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We have studied the spin dynamics in self-organized InAs/GaAs quantum dots by time-resolved photoluminescence performed under strictly resonant excitation. At low temperature, we observe strictly no decay of both the linear and the circular luminescence polarization. This demonstrates that the carrier spins are totally frozen on the exciton lifetime scale.

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Recent progress in nanotechnology has allowed to fabricate new hybrid systems where a single two-level system is coupled to a mechanical nanoresonator [1][2][3][4][5][6]. In such systems the quantum nature of a macroscopic degree of freedom can be revealed and manipulated. This opens up appealing perspectives for quantum information technologies [7], and for the exploration of quantum-classical boundary. Here we present the experimental realization of a monolithic solid-state hybrid system governed by material strain [8] : a quantum dot is embedded within a nanowire featuring discrete mechanical resonances corresponding to flexural vibration modes. Mechanical vibrations result in a time-varying strain field that modulates the quantum dot transition energy. This approach simultaneously offers a large light extraction efficiency [9,10] and a large exciton-phonon coupling strength g 0 . By means of optical and mechanical spectroscopy, we find that g 0 /2π is nearly as large as the mechanical frequency, a criterion which defines the ultrastrong coupling regime [12]. A single quantum two-level system coupled to a micron-size mechanical oscillator constitutes a hybrid system, which connects two different worlds: the classical and the quantum one. This new kind of interaction opens up the possibility of creating macroscopic non-classical states of motion, such as phonon Schrödinger cats or phonon number states. In the case of strain-mediated coupling, it is predicted that the two level system can even be used to cool the mechanical resonator down to its ground state [8] or conversely to achieve phonon lasing [13].Such appealing prospects have recently motivated the development of several kinds of hybrid systems, like for instance: (i) a single spin embedded in a mechanical resonator coupled together by an external magnetic field gradient [4,14,15], (ii) a few elementary charges (single electron or Cooper pair) coupled by electrostatic forces with a vibrating gate [2, 6, 16], or (iii) quantized current loops in superconducting qubits attached to a mechanical oscillator interacting via a magnetic field [17]. However, despite theoretical proposals highlighting the potential of using material strain to mediate a large coupling between Figure 1. Hybrid system and experimental setup. a Scanning electron microscope picture of a representative cone shaped nanowire. The quantum dots (QDs) layer is materialized by the dashed white line. b and c, Nanowire deformation in the first order flexural vibration mode. The stress field is plotted in blue to red color scale: due to its excentric inplane position, the quantum dot (yellow triangle) experiences in b a compressive strain that shifts its transition energy ω0 by + δω and in c a tensile strain that shifts its transition energy by − δω. d Experimental setup: single QD optical measurements are carried out using a spectrometer, and the measurement of the nanowire free-end displacement δx is realized by means of a balanced split photo-diode (SPD). The voltage difference v between th...

Using far-infrared magnetospectroscopy in self-assembled InAs quantum dots, we have investigated the electronic transitions from the ground s levels to the excited p levels. The experiments consist of monitoring, by means of Zeeman tuning of the excited level, a resonant interaction between the discrete ( p, 0 LO phonon) state and the continuum of either (s, 1 LO phonon) or (s, 2 LO phonons). We show that the electrons and the LO phonons are always in a strong coupling regime and form an everlasting mixed electron-phonon mode. PACS numbers: 73.40.Kp, 73.20.Dx, 78.20.Ls Electrons in excited atomic states can relax towards lower lying levels by spontaneous emission of photons. Artificial atoms like semiconductor quantum dots display discrete levels. For electrons (or holes) placed in excited levels the spontaneous emission of photons is inefficient for the relaxation due to the characteristic energy splitting of the dot states (ϳ50 meV in a ϳ20 nm dot). The carriers bound to these artificial atoms are however in interaction with phonons which display a continuum of finite width, unlike photons. It has been shown that the intradot relaxation through acoustical phonons is totally inefficient, the energy mismatch between electron states being much too large [1,2]. In semiconductors, the most powerful energy relaxation channel is (by far) the irreversible emission of longitudinal optical (LO) phonons through the Fröhlich coupling. Despite its effectiveness, this electron-phonon coupling is weak, to the extent that the initial discrete level (e, 0 phonon) irreversibly decays into the continuum (g, 1 phonon) where e and g, respectively, denote an excited state and the ground electronic state. Such a weak coupling is very well described by the Fermi golden rule in bulk, quantum well (2D) or quantum wire (1D) structures. Because the optical phonons show very little dispersion, it has been argued that the LO phonon assisted relaxation in semiconductor quantum dots could be efficient only if the energy separation between the electronic states differs by one (or several) LO phonons. Here we present experimental evidence supported by theoretical modeling that the very idea of an electron emitting LO phonons and relaxing irreversibly to a less excited state (as in bulk, 2D, and 1D heterolayers) is wrong in a quantum dot. What happens in reality is that the electrons and the LO phonons are in a strong coupling regime and form everlasting mixed electron-phonon modes, as recently suggested by Inoshita and Sakaki in the case of one phonon [3]. Using far-infrared (FIR) magnetospectroscopy, we have investigated the g ! e transition in self-assembled doped InAs quantum dots. The experiments consist of monitoring, by means of Zeeman tuning of the dot excited level e, a resonant interaction between the discrete (e, 0 LO phonon) state and the continuum of either (g, 1 LO phonon) or (g, 2 LO phonons). We show that the (e, 0 LO phonon) state does not dissolve when entering into the continuum but forms a hybrid mode with (g, 1, or 2 LO p...