Taking Python to the Next Level

Joey Bernard — Thu, 22 Feb 2018 15:57:52 +0000

A brief intro to simulating quantum systems with QuTiP.

With the reincarnation of Linux Journal, I thought I'd take this article through a quantum leap (pun intended) and look at quantum computing. As was true with the beginning of parallel programming, the next hurdle in quantum computing is developing algorithms that can do useful work while harnessing the full potential of this new hardware.

Unfortunately though, most people don't have a handy quantum computer lying around on which they can develop code. The vast majority will need to develop ideas and algorithms on simulated systems, and that's fine for such fundamental algorithm design.

So, let's take look at one of the Python modules available to simulate quantum systems—specifically, QuTiP. For this short article, I'm focusing on the mechanics of how to use the code rather than the theory of quantum computing.

The first step is installing the QuTiP module. On most machines, you can install it with:


sudo pip install qutip

This should work fine for most people. If you need some latest-and-greatest feature, you always can install QuTiP from source by going to the home page.

Once it's installed, verify that everything worked by starting up a Python instance and entering the following Python commands:


>> from qutip import *
>> about()

You should see details about the version numbers and installation paths.

The first step is to create a qubit. This is the simplest unit of data to be used for quantum calculations. The following code generates a qubit for two-level quantum systems:


>> q1 = basis(2,0)
>> q1
   Quantum object: dims = [[2], [1]], shape = (2, 1), type = ket
   Qobj data =
   [[ 1.]
    [ 0.]]

By itself, this object doesn't give you much. The simulation kicks in when you start applying operators to such an object. For example, you can apply the sigma plus operator (which is equivalent to the raising operator for quantum states). You can do this with one of the operator functions:


>> q2 = sigmap * q1
>> q2
   Quantum object: dims = [[2], [1]], shape = (2, 1), type = ket
   Qobj data =
   [[ 0.]
    [ 0.]]

As you can see, you get the zero vector as a result from the application of this operator.

You can combine multiple qubits into a tensor object. The following code shows how that can work:

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Quantum Computing

Taking Python to the Next Level