quantumlib/qsim

qsimcirq: Applying global phase causes "ValueError: Only up to 6-qubit matrix gates are supported"

rht opened this issue · 4 comments

rht commented

Minimal reproducer:

import numpy as np
import cirq
import qsimcirq

simulator = qsimcirq.QSimSimulator()

circuit = cirq.Circuit([
    cirq.global_phase_operation(np.exp(0.5j * np.pi))
])
simulator.simulate(circuit)

Traceback:

Traceback (most recent call last):
  File "***/minimal_reproducer.py", line 10, in <module>
    simulator.simulate(circuit)
  File "***/venv/lib/python3.10/site-packages/cirq/sim/simulator.py", line 491, in simulate
    return self.simulate_sweep(
  File "***/venv/lib/python3.10/site-packages/cirq/sim/simulator.py", line 506, in simulate_sweep
    return list(self.simulate_sweep_iter(program, params, qubit_order, initial_state))
  File "***/venv/lib/python3.10/site-packages/qsimcirq/qsim_simulator.py", line 519, in simulate_sweep_iter
    options["c"], _ = self._translate_circuit(
  File "***/venv/lib/python3.10/site-packages/qsimcirq/qsim_simulator.py", line 808, in _translate_circuit
    translated, moment_indices = translator_fn(qubit_order)
  File "***/venv/lib/python3.10/site-packages/qsimcirq/qsim_circuit.py", line 413, in translate_cirq_to_qsim
    add_op_to_circuit(qsim_op, time, qubit_to_index_dict, qsim_circuit)
  File "***/venv/lib/python3.10/site-packages/qsimcirq/qsim_circuit.py", line 315, in add_op_to_circuit
    qsim.add_matrix_gate(time, qsim_qubits, m, circuit)
ValueError: Only up to 6-qubit matrix gates are supported.

When I inspected the object associated with the global phase operator, I got something like cirq.MatrixGate(np.array([[(6.123233995736766e-17-1j)]], dtype=np.complex128)).on().
The same circuit works just fine on cirq.Simulator.

Version

cirq-core==1.0.0
qsimcirq==0.14.0
95-martin-orion commented

The error is being raised from this method:

qsim/pybind_interface/pybind_main.cpp

Line 232 in 0041775

Cirq::GateCirq<float> create_matrix_gate(const unsigned time,

In short, we don't have a handler set up for zero-qubit gates (like global phase), and the error message is poorly worded (it should mention zero-qubit gates as well).

@sergeisakov, is there a simple way to add this support? My first thought is to just add case 0: to the switch, but I get the feeling that a no-qubit gate may run into problems elsewhere.

sergeisakov commented

Yes, I think there should be a relatively simple way to add this support. I will do that next week.

  • 🚀2
rht commented

Bump. @sergeisakov any update on this?

sergeisakov commented

#579 should fix this issue.

  • 👍1