Circuits and Gates

MENOS takes circuits from Qiskit's QuantumCircuit, or from the small native menos.Circuit (Native API). This page explains what a circuit may contain.

Gates

The gates MENOS runs natively are listed in Supported Gates: the Pauli gates, Hadamard, the S and T families, the single-qubit rotations rx, ry, rz, p and u, and cx, cz, swap, cp and ccx.

A gate that is not in that list still runs if Qiskit can give it a matrix and it acts on at most five qubits. It is lowered to a dense unitary. For example Qiskit's cswap is not native but runs:

from qiskit import QuantumCircuit

from menos.qiskit import MENOSBackend

qc = QuantumCircuit(3, 3)
qc.x(0)
qc.x(1)
qc.cswap(0, 1, 2)   # swaps qubits 1 and 2 because qubit 0 is 1
qc.measure([0, 1, 2], [0, 1, 2])

counts = MENOSBackend().run(qc, shots=10, seed_simulator=1).result().get_counts()
print(counts)
assert counts == {"101": 10}

A gate that is neither native nor expressible as a matrix on five or fewer qubits is rejected. Use transpile(qc, backend) to lower it into the native set; see Qiskit Integration.

Why the five-qubit limit matters

A unitary is a 2ᵏ × 2ᵏ matrix and the limit keeps that bounded. The same limit (kMaxUnitaryQubits = 5) applies to the native API and to the CUDA kernel. Native gates have no such limit beyond their own arity.

Measurements

Measurements must be terminal: once a qubit is measured, no later operation may act on it. A circuit that measures a qubit and then applies a gate to it is rejected with UnsupportedFeatureError.

operation 1: qubit 0 has a non-terminal measurement (an operation follows a measurement of this qubit)

Measuring several qubits into one classical bit is allowed; the last measurement written to a classical bit wins. A classical register can be any width: counts are keyed by an arbitrary-width bitstring, with no 64-bit ceiling.

Classical bits and counts

A counts key is as wide as the circuit's classical register: one character per classical bit, highest bit on the left. A classical bit that no measurement writes stays 0.

from qiskit import QuantumCircuit

from menos.qiskit import MENOSBackend

qc = QuantumCircuit(3, 3)
qc.x(1)
qc.measure(1, 2)   # qubit 1 -> classical bit 2; bits 0 and 1 are never written

counts = MENOSBackend().run(qc, shots=5, seed_simulator=1).result().get_counts()
print(counts)
assert counts == {"100": 5}

To get short keys, declare a short classical register (QuantumCircuit(3, 1) gives one-character keys).

Global phase

A circuit's global_phase is honoured and part of the statevector MENOS returns. It cannot change counts or expectation values, so it only matters for statevectors. See Conventions.

Dropped instructions

barrier, id and delay are dropped during conversion because they cannot change any output. They do not raise. A noise error attached to id is still applied: it is a separate instruction (see Noise Models).

Not supported

Not supported	What happens
reset	UnsupportedFeatureError
Classical control (if_test, conditional gates)	UnsupportedFeatureError
Mid-circuit measurement	UnsupportedFeatureError
Noise models	Supported for verified noise types; see Noise Models
Unbound parameters	InvalidCircuitError; call assign_parameters first

Size limits

MENOS has no fixed qubit limit. What limits a job is whether some engine can run it inside the memory budget. A dense statevector needs 16 · 2ⁿ bytes; other engines need far less for the right circuits. See Simulation Engines.