feature: add support for braket measure instruction

[ashlhans]

Summary

Add support for Braket's measure instruction.

This solves the bug detailed in Issue #130 where translating a circuit in qiskit to braket returned all the measurements instead of the requested measured qubit.

Details and comments

Now, users can measure a subset of qubits on local simulators or any device that supports partial measurement.

Example:

sim = BraketLocalBackend()
qc = QuantumCircuit(2, 1)
qc.h(0)
qc.cx(0, 1)
qc.measure(0, 0)
task = transpile(qc, sim)
task = sim.run(task)
result = task.result()
counts = result.get_counts()
print(counts)

{'1': 518, '0': 506}

To keep the current functionality of the bug and measure all the qubits, simply change measure(0, 0) to measure_all()

sim = BraketLocalBackend()
qc = QuantumCircuit(2, 1)
qc.h(0)
qc.cx(0, 1)
qc.measure_all()
task = transpile(qc, sim)
task = sim.run(task)
result = task.result()
counts = result.get_counts()
print(counts)

{'00': 503, '11': 521}

List of changes

• Update the amazon-braket-sdk version
• add measure to the Braket gates
• update adapter.py to use Braket's measure
• add tests

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[speller26]

Looking great!

[speller26]

Is this necessary? This dict is only used for gates.

Suggested change

"measure": lambda: [measure.Measure()],

[ashlhans]

This is used to match the qiskit measure to the Braket Measure so without it, the Braket measure isn't added to the Circuit.

I do agree that this might not work well in this particular dict. Maybe there can be a _ADDITIONAL_INSTRUCTION_NAME_TO_BRAKET_NAME dict for things like measure?

[speller26]

That should be handled here:

qiskit-braket-provider/qiskit_braket_provider/providers/adapter.py

Lines 434 to 437 in 0751b4c

	if gate_name == "measure":
	qubit = qubits[0] # qubit count = 1 for measure
	qubit_index = circuit.find_bit(qubit).index
	braket_circuit.measure(qubit_index)

[ashlhans]

Gotcha, that does work for the Qiskit to Braket conversion but the Braket to Qiskit conversion here checks for gates in the _GATE_NAME_TO_QISKIT_GATE dict.

Is there a better way to implement the Braket to Qiskit conversion for measure?

[speller26]

Yeah, it's fine to leave it in the Braket->Qiskit direction; you could do a special check for measure operators, but that's not necessary.

[ashlhans]

Removed the measure instruction from _GATE_NAME_TO_BRAKET_GATE but left it in _GATE_NAME_TO_QISKIT_GATE