Other People Slow Me Down

[del Campo et al. Phys. Rev. Lett. 110, 050403 (2013)]


Would Usain Bolt be fast in a crowd? Does a clock immersed in honey keep time? Interactions with our environment slow us down, and this is true in physics as it is in life. The speed of a clock or a chemical reaction is fundamental to understanding whether a device is useful or whether a process is important, and the effect of the environment can be dramatic. Now researchers at Ulm in Germany have turned their attention to analysing the speed limits in quantum physics, in the case when the environment cannot be ignored [del Campo et al. Phys. Rev. Lett. 110, 050403 (2013)].

Quantum mechanics describes atoms and molecules, and according to the theory, these tiny objects cannot be understood as points, or even as little balls (as we like to draw them), but must be treated more as fuzzy clouds, spread out over space like a dawn mist. The question of speed limits — how fast an object can change its properties — is therefore quite tricky in quantum mechanics. How long does it take for a cloud to change into a different cloud? You need to quantify how much two clouds overlap. Nonetheless the speed limit question has been understood in quantum mechanics for some time, provided that the effect of the environment can be ignored: the meteorological equivalent of a cloud on a clear day. Now try to follow the cloud in a storm, and the situation is complicated, because bits of the cloud are lost, while other bits are added, and the whole system is noisy and necessitates a statistical approach. The Ulm group have succeeded in analysing the speed limit for quantum systems — e.g. atoms and molecules — while they are buffeted by external forces (for instance other molecules or radiation). The result is that the environment causes the quantum systems to slow down. And this has important implications for technologies based on quantum mechanics, which are designed to use the very rapid transformation of special quantum states known as entangled states. These states are the Usain Bolts of the quantum world, with a lot of energy stored up in correlations between quantum particles. It has been suggested that such states could be used in ultra-precise sensors where their rapid oscillations can be used as a kind of quantum clock capable of detecting very small time differences or energy differences. However the work of the Ulm group shows that this super-sensitivity rapidly disappears when an entangled state is subject to external noise. The work makes it possible to predict whether a system will be useful based only on knowledge of the energy in the system, and the coupling to the environment.

This work was undertaken as part of the Q-ESSENCE project.

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