Quantum entanglement forms the core of the second quantum revolution: it is the main feature used to understand forms of quantum matter, and a major resource for current and future quantum technologies. Physically, entangled particles cannot be defined as separate particles with distinct states, but merely as a single system.
Even when the particles are divided by a large distance, variations in one particle also immediately impact the other particle(s). The entanglement of separate particles - whether atoms, photons, or molecules—is part of daily life in the laboratory at present. In many-body physics, following the pioneering research of Li and Haldane, entanglement is commonly characterized by the so-called entanglement spectrum: it is able to trap vital features of collective quantum phenomena, such as topological order, and simultaneously, it allows to quantify the 'quantumness' of a specified state - that is, how difficult it is to just write it down on a classical computer.
Despite its significance, the experimental techniques to measure the entanglement spectrum rapidly reach their limits - until today, these spectra have been measured just in few qubits systems. With a growing number of particles, this work becomes disheartening as the complexity of current methods escalates exponentially.
"Today it is very hard to perform an experiment beyond few particles that allows us to make concrete statements about entanglement spectra," explains Marcello Dalmonte from the International Centre for Theoretical Physics (ICTP) in Trieste, Italy.
Along with Peter Zoller and Benoît Vermersch from the Department of Theoretical Physics at the University of Innsbruck and the Institute of Quantum Optics and Quantum Information (IQOQI) of the Austrian Academy of Sciences, he has currently discovered an unexpectedly simple way to examine quantum entanglement directly. The physicists turn the theory of quantum simulation upside down by no longer mimicking a specific physical system in the quantum simulator, but straightaway mimicking its entanglement Hamiltonian operator, whose spectrum of excitations instantly relates to the entanglement spectrum.
Demonstrate Quantum Advantage
Instead of simulating a specific quantum problem in the laboratory and then trying to measure the entanglement properties, we propose simply turning the tables and directly realizing the corresponding entanglement Hamiltonian, which gives immediate and simple access to entanglement properties, such as the entanglement spectrum. Probing this operator in the lab is conceptually and practically as easy as probing conventional many-body spectra, a well-established lab routine.
Moreover, there are scarcely any limits to this technique in connection with the size of the quantum system. This could also permit the analysis of entanglement spectra in many-particle systems, which is extremely challenging to address with classical computers. Dalmonte, Zoller, and Vermersch describe the fundamentally new technique in a current paper in Nature Physics and demonstrate its tangible realization on several experimental platforms, such as trapped ions, atomic systems, and also solid-state systems based on superconducting quantum bits.