International audienceWe demonstrate the optical addressability of the spin of an individual chromium atom (Cr) embedded in a semiconductor quantum dot. The emission of Cr-doped quantum dots and their evolution in magnetic field reveal a large magnetic anisotropy of the Cr spin induced by local strain. This results in the zero field splitting of the 0, ±1, and ±2 Cr spin states and in a thermalization on the magnetic ground states 0 and ±1. The observed strong spin to strain coupling of Cr is of particular interest for the development of hybrid spin-mechanical devices where coherent mechanical driving of an individual spin in an oscillator is needed. The magneto-optical properties of Cr-doped quantum dots are modeled by a spin Hamiltonian including the sensitivity of the Cr spin to the strain and the influence of the quantum dot symmetry on the carrier-Cr spin coupling

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A Cr atom in a semiconductor host carries a localized spin with an intrinsic large spin to strain coupling particularly promising for the development of hybrid spin-mechanical systems and coherent mechanical spin driving. We demonstrate here that the spin of an individual Cr atom inserted in a semiconductor quantum dot can be controlled optically. We first show that a Cr spin can be prepared by resonant optical pumping. Monitoring the time dependence of the intensity of the resonant fluorescence of the quantum dot during this process permits to probe the dynamics of the optical initialization of the Cr spin. Using this initialization and read-out technique we measured a Cr spin relaxation time at T=5 K of about 2 microseconds. We finally demonstrate that, under a resonant single mode laser field, the energy of any spin state of an individual Cr atom can be independently tuned by using the optical Stark effect.Individual spins in semiconductor nano-structures are promising for the development of quantum information technologies 1-3 . Spins trapped in optically active quantum dots (QDs) have attracted strong interest since their coupling to light enables fast spin control and optical coherent control has been demonstrated for electron 4,5 and hole 6,7 spins confined in QDs. Thanks to their expected longer coherence time, localized spins on individual dopants in semiconductors are also a promissing media for storing quantum information. Optically active QDs containing individual or pairs of magnetic dopants have been realized both in II-VI 8-12 and III-V 13,14 semiconductors. In these systems, since the confined carriers and magnetic atom spins become strongly mixed, an optical excitation of the QD can affect the spin state of the atom offering possibilities for a control of the localized spin 15,16 . The variety of magnetic transition elements that can be incorporated in semiconductors gives a large choice of localized electronic spins, nuclear spins and orbital momentums with optical addressability 8,[17][18][19] . This approach opens a diversity of applications in quantum information and quantum sensing.Among these magnetic atoms, chromium (Cr) is of particular interest 19 . It incorporates in II-VI semiconductors as Cr 2+ carrying an electronic spin S=2 and an orbital momentum L=2. Moreover, most of Cr isotopes have no nuclear spin. This simplifies the spin level structure and its coherent dynamics 20 . With bi-axial strain, the ground state of the Cr is an orbital singlet with spin degeneracy of 5. The orbital momentum of the Cr connects its spin to the local strain through the modification of the crystal field and the spin-orbit coupling. This spin to strain coupling is more than two orders of magnitude larger than for elements without orbital momentum (NV centers in diamond 21 , Mn atoms in II-VI semiconductors 22 ). Cr is therefore a promising qubit for hybrid spin-mechanical systems in which the motion of a mechanical oscillator would be coherently coupled to the spin state of a single atom 21,23,24 . The ...

We studied the spin dynamics of a Cr atom incorporated in a II-VI semiconductor quantum dot using photon correlation techniques. We used recently developed singly Cr-doped CdTe/ZnTe quantum dots (A. Lafuente-Sampietro et al., [1]) to access the spin of an individual magnetic atom. Auto-correlation of the photons emitted by the quantum dot under continuous wave optical excitation reveals fluctuations of the localized spin with a timescale in the 10 ns range. Cross-correlation gives quantitative transfer time between Cr spin states. A calculation of the time dependence of the spin levels population in Cr-doped quantum dots shows that the observed spin dynamics is controlled by the exciton-Cr interaction. These measurements also provide a lower bound in the 20 ns range for the intrinsic Cr spin relaxation time.Diluted magnetic semiconductor systems combining high-quality nano-structures and the magnetic properties of transition metal elements are good candidates for the development of single spin nano-electronic devices [2]. Thanks to their expected long coherence time, localized spin of individual magnetic atoms in a semiconductor host are an interesting media for storing quantum information in the solid state. Optical probing and control of the spin of individual or pairs of magnetic atoms have been obtained in both II-VI [3][4][5][6][7] and III-V [8,9] semiconductors. The variety of magnetic transition elements that could be incorporated in semiconductors gives a large choice of electronic and nuclear spins as well as orbital momentum [10,11]. In this context, growth and optical addressing of II-VI semiconductor quantum dots (QDs) containing an individual Cr atom were achieved recently [1].Cr is incorporated in II-VI compounds as Cr 2+ ions carrying a localized electronic spin S = 2 and an orbital momentum L = 2 [12]. In addition, most of the Cr isotopes have no nuclear spin. In the presence of a large bi-axial strain, the ground state of the Cr is an orbital singlet with spin degeneracy 2S+1=5. The nonzero orbital momentum of Cr and spin-orbit coupling result in a large sensitivity of its spin to local strain. This large spin to strain coupling, at least two orders of magnitude larger than for magnetic elements without orbital momentum (NV centers in diamond [13,14], Mn atoms in II-VI semiconductors [15]) makes Cr a very promising spin qubit for the realization of hybrid spin-mechanical systems in which the motion of a microscopic mechanical oscillator would be coherently coupled to the spin state of a single atom [16]. Large spin to strain coupling also enhances the spin-phonon interaction ultimately responsible for the spin relaxation and decoherence of an isolated magnetic atom in a semiconductor matrix. A too large interaction with phonons could limit the prac- * lucien.besombes@grenoble.cnrs.fr tical use of Cr as a qubit and an investigation of the spin dynamics in Cr-doped QDs is required.We show here how we can use the statistics of the photons emitted by a Cr-doped QD to probe the dynamics of the magnet...