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Electronic flying qubits offer an interesting alternative to photonic qubits: electrons propagate slower, hence easier to control in real time, and Coulomb interaction enables direct entanglement between different qubits. While their coherence time is limited, picosecond-scale control would make them competitive in terms of number of possible coherent operations. The key challenge lies in achieving the dynamical regime, where the injected plasmonic pulse width is shorter than the quantum device dimensions. Here we reach this new regime in a quantum nanoelectronic system by injecting ultrashort single electron plasmonic pulses into a 14-micrometer-long Mach-Zehnder interferometer. Our findings reveal that quantum coherence is preserved for ultrashort plasmonic pulses, exhibiting enhanced contrast of coherent oscillations compared to the DC regime. Moreover, this coherence remains robust even under large bias voltages. This milestone demonstrates the feasibility of flying qubits as a promising alternative to localized qubit architectures, offering reduced hardware footprint, increased connectivity, and potential for scalable quantum information processing.

Spins in semiconductor quantum dots hold great promise as building blocks of quantum processors. Trapping them in SiMOS transistor-like devices eases future industrial scale fabrication. Among the potentially scalable readout solutions, gate-based dispersive radiofrequency reflectometry only requires the already existing transistor gates to readout a quantum dot state, relieving the need for additional elements. In this effort towards scalability, traveling-wave superconducting parametric amplifiers significantly enhance the readout signal-to-noise ratio (SNR) by reducing the noise below typical cryogenic low-noise amplifiers, while offering a broad amplification band, essential to multiplex the readout of multiple resonators. In this work, we demonstrate a 3GHz gate-based reflectometry readout of electron charge states trapped in quantum dots formed in SiMOS multi-gate devices, with SNR enhanced thanks to a Josephson traveling-wave parametric amplifier (JTWPA). The broad, tunable 2GHz amplification bandwidth combined with more than 10dB ON/OFF SNR improvement of the JTWPA enables frequency and time division multiplexed readout of interdot transitions, and noise performance near the quantum limit. In addition, owing to a design without superconducting loops and with a metallic ground plane, the JTWPA is flux insensitive and shows stable performances up to a magnetic field of 1.2T at the quantum dot device, compatible with standard SiMOS spin qubit experiments.

For semiconductor spin qubits, complementary-metal-oxide-semiconductor (CMOS) technology is the ideal candidate for reliable and scalable fabrication. Making the direct leap from academic fabrication to qubits fabricated fully by industrial CMOS standards is difficult without intermediate solutions. With a flexible back-end-of-line (BEOL) new functionalities such as micromagnets or superconducting circuits can be added in a post-CMOS process to study the physics of these devices or achieve proof of concepts. Once the process is established it can be incorporated in the foundry-compatible process flow. Here, we study a single electron spin qubit in a CMOS device with a micromagnet integrated in the flexible BEOL. We exploit the synthetic spin orbit coupling (SOC) to control the qubit via electric field and we investigate the spin-valley physics in the presence of SOC where we show an enhancement of the Rabi frequency at the spin-valley hotspot. Finally, we probe the high frequency noise in the system using dynamical decoupling pulse sequences and demonstrate that charge noise dominates the qubit decoherence in this range., Comment: 11 pages, 10 figures

We investigate experimentally the capacitive coupling between a two-electron singlet-triplet spin qubit and flying electrons propagating in quantum Hall edge channels. After calibration of the spin qubit detector, we assess its charge sensibility and demonstrate experimentally the detection of less than five flying electrons with average measurement. This experiment demonstrates that the spin qubit is an ultrasensitive and fast charge detector with the perspective of a future single shot-detection of a single flying electron. This work opens the route toward quantum electron optics experiments at the single electron level in semiconductor circuits.

Electron flying qubits are envisioned as potential information link within a quantum computer, but also promise -- alike photonic approaches -- a self-standing quantum processing unit. In contrast to its photonic counterpart, electron-quantum-optics implementations are subject to Coulomb interaction, which provide a direct route to entangle the orbital or spin degree of freedom. However, the 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 realise a controlled-phase gate for in-flight quantum manipulations.

Charge noise is one of the main sources of environmental decoherence for spin qubits in silicon, presenting a major obstacle in the path towards highly scalable and reproducible qubit fabrication. Here we demonstrate in-depth characterization of the charge noise environment experienced by a quantum dot in a CMOS-fabricated silicon nanowire. We probe the charge noise for different quantum dot configurations, finding that it is possible to tune the charge noise over two orders of magnitude, ranging from 1 ueV^2 to 100 ueV^2. In particular, we show that the top interface and the reservoirs are the main sources of charge noise and their effect can be mitigated by controlling the quantum dot extension. Additionally, we demonstrate a novel method for the measurement of the charge noise experienced by a quantum dot in the few electron regime. We measure a comparatively higher charge noise value of 40 ueV^2 at the first electron, and demonstrate that the charge noise is highly dependent on the electron occupancy of the quantum dot., Comment: 6 pages 5 figures, supplementary materials included

We propose and demonstrate complete spin state readout of a two-electron system in a double quantum dot probed by an electrometer. The protocol is based on repetitive single shot measurements using Pauli spin blockade and our ability to tune on fast timescales the detuning and the interdot tunnel coupling between the GHz and sub-Hz regime. A sequence of three distinct manipulations and measurements allows establishing if the spins are in S, Tzero, Tplus or Tminus state. This work points at a procedure to reduce the overhead for spin readout, an important challenge for scaling up spin qubit platforms., Comment: 11 pages, 7 figures

The synthesis of single-cycle, compressed optical and microwave pulses sparked novel areas of fundamental research. In the field of acoustics, however, such a generation has not been introduced yet. For numerous applications, the large spatial extent of surface acoustic waves (SAW) causes unwanted perturbations and limits the accuracy of physical manipulations. Particularly, this restriction applies to SAW-driven quantum experiments with single flying electrons, where extra modulation renders the exact position of the transported electron ambiguous and leads to undesired spin mixing. Here, we address this challenge by demonstrating single-shot chirp synthesis of a strongly compressed acoustic pulse. Employing this solitary SAW pulse to transport a single electron between distant quantum dots with an efficiency exceeding 99%, we show that chirp synthesis is competitive with regular transduction approaches. Performing a time-resolved investigation of the SAW-driven sending process, we outline the potential of the chirped SAW pulse to synchronize single-electron transport from many quantum-dot sources. By superimposing multiple pulses, we further point out the capability of chirp synthesis to generate arbitrary acoustic waveforms tailorable to a variety of (opto)nanomechanical applications. Our results shift the paradigm of compressed pulses to the field of acoustic phonons and pave the way for a SAW-driven platform of single-electron transport that is precise, synchronized, and scalable., Comment: To be published in Physical Review X

We demonstrate singlet-triplet readout and parity readout allowing to distinguish T0 and the polarized triplet states. We achieve high fidelity spin readout with an average fidelity above $99.9\%$ for a readout time of $20~\mu$s and $99\%$ for $4~\mu$s at a temperature of $0.5~K$. We initialize a singlet state in a single dot with a fidelity higher than $99\%$ and separate the two electrons while keeping the same spin state with $a \approx 95.6\%$ fidelity., Comment: 13 pages, 12 figures

Recent demonstrations using electron spins stored in quantum dots array as qubits are promising for developing a scalable quantum computing platform. An ongoing effort is therefore aiming at the precise control of the quantum dots parameters in larger and larger arrays which represents a complex challenge. Partitioning of the system with the help of the inter-dot tunnel barriers can lead to a simplification for tuning and offers a protection against unwanted charge displacement. In a triple quantum dot system, we demonstrate a nanosecond control of the inter-dot tunnel rate permitting to reach the two extreme regimes, large GHz tunnel coupling and sub-Hz isolation between adjacent dots. We use this novel development to isolate a sub part of the array while performing charge displacement and readout in the rest of the system. The degree of control over the tunnel coupling achieved in a unit cell should motivate future protocol development for tuning, manipulation and readout including this capability., Comment: 10 pages, 8 figures

Erbium implanted silicon as a quantum technology platform has both telecommunications and integrated circuit processing compatibility. In Si implanted with Er to a concentration of 3x10^17 cm^3 and O to a concentration of 10^20 cm^3, the electron spin coherence time, T2, and the spin-lattice relaxation time, T1, were measured to be 7.5 ls and ~1 ms, respectively, at 5 K. The spin echo decay profile displayed strong modulation, which was consistent with the super-hyperfine interaction between Er3{\th} and a spin bath of 29Si nuclei. The calculated spectral diffusion time was similar to the measured T2, which indicated that T2 was limited by spectral diffusion due to T1-induced flips of neighboring Er3{\th} spins. The origin of the echo is an Er center surrounded by six O atoms with monoclinic C1h site symmetry., Comment: arXiv admin note: substantial text overlap with arXiv:2006.00225

One of the main advantages of silicon spin qubits over other solid-state qubits is their inherent scalability and compatibility with the 300 mm CMOS fabrication technology that is already widely used in the semiconductor industry, whilst maintaining high readout and gate fidelities. We demonstrate detection of a single electron spin using energy-selective readout in a CMOS-fabricated nanowire device with an integrated charge detector. We measure a valley splitting of 0.3 meV and 0.16 meV in two similar devices. The anisotropy of the spin-valley mixing is measured and shown to follow the dependence expected from the symmetry of the local confinement, indicating low disorder in the region of the quantum dot. Finally the charge noise in the spin-valley coupling regime is investigated and found to induce fluctuations in the qubit energy in the range of $0.6GHz/\sqrt{Hz}$., Comment: 4 figures

Surface acoustic waves (SAW) have large potential to realize quantum-optics-like experiments with single flying electrons employing their spin or charge degree of freedom. For such quantum applications, highly efficient trapping of the electron in a specific moving quantum dot (QD) of a SAW train plays a key role. Probabilistic transport over multiple moving minima would cause uncertainty in synchronisation that is detrimental for coherence of entangled flying electrons and in-flight quantum operations. It is thus of central importance to identify the device parameters enabling electron transport within a single SAW minimum. A detailed experimental investigation of this aspect is so far missing. Here we fill this gap by demonstrating time-of-flight measurements for a single electron that is transported via a SAW train between distant stationary QDs. Our measurements reveal the in-flight distribution of the electron within the moving acousto-electric quantum dots of the SAW train. Increasing the acousto-electric amplitude, we observe the threshold necessary to confine the flying electron at a specific, deliberately chosen SAW minimum. Investigating the effect of a barrier along the transport channel, we also benchmark the robustness of SAW-driven electron transport against stationary potential variations. Our results pave the way for highly controlled transport of electron qubits in a SAW-driven platform for quantum experiments., Comment: Main paper: 11 pages, 3 figures, Supplementary material: 5 pages, 4 figures

The ability to shuttle coherently individual electron spins in arrays of quantum dots is a key procedure for the development of scalable quantum information platforms. It allows the use of sparsely populated electron spin arrays, envisioned to efficiently tackle the one- and two-qubit gate challenges. When the electrons are displaced in an array, they are submitted to site-dependent environment interactions such as hyperfine coupling with substrate nuclear spins. Here, we demonstrate that the electron multi-directional displacement in a $3\times 3$ array of tunnel coupled quantum dots enhances the spin coherence time via the motional narrowing phenomenon. More specifically, up to 10 configurations are explored by the electrons to study the impact of the displacement on spin dynamics. An increase of the coherence time by a factor up to 10 is observed in case of fast and repetitive displacement. The physical mechanism responsible for the loss of coherence induced by displacement is quantitatively captured by a simple model and its implications on spin coherence properties during the electron displacement are discussed in the context of large-scale quantum circuits., Comment: 8 pages, 5 figures, 6 supplementary figures

Owing to ever increasing gate fidelities and to a potential transferability to industrial CMOS technology, silicon spin qubits have become a compelling option in the strive for quantum computation. In a scalable architecture, each spin qubit will have to be finely tuned and its operating conditions accurately determined. In this prospect, spectroscopic tools compatible with a scalable device layout are of primary importance. Here we report a two-tone spectroscopy technique providing access to the spin-dependent energy-level spectrum of a hole double quantum dot defined in a split-gate silicon device. A first GHz-frequency tone drives electric-dipole spin resonance enabled by the valence-band spin-orbit coupling. A second lower-frequency tone (approximately 500 MHz) allows for dispersive readout via rf-gate reflectometry. We compare the measured dispersive response to the linear response calculated in an extended Jaynes-Cummings model and we obtain characteristic parameters such as g-factors and tunnel/spin-orbit couplings for both even and odd occupation.

Erbium implanted silicon as a quantum technology platform has both telecommunications and integrated circuit (IC) processing compatibility. The electron spin coherence time of Er implanted Si with an Er concentration of 3X1017 cm-3 is measured to be ~10 {\mu}s at 5 K and the spin echo decay profile displays strong modulation due to super-hyperfine interaction with 29Si nuclei. Three independent measurements: temperature quenching of photoluminescence (PL), PL lifetime and photo-illuminated electron spin resonance (ESR) all indicate the presence of a previously unreported Er related defect state which can facilitate non-radiative relaxation from the Er exited state. This gives an energy level scheme analogous to that of the diamond NV centre, and implies that optical spin polarisation of the Zeeman ground state and high temperature operation of Er qubits in Er implanted Si may be feasible. The collective coupling strength between a superconducting NbN lumped-element microresonator and Er implanted Si with an Er concentration of 1017 cm-3 at 20 mK was ~ 1 MHz.

In the quest for large-scale quantum computing, networked quantum computers offer a natural path towards scalability. Now that nearest neighbor entanglement has been demonstrated for electron spin qubits in semiconductors, on-chip long distance entanglement brings versatility to connect quantum core units. Here we realize the controlled and coherent transfer of a pair of entangled electron spins, and demonstrate their remote entanglement when separated by a distance of 6 microns. Driven by coherent spin rotations induced by the electron displacement, high-contrast spin quantum interferences are observed and are a signature of the preservation of the entanglement all along the displacement procedure. This work opens the route towards fast on-chip deterministic interconnection of remote quantum bits in semiconductor quantum circuits., Comment: Main text of 7 pages with 4 figures. Supplementary information of 4 pages with 3 figures

The recent development of arrays of quantum dots in semiconductor nanostructures highlights the progress of quantum devices toward large scale. However, how to realize such arrays on a scalable platform such as silicon is still an open question. One of the main challenge resides in the detection of charges within the array. It is a prerequisite functionality to initialize a desired charge state and readout spins through spin-to-charge conversion mechanisms. In this paper, we use two methods based on either a single-lead charge detector, or a reprogrammable single electron transistor. Thanks to these methods, we study the charge dynamics and sensitivity by performing single shot detection of the charge. Finally, we can probe the charge stability at any node of a linear array and assess the Coulomb disorder in the structure. We find an electrochemical potential fluctuation induced by charge noise comparable to that reported in other silicon quantum dots., Comment: 10 pages, 6 figures

We fabricated Quantum Dot (QD) devices using a standard SOI CMOS process flow, and demonstrated that the spin of confined electrons could be controlled via a local electrical-field excitation, owing to inter-valley spin-orbit coupling. We discuss that modulating the confinement geometry via an additional electrode may enable switching a quantum bit (qubit) between an electrically-addressable valley configuration and a protected spin configuration. This proposed scheme bears relevance to improve the trade-off between fast operations and slow decoherence for quantum computing on a Si qubit platform. Finally, we evoke the impact of process-induced variability on the operating bias range., Comment: arXiv admin note: substantial text overlap with arXiv:1912.09806

Spins in gate-defined silicon quantum dots are promising candidates for implementing large-scale quantum computing. To read the spin state of these qubits, the mechanism that has provided the highest fidelity is spin-to-charge conversion via singlet-triplet spin blockade, which can be detected in-situ using gate-based dispersive sensing. In systems with a complex energy spectrum, like silicon quantum dots, accurately identifying when singlet-triplet blockade occurs is hence of major importance for scalable qubit readout. In this work, we present a description of spin blockade physics in a tunnel-coupled silicon double quantum dot defined in the corners of a split-gate transistor. Using gate-based magnetospectroscopy, we report successive steps of spin blockade and spin blockade lifting involving spin states with total spin angular momentum up to $S=3$. More particularly, we report the formation of a hybridized spin quintet state and show triplet-quintet and quintet-septet spin blockade. This enables studies of the quintet relaxation dynamics from which we find $T_1 \sim 4 ~\mu s$. Finally, we develop a quantum capacitance model that can be applied generally to reconstruct the energy spectrum of a double quantum dot including the spin-dependent tunnel couplings and the energy splitting between different spin manifolds. Our results open for the possibility of using Si CMOS quantum dots as a tuneable platform for studying high-spin systems., Comment: 7 pages, 3 figures

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., Comment: Accepted for Publication in Nature Communications

The engineering of electron spin qubits in a compact unit cell embedding all quantum functionalities is mandatory for large scale integration. In particular, the development of a high-fidelity and scalable spin readout method remains an open challenge. Here we demonstrate high-fidelity and robust spin readout based on gate reflectometry in a CMOS device comprising one qubit dot and one ancillary dot coupled to an electron reservoir to perform readout. This scalable method allows us to read out a spin with a fidelity above 99% for 1 ms integration time. To achieve such fidelity, we exploit a latched spin blockade mechanism that requires electron exchange between the ancillary dot and the reservoir. We show that the demonstrated high read-out fidelity is fully preserved up to 0.5 K. This results holds particular relevance for the future co-integration of spin qubits and classical control electronics., Comment: 6 pages, 4 figures

The ability to manipulate coherently individual quantum objects organized in arrays is a prerequisite to any scalable quantum information platform. For electron spin qubits, it requires the fine tuning of large arrays of tunnel-coupled quantum dots. The cumulated efforts in linear dot arrays have permitted the recent realization of quantum simulators and multi-electron spin coherent manipulation. However, the two-dimensional scaling of such implementations remains undemonstrated while being compulsory to resolve complex quantum matter problems or process quantum information. Here, we demonstrate the two-dimensional coherent control of individual electron spins in a 3x3 array of tunnel-coupled quantum dots. More specifically, we focus on several key quantum functionalities of such control: charge deterministic displacement, local spin readout, local coherent exchange manipulation between two electron spins trapped in adjacent dots, and coherent multi-directional spin shuttling over distances of several microns. This work lays the foundations for exploiting a two-dimensional array of electron spins for quantum simulation and information processing.

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 new probe for directly investigating real-time dynamics of fractionalisation phenomena in low-dimensional conductors., Comment: In v2 we added 11 pages of Supplementary Information. In v3 we updated the manuscript after it was peer-reviewed and published

Incorporating functional molecules into sensor devices is an emerging field in molecular electronics that aims at exploiting the sensitivity of different molecules to their environment and turning it into an electrical signal. Among the emergent sensors, microelectromechanical systems are promising for their extreme sensitivity to mechanical events. However, in order to bring new functions to these devices, the functionalization of their surface with multifunctional molecules is required. Herein, we present original hybrid devices made of an integrated polymer microelectromechanical resonator functionalized with switchable magnetic molecules. The change of their mechanical properties and geometry induced by the switching of their magnetic state at a molecular level alters the device dynamical behavior, resulting in a change of the resonance frequency. We demonstrate that the device can be operated to sense light or thermal excitation. Moreover, thanks to the collective interaction of the switchable molecules, the device behaves as a nonvolatile memory. Our results open up broad prospects of new flexible photo and thermoactive hybrid devices for molecule based data storage and sensors., Comment: 13 pages 3 figures and supporting informations

Bistability in magnetism has been extensively used, in particular to store information. Here we propose an alternative route by using tetrastable magnetic domains. Using numerical and analytical calculations we show that a spin chain with a canting angle of {\pi}/4 possesses four energy-equivalent states. We discuss the static properties of such chain such as the profile and the energy of the domain walls as they govern the relaxation of the magnetization. The realisation of such spin chain could enable the encoding of the information on four bits which is a potential alternative toward the increase of storage density., Comment: 4 pages, 5 figures, supplementary material available

Electric fields can be used to tune donor spins in silicon using the Stark shift, whereby the donor electron wave function is displaced by an electric field, modifying the hyperfine coupling between the electron spin and the donor nuclear spin. We present a technique based on dynamic decoupling of the electron spin to accurately determine the Stark shift, and illustrate this using antimony donors in isotopically purified silicon-28. We then demonstrate two different methods to use a DC electric field combined with an applied resonant radio-frequency (RF) field to conditionally control donor nuclear spins. The first method combines an electric-field induced conditional phase gate with standard RF pulses, and the second one simply detunes the spins off-resonance. Finally, we consider different strategies to reduce the effect of electric field inhomogeneities and obtain above 90% process fidelities., Comment: 5 pages, 4 figures, 1 supplementary information

A multi-terminal device based on a carbon nanotube quantum dot was used at very low tem- perature to probe a single electronic and nuclear spin embedded in a bis-phthalocyanine Terbium (III) complex (TbPc2). A spin-valve signature with large conductance jumps was found when two molecules were strongly coupled to the nanotube. The application of a transverse field separated the magnetic signal of both molecules and enabled single-shot read-out of the Terbium nuclear spin. The Landau-Zener (LZ) quantum tunneling probability was studied as a function of field sweep rate, establishing a good agreement with the LZ equation and yielding the tunnel splitting \Delta. It was found that ? increased linearly as a function of the transverse field. These studies are an essential prerequisite for the coherent manipulation of a single nuclear spin in TbPc2., Comment: 7 pages, 6 figures, to appear in PRB

Magnetic molecules possess a high potential as building blocks for the design of spintronic devices. Moreover, the use of molecular materials opens the way for the controlled use of bottom-up, e.g. supramolecular, processing techniques combining massively parallel self-fabrication with conventional top-down nanostructuring techniques. The development of solid state spintronic devices based on the giant magnetoresistance (GMR), tunnel magnetoresistance (TMR), and spin valve effects has revolutionized the field of magnetic memory applications. Recently, organic semiconductors were inserted into nanometer sized tunnel junctions allowing enhancement of spin reversal, giant magneto-resistance behaviour was observed in single non-magnetic molecules coupled to magnetic electrodes, and the use of the quantum tunnelling properties of single-molecule magnets (SMMs) in hybrid devices was proposed. Herein, we present an original device in which a non-magnetic molecular quantum dot, made of a single-wall carbon nanotube (SWCNT) contacted with non-magnetic electrodes, is laterally coupled via supramolecular interactions to a TbPc2-SMM (Pc = phthalocyanine), which provides a localized magnetic moment. The conductance through the SWCNT is modulated by sweeping the magnetic field, exhibiting magnetoresistance ratios up to 300% between fully polarized and non-polarized SMMs below 1 K. We thus demonstrate the functionality of a supramolecular spin valve without magnetic leads. Our results open up prospects of circuit-integration and implementation of new device capabilities., Comment: 12 pages, 3 figures