Single-shot readout of electron spins in a semiconductor quantum dot

We report on a method for single-shot readout of spin states in a semiconductor quantum dot that is robust against charge noise and can be used even when the electron temperature exceeds the energy splitting between the states. The spin states are first correlated to different charge states using a spin dependence of the tunnel rates. A subsequent fast measurement of the charge on the dot then reveals the original spin state. The method is analyzed theoretically, and compared to a previously used method. We experimentally demonstrate the method by performing readout of the two-electron spin states, achieving a single-shot visibility of more than 80%. We find very long triplet-to-singlet relaxation times (up to several milliseconds), with a strong dependence on in-plane magnetic field.     

Nonequilibrium transport through a vertical quantum dot in the absence of spin-flip energy relaxation

We investigate nonequilibrium transport in the absence of spin-flip energy relaxation in a few-electron quantum dot artificial atom. Novel nonequilibrium tunneling processes involving high-spin states, which cannot be excited from the ground state because of spin blockade, and other processes involving more than two charge states are observed. These processes cannot be explained by orthodox Coulomb blockade theory. The absence of effective spin relaxation induces considerable fluctuation of the spin, charge, and total energy of the quantum dot. Although these features are revealed clearly by pulse excitation measurements, they are also observed in conventional dc current characteristics of quantum dots.     

Zeeman energy and spin relaxation in a one-electron quantum dot

We have measured the relaxation time, T1, of the spin of a single electron confined in a semiconductor quantum dot (a proposed quantum bit). In a magnetic field, applied parallel to the two-dimensional electron gas in which the quantum dot is defined, Zeeman splitting of the orbital states is directly observed by measurements of electron transport through the dot. By applying short voltage pulses, we can populate the excited spin state with one electron and monitor relaxation of the spin. We find a lower bound on T1 of 50 micros at 7.5 T, only limited by our signal-to-noise ratio. A continuous measurement of the charge on the dot has no observable effect on the spin relaxation.     

Single-shot measurement of triplet-singlet relaxation in a Si/SiGe double quantum dot

We investigate the lifetime of two-electron spin states in a few-electron Si/SiGe double dot. At the transition between the (1,1) and (0,2) charge occupations, Pauli spin blockade provides a readout mechanism for the spin state. We use the statistics of repeated single-shot measurements to extract the lifetimes of multiple states simultaneously. When the magnetic field is zero, we find that all three triplet states have equal lifetimes, as expected, and this time is ~10 ms. When the field is nonzero, the T(0) lifetime is unchanged, whereas the T- lifetime increases monotonically with the field, reaching 3 sec at 1 T.     

Spin Pumping from a Quantum Dot in the Presence of Decoherence

We study the pumped spin current of an interacting quantum dot tunnel coupled to a single lead in the presence of electron spin resonance （ESR） field. The spin decoherence in the dot is included by the Bffttiker approach. Using the nonequilibrium Green＇s function technique, we show that ESR-induced spin flip can generate finite spin current with no charge transport. Both the Coulomb interaction and spin decoherence decrease the amplitude of spin current. The dependence of pumped spin current on the intensity and frequency of ESR field, and the spin decoherence is discussed.     

Electrical control of spin relaxation in a quantum dot

We demonstrate electrical control of the spin relaxation time T1 between Zeeman-split spin states of a single electron in a lateral quantum dot. We find that relaxation is mediated by the spin-orbit interaction, and by manipulating the orbital states of the dot using gate voltages we vary the relaxation rate W identical withT1(-1) by over an order of magnitude. The dependence of W on orbital confinement agrees with theoretical predictions, and from these data we extract the spin-orbit length. We also measure the dependence of W on the magnetic field and demonstrate that spin-orbit mediated coupling to phonons is the dominant relaxation mechanism down to 1 T, where T1 exceeds 1 s.     

Switching spin and charge between edge states in topological insulator constrictions

We show how the coupling between opposite edge states, which overlap in a constriction made of the topological insulator mercury telluride (HgTe), can be employed both for steering the charge flow into different edge modes and for controlled spin switching. Unlike in a conventional spin transistor, the switching does not rely on a tunable Rashba spin-orbit interaction, but on the energy dependence of the edge state wave functions. Based on this mechanism, and supported by extensive numerical transport calculations, we present two different ways to control spin and charge currents, depending on the local gating of the constriction, resulting in a high fidelity spin transistor.     

Measuring the complex admittance of a carbon nanotube double quantum dot

We investigate radio-frequency (rf) reflectometry in a tunable carbon nanotube double quantum dot coupled to a resonant circuit. By measuring the in-phase and quadrature components of the reflected rf signal, we are able to determine the complex admittance of the double quantum dot as a function of the energies of the single-electron states. The measurements are found to be in good agreement with a theoretical model of the device in the incoherent limit. In addition to being of fundamental interest, our results present an important step forward towards noninvasive charge and spin state readout in carbon nanotube quantum dots.     

Spin fractionalization of an even number of electrons in a quantum Dot

We find that Kondo resonant conductance can occur in a quantum dot in the Coulomb blockade regime with an even number of electrons N. The contacts are attached to the dot in a pillar configuration, and a magnetic field B( perpendicular) along the axis is applied. B( perpendicular) lifts the spin degeneracy of the dot energies. Usually, this prevents the system from developing the Kondo effect. Tuning B( perpendicular) to the value B(*) where levels with different total spin cross restores both the degeneracy and the Kondo effect. We analyze a dot charged with N = 2 electrons. Coupling to the contacts is antiferromagnetic due to a spin selection rule and, in the Kondo state, the charge is unchanged while the total spin on the dot is S = 1/2.     

Microwave control of transport through a chaotic mesoscopic dot

We establish analogy between a microwave ionization of Rydberg atoms and a charge transport through a chaotic quantum dot induced by a monochromatic field in a regime with a potential barrier between dot contacts. We show that the quantum coherence leads to dynamical localization of electron excitation in energy so that only a finite number of photons is absorbed inside the dot. The theory developed determines the dependence of localization length on dot and microwave parameters showing that the microwave power can switch the dot between metallic and insulating regimes. ultiphoton ionization and excitation to highly excited states (e.g., Rydberg states)     

Optical pumping and a nondestructive readout of a single magnetic impurity spin in an InAs/GaAs quantum dot

We report on the resonant optical pumping of the | ± 1? spin states of a single Mn dopant in an InAs/GaAs quantum dot which is embedded in a charge tunable device. The experiment relies on a W scheme of transitions reached when a suitable longitudinal magnetic field is applied. The optical pumping is achieved via the resonant excitation of the central Λ system at the neutral exciton X(0) energy. For a specific gate voltage, the redshifted photoluminescence of the charged exciton X- is observed, which allows a nondestructive readout of the spin polarization. An arbitrary spin preparation in the | + 1? or |-1? state characterized by a polarization near or above 50% is evidenced.     

Stimulated and spontaneous optical generation of electron spin coherence in charged GaAs quantum dots

We report on the coherent optical excitation of electron spin polarization in the ground state of charged GaAs quantum dots via an intermediate charged exciton (trion) state. Coherent optical fields are used for the creation and detection of the Raman spin coherence between the spin ground states of the charged quantum dot. The measured spin decoherence time, which is likely limited by the nature of the spin ensemble, approaches 10 ns at zero field. We also show that the Raman spin coherence in the quantum beats is caused not only by the usual stimulated Raman interaction but also by simultaneous spontaneous radiative decay of either excited trion state to a coherent combination of the two spin states.     

Transport through a quantum dot subject to spin and charge bias

Spin and charge transport through a quantum dot coupled to external nonmagnetic leads is analyzed theoretically in terms of the non-equilibrium Green function formalism based on the equation of motion method. The dot is assumed to be subject to spin and charge bias, and the considerations are focused on the Kondo effect in spin and charge transport. It is shown that the differential spin conductance as a function of spin bias reveals a typical zero-bias Kondo anomaly which becomes split when either magnetic field or charge bias are applied. Significantly different behavior is found for mixed charge/spin conductance. The influence of electron-phonon coupling in the dot on tunneling current as well as on both spin and charge conductance is also analyzed.     

Adiabatic charge and spin pumping through interacting quantum dots

In this paper we investigate adiabatic charge and spin pumping through interacting quantum dots using non-equilibrium Green's function techniques and the equation-of-motion method. We treat the electronic correlations inside the dot using a Hartree-Fock approximation and succeed in obtaining closed analytic expressions for the Keldysh Green's functions. These allow us to compute charge and spin currents through the quantum dot. Depending on the parameters of the quantum dot and its coupling to the reservoirs, we show that it can be found in two different regimes: the magnetic regime and the non-magnetic regime. In the magnetic regime we find a non-vanishing spin current in addition to the charge current present in both cases.     

Entanglement switching via the Kondo effect in triple quantum dots

We consider a triple quantum dot system in a triangular geometry with one of the dots connected to metallic leads. Using Wilson’s numerical renormalization group method, we investigate quantum entanglement and its relation to the thermodynamic and transport properties in the regime where each of the dots is singly occupied on average, but with non-negligible charge fluctuations. It is shown that even in the regime of significant charge fluctuations the formation of the Kondo singlets induces switching between separable and perfectly entangled states. The quantum phase transition between unentangled and entangled states is analyzed quantitatively and the corresponding phase diagram is explained by exactly solvable spin model. In the framework of an effective model we also explain smearing of the entanglement transition for cases when the symmetry of the triple quantum dot system is relaxed.     

Transport through artificial single-molecule magnets: Spin-pair state sequential tunneling and Kondo effects

The transport properties of an artificial single-molecule magnet based on a CdTe quantum dot doped with a single Mn+2 ion(S=5/2) are investigated by the non-equilibrium Green function method.We consider a minimal model where the Mn-hole exchange coupling is strongly anisotropic so that spin-flip is suppressed and the impurity spin S and a hole spin s entering the quantum dot are coupled into spin pair states with(2S+1) sublevels.In the sequential tunneling regime,the differential conductance exhibits(2S+1) possible peaks,corresponding to resonance tunneling via(2S+1) sublevels.At low temperature,Kondo physics dominates transport and(2S+1) Kondo peaks occur in the local density of states and conductance.These peaks originate from the spin-singlet state formed by the holes in the leads and on the dot via higher-order processes and are related to the parallel and antiparallel spin pair states.     

Mesoscopic resonating valence bond system on a triple dot

We theoretically introduce a mesoscopic pendulum from a triple dot. The pendulum is fastened through a singly occupied dot (spin qubit). Two other strongly capacitively coupled islands form a double-dot charge qubit with one electron in excess oscillating between the two low-energy charge states (1,0) and (0,1). The triple dot is placed between two superconducting leads. Under realistic conditions, the main proximity effect stems from the injection of resonating singlet (valence) bonds on the triple dot. This gives rise to a Josephson current that is charge- and spin-dependent and, as a consequence, exhibits a distinct resonance as a function of the superconducting phase difference.     

Influence of the image charge effect on the hydrogen-like impurity-bound polaron in a spherical quantum dot in the presence of an electric field

We have investigated the influence of an external electric field on the binding energies and polaronic shifts of the ground and some first few excited states of a hydrogenic impurity in a spherical quantum dot by taking into account the image charge effect. By using Landau–Pekar variational method the general analytical expression is obtained for the impurity bound-polaron energies. It has been numerically identified the conditions (electric field, nominal radius of quantum dot, etc.) in which the bound-polaron states can be existence in GaAs quantum dot. We have shown that the polaronic shifts in the binding energy of 1s-like state are the same in cases with and without image charge effect while they for 2s-like state are not coincide and have different monotonic behavior versus confinement potential. Electron–phonon interaction lifts the degeneracy of the 2px-, 2py-, and 2pz-like states of a donor impurity and reduces their binding energies.     

Spin of the ground quantum state of electrons from first principles in the representation of Feynman path integrals

A method for calculating the spin of the ground quantum state of nonrelativistic electrons and distance between energy levels of quantum states differing in the spin magnitude from first principles is proposed. The approach developed is free from the one-electron approximation and applicable in multielectron systems with allowance for all spatial correlations. The possibilities of the method are demonstrated by the example of calculating the energy gap between spin states in model ellipsoidal quantum dots with a harmonic confining field. The results of computations by the Monte Carlo method point to high sensitivity of the energy gap to the break of spherical symmetry of the quantum dot. For three electrons, the phenomenon of inversion has been revealed for levels corresponding to high and low values of the spin. The calculations demonstrate the practical possibility to obtain spin states with arbitrarily close energies by varying the shape of the quantum dot, which is a key condition for development prospects in technologies of storage systems based on spin qubits.     

Valley-based noise-resistant quantum computation using Si quantum dots

We devise a platform for noise-resistant quantum computing using the valley degree of freedom of Si quantum dots. The qubit is encoded in two polarized (1,1) spin-triplet states with different valley compositions in a double quantum dot, with a Zeeman field enabling unambiguous initialization. A top gate gives a difference in the valley splitting between the dots, allowing controllable interdot tunneling between opposite valley eigenstates, which enables one-qubit rotations. Two-qubit operations rely on a stripline resonator, and readout on charge sensing. Sensitivity to charge and spin fluctuations is determined by intervalley processes and is greatly reduced as compared to conventional spin and charge qubits. We describe a valley echo for further noise suppression.