Demonstration of Largest Entangled Quantum Register of Individually Controllable Systems

In new quantum technologies of the 21st century, entanglement is of fundamental importance. A German-Austrian research team is currently showcasing the largest entangled quantum register of independently controllable systems to date, comprising a total of 20 quantum bits. The physicists in Innsbruck, Vienna, and Ulm are pushing experimental and theoretical approaches to the maximum of what is presently possible.

Some of the new quantum technologies spanning from very precise sensors to universal quantum computers require a great number of quantum bits so as to exploit the benefits of quantum physics. Globally physicists are thus working on executing entangled systems with more and more quantum bits. The record is presently held by Rainer Blatt's research group at the Institute of Experimental Physics at the University of Innsbruck. In 2011, the physicists entangled 14 independently addressable quantum bits for the first time and thus accomplished the largest completely entangled quantum register.

Currently, a research team led by Ben Lanyon and Rainer Blatt at the Institute of Quantum Optics and Quantum Information (IQOQI) of the Austrian Academy of Sciences, along with theorists from the University of Ulm and the Institute of Quantum Optics and Quantum Information in Vienna, has currently realized controlled multi-particle entanglement in a system of 20 quantum bits. The researchers could detect genuine multi-particle entanglement between all adjacent groups of three, four and five quantum bits.

Genuine Multi-Particle Entanglement

Physically, entangled particles cannot be described as separate particles with distinct states, but only as a total system. It is mostly difficult to understand entanglement when many particles are involved. Here a differentiation must be made between the entanglement of separate particles and real, genuine multi-particle entanglement. Genuine multi-particle entanglement can only be comprehended as a property of the complete system of all particles concerned and not be explained by a blend of the subsystems being entangled.

At the Institute of Quantum Optics and Quantum Information in Innsbruck, the group of physicists have currently used laser light to entangle 20 calcium atoms in an ion trap experiment and detected the dynamic propagation of multi-particle entanglement in this system.

The particles are first entangled in pairs. With the methods developed by our colleagues in Vienna and Ulm, we can then prove the further spread of the entanglement to all neighboring particle triplets, most quadruplets and a few quintuplets.

Ben Lanyon​

New Detection Methods

These detection approaches were formulated by Martin Plenio's research group at the University of Ulm and Marcus Huber's team at IQOQI Vienna. "We have chosen a MacGyver approach," says first author Nicolai Friis with a smirk. "We had to find a way to detect multi-particle entanglement with a small number of feasible measurement settings."

The researchers in Vienna and Ulm employed a complementary approach: the group around Huber and Friis used a technique that only necessitates a few measurements and whose results can be easily assessed. In this manner, the entanglement of three particles could be shown in the experiment. The theorists from Ulm used a more complex method based on numerical techniques.

Although this technique is efficient, it also reaches its limits due to the sharp increase in computing effort due to the number of quantum bits. That's why the usefulness of this method also came to an end with the detection of real five-particle entanglement.

Oliver Marty, Martin Plenio's Research Group

A Big Step Towards Application

"There are quantum systems such as ultra-cold gases in which entanglement between a large number of particles has been detected," emphasizes Nicolai Friis. "However, the Innsbruck experiment is able to address and read out every single quantum bit individually." It is thus ideal for practical applications such as quantum information processing or quantum simulations. Rainer Blatt and his team hope to further boost the number of quantum bits in the experiment.

"Our medium-term goal is 50 particles," he says. "This could help us solve problems that the best supercomputers today still fail to accomplish." The physicists in Ulm and Vienna are convinced that the techniques formulated for the ion trap experiment in Innsbruck will be used more extensively. "We want to push the boundaries of our methods even further," say Friis and Marty. "By exploiting symmetries and focusing on certain observables, we can further optimize these methods to detect even more extensive multi-particle entanglement.”

The study was funded by the Austrian Science Fund FWF and the European Union, among others, and published in Physical Review X.