Qiskit is a python SDK for simulating and running quantum algorithms on real quantum computers. As mentioned on qiskit.org:

Qiskit [kiss-kit] is an open-source SDK for working with quantum computers at the level of pulses, circuits and application modules.

I plan on writing a series of tutorials on getting started with quantum computing using qiskit, as I learn as well. Like any other programming tutorial, we start with a simple "Hello, world" program.

With conventional computers, the simplest program that one could perhaps make is printing out "hello, world". However, in contrast to this, in quantum systems, we go ahead and measure a quantum state (this seems to be the most rudimentary but complete program in quantum terms). Let's proceed to do just that!

We import the necessary modules from Qiskit for this operation. To define a quantum circuit, we require the classes `QuantumCircuit`

, `QuantumRegister`

, `ClassicalRegister`

and define a state variable `psi`

as

Notice that the state \(| \psi \rangle\) is normalized. This is important, otherwise we'll end up with an error.

```
from qiskit import QuantumCircuit, QuantumRegister, ClassicalRegister
from math import sqrt
# define an initial state
psi = [1/sqrt(2), 1/sqrt(2)]
```

We then define a function `prepare()`

which takes two parameters: a `QuantumCircuit`

object named `qc`

and the initial state `psi`

. Remember that the quantum circuit that we define, only uses 1 qubit (notice the vector representation of \(| \psi \rangle\)).

```
def prepare(qc, psi):
qc.initialize(psi, [0]) # [0] indicates the zeroth index (or) the first qubit
```

Python by default, calls by reference, so we don't have to worry about returning anything from `prepare()`

.

Let's proceed with making an actual quantum circuit. For this, we define a `QuantumRegister`

object `qr`

, which stores the qubits and a `ClassicalRegister`

object `cr`

which stores the measured values of those qubits.

```
qr = QuantumRegister(1)
cr = ClassicalRegister(1)
```

Combining the two registers, we now make a quantum circuit:

`qc = QuantumCircuit(qr, cr)`

We then call our function `prepare()`

to initialize the first qubit (the only qubit) of our quantum circuit `qc`

.

`prepare(qc, psi)`

How do we go about measuring our state? That's where the `ClassicalRegister`

object `cr`

comes in. We use the `measure`

method under the `qc`

object to actually measure the qubit.

`qc.measure(qr[0], cr[0])`

The above syntax generally means that we're measuring the state of `qr[0]`

(the first qubit) onto the classical register `cr[0]`

where we store the measured value.

Ok so all of this is nice, but one can only describe so much. Let's see how our circuit actually looks like after applying the above operations. For this, we use the `draw()`

method with the `matplotlib`

backend.

`>>> qc.draw(output='mpl')`

To measure the qubit and store the result in the classical register `cr`

, we use the `qasm_simulator`

backend. As we know from basic quantum mechanics,

```
from qiskit import Aer
backend = Aer.get_backend('qasm_simulator')
```

We then transpile our circuit for the `qasm_simulator`

backend to understand and execute the circuit.

```
from qiskit import transpile
qc_compiled = transpile(qc, backend)
```

Now, we simply execute the circuit and gather the measurement results.

```
job = backend.run(qc_compiled, shots=1024) # shots is the number of times we run the experiment
results = job.result()
```

We store the value of the `result`

object and can further access the number of times we end up with a certain value of expected outcome with the `get_counts()`

method.

```
counts = results.get_counts(qc_compiled)
print(counts)
```

`{'0': 518, '1': 506}`

Here we end up with a dictionary of key values `'0'`

and `'1'`

, which indicate the following quantum states:

As expected, we get a 50-50 distribution of the states \(|0\rangle\) and \(|1\rangle\), since our starting state was \(|\psi\rangle\). We can also calculate this probability via the inner-product,

\[ |\langle 0 | \psi \rangle|^2 = \frac{1}{2} \qquad |\langle 1 | \psi \rangle|^2 = \frac{1}{2} \]Let's visualize this in the form of a histogram. For this, we import the `plot_histogram`

function.

```
>>> from qiskit.visualization import plot_histogram
>>> plot_histogram(counts)
```

You can find the entire script in this gist.

Licensed under CC BY-NC-SA 4.0.

Source code under GPLv3.

Copyright (c) 2020- Ashish Panigrahi.

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