```
from qiskit import *
from qiskit.tools.visualization import plot_histogram
import numpy as np
```

Using the NOT gate (expressed as `x`

in Qiskit), the CNOT gate (expressed as `cx`

in Qiskit) and the Toffoli gate (expressed as `ccx`

in Qiskit) create functions to implement the XOR, AND, NAND and OR gates.

An implementation of the NOT gate is provided as an example.

This function takes a binary string input (`'0'`

or `'1'`

) and returns the opposite binary output'.

```
def NOT(input):
q = QuantumRegister(1) # a qubit in which to encode and manipulate the input
c = ClassicalRegister(1) # a bit to store the output
qc = QuantumCircuit(q, c) # this is where the quantum program goes
# We encode '0' as the qubit state |0⟩, and '1' as |1⟩
# Since the qubit is initially |0⟩, we don't need to do anything for an input of '0'
# For an input of '1', we do an x to rotate the |0⟩ to |1⟩
if input=='1':
qc.x( q[0] )
# Now we've encoded the input, we can do a NOT on it using x
qc.x( q[0] )
# Finally, we extract the |0⟩/|1⟩ output of the qubit and encode it in the bit c[0]
qc.measure( q[0], c[0] )
# We'll run the program on a simulator
backend = Aer.get_backend('qasm_simulator')
# Since the output will be deterministic, we can use just a single shot to get it
job = execute(qc,backend,shots=1)
output = next(iter(job.result().get_counts()))
return output
```

Takes two binary strings as input and gives one as output.

The output is `'0'`

when the inputs are equal and `'1'`

otherwise.

```
def XOR(input1,input2):
q = QuantumRegister(2) # two qubits in which to encode and manipulate the input
c = ClassicalRegister(1) # a bit to store the output
qc = QuantumCircuit(q, c) # this is where the quantum program goes
# YOUR QUANTUM PROGRAM GOES HERE
qc.measure(q[1],c[0]) # YOU CAN CHANGE THIS IF YOU WANT TO
# We'll run the program on a simulator
backend = Aer.get_backend('qasm_simulator')
# Since the output will be deterministic, we can use just a single shot to get it
job = execute(qc,backend,shots=1,memory=True)
output = job.result().get_memory()[0]
return output
```

Takes two binary strings as input and gives one as output.

The output is `'1'`

only when both the inputs are `'1'`

.

```
def AND(input1,input2):
q = QuantumRegister(3) # two qubits in which to encode the input, and one for the output
c = ClassicalRegister(1) # a bit to store the output
qc = QuantumCircuit(q, c) # this is where the quantum program goes
# YOUR QUANTUM PROGRAM GOES HERE
qc.measure(q[2],c[0]) # YOU CAN CHANGE THIS IF YOU WANT TO
# We'll run the program on a simulator
backend = Aer.get_backend('qasm_simulator')
# Since the output will be deterministic, we can use just a single shot to get it
job = execute(qc,backend,shots=1,memory=True)
output = job.result().get_memory()[0]
return output
```

Takes two binary strings as input and gives one as output.

The output is `'0'`

only when both the inputs are `'1'`

.

```
def NAND(input1,input2):
q = QuantumRegister(3) # two qubits in which to encode the input, and one for the output
c = ClassicalRegister(1) # a bit to store the output
qc = QuantumCircuit(q, c) # this is where the quantum program goes
# YOUR QUANTUM PROGRAM GOES HERE
qc.measure(q[2],c[0]) # YOU CAN CHANGE THIS IF YOU WANT TO
# We'll run the program on a simulator
backend = Aer.get_backend('qasm_simulator')
# Since the output will be deterministic, we can use just a single shot to get it
job = execute(qc,backend,shots=1,memory=True)
output = job.result().get_memory()[0]
return output
```

Takes two binary strings as input and gives one as output.

The output is `'1'`

if either input is `'1'`

.

```
def OR(input1,input2):
q = QuantumRegister(3) # two qubits in which to encode the input, and one for the output
c = ClassicalRegister(1) # a bit to store the output
qc = QuantumCircuit(q, c) # this is where the quantum program goes
# YOUR QUANTUM PROGRAM GOES HERE
qc.measure(q[2],c[0]) # YOU CAN CHANGE THIS IF YOU WANT TO
# We'll run the program on a simulator
backend = Aer.get_backend('qasm_simulator')
# Since the output will be deterministic, we can use just a single shot to get it
job = execute(qc,backend,shots=1,memory=True)
output = job.result().get_memory()[0]
return output
```

The following code runs the functions above for all possible inputs, so that you can check whether they work.

```
print('\nResults for the NOT gate')
for input in ['0','1']:
print(' Input',input,'gives output',NOT(input))
print('\nResults for the XOR gate')
for input1 in ['0','1']:
for input2 in ['0','1']:
print(' Inputs',input1,input2,'give output',XOR(input1,input2))
print('\nResults for the AND gate')
for input1 in ['0','1']:
for input2 in ['0','1']:
print(' Inputs',input1,input2,'give output',AND(input1,input2))
print('\nResults for the NAND gate')
for input1 in ['0','1']:
for input2 in ['0','1']:
print(' Inputs',input1,input2,'give output',NAND(input1,input2))
print('\nResults for the OR gate')
for input1 in ['0','1']:
for input2 in ['0','1']:
print(' Inputs',input1,input2,'give output',OR(input1,input2))
```