Spin-resolved quantum-dot resonance fluorescence (0806.3707v1)

Abstract: In the quest for physically realizable quantum information science (QIS) primitives, self-assembled quantum dots (QDs) serve a dual role as sources of photonic (flying) qubits and traps for electron spin; the prototypical stationary qubit. Here we demonstrate the first observation of spin-selective, near background-free and transform-limited photon emission from a resonantly driven QD transition. The haLLMark of resonance fluorescence, i.e. the Mollow triplet in the scattered photon spectrum when an optical transition is driven resonantly, is presented as a natural way to spectrally isolate the photons of interest from the original driving field. We go on to demonstrate that the relative frequencies of the two spin-tagged photon states are tuned independent of an applied magnetic field via the spin-selective dynamic Stark effect induced by the very same driving laser. This demonstration enables the realization of challenging QIS proposals such as heralded single photon generation for linear optics quantum computing, spin-photon entanglement, and dipolar interaction mediated quantum logic gates. From a spectroscopy perspective, the spin-selective dynamic Stark effect tunes the QD spin-state splitting in the ground and excited states independently, thus enabling previously inaccessible regimes for controlled probing of mesoscopic spin systems.

• The paper demonstrates independent tuning of spin-correlated photon states using the dynamic Stark effect and the Mollow triplet, enabling efficient spin-photon entanglement.
• The paper reports transform-limited linewidths with a spontaneous emission rate of 356 MHz and dephasing bounded by 18 MHz, confirming theoretical models.
• The paper achieves a tuning range over 14 GHz in photon emission via controlled laser power and detuning, paving the way for advanced quantum information applications.

Overview of Spin-resolved Quantum-dot Resonance Fluorescence

The paper presents a detailed investigation into the spin-resolved resonance fluorescence of quantum dots (QDs), with a particular emphasis on the emission of transform-limited photons and the extraction of spin-state-correlated emitted photons from a self-assembled QD. This research marks a significant advancement in quantum information science (QIS) by demonstrating the potential of self-assembled QDs to act concurrently as sources of photonic flying qubits and as electron spin traps, serving as stationary qubits. The paper focuses on achieving spin-selective, near background-free photon emission from a resonantly driven QD transition, effectively utilizing the Mollow triplet to isolate photons of interest spectrally.

Key Contributions

1. Mollow Triplet and Dynamic Stark Effect: The paper demonstrates resonance fluorescence at the QD transitions, showcasing the Mollow triplet formation, which provides a spectral isolation mechanism for the photons emitted. Furthermore, the paper illustrates the ability to tune the frequencies of spin-correlated photon states independently from an applied magnetic field using the dynamic Stark effect. This capability to independently control the energy splitting of spin states is pivotal for numerous QIS applications, such as heralded single photon generation and spin-photon entanglement.
2. Resonance Fluorescence Spectra: The research investigates the power and frequency control over the resonance fluorescence spectrum. It confirms the consistency of the experimental results with theoretical predictions by fitting the fluorescence spectra to theoretical models, extracting a spontaneous emission rate of 356 MHz and assessing linewidths that are transform-limited, with dephasing mechanisms being bounded by an 18 MHz maximum.
3. Fine-tuning of Photon Emission: The paper successfully manages photon emission frequency across a range of over 14 GHz by manipulating the excitation laser power and detuning. Importantly, this range is substantially larger than the spontaneous emission rate and offers a flexible approach to tuning photon emissions—comparable to traditional methods such as the DC Stark effect.
4. Independence from Magnetic Field Constraints: By applying a nominal magnetic field (50 mT) to break the spin degeneracy of QD states, measurements confirm the spectrally distinct emission linked to spin states and showcase control over photon spectral separation. This feature paves the way for potential spin-photon entanglement schemes and promises an optical control mechanism over spin ground states, termed the optical Zeeman effect.

Implications and Future Directions

The findings of this paper hold substantial theoretical and practical implications. The ability to control photon emission attributes via the dynamic Stark effect opens novel regimes for probing and utilizing mesoscopic spin systems. This approach to spin-photon interactions in QDs can bridge the gap between stationary and flying qubits, enhancing their use in quantum computation and communications.

Future research could rigorously explore these resonance fluorescence properties further to confirm the emissions are transform-limited and to optimize photon collection efficiencies. Investigating the correlations in emissions from QDs coupled with other states, including those in double QD systems, may yield deeper insights into the dynamics of mesoscopic spin systems.

Furthermore, understanding how emission spectra evolve under significant spin pumping and nuclear spin polarization will provide valuable information. Each of these directions holds the potential to refine solid-state QIS capability and deepen our comprehension of the complex interactions within quantum dot systems.