Entanglement, a quantum phenomenon, intricately links the properties of multiple particles, rendering the assignment of a definite state to individual particles impossible. Instead, the collective state of all entangled particles must be considered, influencing the material's properties as a whole, as highlighted in a Nature paper led by Peter Zoller at the University of Innsbruck and the Institute of Quantum Optics and Quantum Information (IQOQI) of the Austrian Academy of Sciences.
Christian Kokail, a first author of the paper, emphasizes the significance of entanglement in determining material properties, stating, “Entanglement of many particles is the feature that makes the difference.” However, quantifying this entanglement poses a formidable challenge.
Addressing this challenge, researchers, including theoretical physicist Rick van Bijnen, introduce an innovative approach to enhance the study and comprehension of entanglement in quantum materials. To characterize large quantum systems and glean entanglement information, traditional methods would demand an impractical number of measurements. The team devises a more efficient description enabling the extraction of entanglement information with significantly fewer measurements.
In a groundbreaking experiment using an ion trap quantum simulator with 51 particles, scientists replicate a real material particle by particle, scrutinizing it within a controlled laboratory setting. Few research groups globally possess the requisite control over numerous particles, a capability demonstrated by the Innsbruck experimental physicists led by Christian Roos and Rainer Blatt.
The experimentalist Manoj Joshi underscores the technical challenges, emphasizing the need for low error rates while controlling 51 ions in the trap, ensuring individual qubit control, and facilitating readout feasibility.
This research not only offers a novel perspective on entanglement but also brings theoretical descriptions into experimental fruition. Kokail, now with the Institute for Theoretical Atomic Molecular and Optical Physics at Harvard, reflects on the collaborative effort, noting the impressive strides made possible by today's resources. The experiment unveils effects previously only theorized, showcasing the remarkable synergy of accumulated knowledge and methodologies in the field.
Shortcut via temperature profiles
In a quantum material, particles can be more or less strongly entangled. Measurements on a strongly entangled particle yield only random results. If the results of the measurements fluctuate very much—i.e., if they are purely random—then scientists refer to this as “hot.” If the probability of a certain result increases, it is a “cold” quantum object. Only the measurement of all entangled objects reveals the exact state.
In systems consisting of very many particles, the effort for the measurement increases enormously. Quantum field theory has predicted that subregions of a system of many entangled particles can be assigned a temperature profile. These profiles can be used to derive the degree of entanglement of the particles.
In the Innsbruck quantum simulator, these temperature profiles are determined via a feedback loop between a computer and the quantum system, with the computer constantly generating new profiles and comparing them with the actual measurements in the experiment.
The temperature profiles obtained by the researchers show that particles that interact strongly with the environment are “hot” and those that interact little are “cold.”
“This is exactly in line with expectations that entanglement is particularly large where the interaction between particles is strong,” says Kokail.
“The methods we have developed provide a powerful tool for studying large-scale entanglement in correlated quantum matter. This opens the door to the study of a new class of physical phenomena with quantum simulators that already are available today,” says Zoller.
“With classical computers, such simulations can no longer be computed with reasonable effort.” The methods developed in Innsbruck will also be used to test new theory on such platforms.
Source: University of Innsbruck