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VBERippleCarryAdder

class qiskit.circuit.library.VBERippleCarryAdder(num_state_qubits, kind='full', name='VBERippleCarryAdder')

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Bases: Adder

The VBE ripple carry adder [1].

This circuit performs inplace addition of two equally-sized quantum registers. As an example, a classical adder circuit that performs full addition (i.e. including a carry-in bit) on two 2-qubit sized registers is as follows:

          ┌────────┐                       ┌───────────┐┌──────┐
   cin_0: ┤0       ├───────────────────────┤0          ├┤0     ├
          │        │                       │           ││      │
     a_0: ┤1       ├───────────────────────┤1          ├┤1     ├
          │        │┌────────┐     ┌──────┐│           ││  Sum │
     a_1: ┤        ├┤1       ├──■──┤1     ├┤           ├┤      ├
          │        ││        │  │  │      ││           ││      │
     b_0: ┤2 Carry ├┤        ├──┼──┤      ├┤2 Carry_dg ├┤2     ├
          │        ││        │┌─┴─┐│      ││           │└──────┘
     b_1: ┤        ├┤2 Carry ├┤ X ├┤2 Sum ├┤           ├────────
          │        ││        │└───┘│      ││           │
  cout_0: ┤        ├┤3       ├─────┤      ├┤           ├────────
          │        ││        │     │      ││           │
helper_0: ┤3       ├┤0       ├─────┤0     ├┤3          ├────────
          └────────┘└────────┘     └──────┘└───────────┘

Here Carry and Sum gates correspond to the gates introduced in [1]. Carry_dg correspond to the inverse of the Carry gate. Note that in this implementation the input register qubits are ordered as all qubits from the first input register, followed by all qubits from the second input register. This is different ordering as compared to Figure 2 in [1], which leads to a different drawing of the circuit.

See also

The following generic gate objects perform additions, like this circuit class, but allow the compiler to select the optimal decomposition based on the context. Specific implementations can be set via the HLSConfig, e.g. this circuit can be chosen via Adder=["ripple_v95"].

ModularAdderGate: A generic inplace adder, modulo 2n2^n. This

is functionally equivalent to kind="fixed".

AdderGate: A generic inplace adder. This

is functionally equivalent to kind="half".

FullAdderGate: A generic inplace adder, with a carry-in bit. This

is functionally equivalent to kind="full".

References:

[1] Vedral et al., Quantum Networks for Elementary Arithmetic Operations, 1995. arXiv:quant-ph/9511018

Parameters

  • num_state_qubits (int) – The size of the register.
  • kind (str) – The kind of adder, can be 'full' for a full adder, 'half' for a half adder, or 'fixed' for a fixed-sized adder. A full adder includes both carry-in and carry-out, a half only carry-out, and a fixed-sized adder neither carry-in nor carry-out.
  • name (str) – The name of the circuit.

Raises

ValueError – If num_state_qubits is lower than 1.

Attributes

name

Type: str

A human-readable name for the circuit.

Example

from qiskit import QuantumCircuit
 
qc = QuantumCircuit(2, 2, name="my_circuit")
print(qc.name)
my_circuit