arXiv subjects

Guido Burkard

Publications and source records attributed to Guido Burkard.

At least 163 records · Page 9Linked to original sources

arXiv2015

Optical manipulation of Berry phase in a solid-state spin qubit

The phase relation between quantum states represents an essential resource for the storage and processing of quantum information. While quantum phases are commonly controlled dynamically by tuning energetic interactions, utilizing geometric phases that accumulate during cyclic evolution may offer superior robustness to noise. To date, demonstrations of geometric phase control in solid-state systems rely on microwave fields that have limited spatial resolution. Here, we demonstrate an all-optical method based on stimulated Raman adiabatic passage to accumulate a geometric phase, the Berry phase, in an individual nitrogen-vacancy (NV) center in diamond. Using diffraction-limited laser light, we guide the NV center's spin along loops on the Bloch sphere to enclose arbitrary Berry phase and characterize these trajectories through time-resolved state tomography. We investigate the limits of this control due to loss of adiabiaticity and decoherence, as well as its robustness to noise intentionally introduced into the experimental control parameters, finding its resilience to be independent of the amount of Berry phase enclosed. These techniques set the foundation for optical geometric manipulation in future implementations of photonic networks of solid state qubits linked and controlled by light.

quant-ph↗

arXiv2015

Long-range photon-mediated gate scheme between nuclear spin qubits in diamond

Defect centers in diamond are exceptional solid-state quantum systems that can have exceedingly long electron and nuclear spin coherence times. So far, single-qubit gates for the nitrogen nuclear spin, a two-qubit gate with a nitrogen-vacancy (NV) center electron spin, and entanglement between nearby nitrogen nuclear spins have been demonstrated. Here, we develop a scheme to implement a universal two-qubit gate between two distant nitrogen nuclear spins. Virtual excitation of an NV center that is embedded in an optical cavity can scatter a laser photon into the cavity mode; we show that this process depends on the nuclear spin state of the nitrogen atom. If two NV centers are simultaneously coupled to a common cavity mode and individually excited, virtual cavity photon exchange can mediate an effective interaction between the nuclear spin qubits, conditioned on the spin state of both nuclei, which implements a universal controlled-$\textit{Z}$ gate. We predict operation times below 100 nanoseconds, which is several orders of magnitude faster than the decoherence time of nuclear spin qubits in diamond.

cond-mat.mes-hall↗

arXiv2015

Spintronics with graphene quantum dots

Thanks to its intrinsic ability to preserve spin coherence, graphene is a prime material for spintronics. In this review article, we summarize recent achievements related to spintronics in graphene quantum dots and motivate this field from a spintronics and a materials science point of view. We focus on theory but also discuss recent experiments. The main sources of spin decoherence are interactions with lattice excitations and the hyperfine interaction with present nuclear spins. We explain effective spin-phonon coupling in detail and present a generic power law for the spin relaxation time T1 as a function of the magnetic field. For specific cases, we discuss spin relaxation in detail. The Heisenberg exchange interaction is paramount for coherent spin qubit operation and addressed in the context of magnetism in graphene nano flakes. Nuclear spins in the host and surrounding material can be considered by several means and the influence of 13C nuclei has been studied in detail. Impressive advances in general spintronics and the fabrication of graphene devices are likely to spark significant advances in spintronics with graphene quantum dots in the near future.

cond-mat.mes-hall↗

arXiv2015

k.p theory for two-dimensional transition metal dichalcogenide semiconductors

We present $\mathbf{k}\cdotp\mathbf{p}$ Hamiltonians parametrised by {\it ab initio} density functional theory calculations to describe the dispersion of the valence and conduction bands at their extrema (the $K$, $Q$, $Γ$, and $M$ points of the hexagonal Brillouin zone) in atomic crystals of semiconducting monolayer transition metal dichalcogenides. We review the parametrisation of the essential parts of the $\mathbf{k}\cdotp\mathbf{p}$ Hamiltonians for MoS$_2$, MoSe$_2$, WS$_2$, and WSe$_2$, including the spin-splitting and spin-polarisation of the bands, and we discuss the vibrational properties of these materials. We then use $\mathbf{k}\cdotp\mathbf{p}$ theory to analyse optical transitions in two-dimensional transition metal dichalcogenides over a broad spectral range that covers the Van Hove singularities in the band structure (the $M$ points). We also discuss the visualisation of scanning tunnelling microscopy maps.

cond-mat.mes-hall↗

arXiv2015

Tuning antiferromagnetism of vacancies with magnetic fields in graphene nanoflakes

Graphene nanoflakes are interesting because electrons are naturally confined in these quasi-zero-dimensional structures, whereas confinement in bulk graphene would require a band gap. Vacancies inside the graphene lattice lead to localized states and the spins of such localized states may be used for spintronics. We perform a tight-binding description of a nanoflake with two vacancies and include a perpendicular magnetic field via a Peierls phase. The tunnel coupling strength and from it the exchange coupling between the localized states can be obtained from the energy splitting between numerically calculated bonding and antibonding energy levels. This allows us to estimate the exchange coupling J, which governs the dynamics of coupled spins. We predict the possibility of switching in situ from J>0 to J=0 by tuning the magnetic field. In the former case, the ground state will be antiferromagnetic with Néel temperatures accessible by experiment.

cond-mat.mes-hall↗

arXiv2015

The asymmetric resonant exchange qubit under the influence of electrical noise

We investigate the influence of electrical charge noise on a resonant exchange (RX) qubit in a triple quantum dot. This RX qubit is a variation of the exchange-only spin qubit which responds to a narrow-band resonant frequency. Our noise model includes uncorrelated charge noise in each quantum dot giving rise to two independent (noisy) bias parameters $\varepsilon$ and $Δ$. We calculate the energy splitting of the two qubit states as a function of these two bias detuning parameters to find "sweet spots", where the qubit is least susceptible to noise. Our investigation shows that such sweet spots exist within the low bias regime, in which the bias detuning parameters have the same magnitude as the hopping parameters. The location of the sweet spots in the $(\varepsilon,Δ)$ plane depends on the hopping strength and asymmetry between the quantum dots. In the regime of weak charge noise, we identify a new favorable operating regime for the RX qubit based on these sweet spots.

cond-mat.mes-hall↗

arXiv2014

Hybrid spin and valley quantum computing with singlet-triplet qubits

The valley degree of freedom in the electronic band structure of silicon, graphene, and other materials is often considered to be an obstacle for quantum computing (QC) based on electron spins in quantum dots. Here we show that control over the valley state opens new possibilities for quantum information processing. Combining qubits encoded in the singlet-triplet subspace of spin and valley states allows for universal QC using a universal two-qubit gate directly provided by the exchange interaction. We show how spin and valley qubits can be separated in order to allow for single-qubit rotations.

cond-mat.mes-hall↗

arXiv2014

Interference of stimulated electronic Raman scattering and linear absorption in coherent control

We consider quantum interference effects in carrier and photocurrent excitation in graphene using coherent electromagnetic field components at frequencies $ω$ and $2ω$. The response of the material at the fundamental frequency $ω$ is presented, and it is shown that one-photon absorption at $ω$ interferes with stimulated electronic Raman scattering (combined $2ω$ absorption and $ω$ emission) to result in a net contribution to the current injection. This interference occurs with a net energy absorption of $\hbarω$ and exists in addition to the previously studied interference occurring with a net energy absorption of $2\hbarω$ under the same irradiation conditions. Due to the absence of a bandgap and the possibility to block photon absorption by tuning the Fermi level, graphene is the perfect material to study this contribution. We calculate the polarization dependence of this all-optical effect for intrinsic graphene and show that the combined response of the material at both $ω$ and $2ω$ leads to an anisotropic photocurrent injection, whereas the magnitude of the injection current in doped graphene, when transitions at $ω$ are Pauli blocked, is isotropic. By considering the contribution to coherent current control from stimulated electronic Raman scattering, we find that graphene offers tunable, polarization sensitive applications. Coherent control due to the interference of stimulated electronic Raman scattering and linear absorption is relevant not only for graphene but also for narrow-gap semiconductors, topological insulators, and metals.

cond-mat.mes-hall↗

arXiv2014

Interplay of spin-orbit and hyperfine interactions in dynamical nuclear polarization in semiconductor quantum dots

We theoretically study the interplay of spin-orbit and hyperfine interactions in dynamical nuclear polarization in two-electron semiconductor double quantum dots near the singlet $(S)$ - triplet $(T_+)$ anticrossing. The goal of the scheme under study is to extend the singlet $(S)$ - triplet $(T_0)$ qubit decoherence time $T_2^{*}$ by dynamically transferring the polarization from the electron spins to the nuclear spins. This polarization transfer is achieved by cycling the electron spins over the $S-T_+$ anticrossing. Here, we investigate, both quantitatively and qualitatively, how this hyperfine mediated dynamical polarization transfer is influenced by the Rashba and Dresselhaus spin-orbit interaction. In addition to $T_2^*$, we determine the singlet return probability $P_s$, a quantity that can be measured in experiments. Our results suggest that the spin-orbit interaction establishes a mechanism that can polarize the nuclear spins in the opposite direction compared to hyperfine mediated nuclear spin polarization. In materials with relatively strong spin-orbit coupling, this interplay of spin-orbit and hyperfine mediated nuclear spin polarizations prevents any notable increase of the $S-T_0$ qubit decoherence time $T_2^{*}$.

cond-mat.mes-hall↗

arXiv2014

Photoinduced pure spin current injection in graphene with Rashba spin-orbit interaction

We propose a photoexcitation scheme for pure spin current generation in graphene subject to a Rashba spinorbit coupling. Although excitation using circularly-polarized light does not result in optical orientation of spins in graphene unless an additional magnetic field is present, we show that excitation with linearly-polarized light at normal incidence yields spin current injection without magnetic field. Spins are polarized within the graphene plane and are displaced in opposite directions, with no net charge displacement. The direction of the spin current is determined by the linear polarization axis of the light, and the injection rate is proportional to the intensity. The technique is tunable via an applied bias voltage and is accessible over a wide frequency range. We predict a spin current polarization as high as 75% for photon frequencies comparable to the Rashba frequency. Spin current injection via optical methods removes the need for ferromagnetic contacts, which have been identified as a possible source of spin scattering in electrical spin injection in graphene.

cond-mat.mes-hall↗

arXiv2014

Quantum limit for nuclear spin polarization in semiconductor quantum dots

A recent experiment [E. A. Chekhovich et al., Phys. Rev. Lett. 104, 066804 (2010)] has demonstrated that high nuclear spin polarization can be achieved in self-assembled quantum dots by exploiting an optically forbidden transition between a heavy hole and a trion state. However, a fully polarized state is not achieved as expected from a classical rate equation. Here, we theoretically investigate this problem with the help of a quantum master equation and we demonstrate that a fully polarized state cannot be achieved due to formation of a nuclear dark state. Moreover, we show that the maximal degree of polarization depends on structural properties of the quantum dot.

cond-mat.mes-hall↗

arXiv2014

Influence of Nuclear Quadrupole Moments on Electron Spin Coherence in Semiconductor Quantum Dots

We theoretically investigate the influence of the fluctuating Overhauser field on the spin of an electron confined to a quantum dot (QD). The fluctuations arise from nuclear angular momentum being exchanged between different nuclei via the nuclear magnetic dipole coupling. We focus on the role of the nuclear electric quadrupole moments (QPMs), which generally cause a reduction in internuclear spin transfer efficiency in the presence of electric field gradients. The effects on the electron spin coherence time are studied by modeling an electron spin echo experiment. We find that the QPMs cause an increase in the electron spin coherence time and that an inhomogeneous distribution of the quadrupolar shift, where different nuclei have different shifts in energy, causes an even larger increase in the electron coherence time than a homogeneous distribution. Furthermore, a partial polarization of the nuclear spin ensemble amplifies the effect of the inhomogeneous quadrupolar shifts, causing an additional increase in electron coherence time, and provides an alternative to the experimentally challenging suggestion of full dynamic nuclear spin polarization.

cond-mat.mes-hall↗

arXiv2014

Mechanically induced spin resonance in a carbon nanotube

The electron spin is a promising qubit candidate for quantum computation and quantum information. Here we propose and analyze a mechanically-induced single electron spin resonance, which amounts to a rotation of the spin about the $x$-axis in a suspended carbon nanotube. The effect is based on the coupling between the spin and the mechanical degree of freedom due to the intrinsic curvature-induced spin-orbit coupling. A rotation about the $z$-axis is obtained by the off-resonant external electric driving field. Arbitrary-angle rotations of the single electron spin about any axis in the $x$-$z$ plane can be obtained with a single operation by varying the frequency and the strength of the external electric driving field. With multiple steps combining the rotations about the $x$- and $z$-axes, arbitrary-angle rotations about arbitrary axes can be constructed, which implies that any single-qubit gate of the electron spin qubit can be performed. We simulate the system numerically using a master equation with realistic parameters.

cond-mat.mes-hall↗

arXiv2014

Spin-orbit coupling, quantum dots, and qubits in monolayer transition metal dichalcogenides

We derive an effective Hamiltonian which describes the dynamics of electrons in the conduction band of transition metal dichalcogenides (TMDC) in the presence of perpendicular electric and magnetic fields. We discuss in detail both the intrinsic and the Bychkov-Rashba spin-orbit coupling (SOC) induced by an external electric field. We point out interesting differences in the spin-split conduction band between different TMDC compounds. An important consequence of the strong intrinsic SOC is an effective out-of-plane $g$-factor for the electrons which differs from the free-electron g-factor $g\simeq 2$. We identify a new term in the Hamiltonian of the Bychkov-Rashba SOC which does not exist in III-V semiconductors. Using first-principles calculations, we give estimates of the various parameters appearing in the theory. Finally, we consider quantum dots (QDs) formed in TMDC materials and derive an effective Hamiltonian which allows us to calculate the magnetic field dependence of the bound states in the QDs. We find that all states are both valley and spin split, which suggests that these QDs could be used as valley-spin filters. We explore the possibility of using spin and valley states in TMDCs as quantum bits, and conclude that, due to the relatively strong intrinsic spin-orbit splitting in the conduction band, the most realistic option appears to be a combined spin-valley (Kramers) qubit at low magnetic fields.

cond-mat.mes-hall↗

arXiv2014

Entanglement Purification with the Exchange Interaction

Entanglement purification allows the creation of qubit pairs of arbitrarily high fidelity with respect to a maximally entangled state, starting from a larger number of low-fidelity pairs. Purification requires quantum memory, a role for which electron spins are well suited. However, using existing recurrence protocols involving symmetric local two-qubit operations for spin qubits turns out to be rather unpractical. We present an efficient purification protocol requiring only a single pulsed Heisenberg- or XY-type exchange interaction between two qubit pairs. In contrast to known protocols, we allow for asymmetric bilateral two-qubit operations where the two communication parties operate differently on their qubits. In the optimal version of our protocol, the local two-qubit interactions in the case of Heisenberg exchange correspond to the $\sqrt{\mathrm{SWAP}}$ gate and its inverse.

quant-ph↗

arXiv2013

Fidelity of optically induced single-spin rotations in semiconductor quantum dots in the presence of nuclear spins

We examine the influence of nuclear spins on the performance of optically induced rotations of single electron spins in semiconductor quantum dots. We consider Raman type optical transitions between electron spin states and take into account the additional effect of the Overhauser field. We calculate average fidelities of rotations around characteristic axes in the presence of nuclear spins analytically with perturbation theory up to second order in the Overhauser field. Moreover, we calculate the fidelity using numerical averaging over the nuclear field distribution, including arbitrary orders of the hyperfine interaction.

cond-mat.mes-hall↗

arXiv2013

Non-monotonic spin relaxation and decoherence in graphene quantum dots with spin-orbit interactions

We investigate the spin relaxation and decoherence in a single-electron graphene quantum dot with Rashba and intrinsic spin-orbit interactions. We derive an effective spin-phonon Hamiltonian via the Schrieffer-Wolff transformation in order to calculate the spin relaxation time T_1 and decoherence time T_2 within the framework of the Bloch-Redfield theory. In this model, the emergence of a non-monotonic dependence of T_1 on the external magnetic field is attributed to the Rashba spin-orbit coupling-induced anticrossing of opposite spin states. A rapid decrease of T_1 occurs when the spin and orbital relaxation rates become comparable in the vicinity of the spin-mixing energy-level anticrossing. By contrast, the intrinsic spin-orbit interaction leads to a monotonic magnetic field dependence of the spin relaxation rate which is caused solely by the direct spin-phonon coupling mechanism. Within our model, we demonstrate that the decoherence time T_2 ~ 2 T_1 is dominated by relaxation processes for the electron-phonon coupling mechanisms in graphene up to leading order in the spin-orbit interaction. Moreover, we show that the energy anticrossing also leads to a vanishing pure spin dephasing rate for these states for a super-Ohmic bath.

cond-mat.mes-hall↗

arXiv2013

Interplay of charge and spin coherence in Landau-Zener-Stückelberg-Majorana interferometry

We study Landau-Zener dynamics in a double quantum dot filled with two electrons, where the spin states can become correlated with charge states and the level velocity can be tuned in a time-dependent fashion. We show that a correct interpretation of experimental data is only possible when finite-time effects are taken into account. In addition, our formalism allows the study of partial adiabatic dynamics in the presence of phonon-mediated hyperfine relaxation and charge-noise-induced dephasing. Our findings demonstrate that charge noise severely impacts the visibility of Landau-Zener-Stückelberg-Majorana interference fringes. This indicates that charge coherence must be treated on an equal footing with spin coherence.

cond-mat.mes-hall↗