Lev Vaidman

Teleportation of quantum states.

The recent result of Bennett [ital et] [ital al]. [Phys. Rev. Lett. 70, 1895 (1993)] of teleportation of an unknown quantum state is obtained in the framework of nonlocal measurements proposed by Aharonov and Albert [Phys. Rev. D 21, 3316 (1980); 24, 359 (1981)]. The latter method is generalized to the teleportation of a quantum state of a system with continuous variables.

Quantum mechanical interaction-free measurements

A novel manifestation of nonlocality of quantum mechanics is presented. It is shown that it is possible to ascertain the existence of an object in a given region of space without interacting with it. The method might have practical applications for delicate quantum experiments.

Quantum cryptography based on orthogonal states.

All existing quantum cryptosystems use non-orthogonal states as the carriers of information. Non-orthogonal states cannot be cloned (duplicated) by an eavesdropper. In result, any eavesdropping attempt must introduce errors in the transmission, and therefore, can be detected by the legal users of the communication channel. Orthogonal states are not used in quantum cryptography, since they can be faithfully cloned without altering the transmitted data. In this Letter we present a cryptographic scheme based on orthogonal states, which also assures the detection of any eavesdropper.

Torque and force on a magnetic dipole

Recent controversies about torque and force on a magnetic dipole are discussed. Three essentially different current loop models are analyzed. Although all models yield the same expression for the torque, N=m×B, the detailed mechanisms that give rise to the torque in each case are very different. The expression for the force on a magnetic dipole is derived and analyzed for all models. The force expression is the same for all current loop models but it differs from the force on a magnetic charge dipole. The expression, obtained for the force on a current loop magnetic dipole, FCL=■(m⋅B)−(d/dt)(m×E/c), differs from what usually appears in the educational literature. The standard ‘‘naive’’ calculation of the force yields the correct expression for the rate of change of the total momentum, dP/dt=∇(m⋅B). However, the current loop in an externa l electric field has an internal ‘‘hidden momentum’’ m×E/c, which is not related to the motion of the center of mass of the dipole. Thus, for the force, defined as mass t...

Methods for reliable teleportation

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 with one another. Teleportation proposals involving atoms and electromagnetic cavities are reviewed and the most feasible methods are described. In particular, the language of nonlocal measurements has been applied; this language has also been used for presenting a method for teleportation of quantum states of systems with continuous variables.

On schizophrenic experiences of the neutron or why we should believe in the many‐worlds interpretation of quantum theory

Abstract This is a philosophical paper in favor of the many‐worlds interpretation (MWI) of quantum theory. The necessity of introducing many worlds is explained by analyzing a neutron interference experiment. The concept of the “measure of existence of a world” is introduced and some difficulties with the issue of probability in the framework of the MWI are resolved.

Measurement of the Schrödinger Wave of a Single Particle

We show that it is possible to measure the Schrodinger wave of a single quantum system. This provides a strong argument for associating physical reality with the quantum state of a single system, and challenges the usual assumption that the quantum state has physical meaning only for an ensemble of identical systems.

Weak-measurement elements of reality

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.

The meaning of protective measurements

Protective measurement, which we have introduced recently, allows one to observe properties of the state of a single quantum system and even the Schrödinger wave itself. These measurements require a protection, sometimes due to an additional procedure and sometimes due to the potential of the system itself The analysis of the protective measurements is presented and it is argued, contrary to recent claims, that they observe the quantum state and not the protective potential. Some other misunderstandings concerning our proposal are also clarified.

Surprising quantum effects

Abstract We consider circumstances wherein a quantum-mechanical system is subjected to a varied sequence of measurements, some of which are substantially more precise than others. Such systems are shown to exhibit paradoxical behavior. The resolution of this paradox turns out to involve bizarre interference effects in the measuring apparatus. The possibilities of observing such behavior in the laboratory are briefly considered.