T-Center Qubit (Silicon Spin-Photon)

The T center is a carbon-hydrogen defect complex (C–C–H) in silicon that functions as a spin-photon interface with native emission at telecom wavelengths (~1326 nm, O-band). Each T center hosts up to four spin qubits (one electron spin, three nuclear spins from ¹³C and ¹H), combining long-lived quantum memory with a photonic interface — all within silicon, compatible with existing photonic integrated circuit fabrication.

Figure

Description

Structure and physics

The T center consists of two substitutional carbon atoms and one hydrogen atom in the silicon lattice. Its bound exciton transition produces photons directly in the telecom O-band, eliminating the need for frequency conversion that plagues diamond-based approaches. The inversion symmetry of the defect provides first-order protection against electric field noise, yielding spectrally stable optical lines.

Spin-photon interface

The electron spin (S = 1/2) provides the primary qubit with optical initialization and readout via spin-dependent fluorescence. Nuclear spins (¹³C, ¹H) serve as long-lived quantum memory registers. Spin-selective optical transitions enable:

• Heralded entanglement between remote T centers via photon interference
• Teleported gates between silicon chips connected by telecom fiber
• Any-to-any connectivity across modules without quantum frequency conversion

Key demonstrations

Bergeron et al. (2020, Nature) achieved the first optical observation of individual T centers in silicon photonic structures with spin-dependent telecom transitions. Photonic Inc. subsequently demonstrated remote entanglement between T centers in separate cryostats connected by standard telecom fiber, and a teleported CNOT gate between silicon spin qubits (2024).

Hamiltonian

Ground-state electron spin:

$H=g_e{\mu }_B\mathbf{B}\cdot \mathbf{S}+{\sum }_i{A}_i\mathbf{S}\cdot {\mathbf{I}}_i$

where $\mathbf{S}$ is the electron spin, ${\mathbf{I}}_i$ are nuclear spins (¹³C, ¹H), and $A_i$ are hyperfine coupling constants. The optical interface is governed by the bound exciton transition:

$H_{\text{opt}}={\omega }_0|e⟩⟨e|+\Omega (t)(|g⟩⟨e|+\text{h.c.})$

at ${\omega }_0\approx 2\pi \times 226$ THz (1326 nm).

Performance Metrics

Metric	Value	Notes	Fidelity reference
Emission wavelength	1326 nm	Telecom O-band, no frequency conversion	bergeron-2020-t-center
Electron spin T₂	~2 ms	In isotopically enriched ²⁸Si	bergeron-2020-t-center
Nuclear spin T₂	>1 s	¹³C nuclear memory	bergeron-2020-t-center
Operating temperature	~1 K	Compatible with standard cryogenics	bergeron-2020-t-center
Remote entanglement	Demonstrated	Between separate cryostats via telecom fiber	photonic-2024-distributed-qc

Scaling Considerations

• Silicon-native: Leverages mature CMOS and silicon photonics fabrication — foundry-compatible.
• Telecom-native: O-band emission means direct fiber coupling, metropolitan-scale networking without frequency conversion.
• QLDPC-compatible: Photonic Inc.’s architecture targets QLDPC codes exploiting the any-to-any connectivity of photon-mediated entanglement.
• Multi-qubit register: Each T center contains up to 4 spin qubits, providing local compute + memory in a single defect.

Linked Papers

• bergeron-2020-t-center
• photonic-2024-distributed-qc

Related Entries

• nv-center-qubit
• siv-color-center-qubit
• silicon-spin-qubit
• dual-rail-photonic-qubit