Superconducting Qubit
Qubits made from superconducting circuits that behave as artificial atoms. The leading technology for near-term quantum computers.
Superconducting qubits are the most widely deployed quantum computing technology. They use superconducting circuits cooled to millikelvin temperatures to create quantum two-level systems.
How They Work
Superconductivity
At very low temperatures (~15 mK), certain materials conduct electricity with zero resistance. Currents can flow indefinitely.
The Josephson Junction
The key element: two superconductors separated by a thin insulator. It creates a non-linear inductor that enables quantum behavior:
- Quantized energy levels (like an atom)
- Controllable transitions between levels
Qubit States
- : Ground state
- : First excited state
- Energy difference: ~5 GHz (microwave range)
Types of Superconducting Qubits
| Type | Principle | Used By |
|---|---|---|
| Transmon | Charge qubit with reduced sensitivity | IBM, Google, Rigetti |
| Flux qubit | Circulating current direction | D-Wave (annealing) |
| Phase qubit | Junction phase | Historical |
| Fluxonium | Enhanced coherence | Research |
The transmon dominates current quantum computers.
Control and Readout
Control
- Microwave pulses at qubit frequency (~5 GHz)
- Pulse shape determines gate operation
- Single-qubit gates: ~20 ns
- Two-qubit gates: ~50-200 ns
Readout
- Coupled to microwave resonator
- Measure resonator frequency shift
- ~1 μs readout time
Operating Conditions
| Parameter | Typical Value |
|---|---|
| Temperature | 10-20 mK |
| Cooling | Dilution refrigerator |
| Frequency | 4-8 GHz |
| T1 time | 50-500 μs |
| T2 time | 50-200 μs |
| Gate fidelity (1Q) | 99.9%+ |
| Gate fidelity (2Q) | 99-99.9% |
Advantages
| Advantage | Details |
|---|---|
| Fabrication | Leverages semiconductor manufacturing |
| Scalability | Lithographic techniques |
| Control | Fast microwave electronics |
| Connectivity | Can couple many qubits |
| Tunability | Frequencies can be adjusted |
Challenges
| Challenge | Issue |
|---|---|
| Cooling | Requires dilution refrigerators |
| Scale | Wiring and control bottlenecks |
| Coherence | Material defects limit T1, T2 |
| Crosstalk | Unwanted qubit interactions |
Major Players
- IBM: Transmon-based, cloud access (IBM Quantum)
- Google: Sycamore processor, quantum supremacy claim
- Rigetti: Full-stack approach
- IQM: European manufacturer
Current State
- Largest systems: 1000+ qubits (IBM)
- Best 2-qubit fidelity: ~99.9%
- Active research on error correction