Can the Lieb-Robinson Bound be Intuited from the Taylor Series of an Operator?

In summary, the Lieb-Robinson bound is a concept that can be understood by looking at the Taylor series for an operator and how it grows in size with each order of it. The "light-cone" in this topic refers to the boundary between the operator and the rest of the system, and it is influenced by the finite speed of propagation of waves. This can be seen as a wave analog of the finite speed of travel for classical particles. However, this is just an intuition and may not fully explain the Lieb-Robinson bound.
  • #1
thatboi
133
18
Hi all,
I was wondering if there was a way to intuit the Lieb-Robinson bound from simply looking at the taylor series for an operator ##A(t) = e^{-iHt}Ae^{iHt}## where ##H## is a k-local Hamiltonian and ##A(t)## initially starts off as a single-site operator. The generic idea is that at each order of ##it##, the operator "grows" in size since it will have non-zero commutator with the local terms in ##H##. My issue though, is how to see the "light-cone" that is frequently used in discussions of this topic because for any non-zero ##t##, ##A(t)## will necessarily have contributions from all order of ##it##, so I am not sure where to draw the boundary between operator and the rest of the system that is typically drawn in the Lieb-Robinson bound.
Any advice would be appreciated.
Thanks!
 
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  • #2
I never heard of Lieb-Robinson bound before, but after a brief googling my intuition is the following. The wave (quantum of classical) propagates with a finite velocity, approximately given by the group velocity
$$v_g=\frac{d\omega}{dk}$$
For example, for a free non-relativistic particle we have ##\omega=k^2/2m## (in units ##\hbar=1##), so
$$v_g=k/m$$
which is finite, provided that ##k## is finite. Thus, if the wave packet has vanishing contributions from infinite ##\omega## and ##k##, i.e. the wave does not have wild oscillations at arbitrarily small time and space scales, then one expects a finite speed of propagation. This is a wave analog of the fact that a classical non-relativistic particle travels with finite speed, provided that its energy and momentum are not infinite.

But that's just my intuition, I'm not certain how much is this related to the actual Lieb-Robinson bound.
 

FAQ: Can the Lieb-Robinson Bound be Intuited from the Taylor Series of an Operator?

What is the Lieb-Robinson bound?

The Lieb-Robinson bound is a theoretical result in quantum mechanics that establishes a maximum speed, often referred to as the "Lieb-Robinson velocity," at which information or correlations can propagate through a quantum system. It is analogous to the speed of light in relativity but applies to non-relativistic quantum systems.

How does the Taylor series of an operator relate to the Lieb-Robinson bound?

The Taylor series of an operator, particularly the exponential of a Hamiltonian, can provide insights into the dynamics of a quantum system. By examining the series, one can understand how local perturbations affect the system over time, which is essential for deriving the Lieb-Robinson bound. The bound essentially emerges from the exponential growth terms in the series, which dictate how fast correlations can spread.

Can the Lieb-Robinson bound be derived directly from the Taylor series of an operator?

While the Taylor series of an operator offers a way to understand the dynamics of quantum systems, deriving the Lieb-Robinson bound directly from it is not straightforward. The bound is typically derived using more sophisticated mathematical techniques that involve norm estimates and commutator bounds, though the Taylor series provides a foundational understanding.

What role do commutators play in understanding the Lieb-Robinson bound?

Commutators are crucial in the derivation of the Lieb-Robinson bound because they measure the non-commutativity of operators, which directly relates to how information propagates in a quantum system. The bound often involves estimating the norm of nested commutators, which helps establish the maximum speed at which correlations can spread.

Is there an intuitive way to understand the Lieb-Robinson bound without deep mathematical formalism?

Intuitively, the Lieb-Robinson bound can be understood as a limit on how quickly information or disturbances can travel through a quantum system. Just as the speed of light limits how fast signals can travel in relativity, the Lieb-Robinson velocity limits the speed of information propagation in quantum systems. This ensures that even in a quantum world, there is a finite speed at which effects can propagate, preserving a form of locality.

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