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Speed limit in the quantum realm
Limit for atoms: For the first time, physicists have determined the maximum speed for the transfer of atomic quantum states. Their experiment confirms that the speed limit for such quantum operations is lower than for simple changes of state. At the same time, it reveals that the atoms arrive safely at their destination faster by “jerking”. This finding is helpful for quantum computers, among other things, as the researchers explain.
Update: interviewabout this experiment and its meaning.
There is a lot going on in the quantum realm that would be physically impossible in our macroscopic world. This includes the transmission of quantum states, for example via entangled photons - a kind of teleportation. But even in the world of the smallest particles, things cannot happen infinitely quickly, as the Soviet physicists Leonid Mandelstam and Igor Tamm theoretically proved in the 1940s.
Limited by the energy blurring
According to this quantum speed limit, quantum objects require a certain minimum time to switch between two distinguishable states - for example, to flip an electron's spin. The reason is a close connection with the energy of these systems: According to Mandelstam and Tamm, the maximum speed of a quantum process depends on the energy uncertainty: the more energetic freedom a particle has, the faster the process can run.
This can be compared to transporting a full glass on a tray: If you want to bring it to a distant table as quickly as possible without spilling anything, you have to compensate for the acceleration: at first you tilt the tray in the direction of movement, when braking against it. This is similar with an atom, which quantum physically corresponds to a matter wave. Its quantum state only remains stable at high transfer speeds if the trough is deep enough.
Atomic transport in the light wave
The place where the speed limit for simple quantum systems such as electrons lies has already been determined experimentally. But for more complex quantum operations such as the non-local transfer of an atom, the maximum speed was unknown. Manolo Lam from the University of Bonn and his colleagues have now determined this for the first time. For their experiment, they cooled cesium atoms down to almost absolute zero and thus brought them into the energetic ground state.
These ultra-cold atoms were then individually locked in “cages made of intersecting laser beams. This hollow of standing light waves corresponds to the tray, the atom to the water glass. “We charged the atom into one of these valleys, then set the light wave in motion and thus shifted the position of the valley,” explains Lam's colleague Andrea Alberti. "Our goal was to get the atom to its destination in the shortest possible time, without it being able, figuratively speaking, to slosh out of the trough of the waves."
Speed limit lower
It showed that there is a maximum speed at which the atom arrives at its destination with an intact basic state. This is around 17 millimeters per second, as the researchers report. If the “conveyor belt” made of light is moved faster, the quantum state is lost and can no longer be read out reliably. “Reliability drops rapidly at faster speeds,” says Lam and his team. "That reveals the existence of the quantum speed limit for the transport of matter waves."
In doing so, the physicists have not only set the speed limit for such quantum operations for the first time. Their result also confirms that a lower speed limit applies to more complex processes with several intermediate states than to simple systems with only two states, such as the spin of an electron. The quantum limit is therefore not only determined by the energy uncertainty, but also by the number of intermediate states, as the researchers explain.
It goes faster if it jerks
Also interesting: the stable transport of the atom is not the fastest if it is moved evenly, but if the conveyor belt of light sometimes falters and sometimes becomes faster. As the measurements showed, this jerking ensures that intermittent jumps in energy and stimuli are balanced out and the atom arrives at its destination in the desired basic state. In principle, this is similar to the inclination of the tray when transporting the water glass.
These findings are important for quantum computing, among other things. Because the calculations that are possible with quantum computers are mostly based on the manipulation of multi-level systems - similar to the atoms transferred in the experiment. "Our study shows the maximum number of operations that can take place in the coherence time," explains Alberti. "This makes it possible to use this time optimally." (Physical Review X, 2021; doi: 10.1103 / PhysRevX.11.011035)
interviewabout this experiment and its meaning.
Source: University of Bonn, American Physical Society (APS)February 22, 2021
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