← Quantum Computing Fundamentals
Day 3 of 14
Quantum Gates & Circuits
Quantum Gates
Just as classical computers use logic gates (AND, OR, NOT), quantum computers use quantum gates. Key gates:
X GATE (NOT): Flips |0⟩ to |1⟩ and vice versa.
H GATE (Hadamard): Puts a qubit into equal superposition. The most fundamental quantum operation.
CNOT GATE (Controlled-NOT): Flips the target qubit if the control qubit is |1⟩. Used to create entanglement.
T GATE: A phase gate used in many quantum algorithms.
Critical difference from classical gates: all quantum gates are reversible. No information is ever destroyed — you can always run a quantum circuit backwards and get back to the input.
Reading a Quantum Circuit
A quantum circuit is read left to right. Each horizontal line is a qubit. Gates are applied in sequence.
Example — Bell State (simplest entangled pair):
1. Start: |00⟩ (both qubits in state 0)
2. Apply H to qubit 1: puts qubit 1 in superposition
3. Apply CNOT (control=qubit 1, target=qubit 2): entangles them
4. Result: (|00⟩ + |11⟩)/√2 — measuring always gives either both 0 or both 1, never mixed
This 2-gate circuit is the foundation of quantum teleportation, superdense coding, and quantum key distribution.
⚡ Today's Action
In IBM Quantum Composer, build the Bell State circuit: H gate on qubit 0, then CNOT with qubit 0 as control and qubit 1 as target. Run and check the results — you should see ~50% |00⟩ and ~50% |11⟩.
💡 Pro Tip
Think of quantum circuits as music notation — time flows left to right, each line is an instrument (qubit), and gates are the notes played on each.