116 research outputs found
GHz bandwidth electro-optics of a single self-assembled quantum dot in a charge-tunable device
The response of a single InGaAs quantum dot, embedded in a miniaturized
charge-tunable device, to an applied GHz bandwidth electrical pulse is
investigated via its optical response. Quantum dot response times of 1.0 \pm
0.1 ns are characterized via several different measurement techniques,
demonstrating GHz bandwidth electrical control. Furthermore a novel optical
detection technique based on resonant electron-hole pair generation in the
hybridization region is used to map fully the voltage pulse experienced by the
quantum dot, showing in this case a simple exponential rise.Comment: 7 pages, 4 figure
Transform-limited single photons from a single quantum dot
A semiconductor quantum dot mimics a two-level atom. Performance as a single
photon source is limited by decoherence and dephasing of the optical
transition. Even with high quality material at low temperature, the optical
linewidths are a factor of two larger than the transform-limit. A major
contributor to the inhomogeneous linewdith is the nuclear spin noise. We show
here that the nuclear spin noise depends on optical excitation, increasing
(decreasing) with increasing resonant laser power for the neutral (charged)
exciton. Based on this observation, we discover regimes where we demonstrate
transform-limited linewidths on both neutral and charged excitons even when the
measurement is performed very slowly
Electrically-tunable hole g-factor of an optically-active quantum dot for fast spin rotations
We report a large g-factor tunability of a single hole spin in an InGaAs
quantum dot via an electric field. The magnetic field lies in the in-plane
direction x, the direction required for a coherent hole spin. The electrical
field lies along the growth direction z and is changed over a large range, 100
kV/cm. Both electron and hole g-factors are determined by high resolution laser
spectroscopy with resonance fluorescence detection. This, along with the low
electrical-noise environment, gives very high quality experimental results. The
hole g-factor g_xh depends linearly on the electric field Fz, dg_xh/dFz = (8.3
+/- 1.2)* 10^-4 cm/kV, whereas the electron g-factor g_xe is independent of
electric field, dg_xe/dFz = (0.1 +/- 0.3)* 10^-4 cm/kV (results averaged over a
number of quantum dots). The dependence of g_xh on Fz is well reproduced by a
4x4 k.p model demonstrating that the electric field sensitivity arises from a
combination of soft hole confining potential, an In concentration gradient and
a strong dependence of material parameters on In concentration. The electric
field sensitivity of the hole spin can be exploited for electrically-driven
hole spin rotations via the g-tensor modulation technique and based on these
results, a hole spin coupling as large as ~ 1 GHz is expected to be envisaged.Comment: 8 pages, 4 figure
High resolution coherent population trapping on a single hole spin in a semiconductor
We report high resolution coherent population trapping on a single hole spin
in a semiconductor quantum dot. The absorption dip signifying the formation of
a dark state exhibits an atomic physics-like dip width of just 10 MHz. We
observe fluctuations in the absolute frequency of the absorption dip, evidence
of very slow spin dephasing. We identify this process as charge noise by,
first, demonstrating that the hole spin g-factor in this configuration
(in-plane magnetic field) is strongly dependent on the vertical electric field,
and second, by characterizing the charge noise through its effects on the
optical transition frequency. An important conclusion is that charge noise is
an important hole spin dephasing process
Decoupling a hole spin qubit from the nuclear spins
A huge effort is underway to develop semiconductor nanostructures as low-noise hosts for qubits. The main source of dephasing of an electron spin qubit in a GaAs-based system is the nuclear spin bath. A hole spin may circumvent the nuclear spin noise. In principle, the nuclear spins can be switched off for a pure heavy-hole spin. In practice, it is unknown to what extent this ideal limit can be achieved. A major hindrance is that p-type devices are often far too noisy. We investigate here a single hole spin in an InGaAs quantum dot embedded in a new generation of low-noise p-type device. We measure the hole Zeeman energy in a transverse magnetic field with 10 neV resolution by dark-state spectroscopy as we create a large transverse nuclear spin polarization. The hole hyperfine interaction is highly anisotropic: the transverse coupling is <1% of the longitudinal coupling. For unpolarized, randomly fluctuating nuclei, the ideal heavy-hole limit is achieved down to nanoelectronvolt energies; equivalently dephasing times up to a microsecond. The combination of large and strong optical dipole makes the single hole spin in a GaAs-based device an attractive quantum platform
Functional Characterization of the HuR:CD83 mRNA Interaction
Maturation of dendritic cells (DC) is characterized by expression of CD83, a surface protein that appears to be necessary for the effective activation of naïve T-cells and T-helper cells by DC. Lately it was shown that CD83 expression is regulated on the posttranscriptional level by interaction of the shuttle protein HuR with a novel posttranscriptional regulatory RNA element (PRE), which is located in the coding region of the CD83 transcript. Interestingly, this interaction commits the CD83 mRNA to efficient nuclear export via the CRM1 pathway. To date, however, the structural basis of this interaction, which potentially involves three distinct RNA recognition motifs (RRM1–3) in HuR and a complex three-pronged RNA stem-loop element in CD83 mRNA, has not been investigated in detail. In the present work we analyzed this interaction in vitro and in vivo using various HuR- and CD83 mRNA mutants. We are able to demonstrate that both, RRM1 and RRM2 are crucial for binding, whereas RRM3 as well as the HuR hinge region contributed only marginally to this protein∶RNA interaction. Furthermore, mutation of uridine rich patches within the PRE did not disturb HuR:CD83 mRNA complex formation while, in contrast, the deletion of specific PRE subfragments from the CD83 mRNA prevented HuR binding in vitro and in vivo. Interestingly, the observed inhibition of HuR binding to CD83 mRNA does not lead to a nuclear trapping of the transcript but rather redirected this transcript from the CRM1- towards the NXF1/TAP-specific nuclear export pathway. Thus, the presence of a functional PRE permits nucleocytoplasmic trafficking of the CD83 transcript via the CRM1 pathway
On-chip picosecond pulse detection and generation using graphene photoconductive switches
We report on the use of graphene for room temperature on-chip detection and generation of pulsed terahertz (THz) frequency radiation, exploiting the fast carrier dynamics of light-generated hot carriers, and compare our results with conventional low-temperature-grown gallium arsenide (LT-GaAs) photoconductive (PC) switches. Coupling of picosecond-duration pulses from a biased graphene PC switch into Goubau line waveguides is also demonstrated. A Drude transport model based on the transient photoconductance of graphene is used to describe the mechanism for both detection and generation of THz radiation
Waveguide Coupled Resonance Fluorescence from On-Chip Quantum Emitter
Resonantly driven quantum emitters offer a very promising route to obtain highly coherent sources of single photons required for applications in quantum information processing (QIP). Realizing this for on-chip scalable devices would be important for scientific advances and practical applications in the field of integrated quantum optics. Here we report on-chip quantum dot (QD) resonance fluorescence (RF) efficiently coupled into a single-mode waveguide, a key component of a photonic integrated circuit, with a negligible resonant laser background and show that the QD coherence is enhanced by more than a factor of 4 compared to off-resonant excitation. Single-photon behavior is confirmed under resonant excitation, and fast fluctuating charge dynamics are revealed in autocorrelation g(2) measurements. The potential for triggered operation is verified in pulsed RF. These results pave the way to a novel class of integrated quantum-optical devices for on-chip quantum information processing with embedded resonantly driven quantum emitters
OH-defects in multiple-doped orthoenstatite at 4–8 GPa: filling the gap between pure and natural systems
Microcavity-integrated graphene photodetector
The monolithic integration of novel nanomaterials with mature and established
technologies has considerably widened the scope and potential of nanophotonics.
For example, the integration of single semiconductor quantum dots into photonic
crystals has enabled highly efficient single-photon sources. Recently, there
has also been an increasing interest in using graphene - a single atomic layer
of carbon - for optoelectronic devices. However, being an inherently weak
optical absorber (only 2.3 % absorption), graphene has to be incorporated into
a high-performance optical resonator or waveguide to increase the absorption
and take full advantage of its unique optical properties. Here, we demonstrate
that by monolithically integrating graphene with a Fabry-Perot microcavity, the
optical absorption is 26-fold enhanced, reaching values >60 %. We present a
graphene-based microcavity photodetector with record responsivity of 21 mA/W.
Our approach can be applied to a variety of other graphene devices, such as
electro-absorption modulators, variable optical attenuators, or light emitters,
and provides a new route to graphene photonics with the potential for
applications in communications, security, sensing and spectroscopy.Comment: 19 pages, 4 figure
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