We propose how to form spin qubits in graphene. A crucial requirement to achieve this goal is to find quantum dot states where the usual valley degeneracy in bulk graphene is lifted. We show that this problem can be avoided in quantum dots based on ribbons of graphene with semiconducting armchair boundaries. For such a setup, we find the energies and the exact wave functions of bound states, which are required for localized qubits. Additionally, we show that spin qubits in graphene can not only be coupled between nearest neighbor quantum dots via Heisenberg exchange interaction but also over long distances. This remarkable feature is a direct consequence of the quasi-relativistic spectrum of graphene.Comment: 10 pages, 9 figure

We theoretically study the interaction of a heavy hole with nuclear spins in a quasi-twodimensional III-V semiconductor quantum dot and the resulting dephasing of heavy-hole spin states. It has frequently been stated in the literature that heavy holes have a negligible interaction with nuclear spins. We show that this is not the case. In contrast, the interaction can be rather strong and will be the dominant source of decoherence in some cases. We also show that for unstrained quantum dots the form of the interaction is Ising-like, resulting in unique and interesting decoherence properties, which might provide a crucial advantage to using dot-confined hole spins for quantum information processing, as compared to electron spins.

We investigate heavy-hole spin relaxation and decoherence in quantum dots in perpendicular magnetic fields. We show that at low temperatures the spin decoherence time is two times longer than the spin relaxation time. We find that the spin relaxation time for heavy holes can be comparable to or even longer than that for electrons in strongly two-dimensional quantum dots. We discuss the difference in the magnetic-field dependence of the spin relaxation rate due to Rashba or Dresselhaus spin-orbit coupling for systems with positive (i.e., GaAs quantum dots) or negative (i.e., InAs quantum dots) g-factor.Spin physics has become one of the most rapidly developing branches of condensed matter physics. Spin physics is very important, not only from a fundamental point of view, but also for the fabrication of novel electronic devices, for the experimental realization of quantum computation, and for the development of spin electronics (spintronics) [1]. Quantum dots (QDs) are most attractive candidates for these applications because of their reduced dimensionality, leading to long-lived spin states and allowing single spin manipulation [2].Recent experiments [3,4,5] show that electrons in QDs have a long spin relaxation time (up to 20 ms [5]) and it is now possible to prepare a single electron spin state with a well-defined orientation, read the spin state out, and store the information about the spin orientation for a long time [5]. There are two main spin relaxation mechanisms for electron spins in QDs: that due to the electron-phonon interaction [6,7,8,9] and that due to the hyperfine interaction with surrounding nuclear spins [10,11,12]. Since the valence band has p symmetry, the hyperfine interaction of holes with lattice nuclei is suppressed with respect to that of the conduction band (electrons). This has led to an increased interest in hole spins as carries of long-lived quantum information. It was shown that in thin quantum wells (QWs) the hole spin relaxation is slower than that in the bulk case [13,14]. Nevertheless, the hole spin relaxation time is several orders of magnitude smaller than that for electrons. This is due to the fact that, in addition to existing spin-orbit (SO) couplings for electrons due to bulk inversion asymmetry (BIA) (Dresselhaus spin-orbit (DSO) coupling [15]) and structure inversion asymmetry (SIA) (the Rashba spinorbit (RSO) coupling [16]) there is strong SO coupling between the heavy-hole (HH) and light-hole (LH) subbands [17].Very recently, investigation of hole spin relaxation in QDs was reported [18,19]. In these works only one SO mechanism was considered, the SO coupling between HHs and LHs. It was shown that the hole spin relaxation time in QDs is longer than that in QWs but still shorter by several orders of magnitude than that for electrons in QDs. Furthermore, it was found that SO coupling between HHs and LHs is negligible for two-dimensional (2D) QDs if the energy splitting between the HH and LH subbands is much larger than the level spacing in those subbands [19]. Up to now ...

We study spin relaxation and decoherence in nanotube quantum dots caused by electron-lattice and spin-orbit interaction and predict striking effects induced by magnetic fields B. For particular values of B, destructive interference occurs resulting in ultralong spin relaxation times T1 exceeding tens of seconds. For small phonon frequencies ω, we find a 1/ √ ω spin-phonon noise spectrum -a dissipation channel for spins in quantum dots -which can reduce T1 by many orders of magnitude. We show that nanotubes exhibit zero-field level splitting caused by spin-orbit interaction. This enables an all-electrical and phase-coherent control of spin.