A new speed limit for moving quantum information has just been hit by physicists

 A new speed limit for moving quantum information has just been hit by physicists


So far, scientific progress towards the future of quantum computing has involved several different breakthroughs in many different (but related) fields, and now there is a new one to report: the discovery of a critical limit of quantum velocity.

A fundamental question is answered by this latest research: how swift can a quantum process be? Knowing whether you want to create a quantum computer or a quantum network is a helpful piece of knowledge, as it shows you some of the weaknesses inherent in the device.

Fortunately, the team behind the new study has produced an easier-to-understand example for those of us who are not quantum physicists, which includes a professional waiter running around with a tray of drinks. How soon will all the drinks be distributed by the waiter without spilling any of the liquid?

The solution, it turns out, is to speed up and slow down at some points carefully, tipping the liquid glasses when appropriate to prevent spillage-only here the scientists used cooled-down cesium atoms instead of champagne, and an optical trap created as the 'drinks tray' by two laser beams.

When two laser beams are directed precisely at each other (physicists call this counter propagation), such a trap - known as an optical lattice - is created, resulting in well-defined interference that is shaped like a bunch of peaks and valleys.

The atoms were put into these valleys for transport, and the two-dimensional lattice was set into motion, not unlike a conveyor belt. The purpose of the analysis was to find out how easily this setup could be transferred without any damage to the atoms.

We loaded the atom into one of these valleys and then set the standing wave in motion, which, according to physicist Andrea Alberti, changed the position of the valley itself.

The setup discusses, completely intact, the physical limitations of bringing quantum information from one location to another. Moving it as quickly as possible helps to protect against external intrusion, but going too quickly and missing key bits of information can (you end up with champagne on the floor, in other words).

What the scientists found was that carefully calibrated accelerations and decelerations were required, rather than sticking to a constant speed throughout, to achieve the optimum overall speed limit for transmitting quantum data.


It is the first time that more complicated transfers have been measured in this way, where systems need to pass through many quantum states along the path. Quantum speed limits have already been developed for simpler states.

Not applicable here is the Mandelstam-Tamm bound limit for simpler states, named after the physicists who discovered it. What it did, however, was to provide a starting point for the researchers: the concept that energy uncertainty (how 'free' particles are to travel between energy states) is crucial to a transfer's maximum speed.


For more complex situations over wider distances, energy instability plays a role alongside the number of intermediate states that the particles must travel through without interruption to successfully reach their target. More complicated quantum systems ultimately have a lower speed limit.

Now that we know the fastest speed at which atoms can be transferred from one position to another without losing their original state, in this analysis, 17 millimeters per second over a distance of 0.5 micrometers, we know how easily we can force similar transfers within quantum computer systems.


Their fragility or their limited coherence time is one of the key problems with quantum states-how long they will remain stable for. This new study takes us closer to learning how we can make the most of that time.

Our research shows the maximum number of operations we can conduct in the time of coherence, Alberti says. This makes it possible to make use of it optimally.

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