In our daily life, which is explained by the laws of classical physics, observing a system just provides information about certain properties, for example location or velocity, without affecting the system itself. In the atomic world, however, the situation is very different as the objects which are under consideration are small and their behavior is governed by the laws of quantum physics. In that domain, any observation (which is often called measurement) will change its properties fundamentally.
Fig. 1: A local measurement is performed on one of the qubits of a chain prepared in its ground state. This causes a collapse to a new state and induces dynamics in the system.
In a recent proposal, published in Physical Review Letters , Professor Abolfazl Bayat from the Institute of Fundamental and Frontier Sciences in collaboration with the researchers of four different countries have come up with the idea to use measurement and its subsequent wave function collapse as a means for kicking a quantum many-body system out of equilibrium. In this protocol, a quantum many-body system of interacting spin-1/2 particles (with each particle having two possibilities, with spin up or down) is initially prepared in an equilibrium state, e.g. the lowest energy eigenstate. The wave function of the system is generally very complex with lots of quantum correlations, so called entanglement, between different parts of the system. In order to induce dynamics, one of the particles is measured locally, as shown in the figure, and as a result the wave function of the whole system changes instantly. The effect of this local measurement and its subsequent wave function collapse affects all parts of the system, even distant ones, immediately. Hence, the system is no longer in equilibrium and starts to evolve, though the interaction between the particles remains the same. Later measurements on the same particle provide information which can be used for various tasks.
This novel method for inducing dynamics in quantum many-body systems provides a wide range of applications in both condensed matter physics and quantum technologies. One application is to perform spectroscopy on many-body systems in order to determine their energy spectrum. This is particularly important for systems which cannot be solved by standard methods, such as the long-range interacting Ising §model with transverse field which is naturally realized in ion traps. Another application is in critical systems for which one can show the emergence of scale invariant dynamics, a feature which is normally observed only for zero temperature equilibrium states. This allows to experimentally measure some critical exponents which are of great interest in condensed matter physics. Furthermore, the proposed protocol allows for observing the scaling behavior in a class of solid state systems with impurities, called Kondo models, even in the absence of criticality. One can also detect the dynamically generated length scale in such systems, known as the Kondo screening length, by just using the dynamics after a local measurement.