In 1935, Einstein, Podolsky, and Rosen (EPR) conceived a gedanken experiment which became a cornerstone of quantum technology and still challenges our understanding of reality and locality today.While the experiment has been realized with small quantum systems, a demonstration of the EPR paradox with massive many-particle systems remains an important challenge, as such systems are particularly closely tied to the concept of local realism in our everyday experience and may serve as probes for new physics at the quantum-to-classical transition.In this work we report an EPR experiment with two spatially separated Bose-Einstein condensates, each containing about 700 rubidium atoms.Entanglement between the condensates results in strong correlations of their collective spins, allowing us to demonstrate the EPR paradox between them.Our results represent the first observation of the EPR paradox with spatially separated, massive many-particle systems.They show that the conflict between quantum mechanics and local realism does not disappear as the system size increases to more than a thousand massive particles.Furthermore, EPR entanglement in conjunction with individual manipulation of the two condensates on the quantum level, as we demonstrate here, constitutes an important resource for quantum metrology and information processing with many-particle systems.
In the usual Bell's-inequality experiments two particles carrying spin or polarization are prepared in an entangled state generated from the decay of an unstable quantum-mechanical system. These particles are then delivered to spin or polarization analyzers. The statistics of the measurements reported by the analyzers are incompatible with our notions of local realism. Here we show that Bell's-inequality violations occur even when the initial state is a direct product state. In fact, the two particles can come from two independent widely separated sources.
Quantum physics violates the basic assumptions of local realism. Particles can be entangled with each other in ways that allow correlations stronger than possible in classical physics. This was first realized by Einstein, Podolsky and Rosen (EPR) in 1935 and led them to conclude that quantum theory was incomplete. It took about 30 years before J. S. Bell found observable consequences of this phenomenon by showing how entangled states violate inequalities for two-particle correlations derived purely from the assumption that the world obeys local realism. Violations of Bell inequalities were first observed experimentally in the Aspect experiment, which will be discussed below. Such experiments are technically challenging and care is needed to avoid any loopholes in the experimental setup that would still permit the description of the experiments by a local realistic theory. Bell inequalities describe violations of local realism on average, meaning that each individual experimental outcome could occur in a local realistic theory. It is the statistics of these outcomes that violates local realism. In contrast, another class of quantum states, so-called Greenberger–Horne–Zeilinger (GHZ) states, which contain three particles, can be shown to violate local realism in each measurement outcome. We will also discuss this class of states and experiments by Zeilinger's group, which demonstrated this type of violation of local realism for the first time. Our aim here is not to discuss the exact relations between quantum entanglement, non-locality and realism, and their physical and philosophical implications. While this is a highly interesting topic which attracts a lot of attention from researchers, such an exploration would go beyond the scope of this book.
In conventional Einstein-Rosen-Podolsky (EPR) experiments that violate local realism, particles are placed in very particular entangled states. We propose here to use two or three spinless Fock-state Bose-Einstein condensates as independent sources in interferometery experiments. While these states do not seem to be entangled, nevertheless we show that interferometers can be constructed that demonstrate a large variety of different violations local reality. We find violations of Bell inequalities, new Greenberger-Horne-Zeilinger (GHZ) contradictions, and Hardy impossibilities. These violations continue to arbitrarily large particle numbers. A necessary condition to observe the quantum effects is that all particles should be observed; if some are missed, the quantum effects disappear.
Although considerable discussion about the completeness of quantum mechanics followed the publication in 1935 of the Gedanken experiment of Einstein, Podolsky, and Rosen (EPR), it was not until Bell's work, published in 3 965, that it was realized that the issue could be rigorously formulated and put to experimental test. An even more provocative demonstration of the incompatibility of quantum mechanics with local realism was recently discovered by Greenberger, Horne, and Zeilinger (GHZ) in a Gedanken experiment employing more than two particles.1 Typical of EPR experiments that have been proposed to date, one has a metastable system that decays into n particles.
The Einstein-Podolsky-Rosen (EPR) paradox gives an argument for the incompleteness of quantum mechanics based on the premises of local realism. A general view is that the argument is compromised, because EPR's premises are falsified by Greenberger-Horne-Zeilinger (GHZ) and Bell experiments. In this paper, we present an EPR argument based on premises not falsifiable by these experiments. We propose macroscopic EPR and GHZ experiments using spins
Sθ defined by two macroscopically distinct states. The analyzers that realize the unitary operations
Uθ determining the measurement settings
θ are devices that create macroscopic superposition states. For a system with two macroscopically distinct states available, macroscopic realism (MR) posits a predetermined outcome for a measurement
Sθ distinguishing between the states. Deterministic macroscopic realism (dMR) posits MR for the system prior to the interaction
Uθ. Weak macroscopic realism (wMR) posits MR for the system after
Uθ, at the time
tf (when the system is prepared for a final "pointer" measurement), the outcome of
Sθ not being changed by interactions that might occur at a remote system
B. The premise also posits that if the outcome for
SθA of a system
A can be predicted by a pointer measurement on a system
B defined after the interaction fixing the setting at
B, then the outcome for
SθA is determined at this time. The GHZ predictions negate dMR but are consistent with wMR. Yet, an EPR paradox arises based on wMR for the set-up proposed by Schrödinger, where one measures two complementary spins simultaneously, "one by direct, the other by indirect" measurement. We revisit the original EPR paradox and find similarly that an EPR argument can be based on a weak form of local realism not falsifiable by GHZ or Bell tests.