Daniel Rohrlich

Quantum nonlocality as an axiom

In the conventional approach to quantum mechanics, indeterminism is an axiom and nonlocality is a theorem. We consider inverting the logical order, making nonlocality an axiom and indeterminism a theorem. Nonlocal “superquantum” correlations, preserving relativistic causality, can violate the CHSH inequality more strongly than any quantum correlations.

Thermodynamics and the measure of entanglement

We point out formal correspondences between thermodynamics and entanglement. By applying them to previous work, we show that entropy of entanglement is the unique measure of entanglement for pure states.

Generic quantum nonlocality

Abstract For any entangled state of two or more systems, quantum theory predicts experimental results that are inconsistent with local realism: they violate a generalized Bell inequality. But some mixtures of entangled states do not violate any generalized Bell inequality.

A path integral to quantize spin

We present a model for a classical spinning particle, characterized by spin magnitude, arbitrary but fixed, and continuously varying direction. A gauge freedom of the model reflects the choice of canonical coordinates in the phase space, which is spherical. We formulate the path integral for the model and find, unexpectedly, that the phase space must be punctured at the poles. It then follows that both the total spin and spin projection along any axis are quantized. The model has rotational invariance and yields the usual quantum mechanics of spin, including commutation relations, in a simple way.

Quantum Cheshire Cats

In this paper we present a quantum Cheshire Cat. In a pre- and post-selected experiment we find the Cat in one place, and its grin in another. The Cat is a photon, while the grin is its circular polarization.

Which states violate Bell's inequality maximally?

Abstract We identify all states of two particles which lead to maximal violation of Bells inequality. They are straightforward extensions of the singlet state of two spinors.

MEASUREMENTS, ERRORS, AND NEGATIVE KINETIC ENERGY

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 weak value, discussed in previous works.

Causality and Nonlocality as Axioms for Quantum Mechanics

Quantum mechanics permits nonlocality—both nonlocal correlations and nonlocal equations of motion—while respecting relativistic causality. Is quantum mechanics the unique theory that reconciles nonlocality and causality? We consider two models, going beyond quantum mechanics, of nonlocality: “superquantum” correlations, and nonlocal “jamming” of correlations. These models are consistent with some definitions of nonlocality and causality.

Quantum nonlocality for each pair in an ensemble

Abstract If violations of Bells inequality turn up in measurements on an ensemble of particle pairs, do they imply that each pair behaves nonlocally, or only the ensemble as a whole? We show that each pair in the ensemble behaves nonlocally when the particles are spins coupled in a singlet state. For spins in a nonsinglet state, however, a model in which some of the pairs behave locally reproduces quantum predictions.

Lattice gauge magnets : local isospin from spin

Abstract We study lattice hamiltonians with continuous local SU(2) gauge invariance, for which the Hilbert space on each link is finite dimensional. As these are gauge-invariant generalizations of quantum magnets, we call them lattice gauge magnets. The generators of gauge transformations are built out of local spin operators. In the simplest version of such a theory, the gauge field components belong to a representation of the algebra SO(5). Long-wavelength excitations of this model are gapless and nonrelativistic at the tree level. In 2 + 1 dimensions, we study a parity violating model, which we argue is equivalent to the topologically massive Yang-Mills theory.