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I would like to know if there is a functionality in qiskit to simplify a sequence of gates in a QASM file. For example, say we have a sequence of two expensive (in fault-tolerant context) gates $TT$. Instead of executing an operation $TT$, it is preferable to re-express $TT$ with a single Clifford gate $S$, i.e., $TT=S$.

Is there a qiskit function that allows the mapping $TT \rightarrow S$ in a QASM file? Also, sequences like $HH$ etc could be simplified as well.

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2 Answers 2

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If your issue is just the mapping $TT \rightarrow S$, you could try the following:

$-$ STEP 1: manually add the gates equivalence $TT = S$ to the qiskit.circuit.equivalence_library.SessionEquivalenceLibrary (it is not included by default)

from qiskit import QuantumCircuit
from qiskit.circuit.equivalence_library import SessionEquivalenceLibrary as sel

tt_qc = QuantumCircuit(1)
tt_qc.t(0)
tt_qc.t(0)
tt_gate = tt_qc.to_gate()

s_qc = QuantumCircuit(1)
s_qc.s(0)

sel.add_equivalence(gate=tt_gate, equivalent_circuit=s_qc)

$-$ STEP 2: build your circuit using the tt_gate previously defined

qc = QuantumCircuit(1)
qc.append(tt_gate, [0])
qc.decompose().draw('mpl')

enter image description here

$-$ STEP 3: transpile your circuit passing the $S$ gate as the basis_gates parameter

from qiskit import transpile

qc = transpile(qc, basis_gates=['s'])
qc.draw('mpl')

enter image description here

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  • $\begingroup$ Thanks a lot Simone. In Step 2, do I have to create the circuit with the tt_gate or it works for any circuit which has two Ts in a sequence? Of course, the latter is the general case that I need. $\endgroup$
    – MonteNero
    Dec 15, 2022 at 22:46
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The following function should do the job correctly. It takes your QASM file as an argument and it returns a Python str object containing your "simplified" (or better transpiled) QASM code:

from qiskit import QuantumCircuit
from qiskit import transpile

def get_simplified_qasm(qasm_file):
    qc = QuantumCircuit.from_qasm_file(qasm_file)
    qc = transpile(qc, optimization_level=2)
    return qc.qasm()

As an example, suppose you have the following circ.qasm file:

OPENQASM 2.0;
include "qelib1.inc";
qreg q[1];
h q[0];
h q[0];

To automatically simplify your circuit so that $HH \rightarrow I$, you can just call the get_simplified_qasm function:

new_qasm = get_simplified_qasm('circ.qasm')
print(new_qasm)

The result, as expected, will be the following:

OPENQASM 2.0;
include "qelib1.inc";
qreg q[1];
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  • $\begingroup$ PS: notice that this will not work for $TT \rightarrow S$ but this happens just because, in general, it's not true that implementing two T gates is more "expensive" than a single S gate but it really depends on the backend (simulator or real quantum device) you are using to run your program. On the other hand, $HH \rightarrow I$ always works because applying two $H$ gates is for sure more "expensive" than doing nothing ($I$) $\endgroup$
    – SimoneGasperini
    Dec 15, 2022 at 19:14
  • $\begingroup$ Thanks for the reply. Having $TT$ mapped to $S$ is the most important functionality. Implementing non-Clifford gates (such as T) in a fault-tolerant manner is very costly. That's why I need $TT \rightarrow S$ mapping. $\endgroup$
    – MonteNero
    Dec 15, 2022 at 20:03

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