Bell correlations in a Bose-Einstein condensate.
Characterizing many-body systems through the quantum correlations between their constituent particles is a major goal of quantum physics. Although entanglement is routinely observed in many systems, we report here the detection of stronger correlations--Bell correlations--between the spins of about 480 atoms in a Bose-Einstein condensate. We derive a Bell correlation witness from a many-particle Bell inequality involving only one- and two-body correlation functions. Our measurement on a spin-squeezed state exceeds the threshold for Bell correlations by 3.8 standard deviations. Our work shows that the strongest possible nonclassical correlations are experimentally accessible in many-body systems and that they can be revealed by collective measurements.
Quantum Atom Optics Laboratory, Department of Physics, University of Basel, Klingelbergstrasse 82, 4056 Basel, Switzerland.
Roman
R
Schmied
Centre for Quantum Technologies, National University of Singapore, 3 Science Drive 2, Singapore 117543. Quantum Optics Theory Group, Department of Physics, University of Basel, Klingelbergstrasse 82, 4056 Basel, Switzerland.
Jean-Daniel
JD
Bancal
Quantum Atom Optics Laboratory, Department of Physics, University of Basel, Klingelbergstrasse 82, 4056 Basel, Switzerland.
Baptiste
B
Allard
Quantum Atom Optics Laboratory, Department of Physics, University of Basel, Klingelbergstrasse 82, 4056 Basel, Switzerland.
Matteo
M
Fadel
Centre for Quantum Technologies, National University of Singapore, 3 Science Drive 2, Singapore 117543. Department of Physics, National University of Singapore, 2 Science Drive 3, Singapore 117542.
Valerio
V
Scarani
Quantum Atom Optics Laboratory, Department of Physics, University of Basel, Klingelbergstrasse 82, 4056 Basel, Switzerland. philipp.treutlein@unibas.ch nicolas.sangouard@unibas.ch.
Philipp
P
Treutlein
Quantum Optics Theory Group, Department of Physics, University of Basel, Klingelbergstrasse 82, 4056 Basel, Switzerland. philipp.treutlein@unibas.ch nicolas.sangouard@unibas.ch.
Nicolas
N
Sangouard