Add Quantum Full Adder circuit for classical integers (#2954)

* requirements: add qiskit

major library required for quantum computing

* quantum: add quantum ripple adder implementation

right now we are essentially performing the same task as a classic ripple adder

Signed-off-by: rupansh-arch <rupanshsekar@hotmail.com>

quantum/ripple_adder_classic.py

@@ -0,0 +1,108 @@
+# https://github.com/rupansh/QuantumComputing/blob/master/rippleadd.py
+# https://en.wikipedia.org/wiki/Adder_(electronics)#Full_adder
+# https://en.wikipedia.org/wiki/Controlled_NOT_gate
+
+from qiskit import Aer, QuantumCircuit, execute
+from qiskit.providers import BaseBackend
+
+
+def store_two_classics(val1: int, val2: int) -> (QuantumCircuit, str, str):
+    """
+    Generates a Quantum Circuit which stores two classical integers
+    Returns the circuit and binary representation of the integers
+    """
+    x, y = bin(val1)[2:], bin(val2)[2:]  # Remove leading '0b'
+
+    # Ensure that both strings are of the same length
+    if len(x) > len(y):
+        y = y.zfill(len(x))
+    else:
+        x = x.zfill(len(y))
+
+    # We need (3 * number of bits in the larger number)+1 qBits
+    # The second parameter is the number of classical registers, to measure the result
+    circuit = QuantumCircuit((len(x) * 3) + 1, len(x) + 1)
+
+    # We are essentially "not-ing" the bits that are 1
+    # Reversed because its easier to perform ops on more significant bits
+    for i in range(len(x)):
+        if x[::-1][i] == "1":
+            circuit.x(i)
+    for j in range(len(y)):
+        if y[::-1][j] == "1":
+            circuit.x(len(x) + j)
+
+    return circuit, x, y
+
+
+def full_adder(
+    circuit: QuantumCircuit,
+    input1_loc: int,
+    input2_loc: int,
+    carry_in: int,
+    carry_out: int,
+):
+    """
+    Quantum Equivalent of a Full Adder Circuit
+    CX/CCX is like 2-way/3-way XOR
+    """
+    circuit.ccx(input1_loc, input2_loc, carry_out)
+    circuit.cx(input1_loc, input2_loc)
+    circuit.ccx(input2_loc, carry_in, carry_out)
+    circuit.cx(input2_loc, carry_in)
+    circuit.cx(input1_loc, input2_loc)
+
+
+def ripple_adder(
+    val1: int, val2: int, backend: BaseBackend = Aer.get_backend("qasm_simulator")
+) -> int:
+    """
+    Quantum Equivalent of a Ripple Adder Circuit
+    Uses qasm_simulator backend by default
+
+    Currently only adds 'emulated' Classical Bits
+    but nothing prevents us from doing this with hadamard'd bits :)
+
+    Only supports adding +ve Integers
+
+    >>> ripple_adder(3, 4)
+    7
+    >>> ripple_adder(10, 4)
+    14
+    >>> ripple_adder(-1, 10)
+    Traceback (most recent call last):
+        ...
+    ValueError: Both Integers must be positive!
+    """
+
+    if val1 < 0 or val2 < 0:
+        raise ValueError("Both Integers must be positive!")
+
+    # Store the Integers
+    circuit, x, y = store_two_classics(val1, val2)
+
+    """
+    We are essentially using each bit of x & y respectively as full_adder's input
+    the carry_input is used from the previous circuit (for circuit num > 1)
+
+    the carry_out is just below carry_input because
+    it will be essentially the carry_input for the next full_adder
+    """
+    for i in range(len(x)):
+        full_adder(circuit, i, len(x) + i, len(x) + len(y) + i, len(x) + len(y) + i + 1)
+        circuit.barrier()  # Optional, just for aesthetics
+
+    # Measure the resultant qBits
+    for i in range(len(x) + 1):
+        circuit.measure([(len(x) * 2) + i], [i])
+
+    res = execute(circuit, backend, shots=1).result()
+
+    # The result is in binary. Convert it back to int
+    return int(list(res.get_counts().keys())[0], 2)
+
+
+if __name__ == "__main__":
+    import doctest
+
+    doctest.testmod()