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.

We report the measurement of extremely slow hole spin relaxation dynamics in small ensembles of self-assembled InGaAs quantum dots. Individual spin orientated holes are optically created in the lowest orbital state of each dot and read out after a defined storage time using spin memory devices. The resulting luminescence signal exhibits a pronounced polarization memory effect that vanishes for long storage times. The hole spin relaxation dynamics are measured as a function of external magnetic field and lattice temperature. We show that hole spin relaxation can occur over remarkably long timescales in strongly confined quantum dots (up to ∼270 µs), as predicted by recent theory. Our findings are supported by calculations that reproduce both the observed magnetic field and temperature dependencies. The results suggest that hole spin relaxation in strongly confined quantum dots is due to spin orbit mediated phonon scattering between Zeeman levels, in marked contrast to higher dimensional nanostructures where it is limited by valence band mixing.

We propose and analyze a new method for manipulation of a heavy hole spin in a quantum dot. Due to spin-orbit coupling between states with different orbital momenta and opposite spin orientations, an applied rf electric field induces transitions between spin-up and spin-down states. This scheme can be used for detection of heavy-hole spin resonance signals, for the control of the spin dynamics in two-dimensional systems, and for determining important parameters of heavy-holes such as the effective g-factor, mass, spin-orbit coupling constants, spin relaxation and decoherence times.PACS numbers: 67.57. Lm,76.60.Es,73.21.La Spintronics, or spin-based electronics, is one of the fascinating and rapidly growing areas in solid state physics and modern technology [1]. Exploiting both the charge and spin degrees of freedom of carriers, it offers wide opportunities for developing devices with unique functionalities. Operation of single charge or spin is the ultimate limit for such devices. Quantum dots (QD) have proven useful for this goal [2], since experiments on readout and coherent manipulation of a single spin in QDs have already been performed successfully [3,4,5,6]. Furthermore, due to supression of the spin-orbit interaction (SOI) in QDs [7,8,9], the electron spin in a QD has long relaxation times T 1 (up to hundreds of milliseconds) [3,4,10]. The spin thus is an attractive candidate as carrier of quantum information [2], if its state stays coherent over sufficiently long times (described by the spin decoherence time T 2 ). At low magnetic fields, in III-V semiconductor QDs there is a rather strong hyperfine interaction between an electron spin and surrounding nuclear spins leading to a significant degradation of the spin coherence [11,12,13]. Very recently, based on the idea of state narrowing via projective measurements [13], several proposals have been made to limit the hyperfine-induced decoherence [14,15], and, therefore, to increase T 2 times for electron spins in QDs, which currently range up to µs [5,6].Electron spin resonance (ESR) provides a powerful tool for coherent manipulation of spins, which is of great importance for spintronics [16]. By applying short resonant microwave pulses, an arbitrary superposition of spin-up and spin-down states is created. Rabi oscillations and spin-echo experiments are based on this approach [5]. In such experiments, the ESR signal can be detected directly by measuring the absorption of radio-frequency (rf) power [16] via charge transport through a QD [6,17] or by using optical detection of magnetic resonance techniques [18,19]. Usually, ESR methods involve magnetic dipole transitions induced by an oscillating magnetic field. However, there has been a strong revival of interest in electric dipole spin resonance (EDSR) controlled by alternating electric fields [20,21,22,23,24,25] that provides the ability to manipulate and detect electron spins at the nanometer scale and may be useful for distinguishing different SOI mechanisms.Recently, the idea to use heavy-hole (HH)...

The spin-orbit splitting of the electron levels in a two-dimensional quantum dot in a perpendicular magnetic field is studied. It is shown that at the point of an accidental degeneracy of the two lowest levels above the ground state the Rashba spin-orbit coupling leads to a level anticrossing and to mixing of spin-up and spin-down states, whereas there is no mixing of these levels due to the Dresselhaus term. We calculate the relaxation and decoherence times of the three lowest levels due to phonons. We find that the spin relaxation rate as a function of a magnetic field exhibits a cusplike structure for Rashba but not for Dresselhaus spin-orbit interaction.

The effect of the surface curvature on the magnetic moment and persistent currents in twodimensional (2D) quantum rings and dots is investigated. It is shown that the surface curvature decreases the spacing between neighboring maxima of de Haas -van Alphen (dHvA) type oscillations of the magnetic moment of a ring and decreases the amplitude and period of AharonovBohm (AB) type oscillations. In the case of a quantum dot, the surface curvature reduces the level degeneracy at zero magnetic fields. This leads to a suppression of the magnetic moment at low magnetic fields. The relation between the persistent current and the magnetic moment is studied. We show that the surface curvature decreases the amplitude and the period of persistent current oscillations.