The "Light Switch" is Dead. Long Live the Spinning Coin.
Been playing around with a bit of Quantum Computing this week. Found a few core concepts
Quantum computing sounds like science fiction, but it is just linear algebra and physics
working in perfect harmony. If you want to understand how a quantum computer actually works,
you need to build the machine in your mind—piece by piece.
Here is the exact 5-step engineering pipeline of a quantum program, from a single particle to working code:
Block 1: Qubits & Superposition (The Raw Material)
Block 2: Quantum Gates (The Steering Wheel)
Block 3: Entanglement (The Cosmic Sync)
Block 4: Quantum Circuits (The Blueprint)
Block 5: Turn Theory into Code (Qiskit / Cirq)
qubits → superposition → gates → entanglement→ circuits->Working Qiskit/Cirq code
🧱 Block 1: Qubits & Superposition (The Raw Material)
What to understand”
A classical bit is a light switch: always 0 or 1.
A qubit is a spinning coin in mid-air: a blend of both possibilities at the same time.
A qubit is a 2‑dimensional complex vector
|0⟩=[1,0],|1⟩=[0,1]
Read as ‘ket zero state is represented by the vector one-zero, and |1⟩=[0,1] is read as the ket one state is represented by the vector zero-one. ’
Superposition is a linear combination
Above is read as ‘Ket psi equals alpha ket zero plus beta ket one.’ This formula
simply means the qubit holds a combination of both states. Instead of being
just ‘0’ or just ‘1’, it is a combination of both.
from qiskit import QuantumCircuit
qc=QuantumCircuit(2)
qc.h(0)
┌───┐
q_0: ┤ H ├
└───┘
q_1: ─────
The Catch (Collapse): You can manipulate a superposition, but you can never
observe it directly. The exact millisecond you measure a qubit, the spinning coin
lands. The state irreversibly collapses into a definite 0 or 1. Measurement collapses
the state and the superposition disappears
Before measurement, a qubit can be in a superposition:
This means:
It is not 0
It is not 1
It is a blend of both at the same time
With probabilities |alpha |^2 and |beta |^2
Before measurement:
A qubit is like a spinning coin in the air — not heads, not tails, but both possibilities.
After measurement:
You catch the coin — now it is definitely heads or tails.
The act of catching (measuring) forces the coin to choose.
⭐ Why does collapse happen?
Because quantum mechanics says:
You can prepare a superposition
You can manipulate a superposition
But you can never observe a superposition directly
Observation forces the system into one definite classical outcome.
⭐ A simple example
Suppose a qubit is in:
This means:
50% chance to get 0
50% chance to get 1
When you measure it:
You get 0 → the state becomes 0⟩
You get 1 → the state becomes 1⟩
The original superposition no longer exists.
Output:
SpaceVector and Bloch sphere representation
Mental Model: A qubit is not a binary switch; it is a rotatable arrow pointing anywhere on a 3D globe called
the Bloch Sphere.
This is the vector:
Which means:
50% chance of 0⟩
50% chance of 1⟩
Equal superposition
Bloch sphere representation(Draws a picture of a qubit on the Bloch sphere)]
🕹️ Block 2: Quantum Gates (The Steering Wheel)
What to understand
In classical coding, gates are logical operations (AND, OR, NOT). In quantum computing, gates
are geometric rotations.
The Math: Gates are unitary matrices that multiply our statevector, rotating our qubit's arrow around the Bloch Sphere.
Key gates:
X gate → flips 0⟩ ↔ 1⟩
H (Hadamard) Gate→ The magic gate. It kicks a stable |0> into a perfect 50/50
superposition. H gate → creates superposition
Z gate → phase flip
S, T gates → phase rotations
RX, RY, RZ → Continuous rotations.Precise degree-by-degree rotations.
Mental model
Gates are rotations, not logic operations.
Qiskit practice
CNOT gate
from qiskit import QuantumCircuit
qc=QuantumCircuit(2)
qc.h(0)
qc.cx(0,1)
print(qc)
┌───┐
q_0: ┤ H ├──■──
└───┘┌─┴─┐
q_1: ─────┤ X ├
└───┘
🔗 Block 3: Entanglement (The Cosmic Sync)
What to understand
Entanglement is when the state of one qubit depends on another, even if separated.
Entanglement is a quantum phenomenon where two qubits share a single combined state, such that measuring one instantly determines the state of the other — even if they are far apart.
Mathematically, an entangled state cannot be factored into separate qubit states.
This is the Bell State, the simplest entangled state.
🧠 Mental Model
Imagine flipping two coins—one in London, one in Tokyo. If they are entangled, the exact instant the
London coin lands on Heads, the Tokyo coin instantly forces itself to land on Heads too. They aren't communicating across space; they are two parts of the
exact same system.
🧪 Qiskit Practice: Create an Entangled Bell State
🗺️ Block 4: Quantum Circuits (The Blueprint)
🎯 What you need to understand
A quantum circuit is:
A set of qubits
A sequence of quantum gates
A final measurement
Executed on a simulator or quantum hardware
Quantum circuits are the “programs” of quantum computing.
🧠 Mental Model
A quantum circuit is like a pipeline where each gate transforms the quantum state.
initialized qubits → sequential gate transformations → final hardware measurement.
Each gate step is simply a matrix multiplication changing the final probability
amplitudes before the system collapses into a classical answer.
🧪 Qiskit Practice: Build a Full Circuit
Output:
💻 Block 5: Turn Theory into Code (Qiskit / Cirq)
🎯 What to install
Run:
pip install qiskit
pip install qiskit[visualization]
You don't need a multi-million dollar lab to run this. You can build a full 2-qubit
entangled Bell State circuit right now in Python using Qiskit or Cirq.
Here is how to initialize a circuit, apply a Hadamard gate to create a superposition,
use a Controlled-NOT (CNOT) gate to entangle the two qubits, and extract the statevector:
🧪 Qiskit Starter Template
Output:
Hope you are also learning with me, let me know if you have any suggestions.
