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Science

Quantum Spookiness Wins, Einstein Loses in Photon Test

A. Watson

Published Jul 25, 1997• 10 citations
Publisher

Abstract

Among quantum theory9s oddest claims is that the states of two particles or photons can influence each other no matter how far apart they are, providing their quantum states are entangled. A measurement on one photon will then influence the outcome of a measurement on its distant twin. Some theorists have questioned this claim, but a group of researchers has enlisted fiber-optic lines belonging to the Swiss telephone system to show that links between quantum entities persist over distances of more than 10 kilometers.

DOI
10.1126/science.277.5325.481

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Among quantum theory9s oddest claims is that the states of two particles or photons can influence each other no matter how far apart they are, providing their quantum states are entangled. A measurement on one photon will then influence the outcome of a measurement on its distant twin. Some theorists have questioned this claim, but a group of researchers has enlisted fiber-optic lines belonging to the Swiss telephone system to show that links between quantum entities persist over distances of more than 10 kilometers.
It is the tenet of quantum mechanics that Einstein rejected as “spooky action at a distance”: if two particles are entangled, then any tinkering with one can cause an instantaneous change in the other – no matter how far apart they may be. But what if quantum mechanics is not quite right – that the change is not instantaneous but instead prompted by a signal transmitted between the two entangled particles? Now an experiment in Switzerland has shown that if such a signal does exist, then it would have to travel at least at the speed of light, and probably thousands of times faster.
Entanglement lies at the heart of quantum mechanics, and has sparked fierce debate between physicists over the years. The phenomenon, whereby two particles become inextricably linked without actually interacting with each other, certainly goes against intuition. For instance, two photons can be created in an experiment such that if one is polarized in the vertical direction, the other is always polarized horizontally. By measuring the polarization state of one entangled photon, we therefore immediately know the state of the other – no matter how far apart the photons are.
Although most physics experiments are carried out with independent particles, it is the collective nature of entangled particles that reveals the most fascinating and unexpected aspects of the quantum world. It was Erwin Schrodinger who first said “entanglement is not one but rather the characteristic trait of quantum mechanics”. One curious aspect of the behavior of a pair of particles in an entangled state is that, though each individual particle exhibits an inherent uncertainty, the joint entity of an entangled pair can exhibit no such uncertainty. As an example, while the time of arrival of an individual particle may be totally random, an entangled pair must always arrive simultaneously. This property offers a unique tool for carrying out absolute measurements. Our goal here is to explore the myriad implications and significance of entanglement and to exploit it for the development of a new type of optical measurement—quantum optical metrology. The existence of unique non-classical correlations between twin photons generated in the nonlinear process of spontaneous parametric origins. The non-classical link between such twin quanta is not diminished by arbitrarily large separations between the twins, even when they lie outside the light cone. Twin states have been used with great effectiveness over the past two decades for carrying out definitive quantum experiments that lead to counterintuitive results; among these are those arising from the EinsteinPodolsky-Rosen (EPR) paradox such as various tests of Bell’s inequalities [1-12], as well as non-local dispersion cancellation, entangled-photon–induced transparency, and entangled-photon spectroscopy with monochromatic light. The availability of these twin beams has made it possible to conduct such experiments without having to resort to
Erwin Schrodinger first used the phrase “quantum entanglement” in 1935 to describe a mechanical phenomenon at the quantum level where two (or more) particles’ quantum states must be defined in relation to one another even if they may be geographically apart. This paradoxical event has confounded us for a very long time. Entangled particle behavior appears to be mysterious, illogical, and akin to sorcery at work. The successful and dependable principle of locality has led us to the achievements of twentieth-century physics. However, some theorists may doubt the locality itself due to the “spooky” and nonlocal effects of the local laws of quantum theory. Could there truly be an instant connection between two subatomic particles located on the extreme edges of the universe? Is there a theory that predicts such a relationship that is fundamentally faulty or lacking? Are the outcomes of the experiments that show this connection being read incorrectly? These questions challenge our most fundamental understanding of space and time. The universe is currently being disassembled by modern physics, and it will never be the same. Utilizing cutting-edge technology, quantum entanglement will have a positive impact on society. The history of quantum entanglement is outlined in this work, along with several hypotheses regarding the puzzling behavior of entangled particles. Quantum entanglement may also be an instance of nonlocality in action, in which what occurs in one location affects what occurs in another. Entanglement can also be compared to a situation in which a light switch is used to control two (or more) light bulbs, causing them to turn on at the same time when the switch is switched on. The key question is whether one of the aforementioned theories actually accounts for the odd behavior of entangled particles, in which case experimental evidence would unquestionably support the theory.
Spooky Action at a Distance In the context of correlation experiments involving pairs of experiments performed at very nearly the same time in very far-apart experimental regions, Einstein famously said [1], But on one supposition we should in my opinion hold absolutely fast: “The real factual situation of the system S2 is independent of what is done with system S1 which is spatially separated from the former.” This demand is incompatible with the basic ideas of standard (Copenhagen/orthodox) quantum mechanics, which makes two relevant claims: (1) Experimenters in the two labs make “local free choices” that determine which experiments will be performed in their respective labs. These choices are free in the sense of not being predetermined by the prior history of the physically described aspects of the universe, and they are localized in the sense that the physical effects of these free choices are inserted into the physically described aspects of the universe only within the laboratory in which the associated experiment is being performed. (2) These choices of what is done with the system being measured in one lab can (because of a measurement-induced global collapse of the quantum state) influence the outcome of the experiment performed at very close to the same time in the very faraway lab. This influence of what is done with the system being measured in one region upon the outcome appearing at very nearly the same time in a very faraway lab was called “spooky action at a distance” by Einstein, and was rejected by him as a possible feature of “reality.” John Bell's Quasi-classical Statistical Theory Responding to the seeming existence in the quantum world of “spooky actions,” John Bell [2] proposed a possible alternative to the standard approach that might conceivably be able to reconcile quantum spookiness with “reality.” This alternative approach rests on the fact that quantum mechanics is a statistical theory.We already have in physics a statistical theory called “classical statistical mechanics.”