Introduction to Quantum Hardware

Introduction to Quantum Hardware#

The physical platforms competing to build scalable quantum computers. Current state-of-the-art, qubit counts, and technology readiness levels.

Building a quantum computer requires controlling individual quantum systems with extraordinary precision. Several physical platforms are being pursued, each with distinct strengths and weaknesses. No single platform has yet proven definitively superior for large-scale, fault-tolerant quantum computation. This chapter surveys the landscape.

The Hardware Zoo#

Quantum computing hardware spans a remarkable range of physical systems, from superconducting circuits cooled to millikelvins, to individual atoms held in vacuum, to photons traveling through waveguides. Each platform encodes a qubit differently:

Platform

Qubit Encoding

Operating Temperature

Approximate Qubit Count (2025)

Key Players

Superconducting

Charge/flux in Josephson circuits

~15 mK

1,000+

IBM, Google, Rigetti

Trapped Ion

Electronic states of ions

Room temp (vacuum)

~50

IonQ, Quantinuum

Spin (Quantum Dots)

Electron/nuclear spin in Si

~100 mK

~12

Intel, UNSW, QuTech

Photonic

Polarization/path of photons

Room temp

216 squeezed modes

Xanadu, PsiQuantum

NV Center

Electron spin in diamond

4 K (or room temp)

~10

QuTech, Element Six

Neutral Atom

Hyperfine states of atoms

~\(\mu\)K

256+

QuEra, Pasqal, Atom

Topological

Non-Abelian anyons

~20 mK

8 (announced 2025)

Microsoft

Quantum Annealer

Flux qubits (Ising spins)

~15 mK

5,000+

D-Wave

These numbers evolve rapidly. The qubit count alone is misleading without considering gate fidelity, connectivity, and coherence time.

Technology Readiness#

Not all qubits are equal. A useful metric is the quantum volume, which accounts for qubit count, connectivity, gate fidelity, and circuit depth. Another metric is the error rate per gate:

  • Trapped ions currently lead in two-qubit gate fidelity (>99.9%)

  • Superconducting qubits lead in qubit count and gate speed (~20 ns gates)

  • Photonic systems operate at room temperature but face challenges with deterministic two-qubit gates

  • Topological qubits promise inherent error protection but remain in early experimental stages

The path to a fault-tolerant quantum computer likely requires millions of physical qubits to encode thousands of logical qubits through quantum error correction. Current devices are in the “noisy intermediate-scale quantum” (NISQ) era, with hundreds to low thousands of physical qubits and error rates that limit circuit depth.

What Makes a Good Qubit?#

A good physical qubit needs several properties simultaneously:

  1. Long coherence time relative to gate duration (the \(T_2/t_\text{gate}\) ratio)

  2. High-fidelity gates for both single-qubit and two-qubit operations

  3. Efficient initialization into a known starting state

  4. Reliable measurement to extract computation results

  5. Scalability to large numbers of qubits with controllable interactions

These criteria were formalized by David DiVincenzo in 2000 and remain the standard framework for evaluating hardware platforms. We discuss them in detail in the next section.

Roadmap for This Part#

This part of the book proceeds as follows. We first review the classical computing background to understand why quantum hardware requires a fundamentally different approach. We then present the DiVincenzo criteria as the evaluation framework. The subsequent chapters survey each major platform in turn, concluding with an extended treatment of topological quantum computing.

A note on timelines

Hardware capabilities change faster than textbooks. The numbers cited here reflect the state of the field as of early 2025. For the latest benchmarks, consult the primary sources listed in each chapter’s Further Reading section.

Further Reading#