Rapid high-temperature initialisation and readout of spins in silicon with 10 THz photons

Each cycle of a quantum computation requires a quantum state initialisation. For semiconductor-based quantum platforms, initialisation is typically performed via slow microwave processes and usually requires cooling to temperatures where only the lowest quantum level is occupied. In silicon, boron atoms are the most common impurities. They bind holes in orbitals including an effective spin-3/2 ground state as well as excited states analogous to the Rydberg series for hydrogen. Here we show that initialisation temperature demands may be relaxed and speeds increased over a thousand-fold by importing, from atomic physics, the procedure of optical pumping via excited orbital states to preferentially occupy a target ground state spin. Spin relaxation within the orbital ground state of unstrained silicon is too fast to measure for conventional pulsed microwave technology, except at temperatures below 2 K, implying a need not only for fast state preparation but also fast state readout. Circularly polarised ~10 THz photon pulses from a free electron laser meet both needs at temperatures above 3 K: a 9 ps pulse enhances the population of one spin eigenstate for the “1s”-like ground state orbital, and the second interrogates this imbalance in spin population. Using parameters given by our data, we calculate that it should be possible to initialise 99% of spins for boron in strained silicon within 250 ps at 3 K. The speedup of both state preparation and measurement gained for THz rather than microwave photons should be explored for the many other solid state quantum systems hosting THz excitations potentially useful as intermediate states.

💡 Research Summary

The authors address a fundamental bottleneck in semiconductor spin‑based quantum computing: the slow and low‑temperature requirement for spin state initialization. In silicon, boron acceptors bind holes that possess an effective spin‑3/2 ground‑state manifold (fourfold degenerate) and a set of hydrogen‑like excited orbitals (1Γ⁻⁶ and 1Γ⁻⁷ doublets). Conventional microwave‑based initialization relies on thermal equilibration, which at millikelvin temperatures can take seconds to minutes, and even at higher temperatures remains limited to microseconds.

Inspired by optical pumping in atomic physics, the team uses a free‑electron laser (FELIX) to generate ~9 ps pulses at ~9.6 THz (≈40 meV photon energy). This frequency simultaneously addresses the two closely spaced transitions from the ground quartet to the excited doublets (separated by only 20 GHz, well within the laser bandwidth). By selecting the circular polarization of the pump pulse (ε⁺ or ε⁻) they exploit the selection rules that couple specific m_J components of the ground state to the excited states. Crucially, one component of the ground quartet becomes a “dark state” – it does not couple to the chosen circular polarization. For ε⁺ pumping, the dark state is |1Γ⁺⁸, m_J = +½⟩; for ε⁻ it is |1Γ⁺⁸, m_J = ‑½⟩.

In a pump‑probe configuration the same THz pulse, after a controllable delay, serves as a probe. The pump excites a fraction of the ground‑state population, which then relaxes back to the manifold within ~36 ps via fast phonon‑mediated cascades. Because the relaxation is essentially random among the four ground sublevels, population accumulates preferentially in the dark state. The probe measures the transient transmission change; when pump and probe share the same circular polarization (same‑circular‑polarization, SCP) the probe sees reduced absorption (the dark state is also dark for the probe). When they have opposite polarizations (opposite‑circular‑polarization, OCP) the probe absorption is enhanced. The difference between SCP and OCP yields a circular dichroism signal that persists for nanoseconds, reflecting the long‑lived spin‑lattice relaxation (T₁) of the ground‑state spins.

Temperature‑dependent measurements reveal T₁ ≈ 1 ns at 2.9 K, decreasing to ≈100 ps at 10 K, with a T² dependence indicative of a two‑phonon Raman process. This sub‑nanosecond spin relaxation regime is inaccessible to conventional pulsed microwave EPR, demonstrating the power of THz pump‑probe spectroscopy for spin dynamics.

The authors also explore alternative excitation pathways (1Γ⁻⁸, 2Γ⁻⁸) and find only partial spin polarization because those pathways do not produce a unique dark state. In unstrained silicon the ground quartet remains essentially degenerate; however, applying uniaxial stress lifts the degeneracy, creating a sizable spin‑orbit splitting (Δ ≈ 0.5 meV). In this strained configuration the dark‑state mechanism becomes even more selective. Using the experimentally extracted rates, the authors model the dynamics and predict that at 3 K, 99 % spin polarization can be achieved within 250 ps—over three orders of magnitude faster than microwave‑based schemes.

Key insights of the work are: (1) THz photons carry energies orders of magnitude larger than the spin splitting, removing the need for millikelvin cooling; (2) circular‑polarization‑controlled optical pumping creates a single, well‑defined dark state, enabling high‑fidelity initialization; (3) the same THz pulse can be used for both initialization and readout, providing a unified, ultrafast control platform; (4) the technique opens a new window on sub‑10 ns spin‑lattice relaxation in solid‑state systems.

Beyond boron‑doped silicon, the authors argue that any solid‑state platform possessing THz‑scale intermediate states—such as other shallow acceptors, defect centers, quantum dots, or 2D materials—could benefit from similar optical pumping schemes. This work therefore establishes a versatile, high‑temperature, high‑speed pathway for initializing and measuring spin qubits, potentially accelerating the development of scalable silicon‑based quantum processors.