Inspecting the circuit - Amazon Braket

Inspecting the circuit

Quantum circuits in Amazon Braket have a pseudo-time concept called Moments. Each qubit can experience a single gate per Moment. The purpose of Moments is to make circuits and their gates easier to address and to provide a temporal structure.

Note

Moments generally do not correspond to the real time at which gates are executed on a QPU.

The depth of a circuit is given by the total number of Moments in that circuit. You can view the circuit depth calling the method circuit.depth as shown in the following example.

from braket.circuits import Circuit

# Define a circuit with parametrized gates
circ = Circuit().rx(0, 0.15).ry(1, 0.2).cnot(0, 2).zz(1, 3, 0.15).x(0)
print(circ)
print('Total circuit depth:', circ.depth)
T  : │     0      │        1         │  2  │
      ┌──────────┐                    ┌───┐ 
q0 : ─┤ Rx(0.15) ├───●────────────────┤ X ├─
      └──────────┘   │                └───┘ 
      ┌──────────┐   │   ┌──────────┐       
q1 : ─┤ Ry(0.20) ├───┼───┤ ZZ(0.15) ├───────
      └──────────┘   │   └────┬─────┘       
                   ┌─┴─┐      │             
q2 : ──────────────┤ X ├──────┼─────────────
                   └───┘      │             
                         ┌────┴─────┐       
q3 : ────────────────────┤ ZZ(0.15) ├───────
                         └──────────┘       
T  : │     0      │        1         │  2  │
Total circuit depth: 3

The total circuit depth of the circuit above is 3 (shown as moments 0, 1, and 2). You can check the gate operation for each moment.

Moments functions as a dictionary of key-value pairs.

  • The key is MomentsKey(), which contains pseudo-time and qubit information.

  • The value is assigned in the type of Instructions().

moments = circ.moments
for key, value in moments.items():
    print(key)
    print(value, "\n")
MomentsKey(time=0, qubits=QubitSet([Qubit(0)]), moment_type=<MomentType.GATE: 'gate'>, noise_index=0, subindex=0)
Instruction('operator': Rx('angle': 0.15, 'qubit_count': 1), 'target': QubitSet([Qubit(0)]), 'control': QubitSet([]), 'control_state': (), 'power': 1) 

MomentsKey(time=0, qubits=QubitSet([Qubit(1)]), moment_type=<MomentType.GATE: 'gate'>, noise_index=0, subindex=0)
Instruction('operator': Ry('angle': 0.2, 'qubit_count': 1), 'target': QubitSet([Qubit(1)]), 'control': QubitSet([]), 'control_state': (), 'power': 1) 

MomentsKey(time=1, qubits=QubitSet([Qubit(0), Qubit(2)]), moment_type=<MomentType.GATE: 'gate'>, noise_index=0, subindex=0)
Instruction('operator': CNot('qubit_count': 2), 'target': QubitSet([Qubit(0), Qubit(2)]), 'control': QubitSet([]), 'control_state': (), 'power': 1) 

MomentsKey(time=1, qubits=QubitSet([Qubit(1), Qubit(3)]), moment_type=<MomentType.GATE: 'gate'>, noise_index=0, subindex=0)
Instruction('operator': ZZ('angle': 0.15, 'qubit_count': 2), 'target': QubitSet([Qubit(1), Qubit(3)]), 'control': QubitSet([]), 'control_state': (), 'power': 1) 

MomentsKey(time=2, qubits=QubitSet([Qubit(0)]), moment_type=<MomentType.GATE: 'gate'>, noise_index=0, subindex=0)
Instruction('operator': X('qubit_count': 1), 'target': QubitSet([Qubit(0)]), 'control': QubitSet([]), 'control_state': (), 'power': 1)

You can also add gates to a circuit through Moments.

from braket.circuits import Instruction, Gate

new_circ = Circuit()
instructions = [Instruction(Gate.S(), 0),
                Instruction(Gate.CZ(), [1, 0]),
                Instruction(Gate.H(), 1)
                ]

new_circ.moments.add(instructions)
print(new_circ)
T  : │  0  │  1  │  2  │
      ┌───┐ ┌───┐       
q0 : ─┤ S ├─┤ Z ├───────
      └───┘ └─┬─┘       
              │   ┌───┐ 
q1 : ─────────●───┤ H ├─
                  └───┘ 
T  : │  0  │  1  │  2  │