(a) Time evolution in the proposed experiment for past-future entanglement extraction. In the first time interval, qubit P interacts with the vacuum field. After a certain time with no interaction, qubit F interacts with the field, getting entangled with qubit P. (b) To activate and deactivate qubit-field coupling, the magnetic flux is varied. Image credit: Sabín, et al. ©2012 American Physical Society
Typically, for two particles to become entangled, they must first physically interact. Then when the particles are physically separated and still share the same quantum state, they are considered to be entangled. But in a new study, physicists have investigated a new twist on entanglement in which two qubits become entangled with each other even though they never physically interact.
“We show that it is possible in a real experiment to entangle two systems that neither interact with each other nor interact with a common resource at the same time, and without the need of measurements,” Sabín told Phys.org. “The trick is to use the correlations between different times – between past and future – contained in the vacuum of a quantum field.”
In quantum theory, the quantum field is the system that contains all particles that are too small to be described classically. Although no particles exist in the vacuum region of a quantum field, physicists have known since the 1970s that this vacuum contains quantum correlations, or entanglement. “The vacuum is globally nothing, but locally is roughly like a cloud consisting of bunches of pairs of particles that die too fast to be detected,” Sabín said. “In quantum field theory, these are called quantum fluctuations. These quantum fluctuations are correlated if we consider different regions of space, and different regions of time as well.” If this vacuum entanglement could be extracted from the vacuum and transferred to actual particles, it could become more than just an odd quantum property and potentially serve as a useful resource for quantum information applications. But experimentally realizing the extraction of vacuum entanglement has been very difficult.
The physicists, Carlos Sabín, Borja Peropadre, Marco del Rey, and Eduardo Martín-Martínez at the Institute of Fundamental Physics at the Spanish National Research Council (CSIC) in Madrid (Sabín is now at the University of Nottingham in the UK, and Martin-Martinez is now at the University of Waterloo in Ontario, Canada), have published a paper on this new kind of entanglement in a recent issue of Physical Review Letters. Via Qubits that never interact could exhibit past-future entanglement.