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arXiv:1206.0592 · Nature Communications

Microcavity controlled coupling of excitonic qubits

Ferdinand Albert, Kanchana Sivalertporn, Jacek Kasprzak, Max Strauß, C. Schneider, Sven Höfling, Martin Kamp, A. Forchel, Stephan Reitzenstein, Egor A. Muljarov, W. Langbein

Published Apr 23, 2013• 59 citations
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Abstract

Controlled non-local energy and coherence transfer enables light harvesting in photosynthesis and non-local logical operations in quantum computing. This process is intuitively pictured by a pair of mechanical oscillators, coupled by a spring, allowing for a reversible exchange of excitation. On a microscopic level, the most relevant mechanism of coherent coupling of distant quantum bits—like trapped ions, superconducting qubits or excitons confined in semiconductor quantum dots—is coupling via the electromagnetic field. Here we demonstrate the controlled coherent coupling of spatially separated quantum dots via the photon mode of a solid state microresonator using the strong exciton–photon coupling regime. This is enabled by two-dimensional spectroscopy of the sample’s coherent response, a sensitive probe of the coherent coupling. The results are quantitatively understood in a rigorous description of the cavity-mediated coupling of the quantum dot excitons. This mechanism can be used, for instance in photonic crystal cavity networks, to enable a long-range, non-local coherent coupling. Controlling coupling between distant quantum objects is important for implementation of quantum technologies. Providing an important step towards using semiconductor structures for hosting optically controlled qubits, this work shows coherent coupling between three quantum dot excitons via a cavity.

DOI
10.1038/ncomms2764

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Vacuum Rabi oscillation is a damped oscillation in which energy can transfer between an atomic excitation and a photon when an atom is strongly coupled to a photonic cavity. This process is challenging to be coherently controlled due to the fact that interaction between the atom and the electromagnetic resonator needs to be modulated in a quick manner compared to vacuum Rabi frequency. This control has been achieved at microwave frequencies, but has remained challenging to be implemented in the optical domain. Here we demonstrated coherent control of energy transfer in a semiconductor quantum dot strongly coupled to a photonic crystal molecule by manipulating the vacuum Rabi oscillation of the system. Instead of using a single photonic crystal cavity, we utilized a photonic crystal molecule consisting two coupled photonic crystal defect cavities to obtain both strong quantum dot-cavity coupling and cavityenhanced AC stark shift. In our system the AC stark shift modulates the coupling interaction between the quantum dot and the cavity by shifting the quantum dot resonance, on timescales (picosecond) shorter than the vacuum Rabi period. We demonstrated the ability to transfer excitation between a quantum dot and cavity, and performed coherent control of light-matter states. Our results provides an ultra-fast approach for probing and controlling light-matter interactions in an integrated nanophotonic device, and could pave the way for gigahertz rate synthesis of arbitrary quantum states of light at optical frequencies.
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Abstract Coupling of quantum emitters in a semiconductor relies, generally, on short-range dipole-dipole or electronic exchange type interactions. Consistently, energy transfer between exciton states, that is, electron-hole pairs bound by Coulomb interaction, is limited to distances of the order of 10 nm. Here, we demonstrate polariton-mediated coupling and energy transfer between excitonic states over a distance exceeding 2 μm. We accomplish this by coupling quantum well-confined excitons through the delocalized mode of two coupled optical microcavities. Use of magnetically doped quantum wells enables us to tune the confined exciton energy by the magnetic field and in this way to control the spatial direction of the transfer. Such controlled, long-distance interaction between coherently coupled quantum emitters opens possibilities of a scalable implementation of quantum networks and quantum simulators based on solid-state, multi-cavity systems.
Generating strong interactions between single quanta of light and matter is central to quantum information science, and a key component of quantum computers and long-distance quantum networks. In quantum information processing, these interactions are required to create elementary logic operations (quantum gates) between stationary matter quantum bits (qubits) and photonic qubits that can be transmitted over long distances. Efficient quantum gates between photonic and matter qubits are a crucial enabler for a broad range of applications that include robust quantum networks, nondestructive quantum measurements, and strong photon-photon interactions. So far these qubit-photon gates have been achieved using single atoms and at microwave frequencies in circuit QED systems. Their implementation with solidstate quantum emitters, however, has remained a difficult challenge. We demonstrate that the qubit state of a photon can be controlled by a single solid-state qubit composed of a quantum dot (QD) strongly coupled to an optical nanocavity. We show that the QD acts as a coherently controllable qubit system that conditionally flips the polarization of a photon reflected from the cavity mode on picosecond timescales. This operation implements a controlled NOT (cNOT) logic gate between the QD and the incident photon, which is a universal quantum operation that can serve as a general light-matter interface for remote entanglements and quantum computations. Our results represent an important step towards an all solid-state implementation of quantum networks and quantum computers, and provide a versatile approach for controlling and probing interactions between a photon and a single quantum emitter on ultra-fast timescales.
Controlling the quantum states of charge (excitons) or spin-polarized carriers in semiconductor quantum dots (QDs) has been the focus of a considerable research effort in recent years due to the strong promise of using this approach to develop solid state quantum computing hardware. The long-term scalability of this type of quantum computing architecture is enhanced by the use of QDs emitting in the telecom band, which would exploit the established photonic infrastructure. This thesis reports the use of all optical infrared experimental techniques to control exciton dynamics in two different QD samples consisting of InAs/GaAs QDs and InAs/InP QDs within a planar microcavity. An infrared quantum control apparatus was developed and used to apply optimized shaping masks to ultrafast pulses from an optical parametric oscillator. Pulse shaping protocols designed to execute a two-qubit controlled-rotation operation on an individual semiconductor QD were demonstrated and characterized. The quantum control apparatus was then implemented in simultaneous single qubit rotations using two uncoupled, distant InAs/GaAs QDs. These optimal control experiments demonstrated high fidelity optical manipulation of exciton states in the two QDs using a single broadband laser pulse, representing a step forward on the path to a scalable QD architecture and showcasing the power of pulse shaping techniques for quantum control on solid state qubits. As an alternative to single QDs, which have very low optical signals, subsets of QDs within an ensemble can be used in quantum computing applications. To investigate the mediation of inhomogeneities in a QD ensemble, pump-probe experiments were performed on InAs/InP QDs within a dielectric Bragg stack microcavity. Two different excitation geometries showed that the angle dependence of the microcavity transmission allowed for the spectral selection of QD subsets with transition energies resonant with the cavity mode. The microcavity mitigated inhomogeneities in the ensemble while providing a basis for addressing QD subsets which could be used as distinguishable quantum bits. This thesis work shows significant advances towards an optical computing architecture using quantum states in semiconductor QDs.