arXiv subjects

Arne Ludwig

Publications and source records attributed to Arne Ludwig.

180 records · Page 10Linked to original sources

arXiv2015

Opposites Attract: Virtual Cluster Embedding for Profit

It is well-known that cloud application performance critically depends on the network. Accordingly, new abstractions for cloud applications are proposed which extend the performance isolation guarantees to the network. The most common abstraction is the Virtual Cluster V C(n, b): the n virtual machines of a customer are connected to a virtual switch at bandwidth b. However, today, not much is known about how to efficiently embed and price virtual clusters. This paper makes two contributions. (1) We present an algorithm called Tetris that efficiently embeds virtual clusters arriving in an online fashion, by jointly optimizing the node and link resources. We show that this algorithm allows to multiplex more virtual clusters over the same physical infrastructure compared to existing algorithms, hence improving the provider profit. (2) We present the first demand-specific pricing model called DSP for virtual clusters. Our pricing model is fair in the sense that a customer only needs to pay for what he or she asked. Moreover, it features other desirable properties, such as price independence from mapping locations.

cs.NI↗

arXiv2015

Single-electron Spin Resonance in a Quadruple Quantum Dot

Electron spins in semiconductor quantum dots are good candidates of quantum bits for quantum information processing. Basic operations of the qubit have been realized in recent years: initialization, manipulation of single spins, two qubit entanglement operations, and readout. Now it becomes crucial to demonstrate scalability of this architecture by conducting spin operations on a scaled up system. Here, we demonstrate single-electron spin resonance in a quadruple quantum dot. A few-electron quadruple quantum dot is formed within a magnetic field gradient created by a micro-magnet. We oscillate the wave functions of the electrons in the quantum dots by applying microwave voltages and this induces electron spin resonance. The resonance energies of the four quantum dots are slightly different because of the stray field created by the micro-magnet and therefore frequency-resolved addressable control of the electron spin resonance is possible.

cond-mat.mes-hall↗

arXiv2015

Nanoscale nonlinear effects in Erbium-implanted Yttrium Orthosilicate

Doping of substrates at desired locations is a key technology for spin-based quantum memory devices. Focused ion beam implantation is well-suited for this task due to its high spacial resolution. In this work, we investigate ion-beam implanted erbium ensembles in Yttrium Orthosilicate crystals by means of confocal photoluminescence spectroscopy. The sample temperature and the post-implantation annealing step strongly reverberate in the properties of the implanted ions. We find that hot implantation leads to a higher activation rate of the ions. At high enough fluences, the relation between the fluence and final concentration of ions becomes non-linear. Two models are developed explaining the observed behaviour.

cond-mat.mtrl-sci↗

arXiv2015

Manipulation of the nuclear spin ensemble in a quantum dot with chirped magnetic resonance pulses

The nuclear spins in nanostructured semiconductors play a central role in quantum applications. The nuclear spins represent a useful resource for generating local magnetic fields but nuclear spin noise represents a major source of dephasing for spin qubits. Controlling the nuclear spins enhances the resource while suppressing the noise. NMR techniques are challenging: the group III and V isotopes have large spins with widely different gyromagnetic ratios; in strained material there are large atom-dependent quadrupole shifts; and nanoscale NMR is hard to detect. We report NMR on 100,000 nuclear spins of a quantum dot using chirped radiofrequency pulses. Following polarization, we demonstrate a reversal of the nuclear spin. We can flip the nuclear spin back and forth a hundred times.We demonstrate that chirped NMR is a powerful way of determining the chemical composition, the initial nuclear spin temperatures and quadrupole frequency distributions for all the main isotopes. The key observation is a plateau in the NMR signal as a function of sweep rate: we achieve inversion at the first quantum transition for all isotopes simultaneously. These experiments represent a generic technique for manipulating nanoscale inhomogeneous nuclear spin ensembles and open the way to probe the coherence of such mesoscopic systems.

cond-mat.mes-hall↗

arXiv2015

Interplay of Electron and Nuclear Spin Noise in GaAs

We present spin noise (SN) measurements on an ensemble of donor-bound electrons in ultrapure GaAs:Si covering temporal dynamics over six orders of magnitude from milliseconds to nanoseconds. The SN spectra detected at the donor-bound exciton transition show the multifaceted dynamical regime of the ubiquitous mutual electron and nuclear spin interaction typical for III-V based semiconductor systems. The experiment distinctly reveals the finite Overhauser-shift of an electron spin precession at zero external magnetic field and a second contribution around zero frequency stemming from the electron spin components parallel to the nuclear spin fluctuations. Moreover, at very low frequencies features related with time-dependent nuclear spin fluctuations are clearly resolved making it possible to study the intricate nuclear spin dynamics at zero and low magnetic fields. The findings are in agreement with the developed model of electron and nuclear SN.

cond-mat.mes-hall↗

arXiv2015

Epitaxial lift-off for solid-state cavity quantum electrodynamics

We present a new approach to incorporate self-assembled quantum dots into a Fabry-Pérot-like microcavity. Thereby a 3$λ$/4 GaAs layer containing quantum dots is epitaxially removed and attached by van der Waals bonding to one of the microcavity mirrors. We reach a finesse as high as 4,100 with this configuration limited by the reflectivity of the dielectric mirrors and not by scattering at the semiconductor - mirror interface, demonstrating that the epitaxial lift-off procedure is a promising procedure for cavity quantum electrodynamics in the solid state. As a first step in this direction, we demonstrate a clear cavity-quantum dot interaction in the weak coupling regime with a Purcell factor in the order of 3. Estimations of the coupling strength via the Purcell factor suggests that we are close to the strong coupling regime.

quant-ph↗

arXiv2015

Strong Coupling of Intersubband Resonance in a High Electron Mobility Transistor Structure to a THz Metamaterial by Ultrawide Electrical Tuning

The interaction between intersubband resonances (ISRs) and metamaterial microcavities can form a strongly coupled system where new resonances form that depend on the coupling strength. Here we present experimental evidence of strong coupling between the cavity resonance of a THz metamaterial and the ISR in a high electron mobility transistor structure with a triangular confinement. The device is electrically switched from an uncoupled to a strongly coupled regime by tuning the ISR with epitaxially grown transparent gates. An asymmetric triangular potential in the heterostructure enables ultrawide electrical tuning of ISR which is an order of magnitude higher as compared to the equivalent square well. For a single triangular well, we achieve a coupling strength of 0.52 THz, with a normalized coupling ratio of 0.26.

cond-mat.mes-hall↗

arXiv2015

Single photoelectron spin detection and angular momentum transfer in a gate defined quantum dot

Recent innovations in fabricating nanoscale confined spin systems have enabled investigation of fundamental quantum correlations between single quanta of photons and matter states. Realization of quantum state transfer from photon polarization to electron spin using gate defined quantum dots (QDs) may give evidence of preserved coherence of angular momentum basis states at the photon-spin interface. The interface would enlarge the concept of quantum information technology, in which single photogenerated electron spins are manipulated with the dots, but this remains a serious challenge. Here, we report the detection of single electron spins generated by polarized single photons via a double QD (DQD) to verify the angular momentum transfer from single photons to single electrons. Pauli spin blockade (PSB) is used to project the photoelectron spin state onto the up or down spin state. Our result promises the realization of coherent quantum state transfer and development of hybrid photon and spin quantum technology.

cond-mat.mes-hall↗

arXiv2014

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.

cond-mat.mes-hall↗

arXiv2014

Quantum manipulation of two-electron spin states in metastable double quantum dots

We studied experimentally the dynamics of the exchange interaction between two antiparallel electron spins in a so-called metastable double quantum dot where coupling to the electron reservoirs can be ignored. We demonstrate that the level of control of such a double dot is higher than in conventional double dots. In particular, it allows to couple coherently two electron spins in an efficient manner following a scheme initially proposed by Loss and DiVincenzo. The present study demonstrates that metastable quantum dots are a possible route to increase the number of coherently coupled quantum dots.

cond-mat.mes-hall↗

arXiv2014

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.

cond-mat.mes-hall↗

arXiv2014

Infrared transmission spectroscopy of charge carriers in self-assembled InAs quantum dots under surface electric fields

We present a study on the intersublevel spacings of electrons and holes in a single layer of InAs self-assembled quantum dots (SAQDs) using Fourier transform infrared (FTIR) transmission spectroscopy without the application of an external magnetic field. Epitaxial, complementary-doped and semi-transparent electrostatic gates are grown within the ultra high vacuum conditions of molecular beam epitaxy to voltage-tune the device, while a two dimensional electron gas (2DEG) serves as back contact. Spacings of the hole sublevels are indirectly calculated using the photoluminescence spectroscopy along with FTIR spectroscopy. The observed spacings fit well to the calculated values for both electrons and holes. Additionally, the intersubband resonances of the 2DEG are enhanced due to the QD layer on top of the device.

cond-mat.mes-hall↗

arXiv2014

A small mode volume tunable microcavity: development and characterization

We report the realization of a spatial and spectrally tunable air-gap Fabry-Perot type microcavity of high finesse and cubic-wavelength-scale mode volume. These properties are attractive in the fields of opto-mechanics, quantum sensing and foremost cavity quantum electrodymanics. The major design feature is a miniaturized concave mirror with atomically smooth surface and radius of curvature as low as 10 micrometer produced by CO2 laser ablation of fused silica. We demonstrate excellent mode-matching of a focussed laser beam to the microcavity mode and confirm from the frequencies of the resonator modes that the effective optical radius matches the physical radius. With these small radii, we demonstrate sub-wavelength beam waists. We also show that the microcavity is sufficiently rigid for practical applications: in a cryostat at 4K, the root-mean-square microcavity length fluctuations are below 5 pm.

cond-mat.mes-hall↗

arXiv2014

Photoluminescence of focused ion beam implanted Er$^{3+}$:Y$_{2}$SiO$_{5}$ crystals

Erbium doped low symmetry Y$_2$SiO$_5$ crystals attract a lot of attention in perspective of quantum information applications. However, only doping of the samples during growth is available up to now, which yields a quite homogeneous doping density. In the present work, we deposit Er$^{3+}$-ions by the focused ion beam technique at Yttrium sites with several fluences in one sample. With a photoluminescence study of these locally doped Er$^{3+}$:Y$_2$SiO$_5$ crystals, we are able to evaluate the efficiency of the implantation process and develop it for the highest efficiency possible. We observe the dependence of the ion activation after the post-implantation annealing on the fluence value.

cond-mat.mtrl-sci↗

arXiv2014

Asymmetry of charge relaxation times in quantum dots: The influence of degeneracy

Using time-resolved transconductance spectroscopy, we study the tunneling dynamics between a two-dimensional electron gas (2DEG) and self-assembled quantum dots (QDs), embedded in a field-effect transistor structure. We find that the tunneling of electrons from the 2DEG into the QDs is governed by a different time constant than the reverse process, i.e., tunneling from the QDs to the 2DEG. This asymmetry is a clear signature of Coulomb interaction and makes it possible to determine the degeneracy of the quantum dot orbitals even when the individual states cannot be resolved energetically because of inhomogeneous broadening. Our experimental data can be qualitatively explained within a master-equation approach.

cond-mat.mes-hall↗

arXiv2013

A frequency-stabilized source of single photons from a solid-state qubit

Single quantum dots are solid-state emitters which mimic two-level atoms but with a highly enhanced spontaneous emission rate. A single quantum dot is the basis for a potentially excellent single photon source. One outstanding problem is that there is considerable noise in the emission frequency, making it very difficult to couple the quantum dot to another quantum system. We solve this problem here with a dynamic feedback technique that locks the quantum dot emission frequency to a reference. The incoherent scattering (resonance fluorescence) represents the single photon output whereas the coherent scattering (Rayleigh scattering) is used for the feedback control. The fluctuations in emission frequency are reduced to 20 MHz, just ~ 5% of the quantum dot optical linewidth, even over several hours. By eliminating the 1/f-like noise, the relative fluctuations in resonance fluorescence intensity are reduced to ~ 10E-5 at low frequency. Under these conditions, the antibunching dip in the resonance fluorescence is described extremely well by the two-level atom result. The technique represents a way of removing charge noise from a quantum device.

cond-mat.mes-hall↗

arXiv2013

A dark-field microscope for background-free detection of resonance fluorescence from single semiconductor quantum dots operating in a set-and-forget mode

Optically active quantum dots, for instance self-assembled InGaAs quantum dots, are potentially excellent single photon sources. The fidelity of the single photons is much improved using resonant rather than non-resonant excitation. With resonant excitation, the challenge is to distinguish between resonance fluorescence and scattered laser light. We have met this challenge by creating a polarization-based dark-field microscope to measure the resonance fluorescence from a single quantum dot at low temperature. We achieve a suppression of the scattered laser exceeding a factor of 10^7 and background-free detection of resonance fluorescence. The same optical setup operates over the entire quantum dot emission range 920-980 nm and also in high magnetic fields. The major development is the outstanding long-term stability: once the dark-field point has been established, the microscope operates for days without alignment. The mechanical and optical design of the microscope is presented, as well as exemplary resonance fluorescence spectroscopy results on individual quantum dots to underline the microscope's excellent performance.

cond-mat.mes-hall↗

arXiv2013

Charge noise and spin noise in a semiconductor quantum device

Solid-state systems which mimic two-level atoms are being actively developed. Improving the quantum coherence of these systems, for instance spin qubits or single photon emitters using semiconductor quantum dots, involves dealing with noise. The sources of noise are inherent to the semiconductor and are complex. Charge noise results in a fluctuating electric field, spin noise in a fluctuating magnetic field at the location of the qubit, and both can lead to dephasing and decoherence of optical and spin states. We investigate noise in an ultra-pure semiconductor using a minimally-invasive, ultra-sensitive, local probe: resonance fluorescence from a single quantum dot. We distinguish between charge noise and spin noise via a crucial difference in their optical signatures. Noise spectra for both electric and magnetic fields are derived. The noise spectrum of the charge noise can be fully described by the fluctuations in an ensemble of localized charge defects in the semiconductor. We demonstrate the "semiconductor vacuum" for the optical transition at frequencies above 50 kHz: by operating the device at high enough frequencies, we demonstrate transform-limited quantum dot optical linewidths.

cond-mat.mes-hall↗