Optical properties of a semiconductor quantum dot with a single magnetic impurity: photoinduced spin orientation Alexander O. Govorov 1 and Alexander V. Kalameitsev 2 1 Department of Physics and Astronomy, Condensed Matter and Surface Science Program, Ohio University, Athens, Ohio 45701-2979, USA 2 Institute of Semiconductor Physics, Novosibirsk, 630090 Russia Received 27 September 2004; published 26 January 2005 We describe the optical resonant manipulation of a single magnetic impurity in a self-assembled quantum dot. We show that using the resonant pumping one can address and manipulate selectively individual spin states of a magnetic impurity. The mechanisms of resonant optical polarization of a single impurity in a quantum dot involve anisotropic exchange interactions and are different than those in diluted semiconductors. A Mn impurity can be prepared in a given spin state and can act as qubit. The limiting factors for the spin manipulation are the electron-hole exchange interaction and finite temperature. DOI: 10.1103/PhysRevB.71.035338 PACS numbers: 78.67.Hc, 42.50.Ct, 75.75.+a I. Introduction The spins of electrons in semiconductors strongly couple with electric and magnetic fields due to the spin-orbit and exchange interactions. Spintronics and quantum computation utilize these interactions to manipulate the electron spins. 1,2 One important class of spintronics materials is diluted mag- netic semiconductors 3 which combine high quality semicon- ductor structures with magnetic properties of impurities. Since many semiconductors efficiently emit and absorb light, the spin states of electrons and magnetic impurities can be manipulated optically by using circularly polarized light pulses. 4 In diluted magnetic semiconductors such as bulk crystals, quantum wells, and dots, photogenerated excitons interact with a large collection of spins of Mn impurities and therefore a large number of degrees of freedom becomes involved. 5–11 In these systems, it is challenging to address individual spins of Mn atoms. At the same time, the quantum computational schemes are based on qubits, pairs of well- controlled quantum states. These elementary blocks, qubits, should be made interacting or decoupled on demand. In a diluted magnetic semiconductor, even a single Mn atom has six spin states I Mn =5/2. Therefore 15 different pairs of states qubits can be defined for a single Mn impurity. In addition, a single Mn impurity can act as two qubits which involve four out of six spin states. Here we study a system which allows us to manipulate optically a single Mn spin. This system is composed of a quantum dot QD and a single Mn impurity. We note that several physical aspects of a QD with a single Mn impurity were recently discussed in Refs. 12 and 13. This paper describes a single Mn impurity embedded into a self-assembled QD. Importantly, such a system permits ef- ficient selective optical control and manipulation of indi- vidual spin states and defining a single qubit for the Mn impurity. This ability comes from the exciton spectrum of a QD with a Mn atom. An exciton in a QD has a well-defined discrete spectrum and, simultaneously, strongly interacts with the Mn spin via the exchange interaction. Since the exciton and Mn spin functions become strongly mixed, the resonant optical excitation strongly affect the spin state of Mn impurity. In particular, we show that one can write spin states of an Mn atom. Since spin relaxation of paramagnetic ions in the absence of carriers i.e., after the exciton recom- bination is an extremely slow process 10–100 s,a single Mn spin is a very promising candidate for spintronics applications. The mechanisms of Mn-spin polarization in a QD are qualitatively different than those in bulk materials because of the discrete character of quantum states. In bulk, the photogenerated spin-polarized electrons transfer their spin to the Mn atoms or induce an effective magnetic field which polarizes the impurity system. 7 In a QD, the spin ori- entation of Mn atom comes from the three-body interactions involving an electron, hole, and Mn spin. The ability to ma- nipulate a pair of chosen states qubit comes from the reso- nant excitation of a certain spin state of the exciton-Mn sys- tem. In addition, we describe the specific optical signatures of a Mn atom embedded into a QD. In contrast to the un- doped self-assembled quantum dots, the optical emission of a laterally asymmetric quantum dot becomes circularly polar- ized due to the exciton-Mn interaction. II. Model We now consider a model of disk-shaped self-assembled QD taking into account only the heavy-hole HH states in the valence band. The QD potential strongly confines the electron and HH envelope wave functions  e r e  and  h r h  and the exchange interactions in the exciton determine the spin state of exciton. According to the conventional model, the Mn-hole and Mn-electron exchange interactions are pro- portional to r eh - R Mn , where R Mn is the Mn position and r eh is the electron hole coordinate. Then, the spin Hamil- tonian takes the form H ˆ spin = H ˆ Mn-hole exc + H ˆ Mn-e exc + H ˆ e-hole exc , 1 which includes three types of exchange interaction. The an- isotropic exchange interaction between the Mn spin and HH is H ˆ Mn-hole exc =  /3| h R Mn | 2 j ˆ h,z I ˆ Mn,z = A h j ˆ h,z I ˆ Mn,z ; the Mn- PHYSICAL REVIEW B 71, 035338 2005 1098-0121/2005/713/0353385/$23.00 ©2005 The American Physical Society 035338-1