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Showing content from https://github.com/qutip/qutip/issues/2680 below:

GitHub · Where software is built

It seems that qutip.gates.phasegate somehow neglects the imaginary part in the phase gate if the angle is such that it points to lower half of the complex plane, i.e. for angles between pi and 2pi. Printing the created Qobj looks okay, but taking the Hermitian transpose .dag() actually doesn't change anything, and .diag() outputs only real numbers.

import numpy as np
import qutip as qt

pg = qt.gates.phasegate(4*np.pi/3)
print(pg)
print(pg.dag())
print(pg.diag())

Quantum object: dims=[[2], [2]], shape=(2, 2), type='oper', dtype=CSR, isherm=True
Qobj data =
[[ 1. +0.j         0. +0.j       ]
 [ 0. +0.j        -0.5-0.8660254j]]
Quantum object: dims=[[2], [2]], shape=(2, 2), type='oper', dtype=CSR, isherm=True
Qobj data =
[[ 1. +0.j         0. +0.j       ]
 [ 0. +0.j        -0.5-0.8660254j]]
[ 1.  -0.5]

Taking the .dag() should change the sign of the imaginary part in [1,1] element. For Qutip version 4.7.3 this works as it should.

QuTiP Version:      5.1.1
Numpy Version:      1.26.4
Scipy Version:      1.14.1
Cython Version:     None
Matplotlib Version: 3.6.3
Python Version:     3.11.11
Number of CPUs:     8
BLAS Info:          Generic
INTEL MKL Ext:      None
Platform Info:      Windows (AMD64)

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