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Fix measurement #290

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Fix measurement #290

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1da8350
Take joint expval in Measure functions, add test
yannick-couzinie Nov 20, 2024
e2bff08
Remove MeasureMultiPauliSum (not correctly implemented)
yannick-couzinie Nov 20, 2024
fc5966d
Adapt examples
yannick-couzinie Nov 20, 2024
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5 changes: 2 additions & 3 deletions examples/PauliSumOp/pauli_sum_op.py
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Original file line number Diff line number Diff line change
Expand Up @@ -44,17 +44,16 @@ def __init__(self):
self.rz0 = tq.RZ(has_params=True, trainable=True)
self.crx0 = tq.CRX(has_params=True, trainable=True)

self.measure = tq.MeasureMultiPauliSum(
self.measure = tq.MeasureMultiQubitPauliSum(
obs_list=[
{'coefficient': [0.2, 0.5]},
{
"wires": [0, 2, 3, 1],
"observables": ["x", "y", "z", "i"],
"coefficient": [1, 0.5, 0.4, 0.3],
},
{
"wires": [0, 2, 3, 1],
"observables": ["x", "x", "z", "i"],
"coefficient": [1, 0.5, 0.4, 0.3],
},
]
)
Expand Down
Empty file removed examples/PauliSumOp/pauli_sum_op_noise.py
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57 changes: 57 additions & 0 deletions test/measurement/test_measuremultipletimes.py
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Original file line number Diff line number Diff line change
@@ -0,0 +1,57 @@
import numpy as np
import torchquantum as tq


def test_non_trivial_pauli_expectation():
class TestCircuit(tq.QuantumModule):
def __init__(self):
super().__init__()

self.meas = tq.measurement.MeasureMultipleTimes([
{'wires': range(2), 'observables': 'xx'},
{'wires': range(2), 'observables': 'yy'},
{'wires': range(2), 'observables': 'zz'},
])

def forward(self, qdev: tq.QuantumDevice):
"""
Prepare and measure the expexctation value of the state
exp(-i pi/8)/sqrt(2) * (cos pi/12,
-i sin pi/12,
-i sin pi/12 * exp(i pi/4),
-i sin pi/12 * exp(i pi/4))
"""
# prepare bell state
tq.h(qdev, 0)
tq.cnot(qdev, [0, 1])

# add some phases
tq.rz(qdev, wires=0, params=np.pi / 4)
tq.rx(qdev, wires=1, params=np.pi / 6)
return self.meas(qdev)

test_circuit = TestCircuit()
qdev = tq.QuantumDevice(bsz=1, n_wires=2) # Batch size 1 for testing

# Run the circuit
meas_results = test_circuit(qdev)[0]

# analytical results for XX, YY, ZZ expval respectively
expval_xx = np.cos(np.pi / 4)
expval_yy = -np.cos(np.pi / 4) * np.cos(np.pi / 6)
expval_zz = np.cos(np.pi / 6)

atol = 1e-6

assert np.isclose(meas_results[0].item(), expval_xx, atol=atol), \
f"Expected {expval_xx}, got {meas_results[0].item()}"
assert np.isclose(meas_results[1].item(), expval_yy, atol=atol), \
f"Expected {expval_yy}, got {meas_results[1].item()}"
assert np.isclose(meas_results[2].item(), expval_zz, atol=atol), \
f"Expected {expval_zz}, got {meas_results[2].item()}"

print("Test passed!")


if __name__ == "__main__":
test_non_trivial_pauli_expectation()
48 changes: 9 additions & 39 deletions torchquantum/measurement/measurements.py
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Original file line number Diff line number Diff line change
Expand Up @@ -23,7 +23,6 @@
"expval",
"MeasureAll",
"MeasureMultipleTimes",
"MeasureMultiPauliSum",
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Why are we no longer adding this as a function to export?

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@yannick-couzinie yannick-couzinie Nov 20, 2024 •
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I removed the function, because it was not correctly implemented. You can check the example pauli_sum_op.py, changing the coefficients has no effect on the outcome of the measurement (see my first post). The coefficient part of the input is not taken into account.

Thinking about it, I think we could implement this easily by using

torchquantum/torchquantum/algorithm/hamiltonian.py

Lines 93 to 112 in 611cc2a

class Hamiltonian(object):
"""Hamiltonian class."""
def __init__(self,
coeffs: List[float],
paulis: List[str],
endianness: str = "big",
) -> None:
"""Initialize the Hamiltonian.
Args:
coeffs: The coefficients of the Hamiltonian.
paulis: The operators of the Hamiltonian, described in strings.
endianness: The endianness of the operators. Default is big. Qubit 0 is the most significant bit.
Example:
.. code-block:: python
coeffs = [1.0, 1.0]
paulis = ["ZZ", "ZX"]
hamil = tq.Hamiltonian(coeffs, paulis)
"""

and using the matrix of the hamiltonian.

Alternatively, we could change expval_joint_analytical to not only take the string of observables, but also coefficient info and multiply the coefficients at the point where the kronecker product is taken here:

torchquantum/torchquantum/measurement/measurements.py

Lines 270 to 271 in 611cc2a

for op in observable[1:]:
hamiltonian = torch.kron(hamiltonian, pauli_dict[op].to(states.device))

But then we would have to change the other expval functions as well to match this. If you think there is a real use case for this measurement and it is worth changing the expval functions we can do that, otherwise just remove as done here.

"MeasureMultiQubitPauliSum",
"gen_bitstrings",
"measure",
Expand Down Expand Up @@ -367,15 +366,14 @@ def forward(self, qdev: tq.QuantumDevice):

observables = []
for wire in range(qdev.n_wires):
observables.append(tq.I())
observables.append("I")

for wire, observable in zip(layer["wires"], layer["observables"]):
observables[wire] = tq.op_name_dict[observable]()
observables[wire] = observable

res = expval(
res = expval_joint_analytical(
qdev_new,
wires=list(range(qdev.n_wires)),
observables=observables,
observable="".join(observables),
)

if self.v_c_reg_mapping is not None:
Expand All @@ -390,42 +388,13 @@ def forward(self, qdev: tq.QuantumDevice):
res = res[:, perm]
res_all.append(res)

return torch.cat(res_all)

return torch.stack(res_all, dim=-1)

def set_v_c_reg_mapping(self, mapping):
self.v_c_reg_mapping = mapping


class MeasureMultiPauliSum(tq.QuantumModule):
"""
similar to qiskit.opflow PauliSumOp
obs list:
list of dict: example
[{'wires': [0, 2, 3, 1],
'observables': ['x', 'y', 'z', 'i'],
'coefficient': [1, 0.5, 0.4, 0.3]
},
{'wires': [0, 2, 3, 1],
'observables': ['x', 'y', 'z', 'i'],
'coefficient': [1, 0.5, 0.4, 0.3]
},
]
"""

def __init__(self, obs_list, v_c_reg_mapping=None):
super().__init__()
self.obs_list = obs_list
self.v_c_reg_mapping = v_c_reg_mapping
self.measure_multiple_times = MeasureMultipleTimes(
obs_list=obs_list, v_c_reg_mapping=v_c_reg_mapping
)

def forward(self, qdev: tq.QuantumDevice):
res_all = self.measure_multiple_times(qdev).prod(-1)

return res_all.sum(-1)


class MeasureMultiQubitPauliSum(tq.QuantumModule):
"""obs list:
list of dict: example
Expand All @@ -449,8 +418,9 @@ def __init__(self, obs_list, v_c_reg_mapping=None):
)

def forward(self, qdev: tq.QuantumDevice):
res_all = self.measure_multiple_times(qdev).prod(-1)

# returns batch x len(obs_list) object, return sum times coefficient
res_all = self.measure_multiple_times(qdev)

return (res_all * torch.tensor(self.obs_list[0]["coefficient"])).sum(-1)


Expand Down
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