Optical RKKY Interaction between Charged Semiconductor Quantum Dots

Abstract

We show how a spin interaction between electrons localized in neighboring quantum dots can be induced and controlled optically. The coupling is generated via virtual excitation of delocalized excitons and provides an efficient coherent control of the spins. This quantum manipulation can be realized in the adiabatic limit and is robust against decoherence by spontaneous emission. Applications to the realization of quantum gates, scalable quantum computers, and to the control of magnetization in an array of charged dots are proposed.Quantum control of an electron spin, either independent of other spins or condition on their states, in a semiconductor nanostructure is a central issue in the emerging fields of spintronics and quantum information processing. The spin of a single electron confined in a semiconductor quantum dot (QD) was proposed [1] as a qubit for the realization of scalable quantum computers. Quantum gates are designed using electric gates to control via overlap the exchange interaction between two electrons in neighboring dots. Optical control was also proposed, in which a cavity mode couples different dots [2], or a dipole-dipole interaction between charged excitons strongly polarized by an external dc field is exploited [3]. Optical control possesses several advantages compared with control by gate voltage. Ultrafast lasers can control quantum systems on the femtosecond time scale, and using shaping techniques the amplitude and phase of the pulses can be designed at will offering a great deal of flexibility and efficiency [4].In this paper we report a theory of an exchange interaction between two electron spins in separate dots in a typical semiconductor QD system by virtual excitation of delocalized exciton states in the host material which interact with the electrons in both dots. This time-dependent effective interaction is driven by the external laser field, and is, thus, controllable. The virtual excitation by an off-resonant laser preserve the coherence of the spin dynamics. This indirect exchange mechanism is analogous to a RKKY interaction [5] between two magnetic impurities mediated by conduction electron or excitons[6], except that the intermediate electron-hole pair is produced by the external light. The optical quantum control of a single exciton in a semiconductor QD has been recently reported in GaAs QDs generated by monolayer fluctuations [7] and InGaAs self-assembled QDs [8,9]. The short radiative recombination lifetime of the exciton (of the order of 100 ps) gives a severe limitation for the application to quantum computation, even with the help of shaping techniques [4]. This can be avoided by doping QDs each with a single conduction electron and by encoding the quantum information in the spin degrees of freedom. Optical control by virtual excitation avoids the fast optical decoherence. Thus, the advantages of a very long spin coherence time in QDs [10] and fast optical control can be combined.Consider two electrons localized in two QDs at R ℓ (ℓ = 1, 2) with wavefunctions φ ℓ (r...

Many-Body Interaction Evidenced through Exciton-Trion-Electron Correlated Dynamics

“…The optical spectrum of moderately doped quantum wells also features a trion (charged exciton) resonance, which is situated only a few meV below the exciton line [2,3]. The way that many-body interactions are modified by the presence of an additional electron gas is clearly a topic of major interest for applications such as transport of light by a charged exciton [4] and for quantum-information science [5][6][7]. Another interest of the trion system is that it constitutes the first step in the transition of the optical susceptibility of semiconductor quantum structures from the undoped case to the case of an arbitrary doping level giving the Fermi edge singularity [8,9].…”

Switching the sign of photon-induced exchange interactions in semiconductor microcavities with finite quality factors

“…The value of the light-matter interaction g depends on the amplitude of the cavity mode in the dot and plays the same role than the Rabi energy Ω in the case of a photoexcited semiconductor 4,7 . We take g = 5 meV which is within the range of Rabi splittings reported in the literature for CdTe nanopillars (larger values have been reported experimentally 16 ).…”

“…This assumption leads to the standard spin selective coupling 4,7,23 that associates photon polarization and fermion spin degrees of freedom. This kind of coupling breaks spin rotational invariance and privileges the axis of the cavity,ẑ.…”

“…For frequencies below the dot gap, an optical coherence between valence and conduction bands is induced, capable of mediating exchange interaction between localized spins 4,7 . This is commonly known as an Optical RKKY (ORKKY) exchange interaction.…”

“…In contrast, there is a number of proposals to control Indirect Exchange Interactions (IEI) of spins sitting several nanometers away, that take advantage of the optical and electrical manipulation of the intermediate fermions afforded in semiconducting hosts 1,2,3,4,5,6,7 . Local spins could be provided by nuclei, by electrons bound to donors 4,5 , or d electrons of magnetic impurities 7 . A variety of phenomena, like the reversible modification of the Curie temperature and coercive fields in (III,Mn)V 8 and (II,Mn,N)VI semiconductors, the induction magnetic order in otherwise paramagnetic (II,Mn)VI semiconductor quantum dots 9 and the entanglement of donor spins in (II,Mn)VI quantum wells 5 have been observed experimentally thanks to the artificial manipulation of IEI.…”