When two quantum objects interact, all the information they carry becomes scrambled. Now, physicists have calculated a fundamental limit for how quickly this can happen.
One striking example of information scrambling happens in black holes, says at the University of Maryland. When objects fall into these super-dense bodies, some of the information they contain reemerges in the black hole鈥檚 emitted radiation 鈥� but in a highly scrambled form.
In fact, physicists have theorised that black holes are the fastest possible scramblers of information. Starting with the basic ingredients of quantum theory, Galitski and his colleagues and , also at the University of Maryland, disproved that idea. They found that even quicker information scramblers could exist in the quantum realm.
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The researchers wanted to calculate the shortest time necessary to fully scramble information contained in some small quantum system as it interacts with a larger one. For instance, if the small system was a few atoms, the information it contains might be the orientation of the atoms鈥� spins.
To calculate how scrambled the information became, Galitski says they used a measure analogous to comparing a drawing in the sand with what that same spot looks like after a few waves hit it. Fully scrambled information was like all the grains of sand that had formed the drawing getting evenly dispersed across the beach. But it was technically challenging to find the shortest amount of time in which this full scrambling could happen 鈥� and to have the calculation apply very generally to almost any type of quantum system.
Vikram says it was mathematician Shou鈥檚 contribution that ultimately proved crucial. The mathematical functions that described the physics at play were complex, but her knowledge of their properties enabled the team to derive an exact speed limit for a quantum system鈥檚 information scrambling.
鈥淗ad you asked me last year, I would have told you that this calculation may not be possible,鈥� says Vikram.
at the University of Texas at Austin says information scrambling is closely related to the strange phenomenon of quantum chaos, which refers to systems that never settle into a simple, even state as time goes on and instead keep changing in intricate ways. Because information scrambling is a symptom by which quantum chaos can often be diagnosed, understanding it better 鈥� and for many types of quantum systems at once 鈥� opens the doors for new insights into quantum chaos itself, he says.
Hunter-Jones says the new work also opens an intriguing question: if black holes are not the best possible information scramblers, then what is?
Vikram says his team鈥檚 calculations point to what energies speedy scramblers may have. But it remains unclear whether physicists should look for them where quantum theory and gravity meet, like with black holes, or in laboratory experiments where quantum systems can be engineered from atoms or light.
at the Institute for Advanced Study in New Jersey says the latter may be a safer bet 鈥� unless some special interactions are missing from mathematical models for how black holes scramble information. He says the limits on black holes鈥� scrambling speed follow from fundamental properties of how gravity makes objects interact. So, the possible existence of even faster quantum scramblers could lead to new insights into how quantum theory and gravity can and cannot combine, which is of great interest to physicists working on a theory of everything.
arXiv
