Trion quantum coherence in site-controlled pyramidal InGaAs quantum dots

Deterministically positioned pyramidal InGaAs quantum dots (QDs) exhibit exceptional quantum properties, making them highly promising candidates for scalable on-chip quantum information processing. In this work, we investigate the coherent dynamics of positively charged excitons under the influence of strong magnetic fields in the Faraday configuration. Pyramidal quantum dots exhibit a fourfold splitting of the charged excitons even in the Faraday configuration, giving rise to an optically addressable double-LambdaΛ system akin to self-assembled quantum dots in oblique magnetic fields. Here, we investigate ultrafast complete coherent control of the trion to ground state transition utilizing advanced optical resonant excitation techniques and we observe quantum coherence over timescales that are similar to other prominent quantum dot platforms. These results pave the way towards establishing site-controlled pyramidal InGaAs QDs as scalable platforms for quantum information processing, expanding the reach of coherent control to new quantum systems.

💡 Research Summary

This work investigates the coherent dynamics of positively charged excitons (trions) in deterministically positioned, site‑controlled pyramidal InGaAs quantum dots (QDs) grown by metal‑organic vapor‑phase epitaxy on (111)‑oriented GaAs recesses. The (111) symmetry endows the structures with C3v point‑group symmetry, which, when a magnetic field is applied in the Faraday configuration, lifts the degeneracy of both the hole ground state and the trion excited state, producing a four‑fold Zeeman splitting. As a result, a double‑Λ level scheme emerges even for a purely longitudinal magnetic field, providing two optically addressable transitions that can be used for spin initialization, manipulation, and read‑out.

The authors first demonstrate ultrafast Rabi oscillations by resonantly driving the |⇓⟩ → |⇑⇓↓⟩ transition with 3 ps Ti:Sapphire pulses. By varying the pulse area (proportional to the square root of the pulse power) they achieve π, 2π, and higher‑order rotations, which are observed as oscillations in the photon counts collected from the spectrally separated |⇑⇓↓⟩ → |⇑⟩ decay channel. The contrast of the oscillations decays with increasing pulse area, a behavior attributed to pulse‑area‑dependent pure dephasing caused by coupling to longitudinal acoustic phonons and to slight deviations from transform‑limited pulse shapes. The experimental data agree well with the microscopic theory of Förster et al. (Phys. Rev. B 31).

Next, Ramsey interference measurements are performed using two identical π/2 pulses separated by a controllable delay Δτ in a Mach‑Zehnder interferometer. The first pulse creates a coherent superposition of the hole ground state and the trion state; the second pulse, arriving after Δτ, rotates the Bloch vector about an axis that is phase‑shifted by ϕ = ωΔτ relative to the first pulse. By scanning Δτ with both coarse (motorized stage) and fine (piezo) adjustments, the authors record high‑visibility Ramsey fringes. Fitting the decay of the fringe contrast yields a dephasing time T2* = 51 ± 9 ps, comparable to values reported for conventional self‑assembled InGaAs QDs.

Finally, the paper demonstrates full SU(2) control of the trion‑hole qubit. Here, both the pulse area (θ) and the fine delay (ϕ) are varied simultaneously, producing a two‑dimensional map of photon counts as a function of θ and ϕ. The resulting multi‑lobed interference pattern confirms that any point on the Bloch sphere can be reached by an appropriate combination of rotation angle and phase, i.e., universal single‑qubit gate operations. Because the experiment lasts several hours, interferometric phase drift is a critical issue; the authors mitigate this by injecting a counter‑propagating, frequency‑stabilized HeNe laser into the same interferometer. Continuous monitoring of the HeNe phase allows post‑processing correction of the drift, reducing the residual RMS phase error to 0.18 rad. This level of phase stability is essential for deterministic quantum gate implementation.

In the discussion, the authors emphasize that site‑controlled pyramidal QDs combine the scalability of deterministic positioning with the high optical quality, narrow linewidths, and spectral homogeneity typical of self‑assembled dots. The observed trion coherence time, while modest, is sufficient for fast optical gate operations, while the underlying hole spin can exhibit much longer lifetimes, opening the possibility of hybrid protocols where rapid optical control is followed by long‑term spin storage. The double‑Λ configuration also enables geometric‑phase gates and entanglement schemes that rely on coherent population trapping.

Overall, this study establishes pyramidal InGaAs QDs as a viable platform for on‑chip quantum information processing. By demonstrating Rabi rotations, Ramsey interferometry, and complete SU(2) control with careful phase‑drift compensation, the work provides a comprehensive toolbox for manipulating solid‑state spin qubits in a scalable, deterministic architecture.