Allow combining circuits with different unique registers

[chriseclectic]

Description

This patch modifies the QuantumCircuit.combine and QuantumCircuit.extend functions (and hence +, and += operators) to allow combing circuits with common and/or distinct quantum and classical registers.

Example 1 (distinct registers)

from qiskit import QuantumRegister, ClassicalRegister, QuantumCircuit

qr1 = QuantumRegister('q1', 2)
cr1 = ClassicalRegister('c1', 2)
qr2 = QuantumRegister('q2', 2)
cr2 = ClassicalRegister('c2', 2)

# Circ 1
qc1 = QuantumCircuit(qr1, cr1)
qc1.h(qr1)

# Circ 2
qc2 = QuantumCircuit(qr2, cr2)
qc2.x(qr2)

# Combine and add additional gates
circ = qc1 + qc2
circ.cx(qr1, qr2)
circ.measure(qr1, cr1)
circ.measure(qr2, cr2)

In this case the resulting qasm equivalents will be

# qc1
OPENQASM 2.0;
include "qelib1.inc";
qreg q1[2];
creg c1[2];
h q1[0];
h q1[1];

# qc2
OPENQASM 2.0;
include "qelib1.inc";
qreg q2[2];
creg c2[2];
x q2[0];
x q2[1];

# circ
OPENQASM 2.0;
include "qelib1.inc";
qreg q1[2];
creg c1[2];
creg c2[2];
qreg q2[2];
h q1[0];
h q1[1];
x q2[0];
x q2[1];
cx q1[0],q2[0];
cx q1[1],q2[1];
measure q1[0] -> c1[0];
measure q1[1] -> c1[1];
measure q2[0] -> c2[0];
measure q2[1] -> c2[1];

Example 2 (common register)

from qiskit import QuantumRegister, ClassicalRegister, QuantumCircuit

qr1 = QuantumRegister('q1', 1)
qr2 = QuantumRegister('q2', 1)
cr = ClassicalRegister('c', 2)

# Circ 1
qc1 = QuantumCircuit(qr1, cr)
qc1.h(qr1)

# Circ 2
qc2 = QuantumCircuit(qr2, cr)
qc2.x(qr2)

# Combine and add additional gates
circ = qc1 + qc2
circ.cx(qr1, qr2)
circ.measure(qr1[0], cr[0])
circ.measure(qr2[0], cr[1])

In this case the resulting qasm equivalents will be

# qc1
OPENQASM 2.0;
include "qelib1.inc";
qreg q1[1];
creg c[2];
h q1[0];

# qc2
OPENQASM 2.0;
include "qelib1.inc";
qreg q2[1];
creg c[2];
x q2[0];

# circ
OPENQASM 2.0;
include "qelib1.inc";
qreg q1[1];
creg c[2];
qreg q2[1];
h q1[0];
x q2[0];
cx q1[0],q2[0];
measure q1[0] -> c[0];
measure q2[0] -> c[1];

Example 3 (raise an error)

from qiskit import QuantumRegister, ClassicalRegister, QuantumCircuit

qr1 = QuantumRegister('reg', 1)
qr2 = QuantumRegister('reg', 2)
cr1 = ClassicalRegister('reg', 2)

qc1 = QuantumCircuit(qr1)
qc2 = QuantumCircuit(qr2)
qc3 = QuantumCircuit(cr1)

qc1 + qc2 # raises QISKitError due to registers having same name but different size
qc2 + qc3 # raises QISKitError due to registers having same name but different type

Motivation and Context

When working with QuantumCircuit objects directly you should be able to combine circuits which are defined on different quantum and/or classical registers. In the case where two circuits have registers with common names these should be the same register or an error is raised.

How Has This Been Tested?

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[ajavadia]

Thanks for the useful patch and the thorough explanation. It looks good to merge to me.
We can now combine circuits across both width and depth.

[awcross1]

Looks good!