Why use adiabatic evolution instead of cooling?

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Discussion Overview

The discussion revolves around the comparison between adiabatic evolution and cooling methods in the context of adiabatic quantum computation. Participants explore the implications of using different Hamiltonians and the challenges associated with decoherence and system interactions with the environment.

Discussion Character

  • Debate/contested
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • One participant questions why adiabatic evolution is preferred over cooling the system directly to the ground state of the problem Hamiltonian, Hp.
  • Another participant raises concerns about how to cool the system without decoherent interactions with the environment, suggesting that such interactions complicate the cooling process.
  • A different viewpoint suggests that coupling the Hamiltonian to a bath temporarily could allow for cooling, but this may not lead to solving the problem as it effectively transitions the system to a different Hamiltonian.
  • Some participants argue that separating the system from the environment after cooling would return the Hamiltonian to its original state, potentially placing the wave function in the ground state.
  • One participant introduces the idea that adiabatic evolution may be fundamentally different due to time complexity considerations, noting that the time required for a system to relax into the ground state may increase exponentially with the number of qubits, while adiabatic evolution may not have the same constraints.
  • There is speculation about the relationship between the spectral gap and the number of qubits, with some participants suggesting that this could affect the efficiency of the cooling versus adiabatic evolution approaches.

Areas of Agreement / Disagreement

Participants express differing views on the feasibility and effectiveness of cooling versus adiabatic evolution, indicating that there is no consensus on which method is superior or under what conditions each method may be preferable.

Contextual Notes

Participants acknowledge the complexity of the problem, including assumptions about decoherence, the nature of Hamiltonians, and the time required for relaxation versus adiabatic evolution. These factors remain unresolved and are subject to further exploration.

mupsi
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Hi everyone,

For adibatic quantum computation one prepares the groundstate of a particular Hamiltonian and than adiabatically evolves the system to a problem Hamiltonian Hp which groundstate encodes the desired solution to a satisfiability problem. If the evolution takes place slowly the system will be in a groundstate Hp with a high probability. My question: why can't I start with Hp to begin with, cool down the system such that the wacefunction relaxes into the goundstate?
 
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How do you cool the system if your Hamiltonian requires that you don't have decoherent interactions with the environment?
 
mfb said:
How do you cool the system if your Hamiltonian requires that you don't have decoherent interactions with the environment?
Why? I could couple the Hamiltonian to a bath for a short time, cool it down and separate the system from the environment. I don't see any problem with that. Decoherence just causes the system to have a Boltzmann-Distribution
 
I think as soon as you do that coupling, your Hamiltonian does something, but not solving the problem. Which means you are doing what you described in post 1: cool to the ground state of one Hamiltonian, then go to a different one.
 
mfb said:
I think as soon as you do that coupling, your Hamiltonian does something, but not solving the problem. Which means you are doing what you described in post 1: cool to the ground state of one Hamiltonian, then go to a different one.

I agree, but when you separate the system from its environment you get the original hamiltonian again and the wave function should be in the groundstate.
 
If you do it adiabatically.
Now we have everything you asked about.
 
mfb said:
If you do it adiabatically.
Now we have everything you asked about.
but that approach is fundamentally different. I think that the argument is time complexity. In general it takes more time for the system to relax into the groundstate (as a function of the number of qubits) than if you perform an adiabatic evolution. The computation time is related to the number of qubits via the inverse of the spectral gap between groundstate and the first excited state (which itself is a function of # qubits) and the spectral gap might (in some cases) only increases polynomially depeding on the computational problem whereas the relaxation time increases exponentially (this is speculative). A further assumption is that the groundstate of the initial hamiltonian is easy to prepare. I could be wrong though.
 

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