Multi-Qubit Circuits

Multi-Qubit Circuits

Beyond One Qubit

Real quantum algorithms use multiple qubits. With n qubits, the state carries amplitudes over $2^n$ basis states. But remember the very first lesson of this track: the power does not come from "trying everything at once." A measurement only ever gives you n bits. The power comes from interference and entanglement, which let algorithms sculpt those $2^n$ amplitudes so the right answers reinforce and the wrong ones cancel.

Creating Multi-Qubit Circuits

qc = QuantumCircuit(2, 2)  # 2 qubits, 2 classical bits

You can apply gates to each qubit independently:

qc.h(0)      # Hadamard on qubit 0
qc.x(1)      # X on qubit 1
qc.measure(0, 0)
qc.measure(1, 1)

State Notation

Note: Qiskit uses little-endian ordering, the rightmost bit is qubit 0.

The Power of Multiple Qubits

With just 50 qubits in superposition, you'd need to store $2^{50}\approx 10^{15}$ complex numbers to simulate the state classically. That's over a petabyte of data!

This exponential scaling is why quantum computers can potentially solve problems that are impossible for classical computers.