphism is defined in terms of a one-round, two-prover interactive proof system in which quantum provers, who are allowed to share entanglement, attempt to convince the verifier that the graphs are isomorphic. Our combinatorial proof leverages the quantum automorphism group of a graph, a notion from noncommutative mathematics.

Abstract Majority vote is a basic method for amplifying correct outcomes that is widely used in computer science and beyond. It can, for example, be used to amplify the correctness of a quantum device whose output is classical. However, when the output of a device is a quantum state, it is not apriori clear how to implement an analogous \emph{quantum} majority vote. To this end, we consider an extension of majority vote to quantum inputs and outputs: given a product state of the form $\ket{\phi_1, \phi_2, \dotsc ,\phi_n}$ where each qubit $\ket{\phi_i}$ is in one of two orthogonal states $\ket{\psi_0}$ or $\ket{\psi_1}$, output the majority state $\ket{\psi_0}$ or $\ket{\psi_1}$. We provide an optimal algorithm for this problem that achieves worst-case fidelity of $1/2 + \Theta(1/\sqrt{n})$. Under the promise that at least $2/3$ of the qubits are in the majority state, the fidelity increases to $1 - \Theta(1/n)$ and approaches one in the limit. More generally, we initiate the study of covariant and symmetric Boolean functions $f: \set{0,1^n} \to \set{0,1}$ with quantum inputs and outputs. We provide a simple linear program of size roughly $n/2$ for computing the optimal worst-case fidelity and show that a generalization of our algorithm is optimal for computing $f$. Our algorithm has complexity $O(n^4 \log n)$ where $n$ is the number of qubits.

Abstract We consider correlations, $p_{n,x}$, arising from measuring a maximally entangled state using $n$ measurements with two outcomes each, constructed from $n$ projections that add up to $xI$. We show that the correlations $p_{n,x}$ robustly self-test the underlying states and measurements. To achieve this, we lift the group-theoretic Gowers-Hatami based approach for proving robust self-tests to a more natural algebraic framework. A key step is to obtain an analogue of the Gowers-Hatami theorem allowing to perturb an ``approximate" representation of the relevant algebra to an exact one. For n=4, the correlations $p_{n,x}$ self-test the maximally entangled state of every odd dimension as well as 2-outcome projective measurements of arbitrarily high rank. The only other family of constant-size self-tests for strategies of unbounded dimension is due to Fu (QIP 2020) who presents such self-tests for an infinite family of maximally entangled states with \emph{even} local dimension. Therefore, we are the first to exhibit a constant-size self-test for measurements of unbounded dimension as well as all maximally entangled states with odd local dimension. In addition, correlations $p_{4,x}$ represent the first self-tests for measurements of rank higher than one.

A pseudo-telepathy game is a nonlocal game which can be won with probability one using some finite-dimensional quantum strategy but not using a classical one. Our central question is whether there exist two-party pseudo-telepathy games which cannot be won with probability one using any maximally entangled state. Towards answering this question, we develop conditions under which maximally entangled states suffice. In particular, we show that maximally entangled states suffice for weak projection games which we introduce as a relaxation of projection games. Our results also imply that any pseudo-telepathy weak projection game yields a device-independent certification of a maximally entangled state. In particular, by establishing connections to the setting of communication complexity, we exhibit a class of games $G_n$ for testing maximally entangled states of local dimension $\Omega(n)$. We leave the robustness of these self-tests as an open question.