Quantum measurements may be helped by coincidence

When it comes to controlling quantum systems — the size most certainly matters. Larger systems — comprised of more particles — quickly become unmanageable. A new method may help physicists tackle larger, more delicate quantum systems.

The repeated measurement of randomly selected transformations of individual particles reveals provides information about the degree of entanglement of a system. (IQOQI Innsbruck/M.R.Knabl)

Scientists have been capable of controlling small quantum systems — investigating their quantum properties — for many years. Such simulations are considered the promising early applications of quantum technologies, advancements that could solve problems where simulations on conventional computers fail.

However, larger quantum systems prove more difficult to deal with experimentally — and as the quantum systems used as quantum simulators must continue to grow — so does the difficulty in manipulating them.

Part of this difficulty is the fact that entanglement becomes more and more difficult to protect from collapse with increasing numbers of particles. This results in an extremely delicate procedure.

Christian Roos from the Institute of Quantum Optics and Quantum Information at the Austrian Academy of Sciences, explains: “In order to operate a quantum simulator consisting of ten or more particles in the laboratory, we must characterize the states of the system as accurately as possible.”

Thus far, quantum state tomography has been used for the characterization of quantum states, with which the system can be completely described. The problem is, as this method involves a great deal of measuring and computing effort it can’t currently be used for systems with more than half a dozen particles.

Christian Roos, together with colleagues from Germany and Great Britain, presented a very efficient method for the characterization of complex quantum states just two years ago. But, only weakly entangled states can be described with this method.

Last year Peter Zoller introduced a method which could deal with this complication and therefore can be used to characterize any entangled state. Working with experimental physicists Rainer Blatt and Christian Roos and their team, they have now demonstrated this method in the laboratory.

Quantum simulations on larger systems

The physicists demonstrated the process in a quantum simulator consisting of several ions arranged in a row in a vacuum chamber. Starting from a simple state, the researchers allowed the individual particles to interact with a little help from laser pulses. That’s how entanglement was generated in the system.

Andreas Elben, part of Zoller’s team, explains: “The new method is based on the repeated measurement of randomly selected transformations of individual particles. The statistical evaluation of the measurement results then provides information about the degree of entanglement of the system.”

Tiff Brydges, a PhD student from the Institute of Quantum Optics and Quantum Information, continues: “We perform 500 local transformations on each ion and repeat the measurements a total of 150 times in order to then be able to use statistical methods to determine information about the entanglement state from the measurement results.”

In the paper, now published in the journal Science, the Innsbruck physicists characterize the dynamic development of a system consisting of ten ions as well as a subsystem consisting of ten ions of a 20-ion chain.

Roos, who hopes that the new method can be successfully applied to quantum systems with up to several dozen particles, says: “In the laboratory, this new method helps us a lot because it enables us to understand our quantum simulator even better and, for example, to assess the purity of the entanglement more precisely.”

For Zoller, the most important aspect of the study was cooperation: “This publication shows once again the fruitful cooperation between the theoretical physicists and the experimental physicists here in Innsbruck.

“At the University of Innsbruck and the Institute for Quantum Optics and Quantum Information of the Austrian Academy of Sciences, young researchers from both fields find very good conditions for research work that is competitive worldwide.”

Roos also hopes for further applications for the new method: “ A second application that we see is in quantum simulation experiments where the technique might help to understand how entanglement spreads in quantum systems when the constituents of the system interact with each other quantum mechanically.”

Original research: http://dx.doi.org/10.1126/science.aau4963