💡 How to Use This Glossary
These definitions assume you've completed Phase 0 (Mental Model Reset). If terms still don't make sense, go back and destroy your classical assumptions first!
Q
Qubit
A quantum bit—the fundamental unit of quantum information. Unlike a classical bit (which is 0 or 1), a qubit can exist in superposition of both states. Physically implemented using two-level quantum systems like electron spin, photon polarization, or superconducting circuits.
📖 See: Lesson 2: What is a Qubit?
Quantum State
The complete description of a quantum system. For a single qubit: |ψ⟩ = α|0⟩ + β|1⟩. It describes what can happen, not what is stored. The state determines probabilities of measurement outcomes.
Quantum Gate
An operation that transforms a quantum state. Unlike classical logic gates, quantum gates are rotations on the Bloch sphere. Common gates: H (Hadamard), X (NOT), Z (phase flip), CNOT (controlled-NOT).
📖 See: Phase 1: Quantum Gates = Rotations
S
Superposition
A quantum state that is a combination of multiple basis states simultaneously. NOT "the qubit is both 0 and 1"—rather, it's in a direction on the Bloch sphere. When measured, it collapses to one outcome probabilistically.
⚠️ Common misconception: Superposition ≠ classical probability. The outcome doesn't exist before measurement.
📖 See: Lesson 3: Superposition Explained
State Vector
The mathematical representation of a quantum state as a complex vector. For n qubits, the state vector has 2n complex amplitudes that sum to 1 (when squared).
M
Measurement
An irreversible physical interaction that forces a quantum system to collapse from superposition into a classical outcome. Measurement is NOT passive observation—it fundamentally changes the state.
📖 See: Phase 0: Measurement Creates Reality
E
Entanglement
A quantum correlation where the joint state of multiple qubits is well-defined, but individual qubits are not. Measuring one entangled qubit instantly affects the measurement statistics of the other, regardless of distance.
⚠️ Does NOT enable faster-than-light communication (no information is transmitted).
📖 See: Phase 1: Entanglement Without Mysticism
B
Bloch Sphere
A geometric representation where every pure single-qubit state corresponds to a point on a sphere's surface. North pole = |0⟩, South pole = |1⟩, equator = superposition states. Quantum gates are rotations on this sphere.
📖 See: Phase 1: The Bloch Sphere
Bell State
One of four maximally entangled two-qubit states. Example: (|00⟩ + |11⟩)/√2. Used to demonstrate quantum correlations and as building blocks for quantum protocols.
C
Collapse
The transition from a superposition state to a definite classical outcome upon measurement. The state "collapses" to one of the basis states with probability determined by the amplitudes.
Circuit
A sequence of quantum gates applied to qubits, followed by measurements. Quantum algorithms are expressed as circuits. Unlike classical circuits, quantum circuits are reversible (until measurement).
Classical Bit
The fundamental unit of classical information. Always in a definite state: 0 or 1. Can be copied, read without disturbance, and exists in a fixed value.
A
Amplitude
A complex number in a quantum state's superposition. The probability of measuring an outcome is |amplitude|². Amplitudes can interfere constructively or destructively—key to quantum algorithms.
I
Interference
When quantum amplitudes combine, they can cancel (destructive interference) or reinforce (constructive interference). This is how quantum algorithms work: amplify correct answers, cancel wrong answers.
O
Oracle
A black-box quantum gate that encodes a problem's solution. Used in algorithms like Deutsch-Jozsa and Grover's to demonstrate quantum advantage without specifying implementation details.
D
Decoherence
Loss of quantum properties due to environmental interaction. Qubits lose superposition and behave classically. This is why quantum computers need extreme cooling and isolation.
H
Hadamard Gate (H)
The most important single-qubit gate. Creates equal superposition from |0⟩ or |1⟩. H|0⟩ = (|0⟩ + |1⟩)/√2. Essential for most quantum algorithms.
P
Phase
The complex angle of a quantum amplitude. Two states with same probabilities but different phases behave differently in interference. Phase is global (invisible to measurement) or relative (affects interference).
📚 Missing a Term?
This glossary covers the essentials. As you progress through lessons, more advanced terms (like fidelity, tomography, variational circuits) will be introduced in context.