Probing Single-Electron Spin Decoherence in Quantum Dots using Charged Excitons

We propose to use optical detection of magnetic resonance (ODMR) to measure the decoherence time T₂ of a single-electron spin in a semiconductor quantum dot. The electron is in one of the spin 1/2 states and a circularly polarized laser can only create an optical excitation for one of the electron spin states due to Pauli blocking. An applied electron spin resonance (ESR) field leads to Rabi spin flips and thus to a modulation of the photoluminescence or, alternatively, of the photocurrent. This allows one to measure the ESR linewidth and the coherent Rabi oscillations, from which the electron spin decoherence can be determined. We study different possible schemes for such an ODMR setup, including cw or pulsed laser excitation. An erratum to this article is available at .     

Spectroscopic investigations on hybrid nanocomposites: CdS : Mn nanocrystals in a conjugated polymer

We have used electron paramagnetic resonance (EPR) spectroscopy for investigating the properties of spins, such as those carried by polarons which carry both spin and charge in poly (meta/para phenylene) PMPP: CdS doped Mn based nanocomposites. To identify the nature of paramagnetic species in PMPP matrix, we have studied the effect of different physical parameters. It was found that we are in presence of trapped polarons and localized spins which concentration has been estimated. Moreover, spin–spin and spin–lattice relaxation rates have been calculated. Then, we discussed the results of optical and EPR study on the hybrid nanocomposite (CdS nanostructures, doped with manganese (II) ions, incorporated in PMPP conjugated polymer matrix). The optical spectra of these nanocomposites were compared to the existing models of energy levels in quantum dots. Moreover, by the use of electronic paramagnetic resonance, conclusions about the location and the symmetry of Mn²⁺ ions have been drawn. The nanocomposite energy gap is in the 3.2–3.3 eV range. The size of the nanoparticle is about 3.3 nm and Mn²⁺ ions are located at or near the nanoparticle surface.     

A case study: Te in ZnSe and Mn-doped ZnSe quantum dots

Photoluminescence (PL) behavior of ZnSe(1-y)Te(y) quantum dots is investigated by varying Te concentration as well as size. The striking effect of quantum confinement is the observation of isoelectronic center-related emission at room temperature in lieu of near-band-edge emission that dominates the optical scenario. ZnSe(0.99)Te(0.01) quantum dots were also doped by Mn(2+) ions. The Mn(2+) ion-related d-d transition is drastically suppressed by Te isoelectronic centers. Incorporation of Mn(2+) at substitutional sites in ZnSe(0.99)Te(0.01) quantum dots is also confirmed by the electron paramagnetic resonance measurements. Effect of Te isoelectronic impurity on the emission behavior is more pronounced than that of Mn(2+) ions. A subtle blueshift in the orange d-d transition is a sign of a decrease in crystal field strength. PL and photoluminescence excitation measurements on Zn(1-x)Se(0.99)Te(0.01)Mn(x) quantum dots indicate that the transition probability from the lowest unoccupied molecular orbital to Te levels is substantially larger than that to Mn(2+) d-d levels.     

Coherent spin transport through dynamic quantum dots

Spin transport and manipulation in semiconductors have been studied intensively with the ultimate goal of realizing spintronic devices. Previous work in GaAs has focused on controlling the carrier density, crystallographic orientation and dimensionality to limit the electron spin decoherence and allow transport over long distances. Here, we introduce a new method for the coherent transport of spin-polarized electronic wave packets using dynamic quantum dots (DQDs) created by the piezoelectric field of coherent acoustic phonons. Photogenerated spin carriers transported by the DQDs in undoped GaAs (001) quantum wells exhibit a spin coherence length exceeding 100 microm, which is attributed to the simultaneous control of the carrier density and the dimensionality by the DQDs during transport. In the absence of an applied magnetic field, we observe the precession of the electron spin induced by the internal magnetic field associated with the spin splitting of the conduction band (Dresselhaus term). The coherent manipulation of the precession frequency is also achieved by applying an external magnetic field.     

Spin–Lattice Relaxation of Mn Ions in Nanostructures with Semiconductor Quantum Dots

We use the combination of nonequilibrium phonon and exciton luminescence techniques to study the spin dynamics in diluted magnetic semiconductor structures with (Cd,Mn)Te and (Cd,Mn)Se quantum dots (QDs). We show that the spin–lattice relaxation (SLR) of Mn ions in these structures differs strongly from the SLR in quantum wells. We explain the results by a model where SLR process in structures with QDs is modified by the spin diffusion on Mn ions from the QD to a wetting layer.     

Direct Observation of sp-d exchange interactions in colloidal Mn2+- and Co2+-doped CdSe quantum dots

The defining attribute of a diluted magnetic semiconductor (DMS) is the existence of dopant-carrier magnetic exchange interactions. In this letter, we report the first direct observation of such exchange interactions in colloidal doped CdSe nanocrystals. Doped CdSe quantum dots were synthesized by thermal decomposition of (Me4N)2[Cd4(SePh)10] in the presence of TMCl2 (TM2+ = Mn2+ or Co2+) in hexadecylamine and were characterized by several analytical and spectroscopic techniques. Using magnetic circular dichroism spectroscopy, successful doping and the existence of giant excitonic Zeeman splittings in both Mn2+- and Co2+-doped wurtzite CdSe quantum dots are demonstrated unambiguously.     

Spin-polarized light-emitting diodes with Mn-doped InAs quantum dot nanomagnets as a spin aligner

We have fabricated and characterized surface-emitting, spin-polarized light-emitting diodes with a Mn-doped InAs dilute magnetic quantum dot spin-injector and contact region grown by low-temperature molecular beam epitaxy, and an In(0.4)Ga(0.6)As quantum dot active region. Energy-dispersive X-ray and electron energy loss spectroscopies performed on individual dots indicate that the Mn atoms incorporate within the dots themselves. Circularly polarized light is observed up to 160 K with a maximum degree of circular polarization of 5.8% measured at 28 K, indicating high-temperature spin injection and device operation.     

A Ge/Si heterostructure nanowire-based double quantum dot with integrated charge sensor

One proposal for a solid-state-based quantum bit (qubit) is to control coupled electron spins on adjacent semiconductor quantum dots. Most experiments have focused on quantum dots made from III-V semiconductors; however, the coherence of electron spins in these materials is limited by hyperfine interactions with nuclear spins. Ge/Si core/shell nanowires seem ideally suited to overcome this limitation, because the most abundant nuclei in Ge and Si have spin zero and the nanowires can be chemically synthesized defect-free with tunable properties. Here, we present a double quantum dot based on Ge/Si nanowires in which we can completely control the coupling between the dots and to the leads. We also demonstrate that charge on the double dot can be detected by coupling it capacitively to an adjacent nanowire quantum dot. The double quantum dot and integrated charge sensor serve as an essential building block to form a solid-state qubit free of nuclear spin.     

Spin Injection Processes in ZnSe-Based Double Quantum Dots of Diluted Magnetic Semiconductors

Spin injection processes in the double quantum dots of ZnSe-based diluted magnetic semiconductors are discussed. Double quantum dots are fabricated from ZnSe-based double quantum wells by electron beam lithography and wet etching. In these samples, the photo-excited carriers in the magnetic dots are injected into the non-magnetic dots. The circular polarization degrees of photoluminescence from the non-magnetic dots are measured by micro-photoluminescence measurement system under the magnetic field up to 5 T. The maximum spin polarization degrees of injected carriers determined from our experiment are 10% for double quantum wells and 15% for double quantum dots. The spin injection efficiency was estimated both from the observed circular polarization degree and the diffusion length of carriers. We concluded that the spin injection efficiency is increased in the double quantum dots.     

Electron spin coherence exceeding seconds in high-purity silicon

Silicon is one of the most promising semiconductor materials for spin-based information processing devices. Its advanced fabrication technology facilitates the transition from individual devices to large-scale processors, and the availability of a (28)Si form with no magnetic nuclei overcomes a primary source of spin decoherence in many other materials. Nevertheless, the coherence lifetimes of electron spins in the solid state have typically remained several orders of magnitude lower than that achieved in isolated high-vacuum systems such as trapped ions. Here we examine electron spin coherence of donors in pure (28)Si material (residual (29)Si concentration <50 ppm) with donor densities of 10(14)-10(15) cm(-3). We elucidate three mechanisms for spin decoherence, active at different temperatures, and extract a coherence lifetime T(2) up to 2 s. In this regime, we find the electron spin is sensitive to interactions with other donor electron spins separated by ~200 nm. A magnetic field gradient suppresses such interactions, producing an extrapolated electron spin T(2) of 10 s at 1.8 K. These coherence lifetimes are without peer in the solid state and comparable to high-vacuum qubits, making electron spins of donors in silicon ideal components of quantum computers, or quantum memories for systems such as superconducting qubits.     

Ultrafast spin dynamics in colloidal ZnO quantum dots

Time-resolved Faraday rotation measurements in the ultraviolet have been performed to reveal the ultrafast spin dynamics of electrons in colloidal ZnO quantum dots. Oscillating Faraday rotation signals are detected at frequencies corresponding to an effective g factor of g = 1.96. Biexponential oscillation decay is observed that is due to (i) rapid depopulation of the fundamental exciton (tau = 250 ps) and (ii) slow electron spin dephasing ( T 2 = 1.2 ns) within a metastable state formed by hole-trapping at the quantum dot surface.     

Enhancement of spin coherence using Q-factor engineering in semiconductor microdisc lasers

Semiconductor microcavities offer unique means of controlling light-matter interactions in confined geometries, resulting in a wide range of applications in optical communications and inspiring proposals for quantum information processing and computational schemes. Studies of spin dynamics in microcavities, a new and promising research field, have revealed effects such as polarization beats, stimulated spin scattering and giant Faraday rotation. Here, we study the electron spin dynamics in optically pumped GaAs microdisc lasers with quantum wells and interface-fluctuation quantum dots in the active region. In particular, we examine how the electron spin dynamics are modified by the stimulated emission in the discs, and observe an enhancement of the spin-coherence time when the optical excitation is in resonance with a high-quality (Q approximately 5,000) lasing mode. This resonant enhancement, contrary to expectations from the observed trend in the carrier-recombination time, is then manipulated by altering the cavity design and dimensions. In analogy with devices based on excitonic coherence, this ability to engineer coherent interactions between electron spins and photons may provide new pathways towards spin-dependent quantum optoelectronics.     

Warping a single Mn acceptor wavefunction by straining the GaAs host

Transition-metal dopants such as Mn determine the ferromagnetism in dilute magnetic semiconductors such as Ga(1-x)Mn(x)As. Recently, the acceptor states of Mn dopants in GaAs were found to be highly anisotropic owing to the symmetry of the host crystal. Here, we show how the shape of such a state can be modified by local strain. The Mn acceptors near InAs quantum dots are mapped at room temperature by scanning tunnelling microscopy. Dramatic distortions and a reduction in the symmetry of the wavefunction of the hole bound to the Mn acceptor are observed originating from strain induced by quantum dots. Calculations of the acceptor-state wavefunction in the presence of strain, within a tight-binding model and within an effective-mass model, agree with the experimentally observed shape. The magnetic easy axes of strained lightly doped Ga(1-x)Mn(x)As can be explained on the basis of the observed local density of states for the single Mn spin.     

Hole spin relaxation in Ge-Si core-shell nanowire qubits

Controlling decoherence is the biggest challenge in efforts to develop quantum information hardware. Single electron spins in gallium arsenide are a leading candidate among implementations of solid-state quantum bits, but their strong coupling to nuclear spins produces high decoherence rates. Group IV semiconductors, on the other hand, have relatively low nuclear spin densities, making them an attractive platform for spin quantum bits. However, device fabrication remains a challenge, particularly with respect to the control of materials and interfaces. Here, we demonstrate state preparation, pulsed gate control and charge-sensing spin readout of hole spins confined in a Ge-Si core-shell nanowire. With fast gating, we measure T(1) spin relaxation times of up to 0.6 ms in coupled quantum dots at zero magnetic field. Relaxation time increases as the magnetic field is reduced, which is consistent with a spin-orbit mechanism that is usually masked by hyperfine contributions.     

Giant fluctuations and gate control of the g-factor in InAs nanowire quantum dots

We study the g-factor of discrete electron states in InAs nanowire based quantum dots. The g values are determined from the magnetic field splitting of the zero bias anomaly due to the spin 1/2 Kondo effect. Unlike to previous studies based on 2DEG quantum dots, the g-factors of neighboring electron states show a surprisingly large fluctuation: g can scatter between 2 and 18. Furthermore electric gate tunability of the g-factor is demonstrated.     

Electrical control of a solid-state flying qubit

Solid-state approaches to quantum information technology are attractive because they are scalable. The coherent transport of quantum information over large distances is a requirement for any practical quantum computer and has been demonstrated by coupling super-conducting qubits to photons. Single electrons have also been transferred between distant quantum dots in times shorter than their spin coherence time. However, until now, there have been no demonstrations of scalable 'flying qubit' architectures-systems in which it is possible to perform quantum operations on qubits while they are being coherently transferred-in solid-state systems. These architectures allow for control over qubit separation and for non-local entanglement, which makes them more amenable to integration and scaling than static qubit approaches. Here, we report the transport and manipulation of qubits over distances of 6?μm within 40?ps, in an Aharonov-Bohm ring connected to two-channel wires that have a tunable tunnel coupling between channels. The flying qubit state is defined by the presence of a travelling electron in either channel of the wire, and can be controlled without a magnetic field. Our device has shorter quantum gates (<1?μm), longer coherence lengths (～86?μm at 70?mK) and higher operating frequencies (～100?GHz) than other solid-state implementations of flying qubits.     

Electron Spin Coherence of Phosphorus Donors in Isotopically Purified 29Si

We investigate spin coherence time of electrons bound to phosphorus donors in silicon single crystals, employing a pulsed electron paramagnetic resonance technique. The samples were isotopically controlled so that they may possess different concentrations (about 5% and 100%) of ²⁹Si, which is the only non-zero-spin (spin-1/2) stable isotope of Si. Both ²⁹Si-concentration dependence and orientation dependence of the electron spin coherence time demonstrate that the decoherence is caused by spectral diffusion due to mutual flip-flops of the environmental nuclear spins. The detail analysis of spin echo decay curves enables the unique assignment of the host sites responsible for electron spin echo envelope modulation.     

Electron Spin Coherence of Phosphorus Donors in Isotopically Purified <Superscript>29</Superscript>Si

We investigate spin coherence time of electrons bound to phosphorus donors in silicon single crystals, employing a pulsed electron paramagnetic resonance technique. The samples were isotopically controlled so that they may possess different concentrations (about 5% and 100%) of ²⁹Si, which is the only non-zero-spin (spin-1/2) stable isotope of Si. Both ²⁹Si-concentration dependence and orientation dependence of the electron spin coherence time demonstrate that the decoherence is caused by spectral diffusion due to mutual flip-flops of the environmental nuclear spins. The detail analysis of spin echo decay curves enables the unique assignment of the host sites responsible for electron spin echo envelope modulation.     

Fabrication of magnetic artificial atoms

We have fabricated gated vertical quantum dots made from a II-VI semiconductor heterostructure containing a paramagnetic quantum well. The absence of a known Schottky gate metal compatible with ZnSe based material precludes the traditional method of using a self-aligning shadow evaporated gate. Instead, we make use of a multi-step electron beam lithography process to surround a pillar with an insulating dielectric and gate. This process allows for the processing of dots with diameters down to 250?nm. Preliminary transport data confirming the magnetic nature of the resulting artificial atom are presented.     

Observation of Rabi splitting from surface plasmon coupled conduction state transitions in electrically excited InAs quantum dots

We demonstrate strong coupling between a surface plasmon and intersublevel transitions in self-assembled InAs quantum dots. The surface plasmon mode exists at the interface between the semiconductor emitter structure and a periodic array of holes perforating a metallic Pd/Ge/Au film that also serves as the top electrical contact for the emitters. Spectrally narrowed quantum-dot electroluminescence was observed for devices with varying subwavelength hole spacing. Devices designed for 9, 10, and 11 μm wavelength emission also exhibit a significant spectral splitting. The association of the splitting with quantum-dot Rabi oscillation is consistent with results from a calculation of spontaneous emission from an interacting plasmonic field and quantum-dot ensemble. The fact that this Rabi oscillation can be observed in an incoherently excited, highly inhomogeneously broadened system demonstrates the utility of intersublevel transitions in quantum dots for investigations of coherent transient and quantum coherence phenomena.