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arXiv:2408.13025 · Nature Communications

Electronic interferometry with ultrashort plasmonic pulses

Seddik Ouacel, Lucas Mazzella, Thomas Kloss, Matteo Aluffi, Thomas Vasselon, Hermann Edlbauer, Junliang Wang, Clément Geffroy, Jashwanth Shaju, Arne Ludwig, Andreas Dirk Wieck, Michihisa Yamamoto, David Pomaranski, Shintaro Takada, Nobu‐Hisa Kaneko, Giorgos Georgiou, Xavier Waintal, Matias Urdampilleta, H. Sellier, Christopher Bäuerle

Published May 19, 2025• 9 citations
arXivHTMLPDF

Abstract

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. Although their coherence time is limited, flying electrons in the form of picosecond plasmonic pulses could be competitive in terms of the number of achievable coherent operations. The key challenge in achieving this critical milestone is the development of a new technology capable of injecting ‘on-demand’ single-electron wavepackets into quantum devices, with temporal durations comparable to or shorter than the device dimensions. Here, we take a significant step towards achieving this regime in a quantum nanoelectronic system by injecting ultrashort single-electron plasmonic pulses into a 14-micrometer-long Mach-Zehnder interferometer. Our results establish that quantum coherence is robust under the on-demand injection of ultrashort plasmonic pulses, as evidenced by the observation of coherent oscillations in the single-electron regime. Building on this, our results demonstrate the existence of a “non-adiabatic" regime that is prominent at high frequencies. This result highlights the potential of flying qubits as a promising alternative to localised qubit architectures, offering advantages such as a reduced hardware footprint, enhanced connectivity, and scalability for quantum information processing. The development of electronic flying qubits requires the ability to generate and control single-electron excitations. Here the authors demonstrate quantum coherence of ultrashort single-electron plasmonic pulses in an electronic Mach-Zehnder interferometer, revealing a non-adiabatic regime at high frequencies.

DOI
10.1038/s41467-025-58939-4

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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. Although their coherence time is limited, flying electrons in the form of picosecond plasmonic pulses could be competitive in terms of the number of achievable coherent operations. The key challenge in achieving this critical milestone is the development of a new technology capable of injecting ‘on-demand’ single-electron wavepackets into quantum devices, with temporal durations comparable to or shorter than the device dimensions. Here, we take a significant step towards achieving this regime in a quantum nanoelectronic system by injecting ultrashort single-electron plasmonic pulses into a 14-micrometer-long Mach-Zehnder interferometer. Our results establish that quantum coherence is robust under the on-demand injection of ultrashort plasmonic pulses, as evidenced by the observation of coherent oscillations in the single-electron regime. Building on this, our results demonstrate the existence of a “non-adiabatic" regime that is prominent at high frequencies. This result highlights the potential of flying qubits as a promising alternative to localised qubit architectures, offering advantages such as a reduced hardware footprint, enhanced connectivity, and scalability for quantum information processing. The development of electronic flying qubits requires the ability to generate and control single-electron excitations. Here the authors demonstrate quantum coherence of ultrashort single-electron plasmonic pulses in an electronic Mach-Zehnder interferometer, revealing a non-adiabatic regime at high frequencies.
The second quantum revolution of the 20th century contributed to a technological expansion in solid-state physics. Modern nanofabrication coupled to material processing techniques have facilitated the development of on-demand single-electron sources. With their remarkable precision, they are currently used for metrology purposes and hold key potential for the recent field of research of electron quantum optics, including a promising candidate for quantum information: the electron flying qubit. These quantum bits are created by short voltage pulses and manipulation of their quantum state occurs on-the-fly. The shortest voltage pulse reported so far attains 27 ps, on-chip, in a cryogenic environment. However, suffering from attenuation and dispersion in coaxial lines, accessibility to electronic quantum regimes at faster time scale remains out-of-reach.On the contrary, empowered by the advent of ultrafast lasers and progress in photonics and plasmonics, photo-conductive-switch technology can allow the generation of electromagnetic pulses with picosecond duration. The large bandwidth of these THz opto-electronic technologies could overcome the technical bottleneck faced by standard electronic equipment. While they have been widely developed for room temperature applications, demonstrations of their integration along-side a quantum architecture in a cryogenic environment are still limited. The realisation of such an experiment would unlock new research directions for studying the dynamics of solid-state electronic quantum devices.In this manuscript, we present the development of an experimental setup to generate picosecond voltage pulses in a cryogenic environment for flying qubit applications. A femtosecond laser generates pulses that are injected into optical fibres and integrated at low temperatures. Fabricated on the surface of Low Temperature grown GaAs (LT-GaAs), a photoconductive-switch is co-integrated to the quantum circuit formed on a high mobility two-Dimensional Electron Gas (2DEG). Owing to the extremely fast opto-electronic response of the LT-GaAs, the photo-conductive device converts optical pulses into voltage pulses with a duration as short as a few picoseconds. Using a THz CoPlanar Waveguide (CPW) circuit, the pulse is conveyed toward the 2DEG, where it is used as an ultra-short single-electron source.To perform pump-probe measurement and resolve the dynamic profile of such pulses, an original experimental setup, involving piezo-electric positioners and alignment protocols at low-temperatures, was implemented. As proof of concept, we first report the characterisation at 300 K of electronic pulses with 1.9 ps duration. In a second step, we cool down the system to 4 K and measure 2.3 ps wide pulses, thus expanding the proof of concept to cryogenic environments. Then, by integrating a 2DEG structure along-side the THz circuit, we were able to excite THz plasmons in a Fabry-Pérot cavity. The characteristic frequency of their fundamental mode was compared to an analytical model that revealed information about the carrier density of the electron gas under illumination. In parallel, extensive developments were carried out on the design of CPWs in order to minimise undesirable signal reflections as well as dispersion losses. Finally, with the prospect of measuring and controlling the propagation of picosecond electron pulse in quantum channels directly with ultrafast THz electrostatic gates, the fabrication of the next generation of devices was initiated. This work will pave the way for on-chip integration of picosecond voltage pulses into quantum nanoelectronics devices and ultrafast control of electronic flying qubits.
Standard approaches to quantum computing require significant overhead to correct for errors. The hardware size for conventional quantum processors in solids often increases linearly with the number of physical qubits, such as for transmon qubits in superconducting circuits or electron spin qubits in quantum dot arrays. While photonic circuits based on flying qubits do not suffer from decoherence or lack of potential scalability, they have encountered significant challenges to overcome photon loss in long delay circuits. Here, we propose an alternative approach that utilizes flying electronic wave packets propagating in solid-state quantum semiconductor circuits. Using a novel time-bin architecture for the electronic wave packets, hardware requirements are drastically reduced because qubits can be created on-demand and manipulated with a common hardware element, unlike the localized approach of wiring each qubit individually. The electronic Coulomb interaction enables reliable coupling and readout of qubits. Improving upon previous devices, we realize electronic interference at the level of a single quantized mode that can be used for manipulation of electronic wavepackets. This important landmark lays the foundation for fault-tolerant quantum computing with a compact and scalable architecture based on electron interferometry in semiconductors.
In this report we review the present state of the art of the control of propagating quantum states at the single-electron level and its potential application to quantum information processing. We give an overview of the different approaches that have been developed over the last few years in order to gain full control over a propagating single-electron in a solid-state system. After a brief introduction of the basic concepts, we present experiments on flying qubit circuits for ensemble of electrons measured in the low frequency (DC) limit. We then present the basic ingredients necessary to realise such experiments at the single-electron level. This includes a review of the various single-electron sources that have been developed over the last years and which are compatible with integrated single-electron circuits. This is followed by a review of recent key experiments on electron quantum optics with single electrons. Finally we will present recent developments in the new physics that has emerged using ultrashort voltage pulses. We conclude our review with an outlook and future challenges in the field.
Electronic interferometers have been identified as possible candidates for building electronic flying qubits. Such a regime requires ultrafast voltage pulses whose duration is shorter than the time of flight through the device. Understanding the corresponding physics in the presence of such short excitations requires a proper treatment of electron-electron interactions. In this article, we take a step in this direction by performing time-resolved simulations of a Mach-Zehnder interferometer treating the interactions at the time-dependent mean-field level. We find that the main effect of the interaction is the renormalization of the pulse velocity. Very importantly, the interference effects appear to be robust to the presence of interactions.
In this report we review the present state of the art of the control of propagating quantum states at the single-electron level and its potential application to quantum information processing. We give an overview of the different approaches that have been developed over the last few years in order to gain full control over a propagating single-electron in a solid-state system. After a brief introduction of the basic concepts, we present experiments on flying qubit circuits for ensemble of electrons measured in the low frequency (DC) limit. We then present the basic ingredients necessary to realise such experiments at the single-electron level. This includes a review of the various single-electron sources that have been developed over the last years and which are compatible with integrated single-electron circuits. This is followed by a review of recent key experiments on electron quantum optics with single electrons. Finally we will present recent developments in the new physics that has emerged using ultrashort voltage pulses. We conclude our review with an outlook and future challenges in the field.
The ability to efficiently control optical quantum interference by the same device is a challenging task that promises to enable key operations in quantum information processing. Here, we tackle this problem by presenting a composite quantum plasmonic metasurface operating as an ultrathin non-unitary beamsplitter capable of achieving both single photon bosonic bunching and fermionic anti-bunching interference just by varying the incident photons frequency or changing the design dimensions. The presented metasurface combines a plasmonic grating corrugated by periodic nanoslits that can achieve extraordinary optical transmission with additional nanostripes on top forming nanocavities characterized by localized gap plasmon resonances. The photon-photon interactions are analyzed based on calculating the quantum coalescence metric which accurately portrays the obtained bunching or anti-bunching quantum interference. The presented quantum interference operation is applicable to both fundamental non-classical light interactions and in the design of more applied quantum optical logic gates used in quantum computing and communications.