Description of the issue
When looping over certain gate operations and measurements, the trace of the density matrix decreases and eventually approaches 0.
How to reproduce the issue
The following script (@kinetic-cipher) reproduces the issue:
import cirq
import numpy as np
q0 = cirq.NamedQubit('q0')
q1 = cirq.NamedQubit('q1')
qc = cirq.Circuit()
qc.append( cirq.CNOT.on(q1,q0) )
qc.append( cirq.H.on(q1) )
qc.append( cirq.measure(q1) )
sim = cirq.DensityMatrixSimulator()
initial_state = None
for _ in range(100):
output = sim.simulate(qc, initial_state=initial_state)
initial_state = output.final_density_matrix
print( np.trace(initial_state) )
The printed output is:
Details
(0.9999999+0j)
(0.99999976+0j)
(0.9999994+0j)
(0.9999988+0j)
(0.9999976+0j)
(0.99999523+0j)
(0.99999034+0j)
(0.99998057+0j)
(0.999961+0j)
(0.99992204+0j)
(0.99984396+0j)
(0.9996879+0j)
(0.99937594+0j)
(0.9987523+0j)
(0.997506+0j)
(0.9950181+0j)
(0.99006104+0j)
(0.98022074+0j)
(0.9608326+0j)
(0.9231992+0j)
(0.8522966+0j)
(0.7264095+0j)
(0.5276707+0j)
(0.27843636+0j)
(0.07752679+0j)
(0.0060104034+0j)
(3.6124948e-05+0j)
(1.3050119e-09+0j)
(1.703056e-18+0j)
(2.9003994e-36+0j)
0j
with error (cirq v0.14.0):
Details
ValueError: probabilities contain NaN
or with error (cirq v1.0.0):
When modifying the same script to use np.complex128 as the dtype of the DensityMatrixSimulator
sim = cirq.DensityMatrixSimulator(dtype=np.complex128)
the density matrix traces get very large.
Details
(1.0000000000000004+0j)
(1.0000000000000013+0j)
(1.0000000000000027+0j)
(1.0000000000000058+0j)
(1.000000000000012+0j)
(1.000000000000024+0j)
(1.0000000000000484+0j)
(1.0000000000000968+0j)
(1.000000000000194+0j)
(1.0000000000003886+0j)
(1.0000000000007776+0j)
(1.0000000000015556+0j)
(1.0000000000031117+0j)
(1.000000000006224+0j)
(1.0000000000124478+0j)
(1.000000000024896+0j)
(1.0000000000497922+0j)
(1.0000000000995848+0j)
(1.00000000019917+0j)
(1.0000000003983405+0j)
(1.0000000007966814+0j)
(1.0000000015933632+0j)
(1.0000000031867269+0j)
(1.0000000063734538+0j)
(1.0000000127469075+0j)
(1.0000000254938157+0j)
(1.0000000509876326+0j)
(1.0000001019752678+0j)
(1.0000002039505465+0j)
(1.0000004079011349+0j)
(1.000000815802436+0j)
(1.000001631605538+0j)
(1.0000032632137386+0j)
(1.0000065264381257+0j)
(1.0000130529188458+0j)
(1.0000261060080706+0j)
(1.000052212697665+0j)
(1.0001044281214957+0j)
(1.0002088671482243+0j)
(1.0004177779219348+0j)
(1.000835730382262+0j)
(1.0016721592097961+0j)
(1.003347114536015+0j)
(1.0067054322477473+0j)
(1.0134558273171237+0j)
(1.0270927139230361+0j)
(1.0549194429937883+0j)
(1.112855031206325+0j)
(1.238446320481231+0j)
(1.5337492887135007+0j)
(2.35238688062917+0j)
(5.533724036156239+0j)
(30.622101708333307+0j)
(937.7131130355101+0j)
(879305.8823587475+0j)
(773178834750.6958+0j)
(5.9780551050644395e+23+0j)
(3.5737142839187013e+47+0j)
(1.2771433783084559e+95+0j)
(1.6310952087571362e+190+0j)
with the same error:
Details
ValueError: probabilities contain NaN
Cirq version
We tested this in cirq version 1.0.0 as well as 0.14.0 and found the error in both cases.
Description of the issue
When looping over certain gate operations and measurements, the trace of the density matrix decreases and eventually approaches 0.
How to reproduce the issue
The following script (@kinetic-cipher) reproduces the issue:
The printed output is:
Details
(0.9999999+0j)
(0.99999976+0j)
(0.9999994+0j)
(0.9999988+0j)
(0.9999976+0j)
(0.99999523+0j)
(0.99999034+0j)
(0.99998057+0j)
(0.999961+0j)
(0.99992204+0j)
(0.99984396+0j)
(0.9996879+0j)
(0.99937594+0j)
(0.9987523+0j)
(0.997506+0j)
(0.9950181+0j)
(0.99006104+0j)
(0.98022074+0j)
(0.9608326+0j)
(0.9231992+0j)
(0.8522966+0j)
(0.7264095+0j)
(0.5276707+0j)
(0.27843636+0j)
(0.07752679+0j)
(0.0060104034+0j)
(3.6124948e-05+0j)
(1.3050119e-09+0j)
(1.703056e-18+0j)
(2.9003994e-36+0j)
0j
with error (cirq v0.14.0):
Details
ValueError: probabilities contain NaNor with error (cirq v1.0.0):
When modifying the same script to use np.complex128 as the dtype of the DensityMatrixSimulator
the density matrix traces get very large.
Details
(1.0000000000000004+0j) (1.0000000000000013+0j) (1.0000000000000027+0j) (1.0000000000000058+0j) (1.000000000000012+0j) (1.000000000000024+0j) (1.0000000000000484+0j) (1.0000000000000968+0j) (1.000000000000194+0j) (1.0000000000003886+0j) (1.0000000000007776+0j) (1.0000000000015556+0j) (1.0000000000031117+0j) (1.000000000006224+0j) (1.0000000000124478+0j) (1.000000000024896+0j) (1.0000000000497922+0j) (1.0000000000995848+0j) (1.00000000019917+0j) (1.0000000003983405+0j) (1.0000000007966814+0j) (1.0000000015933632+0j) (1.0000000031867269+0j) (1.0000000063734538+0j) (1.0000000127469075+0j) (1.0000000254938157+0j) (1.0000000509876326+0j) (1.0000001019752678+0j) (1.0000002039505465+0j) (1.0000004079011349+0j) (1.000000815802436+0j) (1.000001631605538+0j) (1.0000032632137386+0j) (1.0000065264381257+0j) (1.0000130529188458+0j) (1.0000261060080706+0j) (1.000052212697665+0j) (1.0001044281214957+0j) (1.0002088671482243+0j) (1.0004177779219348+0j) (1.000835730382262+0j) (1.0016721592097961+0j) (1.003347114536015+0j) (1.0067054322477473+0j) (1.0134558273171237+0j) (1.0270927139230361+0j) (1.0549194429937883+0j) (1.112855031206325+0j) (1.238446320481231+0j) (1.5337492887135007+0j) (2.35238688062917+0j) (5.533724036156239+0j) (30.622101708333307+0j) (937.7131130355101+0j) (879305.8823587475+0j) (773178834750.6958+0j) (5.9780551050644395e+23+0j) (3.5737142839187013e+47+0j) (1.2771433783084559e+95+0j) (1.6310952087571362e+190+0j)with the same error:
Details
ValueError: probabilities contain NaNCirq version
We tested this in cirq version 1.0.0 as well as 0.14.0 and found the error in both cases.