T-Center Qubit (Silicon Spin-Photon)

Figure

Description

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.

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.

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, and any-to-any connectivity across modules without quantum frequency conversion.

Bergeron et al. (2020) 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).

Motivation

• Telecom-native: O-band emission (1326 nm) means direct fiber coupling and metropolitan-scale networking without frequency conversion.
• Silicon-native: Leverages mature CMOS and silicon photonics fabrication — foundry-compatible path to manufacturing.
• Multi-qubit register: Each T center contains up to 4 spin qubits (1 electron + 3 nuclear), providing local compute + memory in a single defect.
• Any-to-any connectivity: Photon-mediated entanglement enables non-local gate operations between arbitrary modules.
• qLDPC-compatible: Non-local connectivity naturally supports qLDPC codes with their non-planar check structure.

Experimental Status

First optical observation — Bergeron et al. (2020):

• Individual T centers resolved in silicon photonic structures
• Spin-dependent telecom transitions at 1326 nm demonstrated
• Electron spin $T_2$ ~2 ms in isotopically enriched ²⁸Si

Remote entanglement — Photonic Inc. (2024):

• Entanglement between T centers in separate cryostats via telecom fiber
• Teleported CNOT gate between silicon spin qubits
• Demonstrated modular quantum computing architecture

Key Metrics

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

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.

References

Original proposal and demonstration

• L. Bergeron et al., “Silicon-Integrated Telecommunications Photon-Spin Interface,” PRX Quantum 1, 020301 (2020) — arXiv:1912.09178

Distributed quantum computing

• Photonic Inc., “Distributed quantum computing with silicon T centers” (2024) — demonstration of remote entanglement and teleported gates

Linked Papers

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

Related Entries

• nv-center-qubit — Diamond color center; more mature but requires frequency conversion
• siv-color-center-qubit — Diamond SiV/SnV; excellent optical properties but not telecom-native
• silicon-spin-qubit — Silicon spin qubit cousin (no photonic interface)
• dual-rail-photonic-qubit — Photonic encoding that interfaces with T centers
• rare-earth-ion-qubit — rare-earth ions (Er³⁺) as alternative telecom-wavelength solid-state qubit