Synopsis

Prepping an Entanglement Witness

Physics 10, s28
Researchers have devised an improved method for checking whether two particles are entangled.    
Adapted from F. Shahandeh et al., Phys. Rev. Lett. (2017)

Many quantum information applications require the sharing of entangled particles. Verifying this entanglement can be done with an “entanglement witness,” which is a set of measurements on the particles whose combined value falls under some bound when the particles are entangled. Farid Shahandeh and colleagues from the University of Queensland, Australia, show how to optimize entanglement witnessing with a new protocol that can detect more entangled states using fewer measurements than previous methods.

Entanglement witnessing is based on a division of a system’s possible quantum states into two subsets: an enclosed “island” of nonentangled (separable) states surrounded by a region of entangled states. In two dimensions, this division looks like a Venn diagram, with a single loop segregating the nonentangled and entangled regions.

An entanglement witness is basically a set of measurements on the system that divides this Venn diagram along some straight line. States on one side of this line give positive measured values, whereas states on the other give negative values. By carefully choosing the measurements, one can ensure that the whole nonentangled island is on the positive side of the line. Therefore, measuring a negative value implies entanglement (but some entangled states will return positive values).

The new protocol from Shahandeh and colleagues, called ultrafine entanglement witnessing, takes available but unused measurement data from the standard procedure to effectively eliminate portions of the nonentangled island from consideration. This elimination allows the entanglement witness line to be redrawn so that more of the entangled-state region is identified. The team’s procedure relaxes certain constraints on the witnessing measurements, so that the experimental setup for verifying entanglement can be made simpler than in the standard case.

This research is published in Physical Review Letters.

–Michael Schirber

Michael Schirber is a Corresponding Editor for Physics based in Lyon, France.


Subject Areas

Quantum Information

Related Articles

Demonstrating Quantum Communication Under Realistic Conditions
Quantum Information

Demonstrating Quantum Communication Under Realistic Conditions

Researchers achieve secure “real-world” quantum communication along 428 km of optical fiber, the longest terrestrial distance outside of a lab setting. Read More »

Quantum Machine Learning for Data Classification
Quantum Information

Quantum Machine Learning for Data Classification

Quantum machine-learning techniques speed up the task of classifying data delivered by a small network of quantum sensors. Read More »

Microwave Squeezing Beyond 3 dB
Quantum Physics

Microwave Squeezing Beyond 3 dB

Researchers suppress measurement fluctuations of microwaves in a cavity to below that of vacuum. Read More »

More Articles