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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