Error rates in quantum computing refer to the probability that a quantum operation (like a gate, readout, or qubit state preservation) fails or produces incorrect results. These errors are a central challenge because quantum states are fragile and can be easily disrupted by environmental noise, hardware imperfections, or crosstalk. The type of qubit technology used significantly influences these error rates.
Each type of qubit—superconducting, trapped ion, photonic, topological, and others—comes with its own strengths and challenges, especially concerning fidelity and robustness. Below is a comprehensive breakdown of error rates by qubit type and what factors contribute to them.
1. Superconducting Qubits
Technology: These qubits use superconducting circuits that operate at cryogenic temperatures to exhibit quantum behavior. Examples include IBM, Google, and Rigetti systems.
Typical Error Rates
- Single-qubit gate error: ~0.01% to 0.1%
- Two-qubit gate error: ~0.1% to 1%
- Readout error: ~1% to 5%
Error Sources
- Crosstalk between qubits due to microwave control pulses.
- Thermal noise and poor shielding at cryogenic levels.
- Short coherence times (T1 and T2 usually between 20–150 µs).
Strengths
- Fast gate speeds (~10–100 ns).
- Easily scalable with integrated circuits.
Weaknesses
- Frequent recalibration required.
- Higher error rates for two-qubit operations compared to trapped ions.
2. Trapped Ion Qubits
Technology: Qubits are represented by the electronic states of ions trapped using electromagnetic fields and manipulated by lasers. Used by IonQ, Quantinuum.
Typical Error Rates
- Single-qubit gate error: ~0.01% or better
- Two-qubit gate error: ~0.1% to 0.5%
- Readout error: <1%
Error Sources
- Laser alignment drift.
- Motional heating of ions.
- Long gate times (~10–100 µs for two-qubit gates).
Strengths
- High fidelity and long coherence times (up to seconds).
- All-to-all connectivity between qubits.
Weaknesses
- Slower gate speeds.
- More complex laser control systems.
3. Photonic Qubits
Technology: Qubits are encoded in the quantum states of light (photons), such as polarization or phase. Used in Xanadu, PsiQuantum.
Typical Error Rates
- Gate error: Varies greatly depending on technology, typically higher (~1%–10%)
- Readout error: Low, often <1% for detectors
- Loss rate: A key concern; photon loss can be >5% per component
Error Sources
- Photon loss during transmission or through components.
- Detector inefficiencies and dark counts.
- Probabilistic gates—many photonic gates work only with certain probability.
Strengths
- Operates at room temperature.
- Well-suited for quantum networking and communication.
Weaknesses
- Difficult to scale deterministic quantum computation.
- High optical alignment sensitivity.
4. Spin Qubits (Quantum Dots)
Technology: Use the spin state of electrons confined in semiconductor materials like silicon or gallium arsenide. Explored by Intel, HRL, and University of New South Wales.
Typical Error Rates
- Single-qubit gate error: ~0.01% to 1%
- Two-qubit gate error: ~1% to 10%
- Readout error: ~1% to 5%
Error Sources
- Charge noise and spin-orbit coupling.
- Nuclear spin fluctuations in the host material.
Strengths
- Potential compatibility with existing CMOS technology.
- Low power and small size.
Weaknesses
- Still in early development; less robust than superconducting or trapped ion systems.
5. Topological Qubits (Theoretical/Future Tech)
Technology: Designed to encode information in non-local, topologically protected states of matter, such as Majorana zero modes. Pursued by Microsoft and academic researchers.
Typical Error Rates
- Theoretical gate error: Extremely low (~10⁻⁶ or better)
- Experimental implementation: Still not realized for scalable computing
Error Sources
- None measured yet in scalable systems—still under active development.
Strengths
- Intrinsic error correction through topology.
- Potentially extremely robust to decoherence.
Weaknesses
- Not yet practically demonstrated.
- Difficult to engineer stable topological states.
6. Neutral Atom Qubits
Technology: Use individual atoms trapped by optical tweezers. Example: QuEra Computing, Pasqal.
Typical Error Rates
- Single-qubit gate error: ~0.1% to 1%
- Two-qubit gate error: ~1% to 5%
- Readout error: ~2% or lower
Error Sources
- Laser fluctuation and atomic collisions.
- Decoherence due to trapping laser imperfections.
Strengths
- Scalable via reconfigurable atom arrays.
- Long coherence times and good connectivity.
Weaknesses
- Moderate gate fidelities.
- Slower gate speeds (~µs to ms range).
7. Summary Comparison Table
| Qubit Type | 1-Qubit Gate Error | 2-Qubit Gate Error | Readout Error | Speed | Coherence Time | Notes |
|---|---|---|---|---|---|---|
| Superconducting | ~0.01%–0.1% | ~0.1%–1% | ~1%–5% | Fast | Short (~100 µs) | Mature, scalable |
| Trapped Ions | <0.01% | ~0.1%–0.5% | <1% | Slow | Long (~1 s) | High fidelity |
| Photonic | ~1%–10% | Varies | <1% | Fast | N/A | Loss-prone |
| Spin (Quantum Dots) | ~0.01%–1% | ~1%–10% | ~1%–5% | Fast | Moderate | CMOS-compatible |
| Topological | Theoretical | Theoretical | Unknown | TBD | Long | Future tech |
| Neutral Atoms | ~0.1%–1% | ~1%–5% | ~2% | Moderate | Long (~ms) | Flexible geometry |