Ravi Kunjwal

Université Libre de Bruxelles | ULB · Centre for Quantum Information & Communication (QuIC)

Bell scenarios are multipartite scenarios that exclude any communication between parties. This constraint leads to a strict hierarchy of correlation sets in such scenarios, namely, classical, quantum, and nonsignaling. However, without any constraints on communication between the parties, they can realize arbitrary correlations by exchanging only c...

A standard approach to quantifying resources is to determine which operations on the resources are freely available, and to deduce the partial order over resources that is induced by the relation of convertibility under the free operations. If the resource of interest is the nonclassicality of the correlations embodied in a quantum state, i.e., e n...

Quantum theory admits ensembles of quantum nonlocality without entanglement (QNLWE). These ensembles consist of seemingly classical states (they are perfectly distinguishable and nonentangled) that cannot be perfectly discriminated with local operations and classical communication (LOCC). Here, we analyze QNLWE from a causal perspective, and show h...

Causal inequalities are device-independent constraints on correlations realizable via local operations under the assumption of definite causal order between these operations. While causal inequalities in the bipartite scenario require nonclassical resources within the process-matrix framework for their violation, there exist tripartite causal inequ...

The formalism of generalized probabilistic theories (GPTs) was originally developed as a way to characterize the landscape of conceivable physical theories. Thus, the GPT describing a given physical theory necessarily includes all physically possible processes. We here consider the question of how to provide a GPT-like characterization of a particu...

The existence of incompatible measurements is often believed to be a feature of quantum theory which signals its inconsistency with any classical worldview. To prove the failure of classicality in the sense of Kochen-Specker noncontextuality, one does indeed require sets of incompatible measurements. However, a more broadly applicable notion of cla...

The Kochen–Specker (KS) theorem reveals the nonclassicality of single quantum systems. In contrast, Bell's theorem and entanglement concern the nonclassicality of composite quantum systems. Accordingly, unlike incompatibility, entanglement and Bell non-locality are not necessary to demonstrate KS-contextuality. However, here we find that for multiq...

We consider the problem of entanglement-assisted one-shot classical communication. In the zero-error regime, entanglement can increase the one-shot zero-error capacity of a family of classical channels following the strategy of Cubitt et al., Phys. Rev. Lett. 104, 230503 (2010). This strategy uses the Kochen-Specker theorem which is applicable only...

Historically, science has benefited greatly through the mobility of researchers, whether it has been due to large-scale conflict, the search for new opportunities or a lack thereof. Today’s world of strict global immigration policies, exacerbated by the COVID-19 pandemic, places inordinate hurdles on the mobility of all researchers, let alone quant...

Historically, science has benefited greatly through the mobility of researchers, whether it has been due to large-scale conflict, the search for new opportunities or a lack thereof. Today's world of strict global immigration policies, exacerbated by the COVID-19 pandemic, places inordinate hurdles on the mobility of all researchers, let alone quant...

We show a correspondence between quantum nonlocality without entanglement and logically consistent classical processes without causal order. Three parties with access to a noncausal classical process -- the AF/BW process -- can perfectly discriminate the states in an ensemble -- the SHIFT ensemble -- exhibiting quantum nonlocality without entanglem...

The formalism of generalized probabilistic theories (GPTs) was originally developed as a way to characterize the landscape of conceivable physical theories. Thus, the GPT describing a given physical theory necessarily includes all physically possible processes. We here consider the question of how to provide a GPT-like characterization of a particu...

The Kochen-Specker (KS) theorem reveals the nonclassicality of single quantum systems. In contrast, Bell's theorem and entanglement require composite quantum systems. However, here we find that for multiqubit systems entanglement and the violation of Bell inequalities are key to proofs of the Kochen-Specker theorem. First, we show that any logical...

The existence of incompatible measurements is often believed to be a feature of quantum theory which signals its inconsistency with any classical worldview. To prove the failure of classicality in the sense of Kochen-Specker noncontextuality, one does indeed require sets of incompatible measurements. However, a more broadly applicable and more perm...

To make precise the sense in which the operational predictions of quantum theory conflict with a classical worldview, it is necessary to articulate a notion of classicality within an operational framework. A widely applicable notion of classicality of this sort is whether or not the predictions of a given operational theory can be explained by a ge...

A bstract Entanglement entropy of quantum fields in gravitational settings is a topic of growing importance. This entropy of entanglement is conventionally computed relative to Cauchy hypersurfaces where it is possible via a partial tracing to associate a reduced density matrix to the spacelike region of interest. In recent years Sorkin has propose...

Measurements in quantum theory exhibit incompatibility, i.e., they can fail to be jointly measurable. An intuitive way to represent the (in)compatibility relations among a set of measurements is via a hypergraph representing their joint measurability structure: its vertices represent measurements and its hyperedges represent (all and only) subsets...

Measurement incompatibility is necessary but not sufficient for violation of a Bell inequality. The structure of (in)compatibility relations among a set of measurements can be represented by a joint measurability structure, i.e., a hypergraph with its vertices representing measurements and its hyperedges representing all (and only) compatible sets...

We take a resource-theoretic approach to the problem of quantifying nonclassicality in Bell scenarios. The resources are conceptualized as probabilistic processes from the setting variables to the outcome variables having a particular causal structure, namely, one wherein the wings are only connected by a common cause. We term them "common-cause bo...

We consider the problem of entanglement-assisted one-shot classical communication. In the zero-error regime, entanglement can enhance the one-shot zero-error capacity of a family of classical channels following the strategy of Cubitt et al., Phys. Rev. Lett. 104, 230503 (2010). This strategy makes crucial use of the Kochen-Specker theorem which is...

A standard approach to quantifying resources is to determine which operations on the resources are freely available and to deduce the ordering relation among the resources that these operations induce. If the resource of interest is the nonclassicality of the correlations embodied in a quantum state, that is, entanglement, then it is typically pres...

A given set of measurements can exhibit a variety of incompatibility relations and an intuitive way to represent its joint measurability structure is via hypergraphs. The vertices of such a hypergraph denote measurements and the hyperedges denote (all and only) subsets of compatible measurements. Quantum measurements represented by positive operato...

Entanglement entropy of quantum fields in gravitational settings is a topic of growing importance. This entropy of entanglement is conventionally computed relative to Cauchy hypersurfaces where it is possible via a partial tracing to associate a reduced density matrix to the spacelike region of interest. In recent years Sorkin has proposed an alter...

Kochen-Specker (KS) theorem reveals the inconsistency between quantum theory and any putative underlying model of it satisfying the constraint of KS-noncontextuality. A logical proof of the KS theorem is one that relies only on the compatibility relations amongst a set of projectors (a KS set) to witness this inconsistency. These compatibility rela...

One of the most well-motivated and widely applicable notions of classicality for an operational theory is explainability by a generalized-noncontextual ontological model. We here explore what notion of classicality this implies for the generalized probabilistic theory (GPT) that arises from a given operational theory, focusing on prepare-and-measur...

Weak values are quantities accessed through quantum experiments involving weak measurements and postselection. It has been shown that “anomalous” weak values (those lying beyond the eigenvalue range of the corresponding operator) defy classical explanation in the sense of requiring contextuality [M. F. Pusey, Phys. Rev. Lett. 113, 200401 (2014)]. H...

A Bell experiment can be conceptualized as a box, i.e., a process taking classical setting variables to classical outcome variables, which has a common-cause structure, i.e., one that can be realized by implementing local measurements on systems that are prepared by a common-cause mechanism, with no cause-effect relations across the wings of the ex...

It has been shown that observations of weak values outside the eigenvalue range of the corresponding operator defy classical explanation in the sense of requiring contextuality [M. F. Pusey, arXiv:1409.1535]. Here we elaborate on and extend that result in several directions. Firstly, we provide "robust" extensions that account for the failure of re...

A logical proof of the Kochen-Specker theorem is one that relies only on the compatibility relations amongst a set of projectors (called a KS set) rather than their statistics relative to some fixed preparation. These compatibility relations can be abstractly represented by a contextuality scenario. We introduce a framework for obtaining noise-robu...

Ernst Specker considered a particular feature of quantum theory to be especially fundamental, namely that pairwise joint measurability implies global joint measurability for sharp measurements [vimeo.com/52923835 (2009)]. To date, it seemed that Specker's principle failed to single out quantum theory from the space of all general probabilistic theo...

Ernst Specker considered a particular feature of quantum theory to be especially fundamental, namely that pairwise joint measurability of sharp measurements implies their global joint measurability (https://vimeo.com/52923835). To date, Specker's principle seemed incapable of singling out quantum theory from the space of all general probabilistic t...

By generalizing the Cabello-Severini-Winter (CSW) framework, we build a bridge from this graph-theoretic approach for Kochen-Specker (KS) contextuality to a hypergraph-theoretic approach for Spekkens' contextuality, as applied to Kochen-Specker type scenarios. Our generalized framework describes an experiment that requires, besides the correlations...

The Kochen-Specker theorem rules out models of quantum theory wherein sharp measurements are assigned outcomes deterministically and independently of context. This notion of noncontextuality is not applicable to experimental measurements because these are never free of noise and thus never truly sharp. For unsharp measurements, therefore, one must...

The Kochen-Specker theorem rules out models of quantum theory wherein projective measurements are assigned outcomes deterministically and independently of context. This notion of noncontextuality is not applicable to experimental measurements because these are never free of noise and thus never truly projective. For nonprojective measurements, ther...

When it isn't possible to tell two distinct experimental procedures apart purely from their input/output statistics, then it seems a plausible hypothesis that the two procedures must be physically identical. We call such a hypothesis "noncontextuality", an instance of Leibniz's principle of the identity of indiscernibles. Read in the contrapositive...

To make precise the sense in which nature fails to respect classical physics, one requires a formal notion of classicality. Ideally, such a notion should be defined operationally, so that it can be subject to direct experimental test, and it should be applicable in a wide variety of experimental scenarios so that it can cover the breadth of phenome...

Microsoft Excel file containing coincidence counts for each preparation-measurement pair, integrated over all 100 experimental runs.

Microsoft Excel file containing coincidence counts for each preparation-measurement pair, for each of 100 experimental runs.

Supplementary Figures 1-4, Supplementary Tables 1-4, Supplementary Notes 1-4 and Supplementary References

Mathematica notebook which performs the fitting routine (to find characterizations of the primary preparation and measurement procedures given the raw data) and linear optimization (to find characterizations of the secondary procedures given the primary ones) for each of the 100 experimental runs.

The Kochen-Specker theorem demonstrates that it is not possible to reproduce the predictions of quantum theory in terms of a hidden variable model where the hidden variables assign a value to every projector deterministically and noncontextually. A noncontextual value-assignment to a projector is one that does not depend on which other projectors -...

To make precise the sense in which nature fails to respect classical physics, one requires a formal notion of classicality. Ideally, such a notion should be defined operationally, so that it can be subjected to a direct experimental test, and it should be applicable in a wide variety of experimental scenarios, so that it can cover the breadth of ph...

A characterization of noncontextual models which fall within the ambit of Fine's theorem is provided. In particular, the equivalence between the existence of three notions is made explicit: a joint probability distribution over the outcomes of all the measurements considered, a measurement-noncontextual and outcome-deterministic (or KS-noncontextua...

In many a traditional physics textbook, a quantum measurement is defined as a projective measurement represented by a Hermitian operator. In quantum information theory, however, the concept of a measurement is dealt with in complete generality and we are therefore forced to confront the more general notion of positive-operator valued measures (POVM...

The purpose of this note is to clarify the logical relationship between joint measurability and contextuality for quantum observables in view of recent developments [1-4].

Every choice of orthonormal frame in the Hilbert space of a bipartite system corresponds to one set of all mutually commuting density matrices or, equivalently, classical state space of the system; the quantum state space itself can thus be profitably viewed as an orbit of classical state spaces, one for each orthonormal frame. We exploit this conn...

We show that three unsharp dichotomic qubit measurements are enough to violate a generalized-noncontextual inequality in a state-dependent manner. For the case of trine spin axes we calculate the optimal quantum violation of this inequality. We conjecture this to be the optimal quantum violation obtainable from qubit measurements. Besides, we show...