First Author

Schrödinger's cat is a famous example of the apparent contradictions inherent in quantum mechanics. In this ‘thought experiment’, a cat shut in a box stands a 50% chance of being poisoned, and so is deemed to be both dead and alive until the box is opened and the actual state of the cat is observed. This ‘superposition’ of two extremes — dead and alive — for the unobserved cat is a state that experimentalists have strived to create using quantum particles. Such entangled particles could be used in information processing.

On page 639, researchers at the US National Institute of Standards and Technology (NIST) in Boulder, Colorado, reveal a ‘cat state’ created from six atoms. And on page 643, Hartmut Häffner and his colleagues at the Institute for Quantum Optics and Quantum Information in Innsbruck, Austria, reveal a similar entangled state involving eight particles. Dietrich Leibfried, a physicist in the NIST group, talks to Nature about these results.

Entangled states have previously been achieved, although with fewer atoms. What is the significance of doing it with more?

The previous ‘record’ was five photons. But not every run of the experiment produced the desired state — a very large number of failed tries accompanied each successful creation. Also, the process destroyed the result; once you knew you had the correct state it was already gone. With the methods described in this week's papers, you can produce the state on demand, each time the experiment is run.

What is the difference between your approach and that of Häffner?

Both our work and that of Häffner's group is based on manipulating trapped ions with laser light. But there are differences. Our approach is like a conference call where all of the participants communicate with each other at the same time. Häffner uses an approach similar to a phone chain that has to grow in length as the number of participants increases.

What are the implications of these two papers for quantum computing?

Producing cat states is a benchmark for the ability to produce and manipulate fragile quantum states. Such benchmarks can be used to compare the performance of different approaches to practical quantum computing.

Any other implications or applications?

A clock based on a ‘cat state’ of six atoms would reach the same measurement precision six times faster than a clock using six unentangled atoms. This starts to become significant when you think that atomic clocks are typically measured for months to obtain the highest precision.