Physical Qubit

An actual quantum system serving as a qubit, the raw hardware before error correction.

A physical qubit is a real quantum system that implements a qubit. It’s the actual hardware: a superconducting circuit, a trapped ion, a photon, etc.

Physical vs Logical

Physical Qubit Technologies

Quality Metrics

Coherence Times

• T1: Energy relaxation time
• T2: Phase coherence time

Gate Fidelities

• Single-qubit gate: How accurately gates are applied (~99.9%)
• Two-qubit gate: Entangling gate accuracy (~99-99.9%)

Readout Fidelity

• How accurately the qubit state is measured (~99%)

Connectivity

• Which qubits can directly interact
• Nearest-neighbor vs all-to-all

Qubit Counts

When companies announce “X qubits,” they mean physical qubits:

Logical qubit counts are much smaller (single digits to tens).

The Scaling Challenge

Going from physical to useful logical qubits:

Physical qubits:        1,000
  → With error correction
Logical qubits:         1-10
  → For useful computation
Needed logical qubits:  100-10,000
  → Therefore need
Physical qubits:        100,000 - 10,000,000+

This is why quantum computing is hard.

Noise Sources

Every physical qubit platform has specific noise:

See also: Logical Qubit, Qubit, Decoherence, NISQ