On the first three days, we will have talks that are held in person in Vienna by invited speakers; these will also be available online. All other talks and the **poster sessions will be fully online**. The panels will take place in a hybrid form.

For more information on the **panels**, please see the “Speakers” page.

All times are local Vienna time (CEST).

**Antonio ACÍN**(Tuesday, 13:00 – 13:50 CEST, in person)

**Quantum theory needs complex numbers**

Complex numbers are essential in mathematics, but they are not needed to describe physical experiments, expressed in terms of probabilities, hence real numbers. Physics however aims to explain, rather than describe, experiments through theories. While most theories of physics are based on real numbers, quantum theory was the first to be formulated in terms of operators acting on complex Hilbert spaces. However, could a real version of quantum theory, in terms of real operators, lead to the same predictions as standard quantum theory? In fact, previous works showed that such “real quantum theory” can reproduce the outcomes of any multipartite experiment, as long as the parts share arbitrary real quantum states. In the talk, we prove that real and complex quantum theory make different predictions in network scenarios comprising independent states and measurements. We then devise a Bell-type experiment to disprove real quantum theory, in the same way as standard Bell experiments disproved local physics.

**Howard BARNUM**(Wednesday, 15:00 – 15:50 CEST, in person)

**Reflections on general probabilistic characterizations of quantum theory—with two new ones**

The “general probabilistic theories” (GPT), aka “convex operational”, framework formulates potential physical theories in terms of systems having convex, compact state spaces, on which the probabilities of measurement outcomes are given by affine functionals. A major part of the GPT research program has been to find principles, mathematically natural, of physical or information-processing significance, or all three, that narrow down the very wide landscape of possibilities available in the GPT framework to the familiar spaces of quantum density matrices (states) and POVM elements (measurement outcomes).

In this talk I describe two relatively new such characterizations, for finite-dimensional GPT systems, discuss the physical significance of the principles involved, and make some remarks on the interpretation of the GPT framework and its significance, based on the query: “Operational, or Aspirational?” The characterizations proceed by first characterizing the finite dimensional formally real Jordan-algebraic systems: real, complex, and quaternionic quantum theory, systems whose state spaces are balls, and one exceptional case. Complex quantum theory then follows from “local tomography”: that the state of a composite system be determinable from the statistics (including correlations) of local observables. But one may obtain complex quantum theory without considering composite systems, by requiring (4) “energy observability” [3]: the generators of continuous symmetries of the state space (potential reversible dynamics) are observables.

The first characterization, with Joachim Hilgert [1,2], builds on work of HB, Markus Mueller and Cozmin Ududec [3], who showed that three principles characterize irreducible Jordan-algebraic systems (plus finite-dimensional classical state spaces). The principles are

**(1)**a generalized spectral decomposition: every state is a convex combination of perfectly distinguishable pure states,

**(2)**“strong symmetry”: every set of perfectly distinguishable pure states may be taken to any other such set (of the same size) by a symmetry of the state space, and

**(3)**that there is no irreducible three (or more) path interference.

The new result uses only (1) and (2).The second characterization [4] of the same class of Jordan-algebraic systems builds on work of E. Vinberg and uses (1) homogeneity of the cone of unnormalized states: any interior (“maximally impure”) state of this cone may be taken to any other by an affine symmetry of the cone (I’ll discuss its informational interpretation in terms of conditioning on measurement results), and (2) continuous reversible transitivity of the normalized state space: any pure state may be taken to any other pure state by a “continuous symmetry” of the state space (element of the connected identity component of the affine symmetry group). If time permits I’ll discuss some additional results on GPT systems whose unnormalized state cones are homogeneous, which lead to natural conjectures for strengthenings of and alternatives to this result.

[1] H. Barnum and J. Hilgert, “Strongly symmetric spectral convex bodies are Jordan algebra state spaces”, arxiv:1904.03753

[2] H. Barnum and J. Hilgert, “Spectral properties of convex sets”, Journal of Lie Theory 30 (2019) 315-344.

[3] H. Barnum, M. Mueller and C. Ududec, “Higher-order interference and single-system postulates characterizing quantum theory”, New Journal of Physics 16 (2014) 123029. arXiv:1403.4147

[4] H. Barnum and C. Ududec, in preparation.

**Sougato BOSE**(Friday, 13:00 – 13:50 CEST, online)

**Testing the Quantum Nature of Large Masses & Gravity**

I will describe some routes one may take in order to demonstrate the nonclassicality of large masses. To this end, I will describe procedures for preparing quantum superpositions by using spin dependent forces and then evidencing such superpositions, highlighting some of the limitations we face. Based on these developments, I will highlight how to test the quantum nature of gravity in experiments, clarifying and justifying the assumptions on the way. We note a few ways to address principal practical challenges: Decoherence, Screening EM forces and Inertial noise reduction. I will also describe how to use a macroscopic free object as a qubit, and thereby test quantum contextuality and non-Gaussian entanglement.

**Nicolas BRUNNER**(Thursday, 9:50 – 10:40 CEST, online)**Quantum nonlocality in networks**The network configuration may allow for novel forms of nonlocal quantum correlations. This talk will present recent developments in this direction.

**Esteban CASTRO-RUIZ**(Thursday, 9:00 – 9:50 CEST, in person)

**Events, subsystems and quantum reference frames**

Standard quantum mechanics assumes a fixed classical spacetime background. Yet, we expect that gravitating quantum systems lead to situations where this background is not classical anymore. After presenting a simple model of a gravitating quantum clock, I will discuss how quantum reference frames allow us to reason operationally about “superpositions of spacetime backgrounds.” This leads to new interesting features. In particular, the temporal localisation of events becomes relative, depending on the quantum temporal reference frame. A similar effect happens for space, where the notion of entanglement becomes relative, depending on the spatial quantum reference frame. Finally, if I have time, I will argue that the notion of subsystems in quantum reference frames is key to understanding these novel features, and present an algebraic framework for quantum reference frames in which in which relative subsystems play a central role.

**Rafael CHAVES**(Thursday, 13:50 – 14:40 CEST, online)

**Interventions, causality and quantum correlations**Since Bell’s theorem, it is known that the concept of local realism fails to explain quantum phenomena. Indeed, the violation of a Bell inequality has become a synonym of the incompatibility of quantum theory with our classical notion of cause and effect. As recently discovered, however, the instrumental scenario – a tool of central importance in causal inference – allows for signatures of nonclassicality that do not hinge on this paradigm. If, instead of relying on observational data only, we can also intervene in our experimental setup, quantum correlations can violate classical bounds on the causal influence even in scenarios where no violation of a Bell inequality is ever possible. That is, through interventions, we can witness the quantum behaviour of a system that would look classical otherwise. In this talk we will see how the role of instrumental variables in causal inference has to be revisited in the presence of quantum common causes, present a photonic experiment implementing this new and stronger form of nonclassicality and discuss its use in cryptographic protocols.

**Giulio CHIRIBELLA**(Thurday, 10:40 – 11:30 CEST, online)

**Quantum operations with indefinite time direction**The standard operational framework of quantum theory is time-asymmetric. This asymmetry reflects the capabilities of ordinary agents, who are able to deterministically pre-select the states of quantum systems, but not to deterministically post-select the outcomes of quantum measurements. However, the fundamental dynamics of quantum particles is time-symmetric, and is compatible with a broader class of operations where pre-selections and post-selections are combined in general ways that do not presuppose a definite direction of time. In this talk I introduce a framework for quantum operations with indefinite time direction, providing an example, called the quantum time flip, where an unknown, time-symmetric process is accessed in a coherent superposition of two alternative time directions. In certain information-theoretic tasks, a hypothetical agent with access to the quantum flip can in principle outperform all agents who operate in a definite time direction.Related paper: G. Chiribella an Z. Liu, The quantum time flip, https://arxiv.org/abs/2012.03859v2

**Christopher FUCHS**(Thursday, 15:00 – 15:50 CEST, online)

**Quantum mechanics? It’s all fun and games until someone loses an**In this talk, I hope to convey just how ugly real-vector-space quantum mechanics (as opposed to our real-world complex-vector-space quantum mechanics) would be from a QBist perspective.*i*.

**Flaminia GIACOMINI**(Tuesday, 15:50 – 16:40 CEST, in person)

**Einstein’s Equivalence principle for superpositions of gravitational fields**

The Principle of Equivalence, stating that all laws of physics take their special-relativistic form in any local inertial frame, lies at the core of General Relativity. Because of its fundamental status, this principle could be a very powerful guide in formulating physical laws at regimes where both gravitational and quantum effects are relevant. However, its formulation implicitly presupposes that reference frames are abstracted from classical systems (rods and clocks) and that the spacetime background is well defined. Here, we we generalise the Einstein Equivalence Principle to quantum reference frames (QRFs) and to superpositions of spacetimes. We build a unitary transformation to the QRF of a quantum system in curved spacetime, and in a superposition thereof. In both cases, a QRF can be found such that the metric looks locally flat. Hence, one cannot distinguish, with a local measurement, if the spacetime is flat or curved, or in a superposition of such spacetimes. This transformation identifies a Quantum Local Inertial Frame. These results extend the Principle of Equivalence to QRFs in a superposition of gravitational fields. Verifying this principle may pave a fruitful path to establishing solid conceptual grounds for a future theory of quantum gravity.

**Philip GOYAL**(Friday, 15:00 – 15:50 CEST, online)

**What is the nature of identical quantum particles?**In our ordinary conception of the physical world, we tacitly assume that the appearances perceived in the present moment are underpinned by objects that bear properties, that persist through time, and that are reidentifiable on the basis of their characteristic properties.

This view is largely incorporated into the foundations of classical mechanics, but is brought into question by the quantum treatment of assemblies of identical particles. However, no consensus has thus far emerged on what should replace it. For example, although Dirac’s view that that identical particles are indistinguishable is widespread, it conflicts with basic experimental practice. A common alternative view, that identical particles are not individuals at all, suffers other difficulties.

In this talk, drawing on a systematic derivation of the symmetrization postulate [1], we propose a new understanding of identical particles. We adopt an operational approach in which the raw data consists of identical localized events, of which we construct two distinct models, namely a persistence model and a nonpersistence model. These models differ in whether or not it is assumed that successive events are generated by individual persistent entities (‘particles’). We then show that these models can each be described within the Feynman formulation of quantum theory and be synthesized to derive Feynman’s form of the symmetrization postulate.

On this basis, we propose that the quantal behaviour of identical particles reflects a new kind of complementarity—a complementarity of persistence and nonpersistence—, analogous to the way in which the behaviour of an individual electron reflects a complementarity of particle and wave [2].

[1] P. Goyal, Informational approach to the quantum symmetrization postulate, New Journal of Physics 17 013043 (2015)

https://iopscience.iop.org/article/10.1088/1367-2630/17/1/013043

[2] P. Goyal, Persistence and nonpersistence as complementary models of identical quantum particle, New Journal of Physics 21 063031 (2019)

https://iopscience.iop.org/article/10.1088/1367-2630/ab152b

**Daniel GREENBERGER**(Friday, 15:50 – 16:40 CEST, online)

**The Uncertainty Principle Between Mass and Proper Time**

The proper time of an event is a different animal from Newton’s Absolute time, or laboratory time. Pauli has a famous theorem that you can’t make time into a quantum observable, represented by an operator, but in fact that theorem draws the wrong conclusion. And the proper time, and the mass, with the correct definition of mass, do become dynamical variables.

We have dealt with this elsewhere, but here we will be concerned with the fact that one cannot measure them both simultaneously, and that they are controlled and restrained by an uncertainty principle.

**Lucien HARDY**(Thursday, 15:50 – 16:40 CEST, online)

**Time symmetry in operational theories and conditional frames of reference**

Standard operational quantum theory is time asymmetric. This is because operations must be trace non-increasing (in the forward time direction). This is puzzling since Schroedinger’s equation is time symmetric and von Neumann’s model of measurement is time symmetric. Furthermore, probability theory does not care about time direction. In this talk I will show how we can view operational probabilistic theories (like quantum theory) in a time symmetric manner. Furthermore, we will see that we can define different conditional frames of reference corresponding to a forward in time, a backwards in time, and a time symmetric point of view.

**Matthew LEIFER**(Thursday, 16:40 – 17:30 CEST, online)

**What is nonclassical about quantum interference?**In reference to the double slit experiment, Feynman famously called quantum interference “a phenomenon which is impossible,

*absolutely*impossible, to explain in any classical way, and which has in it the heart of quantum mechanics.” The difficulties in understanding quantum interference include wave-particle duality (a photon is neither a wave nor a particle, but behaves like one or the other depending on the experimental setup), the observer dependence of reality (whether a photon behaves like a wave or a particle depends on choices made by the observer), and the necessity for a radical causal explanation (e.g. the particle has to “know” whether or not the other slit is open, so there must be a nonlocal influence).In this talk, I first present a model for a single photon in a Mach-Zehnder interferometer that works when the phase difference between the two paths is either 0 or pi and in which we either have no which path information or complete which path information. Our model is based on the Spekkens’ toy theory and is local, noncontextual, and psi-epistemic. Crucial to this theory is to model the two spatial modes of the interferometer rather than the particle and to recognize that, in a psi-epistemic theory, there must be more than one possible state of reality when a mode is prepared in the vacuum state. Therefore, when a path does not contain a particle, there is still a degree of freedom that can transmit information to the second beam-splitter. As well as the phenomena pointed to by Feynman, the model can reproduce the Elitzur-Vaidman bomb detector, the delayed choice experiment, and the (delayed choice) quantum eraser, among other phenomena.Given this, one might wonder if there is anything nonclassical about quantum interference. We show that the answer is yes when we only have*partial*which-way information. By modelling the detector quantum mechanically, Englert showed that there is a tradeoff between the distinguishability D of the two paths (which path information) and visibility V in a Mach-Zehnder interferometer. In the case where the detector is in a pure state, he showed that V^2+D^2 = 1 and any values of V and D that satisfy this may be obtained. We show that a preparation noncontextual model of the detector can only reproduce the quantum predictions in the extreme cases: V=1, D=0 or V=0, D=1.The lesson of this is that we should evaluate the nonclassicality of quantum phenomena via rigorous no-go theorems, following the example of Bell’s theorem. By doing so, the meaning of “classical” in the given context becomes clear, and we find that the “nonclassical” features of quantum phenomena are often different from those usually discussed.This talk is based on joint work with Lorenzo Catani, Giovanni Scala, David Schmid and Rob Spekkens.

**Lluís MASANES**(Friday, 9:50 – 10:40 CEST, online)

**The measurement postulates are redundant**

The measurement postulates specify: the mathematical structure of quantum measurements (Hermitian operators, POVM, etc), the formula for assigning outcome probabilities (Born’s rule) and the post-measurement state-update rule. The rest of postulates are often referred to as “unitary quantum mechanics”. We will characterise the (possibly empty) family of theories consisting of unitary quantum mechanics supplemented with non-quantum measurement postulates. And we will prove that any such theory has the following problematic feature: the set of mixed state of any finite-dimensional Hilbert space has infinite dimension, which makes the task of state estimation impossible. Therefore, if we disregard theories with this problematic feature (if any such theory exists) then the only measurement postulates compatible with unitary quantum mechanics are the good old quantum measurement postulates. This result transforms our picture of the internal logical structure of quantum mechanics, as it reveals the true identity of the “measurement postulates” as “measurement theorems”.

**Tomasz PATEREK**(Monday,9:00 – 9:50 CEST, in person)

**Mediators and gravitational entanglement**I will review conditions under which two probe quantum objects interacting via unknown mediator can get entangled. It turns out that entanglement can be established even via classical (here meaning continuously dephased) mediator, if the tripartite dynamics starts in a suitable initial state. Accordingly, knowledge of entanglement needs to be augmented with properties of the initial state in order to make conclusion about non-classical mediator from the observations of the probes only. This method will be applied to two probe masses interacting gravitationally.

We will establish experimental parameters required for the observation of gravitational entanglement and will discuss the role mediators play in interpreting results from these experiments. A few words on a possibility of detecting (lack of) mediator will conclude the talk.

**Matthew F. PUSEY**(Friday, 9:00 – 9:50 CEST, online)

**The structure of noncontextuality, in particular for the stabilizer subtheory**

I will give an overview of two recent papers on the generalised notion of noncontextuality due to Spekkens. In the first (arXiv:2005.07161) we show that taking into account transformations imposes a surprisingly rigid structure on noncontextual models of tomographically local theories (such as quantum theory). An example of this rigidity is that the number of ontic states in the noncontextual model must equal the dimension of the operational state space. In the second paper (arXiv:2101.06263) we exploit this structure to classify the noncontextual models of the stabilizer subtheories, finding that there is a unique noncontextual model in odd dimension, and no model at all in even dimension. The unique model in odd dimensions is known as Gross’s discrete Wigner function, which could explain why this representation plays a special role in some forms of quantum computation.

**Renato RENNER**(Tuesday, 13:50 – 14:40 CEST, in person)

**What the Quantum de Finetti Theorem tells us about the black hole information puzzle**

According to Hawking’s calculations, the radiation emitted by a black hole is thermal, in the strong sense that the individual radiation quanta are uncorrelated with each other. Consequently, the entropy of the radiation grows continuously in time and reaches its maximum when the black hole is evaporated completely. Conversely, assuming unitarity of quantum theory, the radiation remaining from a completely evaporated black hole that was formed from matter in a pure state must also be in a pure state, and hence have zero entropy. This (quite drastic) discrepancy between Hawking’s calculation and unitarity is referred to as the “black hole information puzzle”.

Recent calculations of the radiation entropy using gravitational path integrals shed new light on this puzzle, supporting the unitary picture. They involve scenarios consisting of several copies (replica) of a black hole that may be connected by wormholes. In this talk, I will show how these recent results, and in particular their relation to Hawking’s original conclusions, can be understood from an information-theoretic perspective. It turns out that the Quantum de Finetti Theorem, developed originally in the context of quantum foundations and quantum cryptography, plays a key role.

(This talk is based on ongoing work with Jinzhao Wang.)

**Carlo ROVELLI**(Thursday, 13:00 – 13:50 CEST, in person)

**The conceptual basis of quantum theory needed for dealing with quantum spacetime: the quantum gravity perspective**In recent years, part of the Quantum Information community has been extending its interest beyond the conventional non-relativistic domain, working towards an extension of quantum theory capable of taking the quantum nature of clocks, reference frames, causal structures, and the observer itself, into account. These same issues have long been debated within the Quantum Gravity community, leading to a conceptual framework that grounds tentative quantum theories such as Loop Quantum Gravity, which indeed incorporates phenomena such as quantum reference frames, quantum superposition of causal structures, quantum observers. In the talk, I will review this conceptual framework, with the aim of moving towards a reciprocal understanding between different research communities and merge ancient wisdom with fresh novel insights 🙂

**Ana Belén SAINZ**(Wednesday, 13:00 – 13:50 CEST, in person)

**Steering: a modern perspective on this fundamental puzzle**

In this talk we will discuss the phenomenon of Steering — that which puzzled Einstein, Podolsky, Rosen, and Schroedinger back in the 1930’s. We will see the similarities and differences between Bell and steering experiments, and notice hence other aspects in which quantum systems can behave non-classically. We will also take a step further into the realm of post-quantumness: we will discuss how an analogue of the PR-box can be constructed in steering scenarios, and pin down some particular insights on quantum foundations that we can only draw from studying steering. In this talk I’ll also briefly touch upon the topics of the poster presentations by Beata Zjawin (Steering resource theory) and Paulo Cavalcanti (a GPT with post-quantum steering): for details on these please visit the poster sessions!

**Valerio SCARANI**(Wednesday, 13:50 – 14:40 CEST, in person)

**Irreversibility as “irretrodictability”**

Both classical and quantum theory hold that every change is reversible at the fundamental level, although we perceive almost everything as irreversible processes. Yet, physicists are not urging to label that perception as illusory: for most, the Second Law of thermodynamics is untouchable, so aberrant the idea of inexhaustible energy is. Building on a narrative pioneered by Watanabe in the 1960s, we approach the Second Law as statistical inference given partial information. Irreversibility is unavoidable because it translates the asymmetry between prediction and retrodiction on a given process. This viewpoint applies to both classical and quantum systems. Applied to the “fluctuation relations” that have been the preferential tools to address irreversibility in the last two decades, it both simplifies their derivation and vastly expands their scope.

References: F. Buscemi, V. Scarani, Phys. Rev. E 103, 052111 (2021); C. Aw, F. Buscemi, V. Scarani, arXiv:2106.08589 (2021)

**John SELBY**(Thursday, 13:50 – 14:40 CEST, in person)

**A no-go theorem on the nature of the gravitational Field beyond quantum theory**

In this talk I will discuss some recent work with Thomas D. Galley and Flaminia Giacomini, in which we apply the formalism of generalised probabilistic theories to the study of the nature of the gravitational field. Recently, table-top experiments involving massive quantum systems have been proposed to test the interface of quantum theory and gravity. In particular, the crucial point of the debate is whether it is possible to conclude anything on the quantum nature of the

gravitational field. The formalism allows us to study this problem without having to make

any precommitments to any particular model of gravity or ontological notions.

By analysing these experiments within the framework of GPTs we prove that the following are inconsistent

i) the gravitational field is the mediator of an interaction between two systems;

ii) entanglement is generated between the two systems;

iii) the field is classical.

I will discuss the particularly interesting case which is a violation of condition (iii), a violation of which has commonly been viewed as evidence for the quantum nature of the gravitational field. From the perspective of GPTs, however, we see that there are other possibilities. That is, I will discuss other examples of non-classical but non-quantum theories which are nonetheless consistent with conditions (i) and (ii). This leaves an important open question, what evidence do we actually need in order to conclude that the gravitational field is quantum?

**Urbasi SINHA**(Monday, 10:40 – 11:30 CEST, online)

**Revealing new facets of Superposition and Interference**

The superposition principle and the phenomena of interference are two of the fundamental cornerstones of quantum mechanics and its varied applications.

While the superposition principle forms the heart of all modern applications and properties of quantum mechanics such as quantum entanglement and quantum computing, the phenomena of interference forms the basis of several seminal effects and their experimental realizations. One may wonder: Is there anything new to be unearthed in these effects, given the plethora of studies that they have already been a part of? In this talk, we discuss fresh insights in both these “old” phenomena and discover fascinating new facets.

The usual application of the superposition principle to slit based interference experiments has caveat in both classical optics and quantum mechanics where it is often incorrectly assumed that the boundary condition represented by slits opened individually is same as them being opened together. In theory work carried out over the last few years, we have quantified the correction term in terms of the Sorkin parameter [1, 2]. In the first part of this talk, we will discuss the first reported measurement of a deviation from the naïve application of the superposition principle in the microwave domain using antennas as sources and detectors of the electromagnetic waves. This deviation is quantified through the Sorkin parameter which can be as big as 6% in our experiment [3]. Measuring a non-zero Sorkin parameter not only gives experimental verification to the theoretical predictions about the deviation from the superposition principle in interference experiments, it also exemplifies an experimental scenario in which non zero Sorkin parameter need not necessarily imply falsification of Born rule for probabilities in quantum mechanics which has been the basis for several experiments in recent years [4].

In the second part of the talk, we will switch gear to a novel approach to quantum state estimation that we have developed, using interference as a tool. Quantum state tomography (QST) has been the traditional method for characterization of an unknown state. Recently, many direct measurement methods have been implemented to reconstruct the state in a resource efficient way. Here we present an interferometric method, in which any qubit state, whether mixed or pure, can be inferred from the visibility, phase shift, and average intensity of an interference pattern using a single-shot measurement—hence, we call it Quantum State Interferography [5]. This provides us with a “black box” approach to quantum state estimation, wherein, between the incidence and extraction of state information, we are not changing any conditions within the setup, thus giving us a true single shot estimation of the quantum state. An extension of the scheme to pure states involving d−1 interferograms for d-dimensional systems (qudits) is also presented. The scaling gain is even more dramatic in the qudit scenario for our method, where, in contrast, standard QST, scales roughly as d2.

[1] R.Sawant, J.Samuel, A.Sinha, S.Sinha, U.Sinha, Non classical paths in quantum interference experiments. Phys.Rev.Lett.113,

120406 (2014).

[2] A.Sinha, Aravind H.V., U.Sinha, On the Superposition principle in interference experiments. Scientific Reports 5, 10304

(2015).

[3] G. Rengaraj, Prathwiraj U, Surya Narayana Sahoo, R. Somashekhar and U. Sinha, Measuring the deviation from the

superposition principle in interference experiments, New Journal of Physics 20, 2018.

[4] U.Sinha, C.Couteau, T.Jennewein, R.Laflamme, G.Weihs, Ruling out multi-order interference in quantum mechanics. Science

329, 418-421 (2010).

[5] S.N.Sahoo, S.Chakraborti, A.K.Pati, U.Sinha, Phys. Rev. Lett. 125 123601, 2020.

**Aephraim STEINBERG**(Wednesday, 15:50 – 16:40 CEST, online)

**Quantum detective stories: Investigating how much time atoms spend in a “forbidden region” and how much time photons spend “inside” atoms**

One of the most famous tidbits of received wisdom about quantum mechanics is that one cannot ask which path a photon took in an interferometer once it reaches the screen, or in general, that only questions about the specific things you finally measure are well-posed at all. What, then, do present observations tell us about the state of the world in the past? I will describe one-and-a-half recent experiments looking into aspects of this.

The principal experiment I wish to tell you about addresses a century-old controversy: that of the tunneling time. Since the 1930s, and more heatedly since the 1980s, the question of how long a particle spends in a classically forbidden region on those occasions when quantum uncertainty permits it to appear on the far side has been a subject of debate. Using Bose-condensed Rubidium atoms cooled down below a billionth of a degree above absolute zero, we have now measured just how long they spend inside an optical beam which acts as a “tunnel barrier” for them. I will describe these ongoing experiments, as well as proposals we are now refining to study exactly what happens during the time it takes to “collapse” an atom to be in the barrier.

I will also try to say a few words about a more recent experiment, which revisits the common picture that when light slows down in glass, or a cloud of atoms, it is because the photons “get virtually absorbed” before being sent back along their way. It turns out that although it is possible to measure “the average time a photon spends as an atomic excitation,” there seems to be no prior work which directly addresses this, especially in the resonant situation. We carry out an experiment that lets us distinguish between the time spent by transmitted photons and by photons which are eventually absorbed, asking the question “how much time are atoms caused to spend in the excited state by photons which are not absorbed?”

**Gregor WEIHS**(Monday, 9:50 – 10:40 CEST, in person)

**Multipath Interference Tests of Quantum Mechanics**(Gregor Weihs, Sebastian Gstir, Edmond Chan, Robert Keil, Toni Eichelkraut, and Alexander Szameit)

Quantum mechanics can be considered a special case of the class generalized probabilistic physical theories, which can be classified by how they deviate from classical probabilistic theories. Quantum mechanics, for example, deviates because it derives probabilities from wavefunctions and thus exhibits interference. By virtue of Born’s rule, all interference terms stem from pairs of paths. Other probabilistic theories could go beyond that [1] and allow higher-order interference terms, thus violating Born’s rule.Using multipath interferometers [2,3] we were able to tighten the bound on the deviation from ordinary quantum interference to a level of 10^{-5}of the expected, ordinary interference, with a good part of the uncertainty originating from our limited accuracy in determining detector nonlinearity.More recently we have begun to apply our multipath interferometers towards tests for the generalization of quantum mechanics in terms of the underlying numbers, i.e. whether hypercomplex quantum mechanics is allowed or not [4]. For these tests, the achievable interferometer contrast is crucial [5]. Our latest interferometer, an integrated photonic circuit with electrically controllable interferometric shutters, allows us to improve the bound on higher-order interferences and hypercomplex quantum mechanics at the same time.- D. Sorkin,
*Quantum Mechanics as Quantum Measure Theory,*Modern Physics Letters A**9**, 3119 (1994), https://doi.org/10.1142/S021773239400294X - Sinha, C. Couteau, T. Jennewein, R. Laflamme, and G. Weihs,
*Ruling out multi-order interference in quantum mechanics,*Science**329,**418 (2010), https://doi.org/10.1126/science.1190545 - Kauten, R. Keil, T. Kaufmann, et al.,
*Obtaining tight bounds on higher-order interferences with a 5-path interferometer,*New J. Phys.**19**, 033017 (2017), https://doi.org/10.1088/1367-2630/aa5d98 - Peres,
*Proposed Test for Complex versus Quaternion Quantum Theory,*Phys. Rev. Lett.**42**, 683 (1979), https://doi.org/10.1103/PhysRevLett.42.683 - S. Gstir, E. Chan, T. Eichelkraut, et al.,
*Towards probing for hypercomplex quantum mechanics in a waveguide interferometer,*submitted to New J. Phys. (2021), https://arxiv.org/abs/2104.11577

- D. Sorkin,
**Stefan WOLF**(Friday, 13:50 – 14:40 CEST, online), with**Xavier****COITEUX-ROY**

**Boltzmann Cryptography**

In his paper “Basing cryptography on oblivious transfer,” Joe Killian builds secure multiparty computation from inherently noisy communication (the erasure channel). The judoka move — turning informatic limitations into cryptographic possibilities — is in this case remarkably physical. Another famous example of the trick is quantum key distribution (QKD), which is developed upon a physical limitation that is even more fundamental than Killian’s hypothetical censored channel: Heisenberg’s uncertainty principle — a tenet of modern physics. In this presentation we look at a different but as inevitable law of physics: Landauer’s principle (the erasure cost of each bit of information is at least k*T*log2$, where k is Boltzmann’s constant and T is the absolute temperature of the environment). We take the point of view of Maxwell’s demon to construct secret-key establishment and multiparty computation out of the second law of thermodynamics, whose pessimism — expect an inescapable heath death — is folklore. Acting as a magnifying lens, the cryptographic perspective leads us to question (non)contextuality beyond classical and quantum information.

**Marek ŻUKOWSKI**(Wednesday, 16:40 – 17:30 CEST, online)

**Physics and Metaphysics of Wigner’s Friends: Even performed pre-measurements have no results + News flash about “non-locality of a single photon”**

**1.**“The unambiguous account of proper quantum phenomena must, in principle, include a description of all relevant features of experimental arrangement” (Bohr). The measurement process is composed of premeasurement (quantum correlation of the system with the pointer variable) and an irreversible decoherence via interaction with an environment. The system ends up in a probabilistic mixture of the eigenstates of the measured observable. For the premeasurement stage, any attempt to introduce an “outcome” leads, as we show, to a logical contradiction, 1=i. This nullifies claims that a modified concept of Wigner’s friend, who just premeasures, can lead to valid results concerning quantum theory.

**2.**“Non-locality of a single photon” is revisited. An exact, complete local hidden variable model of the 1991 gedanken experiment of Tan-Walls-Collett exists. Also, the claim (Grangier-Potasek-Yurke, 1988) that two mode PDC radiation can lead to violation of local realism in photon number resolving homodyne experiments, is unfounded. Modifications of the experiments, involving switched-off-local-oscillator settings (Hardy’s idea) and unbalanced beam-splitters do not admit local realistic models. A mysterious condition for relation of optimal local oscillator strength and beam-splitter reflectivity emerges.

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