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
121
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.
 

1. What is the Lieb-Robinson Bound?

The Lieb-Robinson Bound is a mathematical theorem that describes the maximum speed at which information can propagate in a quantum system. It states that the commutator of two operators, which represent physical observables, decays exponentially with distance and time.

2. How is the Lieb-Robinson Bound related to the Taylor Series of an Operator?

The Lieb-Robinson Bound can be derived from the Taylor Series expansion of an operator. By taking higher order terms in the expansion, the Lieb-Robinson Bound can be obtained as a limit.

3. Can the Lieb-Robinson Bound be intuitively understood?

Yes, the Lieb-Robinson Bound can be intuitively understood as a limit on the speed at which information can travel in a quantum system. It is similar to the speed of light limit in relativity, where nothing can travel faster than the speed of light.

4. What are the applications of the Lieb-Robinson Bound?

The Lieb-Robinson Bound has many applications in quantum information theory and quantum many-body systems. It is used to study the dynamics of quantum systems, and has implications for quantum computing and quantum communication protocols.

5. Are there any limitations to the Lieb-Robinson Bound?

Yes, the Lieb-Robinson Bound is only applicable to systems with short-range interactions. It also assumes that the system is in a stationary state, and does not account for any external perturbations. Additionally, the bound may not hold for systems with long-range interactions or in highly entangled states.

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