Machine learning enhanced evaluation of semiconductor quantum dots

A key challenge in quantum photonics today is the efficient and on-demand generation of high-quality single photons and entangled photon pairs. In this regard, one of the most promising types of emitters are semiconductor quantum dots, fluorescent nanostructures also described as artificial atoms. The main technological challenge in upscaling to an industrial level is the typically random spatial and spectral distribution in their growth. Furthermore, depending on the intended application, different requirements are imposed on a quantum dot, which are reflected in its spectral properties. Given that an in-depth suitability analysis is lengthy and costly, it is common practice to pre-select promising candidate quantum dots using their emission spectrum. Currently, this is done by hand. Therefore, to automate and expedite this process, in this paper, we propose a data-driven machine-learning-based method of evaluating the applicability of a semiconductor quantum dot as single photon source. For this, first, a minimally redundant, but maximally relevant feature representation for quantum dot emission spectra is derived by combining conventional spectral analysis with an autoencoding convolutional neural network. The obtained feature vector is subsequently used as input to a neural network regression model, which is specifically designed to not only return a rating score, gauging the technical suitability of a quantum dot, but also a measure of confidence for its evaluation. For training and testing, a large dataset of self-assembled InAs/GaAs semiconductor quantum dot emission spectra is used, partially labelled by a team of experts in the field. Overall, highly convincing results are achieved, as quantum dots are reliably evaluated correctly. Note, that the presented methodology can account for different spectral requirements and is applicable regardless of the underlying photonic structure, fabrication method and material composition. We therefore consider it the first step towards a fully integrated evaluation framework for quantum dots, proving the use of machine learning beneficial in the advancement of future quantum technologies.

Curved GaAs cantilever waveguides for the vertical coupling to photonic integrated circuits

We report the nanofabrication and characterization of optical spot-size converter couplers based on curved GaAs cantilever waveguides. Using the stress mismatch between the GaAs substrate and deposited Cr-Ni-Au strips, single-mode waveguides can be bent out-of-plane in a controllable manner. A stable and vertical orientation of the out-coupler is achieved by locking the spot-size converter at a fixed 90 ∘ angle via short-range forces. The optical transmission is characterized as a function of temperature and polarization, resulting in a broad-band chip-to-fiber coupling extending over 150 nm wavelength bandwidth at cryogenic temperatures, with the lower bound for the coupling efficiency into the TE mode being 16±2% in the interval 900-1050 nm. The methods reported here are fully compatible with quantum photonic integrated circuit technology with quantum dot emitters, and open opportunities to design novel photonic devices with enhanced functionality.

Accelerated Adiabatic Passage of a Single Electron Spin Qubit in Quantum Dots

Adiabatic processes can keep the quantum system in its instantaneous eigenstate, which is robust to noises and dissipation. However, it is limited by sufficiently slow evolution. Here, we experimentally demonstrate the transitionless quantum driving (TLQD) of the shortcuts to adiabaticity in gate-defined semiconductor quantum dots (QDs) to greatly accelerate the conventional adiabatic passage for the first time. For a given efficiency of quantum state transfer, the acceleration can be more than twofold. The dynamic properties also prove that the TLQD can guarantee fast and high-fidelity quantum state transfer. In order to compensate for the diabatic errors caused by dephasing noises, the modified TLQD is proposed and demonstrated in experiment by enlarging the width of the counterdiabatic drivings. The benchmarking shows that the state transfer fidelity of 97.8% can be achieved. This work will greatly promote researches and applications about quantum simulations and adiabatic quantum computation based on the gate-defined QDs.

Coherent control of a high-orbital hole in a semiconductor quantum dot

Coherently driven semiconductor quantum dots are one of the most promising platforms for non-classical light sources and quantum logic gates which form the foundation of photonic quantum technologies. However, to date, coherent manipulation of single charge carriers in quantum dots is limited mainly to their lowest orbital states. Ultrafast coherent control of high-orbital states is obstructed by the demand for tunable terahertz pulses. To break this constraint, we demonstrate an all-optical method to control high-orbital states of a hole via a stimulated Auger process. The coherent nature of the Auger process is proved by Rabi oscillation and Ramsey interference. Harnessing this coherence further enables the investigation of the single-hole relaxation mechanism. A hole relaxation time of 161 ps is observed and attributed to the phonon bottleneck effect. Our work opens new possibilities for understanding the fundamental properties of high-orbital states in quantum emitters and for developing new types of orbital-based quantum photonic devices.

Measuring Thermal Diffusivity of Azoheteroarene Thin Layers by Photothermal Beam Deflection and Photothermal Lens Methods

Measurement of thermal properties of thin films is challenging. In particular, thermal characterization is very difficult in semi-transparent samples. Here, we use two photothermal methods to obtain information about the thermal diffusivity as well as thermal conductivity of azoheteroarene functionalized polymer thin layers. The photothermal beam deflection (PBD) method is employed to gather data directly on thermal conductivity and thermal diffusivity, while the thermal lens (TL) method is employed to measure the effective thermal diffusivity. Consequently, the thermal diffusivity of the layers is indirectly estimated from the effective thermal diffusivity using a well-established theoretical relationship. Despite the utilization of distinct methods, our study reveals a remarkable consistency in the highly accurate results obtained from both approaches. This remarkable agreement reaffirms the reliability and mutual compatibility of the employed methods, highlighting their shared ability to provide accurate and congruent outcomes.

Independent Electrical Control of Two Quantum Dots Coupled through a Photonic-Crystal Waveguide

Efficient light-matter interaction at the single-photon level is of fundamental importance in emerging photonic quantum technology. A fundamental challenge is addressing multiple quantum emitters at once, as intrinsic inhomogeneities of solid-state platforms require individual tuning of each emitter. We present the realization of two semiconductor quantum dot emitters that are efficiently coupled to a photonic-crystal waveguide and individually controllable by applying a local electric Stark field. We present resonant transmission and fluorescence spectra in order to probe the coupling of the two emitters to the waveguide. We exploit the single-photon stream from one quantum dot to perform spectroscopy on the second quantum dot positioned 16  μm away in the waveguide. Furthermore, power-dependent resonant transmission measurements reveal signatures of coherent coupling between the emitters. Our work provides a scalable route to realizing multiemitter collective coupling, which has inherently been missing for solid-state deterministic photon emitters.

A Unipolar Quantum Dot Diode Structure for Advanced Quantum Light Sources

Triggered, indistinguishable single photons are crucial in various quantum photonic implementations. Here, we realize a novel n⁺-i-n⁺⁺ diode structure embedding semiconductor quantum dots: the gated device enables spectral tuning of the transitions and deterministic control of the charged states. Blinking-free single-photon emission and high two-photon indistinguishability are observed. The line width's temporal evolution is investigated across over 6 orders of magnitude time scales, combining photon-correlation Fourier spectroscopy, high-resolution photoluminescence spectroscopy, and two-photon interference (visibility of V_TPI,2ns = (85.8 ± 2.2)% and V_TPI,9ns = (78.3 ± 3.0)%). Most of the dots show no spectral broadening beyond ∼9 ns time scales, and the photons' line width ((420 ± 30) MHz) deviates from the Fourier-transform limit by a factor of 1.68. The combined techniques verify that most dephasing mechanisms occur at time scales ≤2 ns, despite their modest impact. The presence of n-doping implies higher carrier mobility, enhancing the device's appeal for high-speed tunable, high-performance quantum light sources.

Cavity-enhanced single-shot readout of a quantum dot spin within 3 nanoseconds

Rapid, high-fidelity single-shot readout of quantum states is a ubiquitous requirement in quantum information technologies. For emitters with a spin-preserving optical transition, spin readout can be achieved by driving the transition with a laser and detecting the emitted photons. The speed and fidelity of this approach is typically limited by low photon collection rates and measurement back-action. Here we use an open microcavity to enhance the optical readout signal from a semiconductor quantum dot spin state, largely overcoming these limitations. We achieve single-shot readout of an electron spin in only 3 nanoseconds with a fidelity of (95.2 ± 0.7)%, and observe quantum jumps using repeated single-shot measurements. Owing to the speed of our readout, errors resulting from measurement-induced back-action have minimal impact. Our work reduces the spin readout-time well below both the achievable spin relaxation and dephasing times in semiconductor quantum dots, opening up new possibilities for their use in quantum technologies.

Controlled Coherent Coupling in a Quantum Dot Molecule Revealed by Ultrafast Four-Wave Mixing Spectroscopy

Semiconductor quantum dot molecules are considered promising candidates for quantum technological applications due to their wide tunability of optical properties and coverage of different energy scales associated with charge and spin physics. While previous works have studied the tunnel-coupling of the different excitonic charge complexes shared by the two quantum dots by conventional optical spectroscopy, we here report on the first demonstration of a coherently controlled interdot tunnel-coupling focusing on the quantum coherence of the optically active trion transitions. We employ ultrafast four-wave mixing spectroscopy to resonantly generate a quantum coherence in one trion complex, transfer it to and probe it in another trion configuration. With the help of theoretical modeling on different levels of complexity, we give an instructive explanation of the underlying coupling mechanism and dynamical processes.

High-speed thin-film lithium niobate quantum processor driven by a solid-state quantum emitter

Scalable photonic quantum computing architectures pose stringent requirements on photonic processing devices. The needs for low-loss high-speed reconfigurable circuits and near-deterministic resource state generators are some of the most challenging requirements. Here, we develop an integrated photonic platform based on thin-film lithium niobate and interface it with deterministic solid-state single-photon sources based on quantum dots in nanophotonic waveguides. The generated photons are processed with low-loss circuits programmable at speeds of several gigahertz. We realize a variety of key photonic quantum information processing functionalities with the high-speed circuits, including on-chip quantum interference, photon demultiplexing, and reprogrammability of a four-mode universal photonic circuit. These results show a promising path forward for scalable photonic quantum technologies by merging integrated photonics with solid-state deterministic photon sources in a heterogeneous approach to scaling up.

Coulomb-mediated antibunching of an electron pair surfing on sound

Electron flying qubits are envisioned as potential information links within a quantum computer, but also promise-like photonic approaches-to serve as self-standing quantum processing units. In contrast to their photonic counterparts, electron-quantum-optics implementations are subject to Coulomb interactions, which provide a direct route to entangle the orbital or spin degree of freedom. However, controlled interaction of flying electrons at the single-particle level has not yet been established experimentally. Here we report antibunching of a pair of single electrons that is synchronously shuttled through a circuit of coupled quantum rails by means of a surface acoustic wave. The in-flight partitioning process exhibits a reciprocal gating effect which allows us to ascribe the observed repulsion predominantly to Coulomb interaction. Our single-shot experiment marks an important milestone on the route to realize a controlled-phase gate for in-flight quantum manipulations.

Photon bound state dynamics from a single artificial atom

The interaction between photons and a single two-level atom constitutes a fundamental paradigm in quantum physics. The nonlinearity provided by the atom leads to a strong dependence of the light-matter interface on the number of photons interacting with the two-level system within its emission lifetime. This nonlinearity unveils strongly correlated quasiparticles known as photon bound states, giving rise to key physical processes such as stimulated emission and soliton propagation. Although signatures consistent with the existence of photon bound states have been measured in strongly interacting Rydberg gases, their hallmark excitation-number-dependent dispersion and propagation velocity have not yet been observed. Here we report the direct observation of a photon-number-dependent time delay in the scattering off a single artificial atom-a semiconductor quantum dot coupled to an optical cavity. By scattering a weak coherent pulse off the cavity-quantum electrodynamics system and measuring the time-dependent output power and correlation functions, we show that single photons and two- and three-photon bound states incur different time delays, becoming shorter for higher photon numbers. This reduced time delay is a fingerprint of stimulated emission, where the arrival of two photons within the lifetime of an emitter causes one photon to stimulate the emission of another.

Integration of GaAs waveguides on a silicon substrate for quantum photonic circuits

We report a method for integrating GaAs waveguide circuits containing self-assembled quantum dots on a Si/SiO₂ wafer, using die-to-wafer bonding. The large refractive-index contrast between GaAs and SiO₂ enables fabricating single-mode waveguides without compromising the photon-emitter coupling. Anti-bunched emission from individual quantum dots is observed, along with a waveguide propagation loss <7 dB/mm, which is comparable with the performance of suspended GaAs circuits. These results enable the integration of quantum emitters with different material platforms, towards the realization of scalable quantum photonic integrated circuits.

Real-Time Observation of Charge-Spin Cooperative Dynamics Driven by a Nonequilibrium Phonon Environment

We report on experimental observations of charge-spin cooperative dynamics of two-electron states in a GaAs double quantum dot located in a nonequilibrium phonon environment. When the phonon energy exceeds the lowest excitation energy in the quantum dot, the spin-flip rate of a single electron strongly enhances. In addition, originated from the spatial gradient of phonon density between the dots, the parallel spin states become more probable than the antiparallel ones. These results indicate that spin is essential for further demonstrations of single-electron thermodynamic systems driven by phonons, which will greatly contribute to understanding of the fundamental physics of thermoelectric devices.

Quantum interference of identical photons from remote GaAs quantum dots

Photonic quantum technology provides a viable route to quantum communication^1,2, quantum simulation³ and quantum information processing⁴. Recent progress has seen the realization of boson sampling using 20 single photons³ and quantum key distribution over hundreds of kilometres². Scaling the complexity requires architectures containing multiple photon sources, photon counters and a large number of indistinguishable single photons. Semiconductor quantum dots are bright and fast sources of coherent single photons^5-9. For applications, a roadblock is the poor quantum coherence on interfering single photons created by independent quantum dots^10,11. Here we demonstrate two-photon interference with near-unity visibility (93.0 ± 0.8)% using photons from two completely separate GaAs quantum dots. The experiment retains all the emission into the zero phonon line-only the weak phonon sideband is rejected; temporal post-selection is not employed. By exploiting quantum interference, we demonstrate a photonic controlled-not circuit and an entanglement with fidelity of (85.0 ± 1.0)% between photons of different origins. The two-photon interference visibility is high enough that the entanglement fidelity is well above the classical threshold. The high mutual coherence of the photons stems from high-quality materials, diode structure and relatively large quantum dot size. Our results establish a platform-GaAs quantum dots-for creating coherent single photons in a scalable way.

Entangling a Hole Spin with a Time-Bin Photon: A Waveguide Approach for Quantum Dot Sources of Multiphoton Entanglement

Deterministic sources of multiphoton entanglement are highly attractive for quantum information processing but are challenging to realize experimentally. In this Letter, we demonstrate a route toward a scaleable source of time-bin encoded Greenberger-Horne-Zeilinger and linear cluster states from a solid-state quantum dot embedded in a nanophotonic crystal waveguide. By utilizing a self-stabilizing double-pass interferometer, we measure a spin-photon Bell state with (67.8±0.4)% fidelity and devise steps for significant further improvements. By employing strict resonant excitation, we demonstrate a photon indistinguishability of (95.7±0.8)%, which is conducive to fusion of multiple cluster states for scaling up the technology and producing more general graph states.

Integrated Whispering-Gallery-Mode Resonator for Solid-State Coherent Quantum Photonics

Tailored photonics cavities enhance light-matter interactions, ultimately enabling a fully coherent quantum interface. Here, we report an integrated microdisk cavity containing self-assembled quantum dots to coherently route photons between different access waveguides. We measure a Purcell factor of F_exp = 6.9 ± 0.9 for a cavity quality factor of about 10,000, allowing us to observe clear signatures of coherent scattering of photons by the quantum dots. We show how this integrated system can coherently reroute photons between the drop and bus ports and how this routing is controlled by detuning the quantum dot and resonator or through the strength of the excitation beam, where a critical photon number less than one photon per lifetime is required. We discuss the strengths and limitations of this approach, focusing on how the coherent scattering and single-photon nonlinearity can be used to increase the efficiency of quantum devices such as routers or Bell-state analyzers.

Charge Tunable GaAs Quantum Dots in a Photonic n-i-p Diode

In this submission, we discuss the growth of charge-controllable GaAs quantum dots embedded in an n-i-p diode structure, from the perspective of a molecular beam epitaxy grower. The QDs show no blinking and narrow linewidths. We show that the parameters used led to a bimodal growth mode of QDs resulting from low arsenic surface coverage. We identify one of the modes as that showing good properties found in previous work. As the morphology of the fabricated QDs does not hint at outstanding properties, we attribute the good performance of this sample to the low impurity levels in the matrix material and the ability of n- and p-doped contact regions to stabilize the charge state. We present the challenges met in characterizing the sample with ensemble photoluminescence spectroscopy caused by the photonic structure used. We show two straightforward methods to overcome this hurdle and gain insight into QD emission properties.

Distant spin entanglement via fast and coherent electron shuttling

In the quest for large-scale quantum computing, networked quantum computers offer a natural path towards scalability. While recent experiments have demonstrated nearest neighbour entanglement for electron spin qubits in semiconductors, on-chip long-distance entanglement could bring more versatility to connect quantum core units. Here, we employ the moving trapping potential of a surface acoustic wave to realize the controlled and coherent transfer of a pair of entangled electron spins between two distant quantum dots. The subsequent electron displacement induces coherent spin rotations, which drives spin quantum interferences. We observe high-contrast interference as a signature of the preservation of the entanglement all along the displacement procedure, which includes a separation of the two spins by a distance of 6 μm. This work opens the route towards fast on-chip deterministic interconnection of remote quantum bits in semiconductor quantum circuits.

Coherent control of individual electron spins in a two-dimensional quantum dot array

The coherent manipulation of individual quantum objects organized in arrays is a prerequisite to any scalable quantum information platform. The cumulated efforts to control electron spins in quantum dot arrays have permitted the recent realization of quantum simulators and multielectron spin-coherent manipulations. Although a natural path to resolve complex quantum-matter problems and to process quantum information, two-dimensional (2D) scaling with a high connectivity of such implementations remains undemonstrated. Here we demonstrate the 2D coherent control of individual electron spins in a 3 × 3 array of tunnel-coupled quantum dots. We focus on several key quantum functionalities: charge-deterministic loading and displacement, local spin readout and local coherent exchange manipulation between two electron spins trapped in adjacent dots. This work lays some of the foundations to exploit a 2D array of electron spins for quantum simulation and information processing.

Coherent Beam Splitting of Flying Electrons Driven by a Surface Acoustic Wave

We develop a coherent beam splitter for single electrons driven through two tunnel-coupled quantum wires by surface acoustic waves (SAWs). The output current through each wire oscillates with gate voltages to tune the tunnel coupling and potential difference between the wires. This oscillation is assigned to coherent electron tunneling motion that can be used to encode a flying qubit and is well reproduced by numerical calculations of time evolution of the SAW-driven single electrons. The oscillation visibility is currently limited to about 3%, but robust against decoherence, indicating that the SAW electron can serve as a novel platform for a solid-state flying qubit.

Experimental Reconstruction of the Few-Photon Nonlinear Scattering Matrix from a Single Quantum Dot in a Nanophotonic Waveguide

Coherent photon-emitter interfaces offer a way to mediate efficient nonlinear photon-photon interactions, much needed for quantum information processing. Here we experimentally study the case of a two-level emitter, a quantum dot, coupled to a single optical mode in a nanophotonic waveguide. We carry out few-photon transport experiments and record the statistics of the light to reconstruct the scattering matrix elements of one- and two-photon components. This provides direct insight to the complex nonlinear photon interaction that contains rich many-body physics.

Coherent Spin-Photon Interface with Waveguide Induced Cycling Transitions

Solid-state quantum dots are promising candidates for efficient light-matter interfaces connecting internal spin degrees of freedom to the states of emitted photons. However, selection rules prevent the combination of efficient spin control and optical cyclicity in this platform. By utilizing a photonic crystal waveguide we here experimentally demonstrate optical cyclicity up to ≈15 through photonic state engineering while achieving high fidelity spin initialization and coherent optical spin control. These capabilities pave the way towards scalable multiphoton entanglement generation and on-chip spin-photon gates.

New signatures of the spin gap in quantum point contacts

One dimensional semiconductor systems with strong spin-orbit interaction are both of fundamental interest and have potential applications to topological quantum computing. Applying a magnetic field can open a spin gap, a pre-requisite for Majorana zero modes. The spin gap is predicted to manifest as a field dependent dip on the first 1D conductance plateau. However, disorder and interaction effects make identifying spin gap signatures challenging. Here we study experimentally and numerically the 1D channel in a series of low disorder p-type GaAs quantum point contacts, where spin-orbit and hole-hole interactions are strong. We demonstrate an alternative signature for probing spin gaps, which is insensitive to disorder, based on the linear and non-linear response to the orientation of the applied magnetic field, and extract a spin-orbit gap ΔE ≈ 500 μeV. This approach could enable one-dimensional hole systems to be developed as a scalable and reproducible platform for topological quantum applications.

In one-dimensional systems, the combination of a strong spin-orbit interaction and an applied magnetic field can give rise to a spin-gap, however experimental identification is difficult. Here, the authors present new signatures for the spin-gap, and verify these experimentally in hole QPCs.

Scalable integrated single-photon source

Photonic qubits are key enablers for quantum information processing deployable across a distributed quantum network. An on-demand and truly scalable source of indistinguishable single photons is the essential component enabling high-fidelity photonic quantum operations. A main challenge is to overcome noise and decoherence processes to reach the steep benchmarks on generation efficiency and photon indistinguishability required for scaling up the source. We report on the realization of a deterministic single-photon source featuring near-unity indistinguishability using a quantum dot in an "on-chip" planar nanophotonic waveguide circuit. The device produces long strings of >100 single photons without any observable decrease in the mutual indistinguishability between photons. A total generation rate of 122 million photons per second is achieved, corresponding to an on-chip source efficiency of 84%. These specifications of the single-photon source are benchmarked for boson sampling and found to enable scaling into the regime of quantum advantage.

Low-noise GaAs quantum dots for quantum photonics

Quantum dots are both excellent single-photon sources and hosts for single spins. This combination enables the deterministic generation of Raman-photons—bandwidth-matched to an atomic quantum-memory—and the generation of photon cluster states, a resource in quantum communication and measurement-based quantum computing. GaAs quantum dots in AlGaAs can be matched in frequency to a rubidium-based photon memory, and have potentially improved electron spin coherence compared to the widely used InGaAs quantum dots. However, their charge stability and optical linewidths are typically much worse than for their InGaAs counterparts. Here, we embed GaAs quantum dots into an n-i-p-diode specially designed for low-temperature operation. We demonstrate ultra-low noise behaviour: charge control via Coulomb blockade, close-to lifetime-limited linewidths, and no blinking. We observe high-fidelity optical electron-spin initialisation and long electron-spin lifetimes for these quantum dots. Our work establishes a materials platform for low-noise quantum photonics close to the red part of the spectrum.

GaAs quantum dots emitting at the near-red part of the spectrum usually suffers from excess charge-noise. With a careful design of a n-i-p-diode structure hosting GaAs quantum dots, the authors demonstrate ultralow-noise behaviour and high-fidelity spin initialisation close to rubidium wavelengths.

Closed-loop control of a GaAs-based singlet-triplet spin qubit with 99.5% gate fidelity and low leakage

Semiconductor spin qubits have recently seen major advances in coherence time and control fidelities, leading to a single-qubit performance that is on par with other leading qubit platforms. Most of this progress is based on microwave control of single spins in devices made of isotopically purified silicon. For controlling spins, the exchange interaction is an additional key ingredient which poses new challenges for high-fidelity control. Here, we demonstrate exchange-based single-qubit gates of two-electron spin qubits in GaAs double quantum dots. Using careful pulse optimization and closed-loop tuning, we achieve a randomized benchmarking fidelity of (99.50±0.04)% and a leakage rate of 0.13% out of the computational subspace. These results open new perspectives for microwave-free control of singlet-triplet qubits in GaAs and other materials.

The exchange interaction between spins poses considerable challenges for high-fidelity control of semiconductor spin qubits. Here, the authors use pulse optimization and closed-loop control to achieve a gate fidelity of 99.5% for exchange-based single-qubit gates of two-electron spin qubits in GaAs.

On-chip deterministic operation of quantum dots in dual-mode waveguides for a plug-and-play single-photon source

A deterministic source of coherent single photons is an enabling device for quantum information processing. Quantum dots in nanophotonic structures have been employed as excellent sources of single photons with the promise of scaling up towards multiple photons and emitters. It remains a challenge to implement deterministic resonant optical excitation of the quantum dot required for generating coherent single photons, since residual light from the excitation laser should be suppressed without compromising source efficiency and scalability. Here, we present a planar nanophotonic circuit that enables deterministic pulsed resonant excitation of quantum dots using two orthogonal waveguide modes for separating the laser and the emitted photons. We report a coherent and stable single-photon source that simultaneously achieves high-purity (g⁽²⁾(0) = 0.020 ± 0.005), high-indistinguishability (V = 96 ± 2%), and >80% coupling efficiency into the waveguide. Such ‘plug-and-play’ single-photon source can be integrated with on-chip optical networks implementing photonic quantum processors.

Resonantly-excited quantum-dot-based single photon sources feature very high purity, but also limited efficiency due to the need to suppress the residual pump. Here, the authors demonstrate a workaround, performing optical pumping and signal collection in two orthogonal modes inside a nanophotonic circuit.

Measurement of Backaction from Electron Spins in a Gate-Defined GaAs Double Quantum dot Coupled to a Mesoscopic Nuclear Spin Bath

Decoherence of a quantum system arising from its interaction with an environment is a key concept for understanding the transition between the quantum and classical world as well as performance limitations in quantum technology applications. The effects of large, weakly coupled environments are often described as a classical, fluctuating field whose dynamics is unaffected by the qubit, whereas a fully quantum description still implies some backaction from the qubit on the environment. Here we show direct experimental evidence for such a backaction for an electron-spin qubit in a GaAs quantum dot coupled to a mesoscopic environment of order 10^{6} nuclear spins. By means of a correlation measurement technique, we detect the backaction of a single qubit-environment interaction whose duration is comparable to the qubit's coherence time, even in such a large system. We repeatedly let the qubit interact with the spin bath and measure its state. Between such cycles, the qubit is reinitialized to different states. The correlations of the measurement outcomes are strongly affected by the intermediate qubit state, which reveals the action of a single electron spin on the nuclear spins.

Radiative Auger process in the single-photon limit

In a multi-electron atom, an excited electron can decay by emitting a photon. Typically, the leftover electrons are in their ground state. In a radiative Auger process, the leftover electrons are in an excited state and a redshifted photon is created^1-4. In a semiconductor quantum dot, radiative Auger is predicted for charged excitons⁵. Here we report the observation of radiative Auger on trions in single quantum dots. For a trion, a photon is created on electron-hole recombination, leaving behind a single electron. The radiative Auger process promotes this additional (Auger) electron to a higher shell of the quantum dot. We show that the radiative Auger effect is a powerful probe of this single electron: the energy separations between the resonance fluorescence and the radiative Auger emission directly measure the single-particle splittings of the electronic states in the quantum dot with high precision. In semiconductors, these single-particle splittings are otherwise hard to access by optical means as particles are excited typically in pairs, as excitons. After the radiative Auger emission, the Auger carrier relaxes back to the lowest shell. Going beyond the original theoretical proposals, we show how applying quantum optics techniques to the radiative Auger photons gives access to the single-electron dynamics, notably relaxation and tunnelling. This is also hard to access by optical means: even for quasi-resonant p-shell excitation, electron relaxation takes place in the presence of a hole, complicating the relaxation dynamics. The radiative Auger effect can be exploited in other semiconductor nanostructures and quantum emitters in the solid state to determine the energy levels and the dynamics of a single carrier.

Real-Time Detection of Single Auger Recombination Events in a Self-Assembled Quantum Dot

Auger recombination is a nonradiative process, where the recombination energy of an electron-hole pair is transferred to a third charge carrier. It is a common effect in colloidal quantum dots that quenches the radiative emission with an Auger recombination time below nanoseconds. In self-assembled QDs, the Auger recombination has been observed with a much longer recombination time on the order of microseconds. Here, we use two-color laser excitation on the exciton and trion transition in resonance fluorescence on a single self-assembled quantum dot to monitor in real-time single quantum events of the Auger process. Full counting statistics on the random telegraph signal give access to the cumulants and demonstrate the tunability of the Fano factor from a Poissonian to a sub-Poissonian distribution by Auger-mediated electron emission from the dot. Therefore, the Auger process can be used to tune optically the charge carrier occupation of the dot by the incident laser intensity, independently from the electron tunneling from the reservoir by the gate voltage. Our findings are not only highly relevant for the understanding of the Auger process but also demonstrate the perspective of the Auger effect for controlling precisely the charge state in a quantum system by optical means.

Terahertz Fano resonances induced by combining metamaterial modes of the same symmetry

Fano resonances are observed in a composite metamaterial that consists of an electric split ring resonator eSRR and an I-shaped resonator ISR. By adjusting the length of the ISR the degree of asymmetry in the line shape of the composite metamaterial can be controlled and even made to be symmetric. In contrast to other methods to create Fano resonances, the individual modes of the eSRR and ISR have the same symmetry and are not evanescently coupled to each other. The transmission is simulated using the finite difference time domain method and a coupled oscillator model is used to obtain nominal values of the Fano asymmetry factor q. Composite metamaterials and individual eSRR and ISR metamaterials are fabricated, and their transmission is measured with terahertz time-domain spectroscopy.

Sound-driven single-electron transfer in a circuit of coupled quantum rails

Surface acoustic waves (SAWs) strongly modulate the shallow electric potential in piezoelectric materials. In semiconductor heterostructures such as GaAs/AlGaAs, SAWs can thus be employed to transfer individual electrons between distant quantum dots. This transfer mechanism makes SAW technologies a promising candidate to convey quantum information through a circuit of quantum logic gates. Here we present two essential building blocks of such a SAW-driven quantum circuit. First, we implement a directional coupler allowing to partition a flying electron arbitrarily into two paths of transportation. Second, we demonstrate a triggered single-electron source enabling synchronisation of the SAW-driven sending process. Exceeding a single-shot transfer efficiency of 99%, we show that a SAW-driven integrated circuit is feasible with single electrons on a large scale. Our results pave the way to perform quantum logic operations with flying electron qubits.

Angular momentum transfer from photon polarization to an electron spin in a gate-defined quantum dot

Gate-defined quantum dots (QDs) are such a highly-tunable quantum system in which single spins can be electrically coupled, manipulated, and measured. However, the spins in gate-defined QDs are lacking its interface to free-space photons. Here, we verify that a circularly-polarized single photon can excite a single electron spin via the transfer of angular momentum, measured using Pauli spin blockade (PSB) in a double QD. We monitor the inter-dot charge tunneling which only occur when the photo-electron spin in one QD is anti-parallel to the electron spin in the other. This allows us to detect single photo-electrons in the spin-up/down basis using PSB. The photon polarization dependence of the excited spin state was finally confirmed for the heavy-hole exciton excitation. The angular momentum transfer observed here is a fundamental step providing a route to instant injection of spins, distributing single spin information, and possibly towards extending quantum communication.

Gate-defined quantum dots offer a way to engineer electrically controllable quantum systems with potential for information processing. Here, the authors transfer angular momentum from the polarization of a single photon to the spin of a single electron in a gate-defined double quantum dot.

Optical Detection of Single-Electron Tunneling into a Semiconductor Quantum Dot

The maximum information of a dynamic quantum system is given by real-time detection of every quantum event, where the ultimate challenge is a stable, sensitive detector with high bandwidth. All physical information can then be drawn from a statistical analysis of the time traces. We demonstrate here an optical detection scheme based on the time-resolved resonance fluorescence on a single quantum dot. Single-electron resolution with high signal-to-noise ratio (4σ confidence) and high bandwidth of 10 kHz make it possible to record the individual quantum events of the transport dynamics. Full counting statistics with factorial cumulants gives access to the nonequilibrium dynamics of spin relaxation of a singly charged dot (γ_{↑↓}=3  ms^{-1}), even in an equilibrium transport measurement.

Contrast of 83% in reflection measurements on a single quantum dot

We report on a high optical contrast between the photon emission from a single self-assembled quantum dot (QD) and the back-scattered excitation laser light. In an optimized semiconductor heterostructure with an epitaxially grown gate, an optically-matched layer structure and a distributed Bragg reflector, a record value of 83% is obtained; with tilted laser excitation even 885%. This enables measurements on a single dot without lock-in technique or suppression of the laser background by cross-polarization. These findings open up the possibility to perform simultaneously time-resolved and polarization-dependent resonant optical spectroscopy on a single quantum dot.

Quantum non-demolition measurement of an electron spin qubit

Measurements of quantum systems inevitably involve disturbance in various forms. Within the limits imposed by quantum mechanics, there exists an ideal projective measurement that does not introduce a back action on the measured observable, known as a quantum non-demolition (QND) measurement^1,2. Here we demonstrate an all-electrical QND measurement of a single electron spin in a gate-defined quantum dot. We entangle the single spin with a two-electron, singlet-triplet ancilla qubit via the exchange interaction^3,4 and then read out the ancilla in a single shot. This procedure realizes a disturbance-free projective measurement of the single spin at a rate two orders of magnitude faster than its relaxation. The QND nature of the measurement protocol^5,6 enables enhancement of the overall measurement fidelity by repeating the protocol. We demonstrate a monotonic increase of the fidelity over 100 repetitions against arbitrary input states. Our analysis based on statistical inference is tolerant to the presence of the relaxation and dephasing. We further exemplify the QND character of the measurement by observing spontaneous flips (quantum jumps)⁷ of a single electron spin. Combined with the high-fidelity control of spin qubits^8-13, these results will allow for various measurement-based quantum state manipulations including quantum error correction protocols¹⁴.

Luminescent Nd2S3 thin films: a new chemical vapour deposition route towards rare-earth sulphides

Neodymium sulphide (Nd2S3) belongs to the exciting class of rare earth sulphides (RES) and is projected to have a serious potential in a wide spectrum of application either in pure form or as dopant. We demonstrate a facile and first growth of Nd2S3 thin films via metal-organic chemical vapour deposition (MOCVD) at moderate process conditions using two new Nd precursors, namely tris(N,N'-diisopropyl-2-dimethylamido-guanidinato)Nd(iii) and tris(N,N'-diisopropyl-acetamidinato)Nd(iii). The promising thermal properties and suitable reactivity of both Nd precursors towards elemental sulphur enabled the formation of high purity γ-Nd2S3. While the process temperature for film growth ranged from 400 °C to 600 °C, the films were crystalline above 500 °C. We also demonstrate that the as-deposited γ-Nd2S3 are luminescent, with the optical bandgap ranging from 2.3 eV to 2.5 eV. The process circumvents post-deposition treatments such as sulfurisation to fabricate the desired Nd2S3, which paves the way for large scale synthesis and also opens up new avenues for exploring the potential of this class of materials with properties for functional applications.

Photon Noise Suppression by a Built-in Feedback Loop

Visionary quantum photonic networks need transform-limited single photons on demand. Resonance fluorescence on a quantum dot provides the access to a solid-state single photon source, where the environment is unfortunately the source of spin and charge noise that leads to fluctuations of the emission frequency and destroys the needed indistinguishability. We demonstrate a built-in stabilization approach for the photon stream, which relies solely on charge carrier dynamics of a two-dimensional hole gas inside a micropillar structure. The hole gas is fed by hole tunneling from field-ionized excitons and influences the energetic position of the excitonic transition by changing the local electric field at the position of the quantum dot. The standard deviation of the photon noise is suppressed by nearly 50% (noise power reduction of 6 dB) and it works in the developed micropillar structure for frequencies up to 1 kHz. This built-in feedback loop represents an easy way for photon noise suppression in large arrays of single photon emitters and promises to reach higher bandwidth by device optimization.

Correction: Luminescent Nd2S3 thin films: a new chemical vapour deposition route towards rare-earth sulphides

Correction for ‘Luminescent Nd₂S₃ thin films: a new chemical vapour deposition route towards rare-earth sulphides’ by Stefan Cwik et al., Dalton Trans., 2019, 48, 2926–2938.

A fast quantum interface between different spin qubit encodings

Single-spin qubits in semiconductor quantum dots hold promise for universal quantum computation with demonstrations of a high single-qubit gate fidelity above 99.9% and two-qubit gates in conjunction with a long coherence time. However, initialization and readout of a qubit is orders of magnitude slower than control, which is detrimental for implementing measurement-based protocols such as error-correcting codes. In contrast, a singlet-triplet qubit, encoded in a two-spin subspace, has the virtue of fast readout with high fidelity. Here, we present a hybrid system which benefits from the different advantages of these two distinct spin-qubit implementations. A quantum interface between the two codes is realized by electrically tunable inter-qubit exchange coupling. We demonstrate a controlled-phase gate that acts within 5.5 ns, much faster than the measured dephasing time of 211 ns. The presented hybrid architecture will be useful to settle remaining key problems with building scalable spin-based quantum computers.

The race to produce a quantum computer has driven the development of many different qubit designs with different benefits and drawbacks. Noiri et al. demonstrate a hybrid device with two coupled semiconductor spin qubits of different designs, which should allow each qubit’s advantages to be exploited.

Unveiling the bosonic nature of an ultrashort few-electron pulse

Quantum dynamics is very sensitive to dimensionality. While two-dimensional electronic systems form Fermi liquids, one-dimensional systems—Tomonaga–Luttinger liquids—are described by purely bosonic excitations, even though they are initially made of fermions. With the advent of coherent single-electron sources, the quantum dynamics of such a liquid is now accessible at the single-electron level. Here, we report on time-of-flight measurements of ultrashort few-electron charge pulses injected into a quasi one-dimensional quantum conductor. By changing the confinement potential we can tune the system from the one-dimensional Tomonaga–Luttinger liquid limit to the multi-channel Fermi liquid and show that the plasmon velocity can be varied over almost an order of magnitude. These results are in quantitative agreement with a parameter-free theory and demonstrate a powerful probe for directly investigating real-time dynamics of fractionalisation phenomena in low-dimensional conductors.

Electronic excitations in low-dimensional quantum nanoelectronic devices are collective waves that are strongly affected by the Coulomb interaction. Here, the authors demonstrate that they are able to prepare these collective excitations down to the single electron level and control their propagation.

Quantum Optics with Near-Lifetime-Limited Quantum-Dot Transitions in a Nanophotonic Waveguide

Establishing a highly efficient photon-emitter interface where the intrinsic linewidth broadening is limited solely by spontaneous emission is a key step in quantum optics. It opens a pathway to coherent light-matter interaction for, e.g., the generation of highly indistinguishable photons, few-photon optical nonlinearities, and photon-emitter quantum gates. However, residual broadening mechanisms are ubiquitous and need to be combated. For solid-state emitters charge and nuclear spin noise are of importance, and the influence of photonic nanostructures on the broadening has not been clarified. We present near-lifetime-limited linewidths for quantum dots embedded in nanophotonic waveguides through a resonant transmission experiment. It is found that the scattering of single photons from the quantum dot can be obtained with an extinction of 66 ± 4%, which is limited by the coupling of the quantum dot to the nanostructure rather than the linewidth broadening. This is obtained by embedding the quantum dot in an electrically contacted nanophotonic membrane. A clear pathway to obtaining even larger single-photon extinction is laid out; i.e., the approach enables a fully deterministic and coherent photon-emitter interface in the solid state that is operated at optical frequencies.

Superresolution Microscopy of Single Rare-Earth Emitters in YAG and H3 Centers in Diamond

We demonstrate superresolution imaging of single rare-earth emitting centers, namely, trivalent cerium, in yttrium aluminum garnet crystals by means of stimulated emission depletion (STED) microscopy. The achieved all-optical resolution is ≈50  nm. Similar results were obtained on H3 color centers in diamond. In both cases, STED resolution is improving slower than the conventional inverse square-root dependence on the depletion beam intensity. In the proposed model of this effect, the anomalous behavior is caused by excited state absorption and the interaction of the emitter with nonfluorescing crystal defects in its local surrounding.

Non-universal transmission phase behaviour of a large quantum dot

The electron wave function experiences a phase modification at coherent transmission through a quantum dot. This transmission phase undergoes a characteristic shift of π when scanning through a Coulomb blockade resonance. Between successive resonances either a transmission phase lapse of π or a phase plateau is theoretically expected to occur depending on the parity of quantum dot states. Despite considerable experimental effort, this transmission phase behaviour has remained elusive for a large quantum dot. Here we report on transmission phase measurements across such a large quantum dot hosting hundreds of electrons. Scanning the transmission phase along 14 successive resonances with an original two-path interferometer, we observe both phase lapses and plateaus. We demonstrate that quantum dot deformation alters the sequence of phase lapses and plateaus via parity modifications of the involved quantum dot states. Our findings set a milestone towards an comprehensive understanding of the transmission phase of quantum dots.

Higher-order spin and charge dynamics in a quantum dot-lead hybrid system

Understanding the dynamics of open quantum systems is important and challenging in basic physics and applications for quantum devices and quantum computing. Semiconductor quantum dots offer a good platform to explore the physics of open quantum systems because we can tune parameters including the coupling to the environment or leads. Here, we apply the fast single-shot measurement techniques from spin qubit experiments to explore the spin and charge dynamics due to tunnel coupling to a lead in a quantum dot-lead hybrid system. We experimentally observe both spin and charge time evolution via first- and second-order tunneling processes, and reveal the dynamics of the spin-flip through the intermediate state. These results enable and stimulate the exploration of spin dynamics in dot-lead hybrid systems, and may offer useful resources for spin manipulation and simulation of open quantum systems.

Coherent long-distance displacement of individual electron spins

Controlling nanocircuits at the single electron spin level is a possible route for large-scale quantum information processing. In this context, individual electron spins have been identified as versatile quantum information carriers to interconnect different nodes of a spin-based semiconductor quantum circuit. Despite extensive experimental efforts to control the electron displacement over long distances, maintaining electron spin coherence after transfer remained elusive up to now. Here we demonstrate that individual electron spins can be displaced coherently over a distance of 5 µm. This displacement is realized on a closed path made of three tunnel-coupled lateral quantum dots at a speed approaching 100 ms⁻¹. We find that the spin coherence length is eight times longer than expected from the electron spin coherence without displacement, pointing at a process similar to motional narrowing observed in nuclear magnetic resonance experiments. The demonstrated coherent displacement will open the route towards long-range interaction between distant spin qubits.

The spin states of electrons in quantum dots have well-established potential for use as qubits but some proposed developments require the ability to move the quantum spin state across a larger device. Here, the authors experimentally demonstrate coherent shuttling of spins in a ring of three dots.

Two-dimensional coherent spectroscopy of a THz quantum cascade laser: observation of multiple harmonics

Two-dimensional spectroscopy is performed on a terahertz (THz) frequency quantum cascade laser (QCL) with two broadband THz pulses. Gain switching is used to amplify the first THz pulse and the second THz pulse is used to probe the system. Fourier transforms are taken with respect to the delay time between the two THz pulses and the sampling time of the THz probe pulse. The two-dimensional spectrum consists of three peaks at (ω_τ = 0, ω_t = ω₀), (ω_τ = ω₀, ω_t = ω₀), and (ω_τ = 2ω₀, ω_t = ω₀) where ω₀ denotes the lasing frequency. The peak at ω_τ = 0 represents the response of the probe to the zero-frequency (rectified) component of the instantaneous intensity and can be used to measure the gain recovery.

Robust Single-Shot Spin Measurement with 99.5% Fidelity in a Quantum Dot Array

We demonstrate a new method for projective single-shot measurement of two electron spin states (singlet versus triplet) in an array of gate-defined lateral quantum dots in GaAs. The measurement has very high fidelity and is robust with respect to electric and magnetic fluctuations in the environment. It exploits a long-lived metastable charge state, which increases both the contrast and the duration of the charge signal distinguishing the two measurement outcomes. This method allows us to evaluate the charge measurement error and the spin-to-charge conversion error separately. We specify conditions under which this method can be used, and project its general applicability to scalable quantum dot arrays in GaAs or silicon.

Detection and Control of Spin-Orbit Interactions in a GaAs Hole Quantum Point Contact

We investigate the relationship between the Zeeman interaction and the inversion-asymmetry-induced spin-orbit interactions (Rashba and Dresselhaus SOIs) in GaAs hole quantum point contacts. The presence of a strong SOI results in the crossing and anticrossing of adjacent spin-split hole subbands in a magnetic field. We demonstrate theoretically and experimentally that the anticrossing energy gap depends on the interplay between the SOI terms and the highly anisotropic hole g tensor and that this interplay can be tuned by selecting the crystal axis along which the current and magnetic field are aligned. Our results constitute the independent detection and control of the Dresselhaus and Rashba SOIs in hole systems, which could be of importance for spintronics and quantum information applications.