Resonant optical control of the spin of a single Cr atom in a quantum dot

Abstract

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 ...

Influence of nonequilibrium phonons on the spin dynamics of a single Cr atom

“…Spin-phonon coupling is expected to be efficient for magnetic elements carrying an orbital momentum where a modulation of the crystal field by the strain field of phonons combined with the spin-orbit coupling can induce spin transitions. A spin relaxation time in the µs range was recently observed for an isolated Cr spin but the mechanisms controlling the spin relaxation were not identified in these optical pumping experiments 18,19 .…”

“…The spin states S z =±1 of a Cr atom inserted in a QD can be pumped under resonant optical excitation 18 . This initialization of the Cr spin can be detected in the resonant PL of the QD.…”

Charge fluctuations of a Cr atom probed in the optical spectra of a quantum dot

“…Magnetic CdTe/ZnTe QDs containing an individual Cr 2+ ion (Cr atom in the d 4 configuration) were recently studied 17,19 . The photoluminescence (PL) of neutral Cr-doped QDs is dominated by three main excitonic emission lines corresponding to the spin states S z =0 and S z =±1 of the Cr exchange-coupled with the spins of the confined electron and hole.…”

Readout of a dopant spin in the anisotropic quantum dot with a single magnetic ion

“…This particular feature has been exploited in a number of studies concerning various types of dopants in different semiconductor systems [7,[9][10][11][12][13][14][15][16][17]. Such an approach is naturally limited to the case of the non-resonant excitation, since in the resonant case the directly excited state is vastly over-represented in the photoluminescence spectrum [18,19]. * Electronic address: Aleksander.Rodek@fuw.edu.pl † Electronic address: Tomasz.…”