Coherent control and suppressed nuclear feedback of a single quantum dot hole qubit (1106.5676v1)

Abstract: Future communication and computation technologies that exploit quantum information require robust and well-isolated qubits. Electron spins in III-V semiconductor quantum dots, while promising candidates, see their dynamics limited by undesirable hysteresis and decohering effects of the nuclear spin bath. Replacing electrons with holes should suppress the hyperfine interaction and consequently eliminate strong nuclear effects. Using picosecond optical pulses, we demonstrate coherent control of a single hole qubit and examine both free-induction and spin-echo decay. In moving from electrons to holes, we observe significantly reduced hyperfine interactions, evidenced by the reemergence of hysteresis-free dynamics, while obtaining similar coherence times, limited by non-nuclear mechanisms. These results demonstrate the potential of optically controlled, quantum dot hole qubits.

• The paper shows that replacing electron with hole spins in quantum dots suppresses hyperfine interactions by over 30-fold, significantly reducing nuclear feedback.
• Researchers used picosecond optical pulses and a p-i-n diode in Voigt geometry to achieve SU(2) coherent control, validated by Rabi oscillations and Ramsey interferometry.
• Experimental measurements reveal a T2* of 2.3 ns and a spin-echo T2 of 1.1 μs, highlighting potential improvements in mitigating electric field noise for scalable quantum computing.

Coherent Control and Suppressed Nuclear Feedback of a Single Quantum Dot Hole Qubit

This essay presents a detailed analysis of the paper titled "Coherent Control and Suppressed Nuclear Feedback of a Single Quantum Dot Hole Qubit." The paper explores advancements in quantum information technologies, specifically focusing on quantum dots (QDs) as building blocks for qubits, with an emphasis on reducing decohering effects through the utilization of hole spins.

The investigation addresses a critical challenge in the application of quantum technologies: the decoherence of electron spins in III-V semiconductor QDs due to interactions with the nuclear spin bath via hyperfine interactions. To counteract these limitations, the paper proposes the replacement of electron spins with hole spins. This substitution aims to suppress the hyperfine interaction, which is typically stronger for electrons due to their contact hyperfine interactions with the nuclei. As a result, the nuclear feedback is expected to be significantly reduced when using holes, thereby enhancing qubit coherence.

The authors demonstrate their approach using picosecond optical pulses for coherent control of a single hole qubit. Experimental procedures highlight a comparative reduction in hyperfine interaction effects, with the observed reemergence of hysteresis-free dynamics in hole qubits akin to those previously seen in electron qubits but without the hysteresis. Methodologically, the experiments involved the use of a p-i-n diode to deterministically charge QDs with holes and applying magnetic fields in Voigt geometry for effective qubit manipulation.

Results from the Rabi oscillations and Ramsey interferometry confirm the potential for coherent manipulation. The coherent control tests revealed an SU(2) control capability over the qubit, a fundamental requirement for practical quantum computation. Notably, the paper presents a profound reduction in observed nuclear feedback, with at least a factor of 30 compared to electron spins, aligning with other studies that measure reduced hyperfine interactions in hole spins.

Furthermore, the coherence properties of the hole spins were quantitatively analyzed. The measured $T_2^*$, representing the dephasing time due to low-frequency noise, was found to be 2.3 ns, whereas the spin-echo measurements yielded a $T_2$ of 1.1 $\mu$s. These measurements elucidate that non-nuclear mechanisms might still impose limitations due to electric field fluctuations and spectral diffusion contributing to decoherence.

The implications of this research are significant for quantum information technologies. By demonstrating coherent control of hole spins with reduced nuclear feedback, the paper lays the groundwork for higher fidelity qubit operations, allowing up to 50,000 operations within their coherence timeframe. The paper suggests that further studies on electric field fluctuations and potential isotopic or device engineering could extend the coherence of hole qubits and enhance their applicability in scalable quantum computing systems.

In conclusion, the research expounded in this paper substantiates the theoretical expectations about the suppressed hyperfine interactions intrinsic to quantum dot hole spins. It highlights a noticeable shift from nuclear to electric field-induced decoherence mechanisms, suggesting new avenues for improving qubit fidelity and scalability. Future investigations may explore advanced dynamical decoupling techniques and devise engineering methods to mitigate non-nuclear noise sources. These developments could eventually broaden the practical utilization of hole qubits in quantum computation and communication infrastructures.