The Quantum Computing Stack

Before diving into individual terms, it helps to understand how quantum computing is organized. Like classical computing, quantum computing has “layers” or a “stack” that builds from physics up to applications.

The Layers

1. Physical Layer (The Hardware)

At the bottom is actual quantum hardware. This includes:

  • Qubits: The physical quantum systems that store information (superconducting circuits, trapped ions, photons, etc.)
  • Control systems: Lasers, microwave pulses, and electronics that manipulate qubits
  • Cooling systems: Dilution refrigerators that keep superconducting qubits near absolute zero

Key terms: Superconducting Qubit, Trapped Ion, Photonic Qubit

2. Quantum Gate Layer

Above the physics, we have quantum operations:

  • Gates: The quantum equivalent of logic gates (AND, OR, NOT in classical computing)
  • Circuits: Sequences of gates that perform computations
  • Measurement: Reading out the result of a computation

Key terms: Quantum Gate, Hadamard Gate, CNOT Gate, Quantum Circuit

3. Error Correction Layer

Quantum systems are noisy. This layer deals with that:

  • Error detection: Finding when errors occur
  • Error correction: Fixing errors without destroying quantum information
  • Logical qubits: Error-protected qubits built from many physical qubits

Key terms: Quantum Error Correction, Surface Code, Logical Qubit, Fault Tolerance

4. Algorithm Layer

This is where computation happens:

  • Quantum algorithms: Procedures that leverage quantum effects for computational advantage
  • Variational methods: Hybrid quantum-classical approaches for near-term devices

Key terms: Shor’s Algorithm, Grover’s Algorithm, VQE, QAOA

5. Application Layer

At the top, real-world problems:

  • Cryptography: Secure communication, key distribution
  • Chemistry: Molecular simulation, drug discovery
  • Optimization: Supply chain, finance, logistics
  • Machine learning: Quantum-enhanced AI

Key terms: Quantum Key Distribution, Quantum Simulation, Quantum Machine Learning

Where Are We Today?

As of 2024-2025, we’re in the NISQ era (Noisy Intermediate-Scale Quantum). This means:

  • We have quantum computers with 50-1000+ qubits
  • These qubits are “noisy” (error-prone) and lack full error correction
  • We’re exploring what useful tasks these imperfect machines can do
  • Full fault-tolerant quantum computing is still years away

Key terms: NISQ, Quantum Advantage, Fault-Tolerant Quantum Computing

The Path Forward

Today: NISQ devices (noisy, limited)
       ↓
Next:  Early fault-tolerant systems (100s of logical qubits)
       ↓
Goal:  Large-scale fault-tolerant quantum computers (millions of qubits)

Understanding this stack helps contextualize every term you’ll encounter. When someone mentions “T1 times,” you know that’s about physical hardware. When they discuss “logical qubits,” that’s the error correction layer. When they’re excited about “quantum advantage,” that’s about the algorithm/application layers proving real value.

Next: From Classical to Quantum - Understanding what makes quantum different.