Physicists discover an infinite number of quantum speed limits

June 3, 2016 by Lisa Zyga report
Illustration of geometric quantum speed limits. The ultimate speed limit arises from the length of the shortest geodesic path between two states, which here is the solid red line. This distance corresponds to the minimum time needed for a system to evolve from one state to another. Credit: Diego Paiva Pires et al. ©2016 American Physical Society

(Phys.org)—In order to determine how fast quantum technologies can ultimately operate, physicists have established the concept of "quantum speed limits." Quantum speed limits impose limitations on how fast a quantum system can transition from one state to another, so that such a transition requires a minimum amount of time (typically on the order of nanoseconds). This means, for example, that a future quantum computer will not be able to perform computations faster than a certain time determined by these limits.

Although physicists have been investigating different quantum speed limits for different types of quantum systems, it has not been clear what the best way to do this is, or how many different quantum speed limits there are.

Now in a new paper published in Physical Review X, Diego Paiva Pires et al., from the UK and Brazil, have used techniques from information geometry to show that there are an infinite number of quantum speed limits. They also develop a way to determine which of these speed limits are the strictest, or in other words, which speed limits offer the tightest lower bounds. As the researchers explain, the search for the ultimate quantum speed limits is closely related to the very nature of time itself.

"In recent years, there has been an intense theoretical and experimental research activity to understand, on one hand, a fundamental concept in quantum mechanics such as time, and to devise, on the other hand, efficient schemes for the implementation of quantum technologies," coauthor Gerardo Adesso, at the University of Nottingham, told Phys.org. "A basic question that combines and underpins both areas of research is: 'How fast can a quantum system evolve in time?' Establishing general and tight quantum speed limits is crucial to assess how fast quantum technologies can ultimately be, and can accordingly guide in the design of more efficient protocols operating at or close to the ultimate bounds."

In order to determine how fast a quantum system can evolve from one state to another, it's necessary to be able to distinguish between the two states, and there are multiple ways to do this. In the new study, the physicists used a general method based on information geometry. From a geometric perspective, two distinguishable states can be represented by two points on the surface of some shape, such as a sphere or other manifold. Previous research has shown that there are an of corresponding metrics that can be used to measure the distinguishability of two quantum states.

In the new study, the physicists have shown that each of these metrics corresponds to a different quantum speed limit. The "strictest" quantum speed limit is determined by the metric that gives the shortest distance (also known as a 'geodesic') between the two points, or states, as measured along the manifold's curved surface.

"A different quantum speed limit arises from each of these metrics in such a way that the tightest bound for a given dynamics is specified by the metric whose geodesic is best tailored to the given dynamical path," explained coauthor Marco Cianciaruso, also at Nottingham.

Overall, the new approach unifies most of the previous results by interpreting them under a single, new framework. On one hand, the researchers could derive the tightest bounds to date on quantum speed limits for some relevant instances, such as quantum bits undergoing dissipative and decohering evolutions. On the other hand, they could also show that bounds that have been previously proposed for other instances are truly the optimal bounds—no tighter bounds will ever be found.

In the future, the researchers plan to experimentally investigate the quantum speed limits derived here using nuclear magnetic resonance (NMR) techniques.

"Our findings are expected to have an impact on the fields of quantum information, computation, simulation, and metrology," said Diogo Soares-Pinto at the Sao Carlos Institute of Physics, who supervised the project.

Explore further: Physicists quantify the usefulness of 'quantum weirdness'

More information: Diego Paiva Pires et al. "Generalized Geometric Quantum Speed Limits." Physical Review X. DOI: 10.1103/PhysRevX.6.021031

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BrettC
3.8 / 5 (4) Jun 03, 2016
An infinite number of limitations? That's nearly an Oxymoron!
Whydening Gyre
4.3 / 5 (7) Jun 03, 2016
It does present a metaphorical paradox of sorts, doesn't it....
Hyperfuzzy
1.7 / 5 (11) Jun 03, 2016
Funny! In fact I wonder what are these people thinking, QM is reality? Who Taught these PhD's, children? It describes some other world! Look at the causal effects without QM. You don't need it. Take a look at reality. Come on, we can anticipate controls when we use the correct physic. This is BS! Oh so this defines reality? Ok define the dwell time to initiate a process with total definition and thus we know exactly what is happening after a given setup period. So dude learn to be a part of the measurement at every point. But minimum time to change state? What are you talking about? Define the two states? Dude all states are not equal. In fact I can think of a few that cannot be reached or states that occur instantly give a set of conditions. But this is a stupid filter from undefined state to undefined state, some QM state. Dude, it is what it is!
ogg_ogg
3 / 5 (7) Jun 03, 2016
I wonder if this is related to the problem with a discrete space-time: how do you place a lower limit on the size of an elementary 'particle' of space-time when its dimensions must be relative to the velocity of the observer? I've just read that the LHC accelerates protons to near the speed of light & that their Lorentz factor is over 6,000. Meaning their diameter varies over 3, almost 4, orders of magnitude. So, how small is a proton? The only absolute limit seems to be 0.
Steelwolf
1.5 / 5 (2) Jun 04, 2016
So, it is almost sounding like the closer they get to finding out EXACTLY what happens when an Electron hops orbit into another one, something we have known many parts about why and what the preconditions have to be, the increasing of energy though photonic addition, magnetic amplification or even friction: The Energy needed to hop to the next orbital, which we know a lot of the configuration for, but the little bit between one orbit an another many people considered 'tunneling'.

This paper admits the possibility of, at the intensely tiny time that it takes, there may be a decoherence of that electron, into another state, whereby it travels and reassembles in a higher orbital state. Now, if one considers that groups and bands of stars are going to want to find the right speed for the obits of their particles, it is possible that part of the 'electron' be stars novaeing to the point that the gas and stellar cores of the globular cluster moves to a higher energy orbital.
Fascinating
Hyperfuzzy
1.5 / 5 (2) Jun 04, 2016
an orbiting electron experiences an EM wave move over it, it oscillates in kind, it ends up in a new state, what is the minimum time for change. I think it's calculable and has has no dependency upon QM.
torbjorn_b_g_larsson
4.2 / 5 (5) Jun 04, 2016
Just to be clear, which the article may not have been, the existence of infinitely parametrized families of measures is needed in order to chose the one that fits a particular problem, same as differential equations have similar solution families but only one that goes through the desired point.

The image illustration is also poorly described since the alleged geodesic is not the shortest path as it should be. Indeed, while the paper is dense theoretic, it has a clear image caption where the geodesic is in red with a generic quantum state evolution in blue. The trick is to get the blue path to coincide with a geodesic.

And that is as far as I will dig into the paper.

@ogg: Relativistic deformation is not the invariant size. But yes, the Planck size is not the size of a geometry.

@Steelworlf, Hyperfuzzy: An electron transfers to another orbit described by its distribution. If it tunnels that part is instantaneous, the rest time is by EM theory of the photon et cetera.
ski137
not rated yet Jun 04, 2016
Finally we have a use for Planck time.
beshir75
not rated yet Jun 06, 2016
I mean that's not new, that the fastest way is the shortest distance between two points. The more important thing is that the conditions and the right timing are perfect when strike that action.
Nik_2213
not rated yet Jul 17, 2016
Slightly OT: We quip that the shortest distance between two points may be a bee-line, but the fastest is a Feline !!

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