26 research outputs found
Single-shot readout of hole spins in Ge
The strong atomistic spin orbit coupling of holes makes single-shot spin
readout measurements difficult because it reduces the spin lifetimes. By
integrating the charge sensor into a high bandwidth radio-frequency
reflectometry setup we were able to demonstrate single-shot readout of a
germanium quantum dot hole spin and measure the spin lifetime. Hole spin
relaxation times of about 90 s at 500\,mT are reported. By analysing
separately the spin-to-charge conversion and charge readout fidelities insight
into the processes limiting the visibilities of hole spins has been obtained.
The analyses suggest that very high hole visibilities are feasible at realistic
experimental conditions underlying the potential of hole spins for the
realization of viable qubit devices
Ge hole spin qubit
Holes confined in quantum dots have gained considerable interest in the past
few years due to their potential as spin qubits. Here we demonstrate double
quantum dot devices in Ge hut wires. Low temperature transport measurements
reveal Pauli spin blockade. We demonstrate electric-dipole spin resonance by
applying a radio frequency electric field to one of the electrodes defining the
double quantum dot. Next, we induce coherent hole spin oscillations by varying
the duration of the microwave burst. Rabi oscillations with frequencies
reaching 140MHz are observed. Finally, Ramsey experiments reveal dephasing
times of 130ns. The reported results emphasize the potential of Ge as a
platform for fast and scalable hole spin qubit devices
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Optical properties of individual site-controlled Ge quantum dots
We report photoluminescence (PL) experiments on individual SiGe quantum dots (QDs) that were epitaxially grown in a site-controlled fashion on pre-patterned Si(001) substrates. We demonstrate that the PL line-widths of single QDs decrease with excitation power to about 16 meV, a value that is much narrower than any of the previously reported PL signals in the SiGe/Si heterosystem. At low temperatures, the PL-intensity becomes limited by a 25 meV high potential-barrier between the QDs and the surrounding Ge wetting layer (WL). This barrier impedes QD filling from the WL which collects and traps most of the optically excited holes in this type-II heterosystem.
This work was supported by the Austrian Science Funds (FWF) via Schrödinger Scholarship J3328-N19 and the Project Nos. F2502-N17 and F2512-N17 of SFB025: IRON. M.G. and O.G.S. acknowledge support from the Center for Advancing Electronics Dresden, CfAED. T.T. was supported by the ICR-KU International Short-term Exchange Program for Young Researchers. The authors thank T. Fromherz and F. Hackl for helpful discussions
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Photoluminescence investigation of strictly ordered Ge dots grown on pit-patterned Si substrates
We investigate the optical properties of ordered Ge quantum dots (QDs) by means of micro-photoluminescence spectroscopy (PL). These were grown on pit-patterned Si(001) substrates with a wide range of pit-periods and thus inter QD-distances (425–3400 nm). By exploiting almost arbitrary inter-QD distances achievable in this way we are able to choose the number of QDs that contribute to the PL emission in a range between 70 and less than three QDs. This well-defined system allows us to clarify, by PL-investigation, several points which are important for the understanding of the formation and optical properties of ordered QDs. We directly trace and quantify the amount of Ge transferred from the surrounding wetting layer (WL) to the QDs in the pits. Moreover, by exploiting different pit-shapes, we reveal the role of strain-induced activation energy barriers that have to be overcome for charge carriers generated outside the dots. These need to diffuse between the energy minimum of the WL in and between the pits, and the one in the QDs. In addition, we demonstrate that the WL in the pits is already severely intermixed with Si before upright QDs nucleate, which further enhances intermixing of ordered QDs as compared to QDs grown on planar substrates. Furthermore, we quantitatively determine the amount of Ge transferred by surface diffusion through the border region between planar and patterned substrate. This is important for the growth of ordered islands on patterned fields of finite size. We highlight that the Ge WL-facets in the pits act as PL emission centres, similar to upright QDs
Free-running Sn precipitates : An efficient phase separation mechanism for metastable GeSn epilayers
g-Factor Tuning and Manipulation of Spins by an Electric Current
We investigate the Zeeman splitting of two-dimensional electrons in an
asymmetric silicon quantum well, by electron-spin-resonance (ESR) experiments.
Applying a small dc current we observe a shift in the resonance field due to
the additional current-induced Bychkov-Rashba (BR) type of spin-orbit (SO)
field. This finding demonstrates SO coupling in the most straightforward way:
in the presence of a transverse electric field the drift velocity of the
carriers imposes an effective SO magnetic field. This effect allows selective
tuning of the g-factor by an applied dc current. In addition, we show that an
ac current may be used to induce spin resonance very efficiently.Comment: 4 pages, 4 figure
Single SiGe Quantum Dot Emission Deterministically Enhanced in a High-Q Photonic Crystal Resonator
We report the resonantly enhanced radiative emission from a single SiGe
quantum dot (QD), which is deterministically embedded into a bichromatic
photonic crystal resonator (PhCR) at the position of its largest modal electric
field by a scalable method. By optimizing our molecular beam epitaxy (MBE)
growth technique, we were able to reduce the amount of Ge within the whole
resonator to obtain an absolute minimum of exactly one QD, accurately
positioned by lithographic methods relative to the PhCR, and an otherwise flat,
a few monolayer thin, Ge wetting layer (WL). With this method, record quality
(Q) factors for QD-loaded PhCRs up to are achieved. A comparison
with control PhCRs on samples containing a WL but no QDs is presented, as well
as a detailed analysis of the dependence of the resonator-coupled emission on
temperature, excitation intensity, and emission decay after pulsed excitation.
Our findings undoubtedly confirm a single QD in the center of the resonator as
a potentially novel photon source in the telecom spectral range