Control of spin blockade by ac magnetic fields in triple quantum dots

Abstract

We analyze coherent spin phenomena in triple quantum dots in triangular configuration under crossed dc and ac magnetic fields. In particular, we discuss the interplay between Aharonov-Bohm current oscillations, coherent electron trapping and spin blockade under two-electron spin resonance configurations. We demonstrate an unexpected antiresonant behaviour in the current, allowing for both removal and restoration of maximally entangled spin blockaded states by tuning the ac field frequency. Our theoretical predictions indicate how to manipulate spin qubits in a triangular quantum dot array. PACS numbers:Electronic transport through mesoscopic systems can become correlated not only by charge interaction but also by the spin degree of freedom. A dramatic combination of both can be found in systems where strong Coulomb interaction limits the population to a small number of electrons (Coulomb blockade) and where Pauli exclusion principle avoids certain internal transitions -spin blockade (SB). This was first observed as a rectification effect in the current through a double quantum dot (DQD) 1 . Recent experiments have taken advantage of SB to achieve qubit operations in a double dot by electric gate control 2 or by electron spin resonance (ESR) 3 . It consists in inducing transitions between the electron's spinup and spin-down states, which are splitted by the Zeeman energy coming from a dc magnetic field, B dc . Different mechanisms have been considered: crossed dc and ac magnetic fields (B ac ), where the ac frequency is resonant with the Zeeman splitting 4,5 , effective B ac induced by ac electric fields in the presence of spin-orbit interaction 6 , slanting Zeeman fields 7 or hyperfine interaction 8 .Lately, a next step towards quantum dot arrays has been reached: tunnel spectroscopy measurements with triple quantum dots (TQD), both in series 9 and in triangular configurations 10 have been achieved. Theoretical works in these systems 11,12 analyze their eigenstates and stability diagram, as well as the effect of a magnetic field penetrating the structure. TQDs with strongly correlated electrons have also been investigated in the Kondo regime 13 and have been proposed as spin entanglers 14 . Additionally, these systems show a more peculiar property which is intrinsic to three-level systems, namely coherent population trapping, which is a well-known effect in quantum optics and which was observed in threelevel atoms excited by two resonant laser fields 15 . There, the electronic wave function evolves towards an eigenstate superposition, a so-called dark state, which is decoupled from the laser fields and therefore it manifests as an antiresonance in the emission spectrum. An analogy in transport has been made when coherent superpositions avoid transport by interference between tunneling events. These dark states can be achieved by driv-FIG. 1: (Color online) Coherent processes in a triangular TQD, with one electron confined in the dot connected to the drain. ∆1 = ∆2 = ∆3, where ∆i is the Zeeman splitting in dot i. ...

Functional renormalization group study of parallel double quantum dots: Effects of asymmetric dot-lead couplings

“…The same value of the conductance is obtained close to half filling due to maximum of g ↑ at V g = V (1) g . On the other hand, the parameter q ↑ diverges when V g → V (2) g (red filled triangle) because of the crossing of the odd energy level o,σ the Fermi level of the lead, which is put to zero.…”

“…Nanostructures based on quantum dots attract growing interest due to an opportunity of tuning of their transport and magnetic properties [1][2][3][4][5][6][7], which makes them promising candidates for quantum spintronics and quantum information processing applications [8][9][10][11][12][13][14][15]. At appropriate conditions, these systems consist of discrete energy levels of quantum dots, which are hybridized with the leads, having continuous energy bands.…”

Photon assisted long-range tunneling

“…In transport through quantum dots, longrange coupling has found interest for the prediction of the interference of spin superexchange trajectories 34 , the generation of entangled currents 35 , or the influence of dephasing 36 . It is also involved in the occurrence of dark states [37][38][39][40][41] .…”

Dark Bell states in tunnel-coupled spin qubits

“…In quantum dot arrays, dark superpositions are essential to proposals of coherent-state transfer 4 or current switching by coherent population trapping. 5 However, their detection in transport has been elusive for requiring exquisite control of complicated multidot arrangements 6,7 or many level configurations. 8 Here we introduce a dark state based on collective spin dynamics rather than single-electron interference.…”