arXiv subjects

A. D. Wieck

Publications and source records attributed to A. D. Wieck.

At least 37 records · Page 2Linked to original sources

arXiv2019

Detection and amplification of spin noise using scattered laser light in a quantum-dot microcavity

Fundamental properties of the spin-noise signal formation in a quantum-dot microcavity are studied by measuring the angular characteristics of the scattered light intensity. A distributed Bragg reflector microcavity was used to enhance the light-matter interaction with an ensemble of n-doped (In,Ga)As/GaAs quantum dots, which allowed us to study subtle effects of the noise signal formation. Detecting the scattered light outside of the aperture of the transmitted light, we measured the basic electron spin properties, like g-factor and spin dephasing time. Further, we investigated the influence of the microcavity on the scattering distribution and possibilities of signal amplification by additional resonant excitation.

cond-mat.mes-hall↗

arXiv2018

A fast quantum interface between different spin qubit encodings

Single-spin qubits in semiconductor quantum dots proposed by Loss and DiVincenzo (LD qubits) hold promise for universal quantum computation with demonstrations of a high single-qubit gate fidelity above 99.9 % and two-qubit gates in conjunction with a long coherence time. However, initialization and readout of a qubit is orders of magnitude slower than control, which is detrimental for implementing measurement-based protocols such as error-correcting codes. In contrast, a singlet-triplet (ST) qubit, encoded in a two-spin subspace, has the virtue of fast readout with high fidelity and tunable coupling to the electric field. Here, we present a hybrid system which benefits from the different advantages of these two distinct spin-qubit implementations. A quantum interface between the two codes is realized by electrically tunable inter-qubit exchange coupling. We demonstrate a controlled-phase (CPHASE) gate that acts within 5.5 ns, much faster than the measured dephasing time of 211 ns. The presented hybrid architecture will be useful to settle remaining key problems with building scalable spin-based quantum computers.

cond-mat.mes-hall↗

arXiv2018

Decay and revival of electron spin polarization in an ensemble of (In,Ga)As quantum dots

The periodic optical orientation of electron spins in (In,Ga)As/GaAs quantum dots leads to the formation of electron spin precession modes about an external magnetic field which are resonant with the pumping periodicity. As the electron spin is localized within a nuclear spin bath, its polarization imprints onto the spin polarization of the bath. The latter acts back on the electron spin polarization. We implement a pulse protocol where a train of laser pulses is followed by a long, dark gap. It allows us to obtain a high-resolution precession mode spectrum from the free evolution of the electron spin polarization. Additionally, we vary the number of pump pulses in a train to investigate the build-up of the precession modes. To separate out nuclear effects, we suppress the nuclear polarization by using a radio-frequency field. We find that a long-living nuclear spin polarization imprinted by the periodic excitation significantly speeds up the buildup of the electron spin polarization and induces the formation of additional electron spin precession modes. To interpret these findings, we extend an established dynamical nuclear polarization model to take into account optically detuned quantum dots for which nuclear spins activate additional electron spin precession modes.

cond-mat.mes-hall↗

arXiv2018

Spin Inertia of Resident and Photoexcited Carriers in Singly-Charged Quantum Dots

The spin dynamics in a broad range of systems can be studied using circularly polarized optical excitation with alternating helicity. The dependence of spin polarization on the frequency of helicity alternation, known as the spin inertia effect, is used here to study the spin dynamics in singly-charged (In,Ga)As/GaAs quantum dots (QDs) providing insight into spin generation and accumulation processes. We demonstrate that the dependence of spin polarization in $n$- and $p$-type QDs on the external magnetic field has a characteristic V- and M-like shape, respectively. This difference is related to different microscopic mechanisms of resident carriers spin orientation. It allows us to determine the parameters of the spin dynamics both for the ground and excited states of singly-charged QDs.

cond-mat.mes-hall↗

arXiv2018

Theory of spin inertia in singly-charged quantum dots

The spin inertia measurement is a recently emerged tool to study slow spin dynamics, which is based on the excitation of the system by a train of circularly polarized pulses with alternating helicity. Motivated by the experimental results reported in E. A. Zhukov et al., arXiv:1806.11100 we develop the general theory of spin inertia of localized charge carriers. We demonstrate that the spin inertia measurement in longitudinal magnetic field allows one to determine the parameters of the spin dynamics of resident charge carriers and of photoexcited trions, such as the spin relaxation times, longitudinal g-factors, parameters of hyperfine interaction and nuclear spin correlation times.

cond-mat.mes-hall↗

arXiv2018

Photon noise suppression by a built-in feedback loop

Visionary quantum photonic networks need transform-limited single photons on demand. Resonance fluorescence on a quantum dot provides the access to a solid-state single photon source, where the environment is unfortunately the source of spin and charge noise that leads to fluctuations of the emission frequency and destroys the needed indistinguishability. We demonstrate a built-in stabilization approach for the photon stream, which relies solely on charge carrier dynamics of a two-dimensional hole gas inside a micropillar structure. The hole gas is fed by hole tunneling from field-ionized excitons and influences the energetic position of the excitonic transition by changing the local electric field at the position of the quantum dot. The standard deviation of the photon noise is suppressed by nearly 50 percent (noise power reduction of 6 dB) and it works in the developed micropillar structure for frequencies up to 1 kHz. This built-in feedback loop represents an easy way for photon noise suppression in large arrays of single photon emitters and promises to reach higher bandwidth by device optimization.

cond-mat.mes-hall↗

arXiv2017

Field effect induced mesoscopic devices in depleted two dimensional electron systems

Nanoelectronic devices embedded in the two-dimensional electron system (2DES) of a GaAs/AlGaAs heterostructure enable a large variety of applications from fundamental research to high speed transistors. Electrical circuits are thereby commonly defined by creating barriers for carriers by selective depletion of a pre-existing 2DES. Here we explore an alternative approach: we deplete the 2DES globally by applying a negative voltage to a global top gate and screen the electric field of the top gate only locally using nanoscale gates placed on the wafer surface between the plane of the 2DES and the top gate. Free carriers are located beneath the screen gates and their properties can be controlled by means of geometry and applied voltages. This method promises considerable advantages for the definition of complex circuits by the electric field effect as it allows to reduce the number of gates and simplify gate geometries. Examples are carrier systems with ring topology or large arrays of quantum dots. Here, we present a first exploration of this method pursuing field effect, Hall effect and Aharonov-Bohm measurements to study electrostatic, dynamic and coherent properties.

cond-mat.mes-hall↗

arXiv2017

Super-resolution microscopy of single rare-earth emitters

We demonstrate super-resolution imaging of single rare-earth emitting centers, namely, trivalent cerium, in yttrium aluminum garnet (YAG) crystals by means of stimulated emission depletion (STED) microscopy. The achieved all-optical resolution is $\approx$ 80nm. Similar results were obtained on H3 color centers in diamond with resolution of $\approx$ 60nm. In both cases, STED resolution is improving slower than the inverse square-root of the depletion beam intensity. This is caused by excited state absorption (ESA) and interaction of the emitter with non-fluorescing crystal defects in its near surrounding.

physics.optics↗

arXiv2017

Non-universal transmission phase behaviour of a large quantum dot

The electron wave function experiences a phase modification at coherent transmission through a quantum dot. This transmission phase undergoes a characteristic shift of $π$ when scanning through a Coulomb-blockade resonance. Between successive resonances either a transmission phase lapse of $π$ or a phase plateau is theoretically expected to occur depending on the parity of the corresponding quantum dot states. Despite considerable experimental effort, this transmission phase behaviour has remained elusive for a large quantum dot. Here we report on transmission phase measurements across such a large quantum dot hosting hundreds of electrons. Using an original electron two-path interferometer to scan the transmission phase along fourteen successive resonances, we observe both phase lapses and plateaus. Additionally, we demonstrate that quantum dot deformation alters the sequence of transmission phase lapses and plateaus via parity modifications of the involved quantum dot states. Our findings set a milestone towards a comprehensive understanding of the transmission phase of quantum dots.

cond-mat.mes-hall↗

arXiv2017

Mesoscopic phase behavior in a quantum dot around crossover between single-level and multilevel transport regimes

The transmission phase across a quantum dot (QD) is expected to show mesoscopic behavior, where the appearance of a phase lapse between Coulomb peaks (CPs) as a function of the gate voltage depends on the orbital parity relation between the corresponding CPs. On the other hand, such mesoscopic behavior has been observed only in a limited QD configuration (a few-electron and single-level transport regime) and universal phase lapses by $π$ between consecutive CPs have been reported for all the other configurations. Here, we report on the measurement of a transmission phase across a QD around the crossover between single-level and multilevel transport regimes employing an original two-path quantum interferometer. We find mesoscopic behavior for the studied QD. Our results show that the universal phase lapse, a longstanding puzzle of the phase shift, is absent for a standard QD, where several tens of successive well-separated CPs are observed.

cond-mat.mes-hall↗

arXiv2017

Fast spin information transfer between distant quantum dots using individual electrons

Transporting ensembles of electrons over long distances without losing their spin polarization is an important benchmark for spintronic devices. It requires usually to inject and to probe spin polarized electrons in conduction channels using ferromagnetic contacts or optical excitation. Parallel to this development, an important effort has been dedicated to the control of nanocircuits at the single electron level. The detection and the coherent manipulation of the spin of a single electron trapped in a quantum dot are now well established. Combined with the recent control of the displacement of individual electrons between two distant quantum dots, these achievements permit to envision the realization of spintronic protocols at the single electron level. Here, we demonstrate that spin information carried by one or two electrons can be transferred between two quantum dots separated by a distance of 4 micrometers with a classical fidelity of 65 %. We show that it is presently limited by spin flips occurring during the transfer procedure prior to and after the electron displacement. Being able to encode and control information in the spin degree of freedom of a single electron while being transferred over distances of a few microns on nanosecond timescales paves the way towards "quantum spintronics" devices where large scale spin-based quantum information processing could be implemented.

cond-mat.mes-hall↗

arXiv2017

A triangular triple quantum dot with tunable tunnel couplings

A two-dimensional arrangement of quantum dots with finite inter-dot tunnel coupling provides a promising platform for studying complicated spin correlations as well as for constructing large-scale quantum computers. Here, we fabricate a tunnel-coupled triangular triple quantum dot with a novel gate geometry in which three dots are defined by positively biasing the surface gates. At the same time, the small area in the center of the triangle is depleted by negatively biasing the top gate placed above the surface gates. The size of the small center depleted area is estimated from the Aharonov-Bohm oscillation measured for the triangular channel but incorporating no gate-defined dots, with a value consistent with the design. With this approach, we can bring the neighboring gate-defined dots close enough to one another to maintain a finite inter-dot tunnel coupling. We finally confirm the presence of the inter-dot tunnel couplings in the triple quantum dot from the measurement of tunneling current through the dots in the stability diagram. We also show that the charge occupancy of each dot and that the inter-dot tunnel couplings are tunable with gate voltages.

cond-mat.mes-hall↗

arXiv2017

Nonequilibrium spin noise in a quantum dot ensemble

The spin noise in singly charged self-assembled quantum dots is studied theoretically and experimentally under the influence of a perturbation, provided by additional photoexcited charge carriers. The theoretical description takes into account generation and relaxation of charge carriers in the quantum dot system. The spin noise is measured under application of above barrier excitation for which the data are well reproduced by the developed model. Our analysis demonstrates a strong difference of the recharging dynamics for holes and electrons in quantum dots.

cond-mat.mes-hall↗

arXiv2017

Detection and control of spin-orbit interactions in a GaAs hole quantum point contact

We investigate the relationship between the Zeeman interaction and the inversion asymmetry induced spin orbit interactions (Rashba and Dresselhaus SOIs) in GaAs hole quantum point contacts. The presence of a strong SOI results in crossing and anti-crossing of adjacent spin-split hole subbands in a magnetic field. We demonstrate theoretically and experimentally that the anti-crossing energy gap depends on the interplay between the SOI terms and the highly anisotropic hole g tensor, and that this interplay can be tuned by selecting the crystal axis along which the current and magnetic field are aligned. Our results constitute independent detection and control of the Dresselhaus and Rashba SOIs in hole systems, which could be of importance for spintronics and quantum information applications.

cond-mat.mes-hall↗

arXiv2017

Spin dynamics of quadrupole nuclei in InGaAs quantum dots

Photoluminescence polarization is experimentally studied for samples with (In,Ga)As/GaAs selfassembled quantum dots in transverse magnetic field (Hanle effect) under slow modulation of the excitation light polarization from fractions of Hz to tens of kHz. The polarization reflects the evolution of strongly coupled electron-nuclear spin system in the quantum dots. Strong modification of the Hanle curves under variation of the modulation period is attributed to the peculiarities of the spin dynamics of quadrupole nuclei, which states are split due to deformation of the crystal lattice in the quantum dots. Analysis of the Hanle curves is fulfilled in the framework of a phenomenological model considering a separate dynamics of a nuclear field BNd determined by the +/- 12 nuclear spin states and of a nuclear field BNq determined by the split-off states +/- 3/2, +/- 5/2, etc. It is found that the characteristic relaxation time for the nuclear field BNd is of order of 0.5 s, while the relaxation of the field BNq is faster by three orders of magnitude.

cond-mat.mes-hall↗

arXiv2017

Coherent long-distance displacement of individual electron spins

Controlling nanocircuits at the single electron spin level is a possible route for large-scale quantum information processing. In this context, individual electron spins have been identified as versatile quantum information carriers to interconnect different nodes of a spin-based semiconductor quantum circuit. Despite important experimental efforts to control the electron displacement over long distances, keeping the electron spin coherence after transfer remained up to now elusive. Here we demonstrate that individual electron spins can be displaced coherently over a distance of 5 micrometers. This displacement is realized on a closed path made of three tunnel-coupled lateral quantum dots. Using fast quantum dot control, the electrons tunnel from one dot to another at a speed approaching 100 m/s. We find that the spin coherence length is 8 times longer than expected from the electron spin coherence without displacement. Such an enhanced spin coherence points at a process similar to motional narrowing observed in nuclear magnetic resonance experiments6. The demonstrated coherent displacement will enable long-range interaction between distant spin-qubits and will open the route towards non-abelian and holonomic manipulation of a single electron spin.

cond-mat.mes-hall↗

arXiv2016

Low-temperature behavior of transmission phase shift across a Kondo correlated quantum dot

We study the transmission phase shift across a Kondo correlated quantum dot in a GaAs heterostructure at temperatures below the Kondo temperature ($T < T_{\rm K}$), where the phase shift is expected to show a plateau at $π/2$ for an ideal Kondo singlet ground state. Our device is tuned such that the ratio $Γ/U$ of level width $Γ$ to charging energy $U$ is quite large ($\lesssim 0.5$ rather than $\ll 1$). This situation is commonly used in GaAs quantum dots to ensure Kondo temperatures large enough ($\simeq 100$ mK here) to be experimentally accessible; however it also implies that charge fluctuations are more pronounced than typically assumed in theoretical studies focusing on the regime $Γ/U \ll 1$ needed to ensure a well-defined local moment. Our measured phase evolves monotonically by $π$ across the two Coulomb peaks, but without being locked at $π/2$ in the Kondo valley for $T \ll T_{\rm K}$, due to a significant influence of large $Γ/U$. Only when $Γ/U$ is reduced sufficiently does the phase start to be locked around $π/2$ and develops into a plateau at $π/2$. Our observations are consistent with numerical renormalization group calculations, and can be understood as a direct consequence of the Friedel sum rule that relates the transmission phase shift to the local occupancy of the dot, and thermal average of a transmission coefficient through a resonance level near the Fermi energy.

cond-mat.mes-hall↗

arXiv2016

Footprints of hyperfine, spin-orbit, and decoherence effects in Pauli spin blockade

We detect in real time inter-dot tunneling events in a weakly coupled two electron double quantum dot in GaAs. At finite magnetic fields, we observe two characteristic tunneling times, T_d and T_b, belonging to, respectively, a direct and a blocked (spin-flip-assisted) tunneling. The latter corresponds to lifting of a Pauli spin blockade and the tunneling times ratio eta=T_b/T_d characterizes the blockade efficiency. We find pronounced changes in the behavior of eta upon increasing the magnetic field, with eta increasing, saturating and increasing again. We explain this behavior as due to the crossover of the dominant blockade lifting mechanism from the hyperfine to spin-orbit interactions and due to a change in the contribution of the charge decoherence.

cond-mat.mes-hall↗