arXiv subjects

Antonio Badolato

Publications and source records attributed to Antonio Badolato.

At least 19 recordsLinked to original sources

arXiv2017

Ultra-wide-band slow light in photonic crystal coupled-cavity waveguides

Slow light propagation in structured materials is a highly promising approach for realizing on-chip integrated photonic devices based on enhanced optical nonlinearities. One of the most successful research avenues consists in engineering the band dispersion of light-guiding photonic crystal (PC) structures. The primary goal of such devices is to achieve slow-light operation over the largest possible bandwidth, with large group index, minimal index dispersion, and constant transmission spectrum. Here, we report on the experimental demonstration of to date record high GBP in silicon-based coupled-cavity waveguides (CCWs) operating at telecom wavelengths. Our results rely on novel CCW designs, optimized using a genetic algorithm, and refined nanofabrication processes.

physics.optics↗

arXiv2016

Cryogenic photoluminescence imaging system for nanoscale positioning of single quantum emitters

We report a photoluminescence imaging system for locating single quantum emitters with respect to alignment features. Samples are interrogated in a 4~K closed-cycle cryostat by a high numerical aperture (NA=0.9, 100$\times$ magnification) objective that sits within the cryostat, enabling high efficiency collection of emitted photons without image distortions due to the cryostat windows. The locations of single InAs/GaAs quantum dots within a $>50$~$μ$m~$\times$~50~$μ$m field of view are determined with $\approx4.5$~nm uncertainty (one standard deviation) in a 1~s long acquisition. The uncertainty is determined through a combination of a maximum likelihood estimate for localizing the quantum dot emission, and a cross-correlation method for determining the alignment mark center. This location technique can be an important step in the high-throughput creation of nanophotonic devices that rely upon the interaction of highly confined optical modes with single quantum emitters.

quant-ph↗

arXiv2016

Combined atomic force microscopy and photoluminescence imaging to select single InAs/GaAs quantum dots for quantum photonic devices

We report on a combined photoluminescence imaging and atomic force microscopy study of single, isolated self-assembled InAs quantum dots (density <0.01 um^(-2) capped by a 95 nm GaAs layer, and emitting around 950 nm. By combining optical and scanning probe characterization techniques, we determine the position of single quantum dots with respect to comparatively large (100 nm to 1000 nm in-plane dimension) topographic features. We find that quantum dots often appear (>25% of the time) in the vicinity of these features, but generally do not exhibit significant differences in their non-resonantly pumped emission spectra in comparison to quantum dots appearing in defect-free regions. This behavior is observed across multiple wafers produced in different growth chambers. Our characterization approach is relevant to applications in which single quantum dots are embedded within nanofabricated photonic devices, where such large surface features can affect the interaction with confined optical fields and the quality of the single-photon emission. In particular, we anticipate using this approach to screen quantum dots not only based on their optical properties, but also their surrounding surface topographies.

cond-mat.mes-hall↗

arXiv2015

Nanoscale optical positioning of single quantum dots for bright and pure single-photon emission

Self-assembled, epitaxially-grown InAs/GaAs quantum dots are promising semiconductor quantum emitters that can be integrated on a chip for a variety of photonic quantum information science applications. However, self-assembled growth results in an essentially random in-plane spatial distribution of quantum dots, presenting a challenge in creating devices that exploit the strong interaction of single quantum dots with highly confined optical modes. Here, we present a photoluminescence imaging approach for locating single quantum dots with respect to alignment features with an average position uncertainty < 30 nm (< 10 nm when using a solid immersion lens), which represents an enabling technology for the creation of optimized single quantum dot devices. To that end, we create quantum dot single-photon sources, based on a circular Bragg grating geometry, that simultaneously exhibit high collection efficiency (48 % +/- 5 % into a 0.4 numerical aperture lens, close to the theoretically predicted value of 50 %), low multiphoton probability (g(2)(0) <1 %), and a significant Purcell enhancement factor (~ 3).

cond-mat.mes-hall↗

arXiv2014

Multiple time scale blinking in InAs quantum dot single-photon sources

We use photon correlation measurements to study blinking in single, epitaxially-grown self-assembled InAs quantum dots situated in circular Bragg grating and microdisk cavities. The normalized second-order correlation function g(2)(τ) is studied across eleven orders of magnitude in time, and shows signatures of blinking over timescales ranging from tens of nanoseconds to tens of milliseconds. The g(2)(τ) data is fit to a multi-level system rate equation model that includes multiple non-radiating (dark) states, from which radiative quantum yields significantly less than 1 are obtained. This behavior is observed even in situations for which a direct histogramming analysis of the emission time-trace data produces inconclusive results.

physics.optics↗

arXiv2012

Scalable qubit architecture based on holes in quantum dot molecules

Spins confined in quantum dots are a leading candidate for solid-state quantum bits that can be coherently controlled by optical pulses. There are, however, many challenges to developing a scalable multibit information processing device based on spins in quantum dots, including the natural inhomogeneous distribution of quantum dot energy levels, the difficulty of creating all-optical spin manipulation protocols compatible with nondestructive readout, and the substantial electron-nuclear hyperfine interaction-induced decoherence. Here, we present a scalable qubit design and device architecture based on the spin states of single holes confined in a quantum dot molecule. The quantum dot molecule qubit enables a new strategy for optical coherent control with dramatically enhanced wavelength tunability. The use of hole spins allows the suppression of decoherence via hyperfine interactions and enables coherent spin rotations using Raman transitions mediated by a hole-spin-mixed optically excited state. Because the spin mixing is present only in the optically excited state, dephasing and decoherence are strongly suppressed in the ground states that define the qubits and nondestructive readout is possible. We present the qubit and device designs and analyze the wavelength tunability and fidelity of gate operations that can be implemented using this strategy. We then present experimental and theoretical progress toward implementing this design.

cond-mat.mes-hall↗

arXiv2012

Improving the performance of bright quantum dot single photon sources using amplitude modulation

Single epitaxially-grown semiconductor quantum dots have great potential as single photon sources for photonic quantum technologies, though in practice devices often exhibit non-ideal behavior. Here, we demonstrate that amplitude modulation can improve the performance of quantum-dot-based sources. Starting with a bright source consisting of a single quantum dot in a fiber-coupled microdisk cavity, we use synchronized amplitude modulation to temporally filter the emitted light. We observe that the single photon purity, temporal overlap between successive emission events, and indistinguishability can be greatly improved with this technique. As this method can be applied to any triggered single photon source, independent of geometry and after device fabrication, it is a flexible approach to improve the performance of solid-state systems, which often suffer from excess dephasing and multi-photon background emission.

quant-ph↗

arXiv2012

Two-photon interference using background-free quantum frequency conversion of single photons from a semiconductor quantum dot

We show that quantum frequency conversion (QFC) can overcome the spectral distinguishability common to inhomogeneously broadened solid-state quantum emitters. QFC is implemented by combining single photons from an InAs quantum dot (QD) at 980 nm with a 1550 nm pump laser in a periodically-poled lithium niobate (PPLN) waveguide to generate photons at 600 nm with a signal-to-background ratio exceeding 100:1. Photon correlation and two-photon interference measurements confirm that both the single photon character and wavepacket interference of individual QD states are preserved during frequency conversion. Finally, we convert two spectrally separate QD transitions to the same wavelength in a single PPLN waveguide and show that the resulting field exhibits non-classical two-photon interference.

quant-ph↗

arXiv2012

Background-free quantum frequency conversion of single photons from a semiconductor quantum dot

We demonstrate background-free quantum frequency conversion of single photons from an epitaxially-grown InAs quantum dot. Single photons at \approx 980 nm are combined with a pump laser near 1550 nm inside a periodically-poled lithium niobate (PPLN) waveguide, generating single photons at \approx 600 nm. The large red-detuning between the pump and signal wavelengths ensures nearly background-free conversion, avoiding processes such as upconversion of anti-Stokes Raman scattered pump photons in the PPLN crystal. Second-order correlation measurements on the single photon stream are performed both before and after conversion, confirming the preservation of photon statistics during the frequency conversion process.

quant-ph↗

arXiv2011

Bright single photon emission from a quantum dot in a circular Bragg grating microcavity

Bright single photon emission from single quantum dots in suspended circular Bragg grating microcavities is demonstrated. This geometry has been designed to achieve efficient (> 50 %) single photon extraction into a near-Gaussian shaped far-field pattern, modest (~10x) Purcell enhancement of the radiative rate, and a spectral bandwidth of a few nanometers. Measurements of fabricated devices show progress towards these goals, with collection efficiencies as high as ~10% demonstrated with moderate spectral bandwidth and rate enhancement. Photon correlation measurements are performed under above-bandgap excitation (pump wavelength = 780 nm to 820 nm) and confirm the single photon character of the collected emission. While the measured sources are all antibunched and dominantly composed of single photons, the multi-photon probability varies significantly. Devices exhibiting tradeoffs between collection efficiency, Purcell enhancement, and multi-photon probability are explored and the results are interpreted with the help of finite-difference time-domain simulations. Below-bandgap excitation resonant with higher states of the quantum dot and/or cavity (pump wavelength = 860 nm to 900 nm) shows a near-complete suppression of multi-photon events and may circumvent some of the aforementioned tradeoffs.

quant-ph↗

arXiv2011

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.

cond-mat.mes-hall↗

arXiv2011

Ultrafast all-optical switching by single photons

An outstanding goal in quantum optics is the realization of fast optical non-linearities at the single-photon level. Such non-linearities would allow for the realization of optical devices with new functionalities such as a single-photon switch/transistor or a controlled-phase gate, which could form the basis of future quantum optical technologies. While non-linear optics effects at the single-emitter level have been demonstrated in different systems, including atoms coupled to Fabry-Perot or toroidal micro-cavities, super-conducting qubits in strip-line resonators or quantum dots (QDs) in nano-cavities, none of these experiments so far has demonstrated single-photon switching on ultrafast timescales. Here, we demonstrate that in a strongly coupled QD-cavity system the presence of a single photon on one of the fundamental polariton transitions can turn on light scattering on a transition from the first to the second Jaynes-Cummings manifold with a switching time of 20 ps. As an additional device application, we use this non-linearity to implement a single-photon pulse-correlator. Our QD-cavity system could form the building-block of future high-bandwidth photonic networks operating in the quantum regime.

cond-mat.mes-hall↗

arXiv2011

Strongly correlated photons on a chip

Optical non-linearities at the single-photon level are key ingredients for future photonic quantum technologies. Prime candidates for the realization of strong photon-photon interactions necessary for implementing quantum information processing tasks as well as for studying strongly correlated photons in an integrated photonic device setting are quantum dots embedded in photonic crystal nanocavities. Here, we report strong quantum correlations between photons on picosecond timescales. We observe (a) photon antibunching upon resonant excitation of the lowest-energy polariton state, proving that the first cavity photon blocks the subsequent injection events, and (b) photon bunching when the laser field is in two-photon resonance with the polariton eigenstates of the second Jaynes-Cummings manifold, demonstrating that two photons at this color are more likely to be injected into the cavity jointly, than they would otherwise. Together,these results demonstrate unprecedented strong single-photon non-linearities, paving the way for realizing a single-photon transistor or a quantum optical Josephson interferometer.

cond-mat.mes-hall↗

arXiv2009

Resolution of the mystery of counter-intuitive photon correlations in far off-resonance emission from a quantum dot-cavity system

Cavity quantum-electrodynamics experiments using an atom coupled to a single radiation-field mode have played a central role in testing foundations of quantum mechanics, thus motivating solid-state implementations using single quantum dots coupled to monolithic nano-cavities. In stark contrast to their atom based counterparts, the latter experiments revealed strong cavity emission, even when the quantum dot is far off resonance. Here we present experimental and theoretical results demonstrating that this effect arises from the mesoscopic nature of quantum dot confinement, ensuring the presence of a quasi-continuum of transitions between excited quantum dot states that are enhanced by the cavity mode. Our model fully explains photon correlation measurements demonstrating that photons emitted at the cavity frequency are essentially uncorrelated with each other even though they are generated by a single quantum dot.

cond-mat.mes-hall↗

arXiv2008

Quantum dot spectroscopy using cavity QED

Cavity quantum electrodynamics has attracted substantial interest, both due to its potential role in the field of quantum information processing and as a testbed for basic experiments in quantum mechanics. Here, we show how cavity quantum electrodynamics using a tunable photonic crystal nanocavity in the strong coupling regime can be used for single quantum dot spectroscopy. From the distinctive avoided crossings observed in the strongly coupled system we can identify the neutral and single positively charged exciton as well as the biexciton transitions. Moreover we are able to investigate the fine structure of those transitions and to identify a novel cavity mediated mixing of bright and dark exciton states, where the hyperfine interactions with lattice nuclei presumably play a key role. These results are enabled by a deterministic coupling scheme which allowed us to achieve unprecedented coupling strengths in excess of 0.15 meV.

cond-mat.mes-hall↗

arXiv2007

Optical detection of single electron spin resonance in a quantum dot

We demonstrate optically detected spin resonance of a single electron confined to a self-assembled quantum dot. The dot is rendered dark by resonant optical pumping of the spin with a coherent laser. Contrast is restored by applying a radio frequency (rf) magnetic field at the spin resonance. The scheme is sensitive even to rf fields of just a few micro-T. In one case, the spin resonance behaves exactly as a driven 3-level quantum system (a lambda-system) with weak damping. In another, the dot exhibits remarkably strong (67% signal recovery) and narrow (0.34 MHz) spin resonances with fluctuating resonant positions, evidence of unusual dynamic processes of non-Markovian character.

cond-mat.other↗

arXiv2007

Strong Electron-Hole Exchange in Coherently Coupled Quantum Dots

We have investigated few-body states in vertically stacked quantum dots. Due to small inter-dot tunneling rate, the coupling in our system is in a previously unexplored regime where electron-hole exchange is the dominant spin interaction. By tuning the gate bias, we are able to turn this coupling off and study a complementary regime where total electron spin is a good quantum number. The use of differential transmission allows us to obtain unambiguous signatures of the interplay between electron and hole spin interactions. Small tunnel coupling also enables us to demonstrate all-optical charge sensing, where conditional exciton energy shift in one dot identifies the charging state of the coupled partner.

cond-mat.mes-hall↗

arXiv2007

Optical investigations of quantum-dot spin dynamics

We have performed all-optical measurements of spin relaxation in single self-assembled InAs/GaAs quantum dots (QD) as a function of static external electric and magnetic fields. To study QD spin dynamics we measure the degree of resonant absorption which results from a competition between optical spin pumping induced by the resonant laser field and spin relaxation induced by reservoirs. Fundamental interactions that determine spin dynamics in QDs are hyperfine coupling to QD nuclear spin ensembles, spin-phonon coupling and exchange-type interactions with a nearby Fermi sea of electrons. We show that the strength of spin relaxation generated by the three fundamental interactions can be changed by up to five orders of magnitude upon varying the applied electric and magnetic fields. We find that the strength of optical spin pumping that we use to study the spin relaxation is determined predominantly by hyperfine-induced mixing of single-electron spin states at low magnetic fields and heavy-light hole mixing at high magnetic fields. Our measurements allow us to determine the rms value of the hyperfine (Overhauser) field to be ~15 mTesla with an electron g-factor of g_e=0.6 and a hole mixing strength of |epsilon|^2 = 0.0005.

cond-mat.mes-hall↗