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Is it possible that a measurement of a spin component of a spin-1/2 particle yields the value 100? In 1988 Aharonov, Albert and Vaidman argued that upon pre- and postselection of particular spin states, weakening the coupling of a standard measurement procedure ensures this paradoxical result. This theoretical prediction, called weak value, was realized in numerous experiments, but its meaning remains very controversial, since its "anomalous" nature, i.e. the possibility to exceed the eigenvalues range, as well as its "quantumness" are debated. We address these questions by presenting the first experiment measuring anomalous weak values with just a single click, without any statistics. The measurement uncertainty is significantly smaller than the gap between the measured weak value and the nearest eigenvalue. Beyond clarifying the meaning of weak values, this result represents a breakthrough in understanding quantum measurement foundations, paving the way to further applications of weak values to quantum photonics., Comment: Revised version to appear in Light: Science & Applications

Quantum mechanics, one of the keystones of modern physics, exhibits several peculiar properties, differentiating it from classical mechanics. One of the most intriguing is that variables might not have definite values. A complete quantum description provides only probabilities for obtaining various eigenvalues of a quantum variable. These and corresponding probabilities specify the expectation value of a physical observable, which is known to be a statistical property of an ensemble of quantum systems. In contrast to this paradigm, we demonstrate a unique method allowing to measure the expectation value of a physical variable on a single particle, namely, the polarisation of a single protected photon. This is the first realisation of quantum protective measurements., Comment: Nature Physics, in press (this version corresponds to the one initially submitted to Nature Physics)

Svensson [Found. Phys. 45, 1645 (2015)] argued that the concept of the weak value of an observable of a pre- and post-selected quantum system cannot be applied when the expectation value of the observable in the initial state vanishes. Svensson's argument is analyzed and shown to be inconsistent using several examples., Comment: 2 pages, comment on Found. Phys. 45, 1645 (2015)

The assumptions of Ferrie and Combes are irrelevant for realistic experimental situations. This undermines their conclusions., Comment: 1 page. Comment on arXiv:1307.4016

Interesting objections to conclusions of our experiment with nested interferometers raised by Salih in a recent Commentary are analysed and refuted., Comment: Published version (Frontiers in Physics) to revised version of the Commentary

The interaction-free measurements proposed by Elitzur and Vaidman (EV IFM) is a quantum mechanical method to find an object that interacts with other systems solely via its explosion without exploding it. In this method, an object can be found without ``touching it'', i.e. without any particle being at its vicinity., Comment: An entry in the Compendium of Quantum Physics: Concepts, Experiments, History and Philosophy, ed. F. Weinert, K. Hentschel, D. Greenberger and B. Falkenburg

A review of protective measurements. Protective measurement is a method for measuring an expectation value of an observable on a single quantum system. The quantum state of the system can be protected by a potential, when the state is a nondegenerate energy eigenstate with a known gap to neighboring states, or via Zeno effect by frequent projection measurements., Comment: An entry in the Compendium of Quantum Physics: Concepts, Experiments, History and Philosophy, ed. F. Weinert, K. Hentschel, D. Greenberger and B. Falkenburg

We discuss experimental situations that consist of multiple preparation and measurement stages. This leads us to a new approach to quantum mechanics. In particular, we introduce the idea of multi-time quantum states which are the appropriate tools for describing these experimental situations. We also describe multi-time measurements and discuss their relation to multi-time states. A consequence of our new formalism is to put states and operators on an equal footing. Finally we discuss the implications of our new approach to quantum mechanics for the problem of the flow of time., Comment: 17 pages, 8 figures

We analyze classically defined games for which a quantum team has an advantage over any classical team. The quantum team has a clear advantage in games in which the players of each team are separated in space and the quantum team can use unusually strong correlations of the Einstein-Podolsky-Rosen (EPR) type. We present an example of a classically defined game played at one location for which quantum players have a real advantage., Comment: 4 pages, revised version, to be published in PRA

We present a family of nonlocal games in which the inputs the players receive are continuous. We study three representative members of the family. For the first two a team sharing quantum correlations (entanglement) has an advantage over any team restricted to classical correlations. We conjecture that this is true for the third member of the family as well., Comment: Journal version, slight modifications

We consider quantum ensembles which are determined by pre- and post-selection. Unlike the case of only pre-selected ensembles, we show that in this case the probabilities for measurement outcomes at intermediate times satisfy causality only rarely; such ensembles can in general be used to signal between causally disconnected regions. We show that under restrictive conditions, there are certain non-trivial bi-partite ensembles which do satisfy causality. These ensembles give rise to a violation of the CHSH inequality, which exceeds the maximal quantum violation given by Tsirelson's bound, $B_{\rm CHSH}\le 2\sqrt2$, and obtains the Popescu-Rohrlich bound for the maximal violation, $B_{\rm CHSH}\le 4$. This may be regarded as an a posteriori realization of super-correlations, which have recently been termed Popescu-Rohrlich boxes., Comment: 5 pages

A dialog with Asher Peres regarding the meaning of quantum teleportation is briefly reviewed. The Braunstein-Kimble method for teleportation of light is analyzed in the language of quantum wave functions. A pictorial example of continuous variable teleportation is presented using computer simulation., Comment: 11 pages, 8 figures

Several situations, in which an empty wave causes an observable effect, are reviewed. They include an experiment showing ``surrealistic trajectories'' proposed by Englert et al. and protective measurement of the density of the quantum state. Conditions for observable effects due to empty waves are derived. The possibility (in spite of the existence of these examples) of minimalistic interpretation of Bohmian Quantum Mechanics in which only Bohmian positions supervene on our experience is discussed., Comment: 15 pages, contribution to the Jim Cushing Memorial Found. Phys. Issue

It is shown that a quantum shutter, pre- and post-selected in particular quantum states, can close simultaneously arbitrary number of slits preventing the passage of a single photon in an arbitrary state. A set of K pre- and post-selected shutters can close the slits preventing the passage of K or less photons. This result indicates that the surprising properties of pre- and post-selected quantum systems are even more robust than previously expected., Comment: 3 pages RevTex4

According to recent reports, the last loopholes in testing Bell's inequality are closed. It is argued that the really important task in this field has not been tackled yet and that the leading experiments claiming to close locality and detection efficiency loopholes, although making a very significant progress, have conceptual drawbacks. The important task is constructing quantum devices which will allow winning games of certain correlated replies against any classical team. A novel game of this type is proposed., Comment: 3 pages, to be published in PLA. There is some overlap with quant-ph/0102139, but introduction of "impossible necklace" game merits new submission

According to recent reports, the last loopholes in testing Bell's inequality are closed. It is argued that the really important task in this field has not been tackled yet and that the leading experiments claiming to close locality and detection efficiency loopholes, although making a very significant progress, have conceptual drawbacks., Comment: 2 pages

Two recent works suggest a possibility of sending signals to a space-like separated region, contrary to the spirit of special relativity. In the first work [J. Grunhaus, S. Popescu, and D. Rohrlich, Phys. Rev. A 53, 3781 (1996)] it has been shown that sending signals to particular union of space-like separated region cannot cause causality paradoxes. Another work [Y. Aharonov and L. Vaidman, Phys. Rev. A 61, 052108 (2000)] showed that the relative phase of quantum superposition of a particle in two separate locations can be measured locally. Together with the possibility of changing the relative phase in a nonlocal way using potential effect we, apparently, have a method of sending signals to space-like separated regions. These arguments are critically analyzed in this paper., Comment: 6 pages. Contribution to the "Mysteries and Paradoxes in Quantum Mechanics", Garda Lake 2000

Interaction-free measurements introduced by Elitzur and Vaidman [Found. Phys. 23, 987 (1993)] allow finding infinitely fragile objects without destroying them. Paradoxical features of these and related measurements are discussed. The resolution of the paradoxes in the framework of the Many-Worlds Interpretation is proposed., Comment: 8 pages, 7 eps figures. Contribution to the "Mysteries and Paradoxes in Quantum Mechanics", Garda Lake 2000

In 1993 Elitzur and Vaidman introduced the concept of interaction-free measurements which allowed finding objects without ``touching'' them. In the proposed method, since the objects were not touched even by photons, thus, the interaction-free measurements can be called as ``seeing in the dark''. Since then several experiments have been successfully performed and various modifications were suggested. Recently, however, the validity of the term ``interaction-free'' has been questioned. The criticism of the name is briefly reviewed and the meaning of the interaction-free measurements is clarified., Comment: 11 pages, 3 eps figures. Contribution to the ICQO 2000, Raubichi, Belarus

Byrne and Hall (1999) criticized the argument of Chalmers (1996) in favor of the Everett-style interpretation. They claimed to show ``the deep and underappreciated flaw in ANY Everett-style interpretation''. I will argue that it is possible to interpret Chalmers's writing in such a way that most of the criticism by Byrne and Hall does not apply. In any case their general criticism of the many-worlds interpretation is unfounded. The recent recognition that the Everett-style interpretations are good (if not the best) interpretations of quantum mechanics has, therefore, not been negated., Comment: 6 pages

Various aspects of nonlocality of a quantum wave are discussed. In particular, the question of the possibility of extracting information about the relative phase in a quantum wave is analyzed. It is argued that there is a profound difference in the nonlocal properties of the quantum wave between fermion and boson particles. The phase of the boson quantum state can be found from correlations between results of measurements in separate regions. These correlations are identical to the Einstein-Podolsky-Rosen (EPR) correlations between two entangled systems. An ensemble of results of measurements performed on fermion quantum waves does not exhibit the EPR correlations and the relative phase of fermion quantum waves cannot be found from these results. The existence of a physical variable (the relative phase) which cannot be measured locally is the nonlocality aspect of the quantum wave of a fermion., Comment: 12 pages

This paper is the answer to the paper by Kastner [Found. Phys., to be published, quant-ph/9807037] in which she continued the criticism of the counterfactual usage of the Aharonov-Bergman-Lebowitz rule in the framework of the time-symmetrized quantum theory, in particular, by analyzing the three-box ``paradox''. It is argued that the criticism is not sound. Paradoxical features of the three-box example has been explained. It is explained that the elements of reality in the framework of time-symmetrized quantum theory are counterfactual statements and, therefore, even conflicting elements of reality can be associated with a single particle. It is shown how such ``counterfactual'' elements of reality can be useful in the analysis of a physical experiment (the three-box example). The validity of Kastner's application of the consistent histories approach to the time-symmetrized counterfactuals is questioned., Comment: 10 pages

Recently, several authors have criticized the time-symmetrized quantum theory originated by the work of Aharonov et al. (1964). The core of this criticism was a proof, appearing in various forms, which showed that the counterfactual interpretation of time-symmetrized quantum theory cannot be reconciled with standard quantum theory. I, (Vaidman, 1996a, 1997) have argued that the apparent contradiction is due to a logical error and introduced consistent time-symmetrized quantum counterfactuals. Here I repeat my arguments defending the time-symmetrized quantum theory and reply to the criticism of these arguments by Kastner (1999)., Comment: 22 pages

Since its discovery in 1993, we witness an intensive theoretical and experimental effort centered on teleportation. Very recently it was claimed in the press that ``quantum teleportation has been achieved in the laboratory'' (T. Sudbery, Nature, 390, p. 551). Here, I briefly review this research focusing on the connection to nonlocal measurements, and question Sudbery's statement. A philosophical inquiry about the paradoxical meaning of teleportation in the framework of the many-worlds interpretation is added., Comment: Invited Talk in the Conference: Mysteries, Puzzles and Paradoxes in Quantum Mechanics, 13 pages

Recent experimental results and proposals towards implementation of quantum teleportation are discussed. It is proved that reliable (theoretically, 100% probability of success) teleportation cannot be achieved using the methods applied in recent experiments, i.e., without quantum systems interacting one with the other. Teleportation proposal involving atoms and electro-magnetic cavities are reviewed and the most feasible methods are described. In particular, the language of nonlocal measurements has been applied which has also been used for presenting a method for teleportation of quantum states of systems with continuous variables., Comment: 11 pages, 5eps figures

Three arguments based on the Greenberger-Horne-Zeilinger (GHZ) proof of the nonexistence of local hidden variables are presented. The first is a description of a simple game which a team that uses the GHZ method will always win. The second uses counterfactuals in an attempt to show that quantum theory is nonlocal in a stronger sense than is implied by the nonexistence of local hidden variables and the third describes peculiar features of time-symmetrized counterfactuals in quantum theory., Comment: 14 pages, 2eps figures

Recently, several authors have criticized the time-symmetrized quantum theory originated by the work of Aharonov et al. (1964). The core of this criticism was a proof, appearing in various forms, which showed that the counterfactual interpretation of time-symmetrized quantum theory cannot be reconciled with standard quantum theory. I argue here that the apparent contradiction is due to a logical error. I analyze the concept of counterfactuals in quantum theory and introduce time-symmetrized counterfactuals. These counterfactuals do not lead to any contradiction with the predictions of quantum theory. I discuss applications of time-symmetrized counterfactuals to several surprising examples and show the usefulness of the time-symmetrized quantum formalism., Comment: This is a preprint from July 97 and it has partial overlap with quant-ph/9609007 and quant-ph/9802042. It is submitted now, in part because the paper (accepted for publication) by R. Kastner, quant-ph/9806002 addresses this manuscript

Alter and Yamamoto [Phys. Rev. A 53, R2911 (1996)] claimed to consider ``protective measurements'' [Phys. Lett. A 178, 38 (1993)] which we have recently introduced. We show that the measurements discussed by Alter and Yamamoto ``are not'' the protective measurements we proposed. Therefore, their results are irrelevant to the nature of protective measurements., Comment: 2 pages LaTeX

In several situations, most notably when describing metastable states, a system can evolve according to an effective non hermitian Hamiltonian. To each eigenvalue of a non hermitian Hamiltonian is associated an eigenstate $\vert\phi\rangle$ which evolves forward in time and an eigenstate $\langle{\psi}\vert$ which evolves backward in time. Quantum measurements on such systems are analyzed in detail with particular emphasis on adiabatic measurements in which the measuring device is coupled weakly to the system. It is shown that in this case the outcome of the measurement of an observable $A$ is the weak value $\langle{\psi}\vert A\vert\phi\rangle / \langle{\psi}\vert{\phi}\rangle $ associated to the two-state vector $\langle{\psi}\vert$ $\vert\phi\rangle$ corresponding to one of the eigenvalues of the non hermitian Hamiltonian. The possibility of performing such measurements in a laboratory is discussed., Comment: RevTeX, 4 pages

A recent result about measurability of a quantum state of a single quantum system is generalized to the case of a single pre- and post-selected quantum system described by a two-state vector. The protection required for such measurement is achieved by coupling the quantum system to a pre- and post-selected protected device yielding a nonhermitian effective Hamiltonian., Comment: TeX, 11 pages

A brief review of the attempts to define ``elements of reality'' in the framework of quantum theory is presented. It is noted that most definitions of elements of reality have in common the feature to be a definite outcome of some measurement. Elements of reality are extended to pre- and post-selected systems and to measurements which fulfill certain criteria of weakness of the coupling. Some features of the newly introduced concepts are discussed., Comment: LaTeX, 11 pages

In a recent article [D. Suter, Phys. Rev. {\bf A 51}, 45 (1995)] Suter has claimed to present an optical implementation of the quantum time-translation machine which ``shows all the features that the general concept predicts and also allows, besides the quantum mechanical, a classical description.'' It is argued that the experiment proposed and performed by Suter does not have the features of the quantum time-translation machine and that the latter has no classical analog., Comment: 7 pages, LaTeX

Comment on [R.L. Ingraham, Phys. Rev. A 50, 4502 (1994)]. Ingraham suggested ``a delayed-choice experiment with partial, controllable memory erasing''. It is shown that he cannot be right since his predictions contradict relativistic causality. A subtle quantum effect which was overlooked by Ingraham is explained., Comment: 4 pages, LaTeX

It is shown that the ``retrodiction paradox'' recently introduced by Peres arises not because of the fallacy of the time-symmetric approach as he claimed, but due to an inappropriate usage of retrodiction., Comment: 4 pages

Comment on L. Hardy, Phys. Rev. Lett. {\bf 73}, 2279 (1994). It is argued that the experiment proposed by Hardy should not be considered as a single photon experiment., Comment: 5 pages, LaTeX

We investigate the phase accumulated by a charged particle in an extended quantum state as it encircles one or more magnetic fluxons, each carrying half a flux unit. A simple, essentially topological analysis reveals an interplay between the Aharonov-Bohm phase and Berry's phase., Comment: 10 pages, TAUP 2110-93. TeX

An analysis of errors in measurement yields new insight into the penetration of quantum particles into classically forbidden regions. In addition to ``physical" values, realistic measurements yield ``unphysical" values which, we show, can form a consistent pattern. An experiment to isolate a particle in a classically forbidden region obtains negative values for its kinetic energy. These values realize the concept of a {\it weak value}, discussed in previous works., Comment: 22 pp, TAUP 1850-90