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How can you measure qubits in QuTiP?

As far as I have seen you can define a Hamiltonian and let it evolve in time. It is also possible to define a quantum circuit, however, measuring and running it is not possible.

Does anyone know how to do this?

A simple circuit could be

H q[0]
CNOT q[0],q[1]
Measure q[0], q[1]
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QuTiP is not really meant for this I think. As said on the home page :

QuTiP is open-source software for simulating the dynamics of open quantum systems.

Simulating dynamics of open quantum systems by definition means you are interested in the quantum state as a result of your algorithm.

I tried looking at the Notebook examples provided in this Github but could not find measurement examples somewhere. You have a possibility to get expectation values though (see this notebook).

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Main purpose of Qutip is to explore dynamics of quantum systems and therefore density matrices are the tool to use. According to this answer on Quantum computing, we can model a measurement operator Pi on a density matrix. In the case of the measurement of a single qubit in the computational basis, you have $$P_0=|0\rangle\langle 0|\qquad P_1=|1\rangle\langle 1|$$

If you want to talk about n qubits where you measure just the first one, then you use the measurement operators

$$P_0=|0\rangle\langle 0|\otimes\mathbb{I}^{\otimes(n-1)}\qquad P_1=|1\rangle\langle 1|\otimes\mathbb{I}^{\otimes(n-1)}$$

Implementation with the Qutip dag method. First we set up a two level quantum system with the basis method use a vector v0 for the zero vector and v1 for one vector.

v0 = qp.basis(2, 0)

Calculate outer product with the dag method this will give a density operator

P0 = v0 * v0.dag()

expand for multiqubit gate

M0 = qp.gate_expand_1toN(P0, self.activeQubits, qubitNum)

Also

v1 = qp.basis(2, 1)

You can find a basic qubit quantum simulator running on Qutip in the SimulaQron software.

SimulaQron crudeSimulator

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Here. Scroll down to the stochastic solver, and you'll find an attribute for storing measurements.

It's certainly not the emphasis of the package, as cnada pointed out, but it's there.

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