Cavity-Free Flying Electrons in Quantum Architectures

• Cavity-free flying electrons are quantum-coherent, single-electron excitations that propagate in engineered nanostructures like SAW-driven dots and quantum Hall edge channels.
• They achieve near-unity emission and detection efficiencies with ballistic guidance over micrometer distances, ensuring robust coherent control.
• These systems enable ultrafast quantum gates, entanglement via beam splitting and Coulomb interactions, and photon–electron interactions, paving the way for scalable quantum technologies.

Cavity-free flying electrons are quantum-coherent, single-electron excitations propagating through engineered nanostructures or optical fields without confinement by an electromagnetic cavity. Unlike conventional cavity quantum electrodynamics (QED) platforms—which rely on stationary photons in resonant cavities to mediate coherent quantum operations—cavity-free flying electron systems use ballistic electronic wavepackets moving in low-dimensional channels, surface-acoustic-wave (SAW) potentials, edge states, or free space. These electrons serve as mobile quantum bits (qubits), quantum channels, or probes for quantum optics, quantum information, and ultrafast electron–photon interactions. Recent advances demonstrate near-unity efficiency in emission, guidance, and detection, coherent control and entangling operations, and strong coupling to light and matter, all achieved without photonic or microwave cavities.

1. Physical Mechanisms and Architectures for Cavity-Free Flying Electrons

Cavity-free flying electron platforms are realized in both solid-state nanostructures and vacuum/optical environments:

• SAW-driven moving quantum dots (MQDs): Applying a microwave burst to an interdigital transducer (IDT) on a piezoelectric GaAs substrate generates a SAW at frequency $f_0\sim2.6$ GHz and wavelength $\lambda\sim1\,\mu$m. The SAW produces a moving electrostatic potential $V(x,t)=V_0\sin(kx-\omega t)$ that traps and transports single electrons at phase velocity $v_\text{SAW}\sim2.6$–$3\,\mu$m/ns in a depleted 2DEG channel (Hermelin et al., 2011).
• Quantum Hall edge channels: In high-mobility 2DEGs subject to perpendicular magnetic fields, chiral one-dimensional edge channels (edge magnetoplasmons) propagate at $v_F\sim10^5$ m/s. Single electrons can be launched as short wavepackets by quantum-dot pumps or Lorentzian voltage pulses ("Levitons") (Edlbauer et al., 2022, Thalineau et al., 2014, Thiney et al., 2022).
• Tunnel-coupled quantum wires and rails: Gate-defined parallel quantum nanowires or rails allow for path-encoded flying qubits and in-flight beam-splitting and interference operations, as realized in SAW-driven tunnel-coupled circuits (Ito et al., 2020, Wang et al., 2022).
• Free-space and nanophotonic Bloch grating platforms: A dielectric grating illuminated by a femtosecond laser pulse excites a quantized electromagnetic near-field. Free electrons traversing the grating interact with the mode via phase-matched, cavity-free Jaynes–Cummings–type Hamiltonians (Ding, 14 Nov 2025). Topologically protected, dispersion-free "half-electron" bound states arise when a twisted optical phase profile creates a Dirac mass domain wall in the electron’s effective Hamiltonian (Pan et al., 1 Jan 2024).
• Direct laser acceleration (DLA) with flying-focus pulses: In vacuum or plasma, spatiotemporally structured laser fields accelerate electrons to ultrarelativistic energies without recourse to optical cavities or static structures (Meir et al., 29 Oct 2025).

2. Single-Electron Sources, Guidance, and Detection without Cavities

Central to cavity-free flying-electron platforms are high-fidelity, on-demand single-electron sources and detectors:

• Single-electron emission: SAW-driven sources prepare a single electron in a quantum dot (QD), which is then loaded into a moving SAW minimum and emitted into a one-dimensional channel with quantum efficiency $\eta_\text{emit}\sim96\%$ (Hermelin et al., 2011). Quantum-dot turnstile pumps and Levitons achieve sub-10 ps timing and emission energies decoupled from the Fermi sea (Edlbauer et al., 2022).
• Ballistic guidance: In SAW platforms, the electron is confined in a $300$ nm-wide channel, maintained several meV above the Fermi energy to suppress backscattering or inelastic relaxation. Transport occurs over $2$–$40\,\mu$m, orders of magnitude below the mean free path, ensuring ballisticity (Wang et al., 2022).
• Single-electron detection: Gate-defined QDs at the channel termini serve as single-charge detectors via quantum point contact (QPC) charge sensing, achieving detection efficiency $\eta_\text{det}\sim92\%$ (Hermelin et al., 2011). Alternatively, a two-electron singlet–triplet (S–T$_0$) qubit, capacitively coupled to the channel, detects passing electrons by a phase shift in its quantum oscillation ($\sim1\,\mu$eV per electron), with $>90\%$ detection fidelity and nanosecond time resolution (Thiney et al., 2022, Thalineau et al., 2014).
• Key experimental metrics:

| Platform | Emission Efficiency | Detection Fidelity | Transport Length | Coherence Time | |--------------------|--------------------|-------------------|------------------|--------------------------| | SAW (GaAs) | $\sim96\%$ | $\sim92\%$ | $3$–$40\,\mu$m | $T_2^*\sim25$ ns (spin) | | Edge channels | $>90\%$ (Levitons) | $>90\%$ | $>100\,\mu$m | $\sim$ns |

3. Coherent Operations, Flying-Qubit Gates, and Interactions

Cavity-free flying-electron architectures realize coherent single- and two-qubit operations through spatially engineered gate potentials or optically-induced Floquet interactions:

• Flying-qubit encoding and beam splitting: The quantum state may be encoded in the path (wire index) degree of freedom—e.g., in tunnel-coupled wires, the electron tunnels coherently between parallel rails, described by a two-site Hubbard model $H_0 = -\tau\sigma_x + (\epsilon/2)\sigma_z$. Gate-controlled tunnel coupling and detuning generate Rabi-like oscillations between paths, observable as current oscillations with visibility limited by initialization errors (max~3%) (Ito et al., 2020).
• Coulomb-mediated entanglement and antibunching: Synchronised injection of two single electrons into adjacent rails coupled by a tunnel barrier yields strong antibunching: the probability that both electrons exit in separate rails ($P_{11}$) is enhanced to $80\%$ compared to the noninteracting $50\%$ limit, set by Coulomb repulsion. The observed quantitive shift ($U_C\approx0.5$ meV, phase $\varphi\sim\pi$ over $12$ nm) enables direct controlled-phase gates in-flight (Wang et al., 2022).
• Spin coherence and transfer: Electron spins preserve coherence during transit, provided the SAW or edge-channel transfer time is $\ll T_2^*$. For SAW moving dots (GaAs), $T_2^*\sim25$ ns, and a $3\,\mu$m transfer is completed in $\sim1$ ns, enabling coherent spin shuttling and teleportation primitives (Hermelin et al., 2011).
• Photon–electron quantum optics: Grating-based architectures realize Jaynes–Cummings and Tavis–Cummings Hamiltonians without a cavity. Free electrons traversing a phase-matched Bloch mode exchange quanta with the mode: single-qubit ($R_x$) rotations occur in $T_\pi\sim43$ fs at $g/\omega_L\sim3.85\times10^{-4}$; iSWAP gates are implemented dispersively in $7.8$ ps with fidelity $F>99\%$ (Ding, 14 Nov 2025).

4. Scaling Laws, Strong Coupling, and Nonclassical State Preparation

Optimization of electron–photon or electron–mode coupling in cavity-free platforms adheres to universal scaling principles:

• Coupling probability: For a bosonic mode of frequency $\omega_i$ and effective size $D$, the excitation probability by a single electron is

$$P_i = \left(\frac{e v}{\hbar\omega_i}\right)^2 \left| \int dz\, E_{i,z}(z)\, e^{-i\omega_i z/v} \right|^2,$$

which scales through the dimensionless phase $\varphi_i=\omega_i D/v$.

• Order-unity coupling regimes: For small $D$ (10–50 nm), low-energy modes ($\hbar\omega_i<1$ eV) and $v/c\sim0.02$–$0.2$ ($E\sim10$–100 eV), $P\sim1$ (quasistatic 3D modes). Waveguides phase-matched over long effective length $L_\text{eff}\sim100$ µm–mm allow $P>1$, i.e., multiple photons per electron, enabling highly nonclassical Fock-state preparation (Giulio et al., 23 Mar 2024).
• Topological states: Spatial–temporal engineering of the light field yields a Jackiw–Rebbi zero-mode—a "half-electron" ($e/2$) bound to a propagating domain wall of the optically induced Dirac mass. This state is robust, non-dispersive, and forms via phase kinks in femtosecond, twisted laser beams, without cavities or static traps (Pan et al., 1 Jan 2024).

5. Detection, Readout, and Device Integration

Cavity-free schemes achieve rapid, high-sensitivity, and minimally destructive single-electron detection:

• Spin-qubit sensors: Charge detection is achieved by capacitive coupling between a flying electron and a stationary S–T$_0$ or singlet–triplet qubit. The induced detuning shift ($\Delta\epsilon\sim1\,\mu$eV) produces a phase shift $\sim0.035$ rad per electron on a $0.2$ ns timescale. Signal-to-noise ratios $>$1 per shot require tens of electrons; device optimization (reduced mutual capacitance, steeper exchange response, faster/readout) aims for single-shot, single-electron detection (Thiney et al., 2022, Thalineau et al., 2014).
• QPC and quantum-dot detectors: For electrons shuttled by SAWs, single-electron presence is detected through charge steps in the conductance of a proximal QPC with $>90\%$ efficiency and sub-millisecond timing (Hermelin et al., 2011).
• Error rates and back-action: For present spin qubit detectors, error probabilities $p_\text{err}\sim5$–$10\%$ per shot are achieved. Back-action on the flying electron is negligible (induced phase shift $<0.01$ rad), due to small mutual capacitance and short interaction windows (Thalineau et al., 2014).

6. Applications, Scalability, and Distinctions from Cavity-Based Schemes

Cavity-free flying-electron technology opens multiple avenues in quantum science distinct from cavity-based platforms:

• On-chip, on-demand quantum optics: Real-time electron Hong–Ou–Mandel and Hanbury Brown–Twiss interferometry, quantum entanglement distribution, and deterministic electron quantum optics are fully implemented on a single chip, independent of macroscopic cavities (Hermelin et al., 2011, Wang et al., 2022, Ito et al., 2020).
• Ultrafast quantum gates and collective control: Grating-based Jaynes–Cummings architectures provide sub-100 fs single-qubit and ps-scale two-qubit gates, natively supporting Tavis–Cummings–type collective interactions for digital and analog quantum simulation (Ding, 14 Nov 2025).
• Direct laser acceleration: Flying-focus DLA with spatiotemporally structured pulses enhances electron energies (cutoff $170$ MeV), collimation, charge yield ($80\times$ increase), and x-ray emission ($3\times$ improvement) over Gaussian pulses, with full avoidance of guiding cavities or waveguides (Meir et al., 29 Oct 2025).
• Scalability and implementation: All-electrical, lithographically defined semiconductor systems are integrable with stationary spin/charge qubits and scalable to circuits with interferometers, entangling gates, and high-fidelity routers. Free-space modules using programmable nanogratings or twisted light fields are compatible with ultrafast electron microscopy and quantum sensing (Giulio et al., 23 Mar 2024).
• Key distinctions from cavity-based methods:

| Feature | Cavity-Based QED | Cavity-Free Flying Electrons | |-------------------------------|---------------------------|---------------------------------------------| | Confinement | Stationary EM field, high-Q| Ballistic electron (no cavity) | | Information carrier | Photon qubit | Electron path/spin/momentum qubit | | Coupling | Strong matter–photon | Gate / field–induced electron–mode, Coulomb | | Emission/Detection | Spontaneous/induced | On-demand, >90% efficiency | | Scalability | Limited by cavity system | Lithographic, on-chip, or free-space | | Coherence times | $\mu$s–ms (photonic) | ns (spin), $\mu$s (charge), fs (optical) | | Application scope | cQED, quantum networks | Electron quantum optics, quantum information, ultrafast dynamics |

7. Outlook and Challenges

Future directions for cavity-free flying-electron platforms include:

• Universal quantum information processing: Integration of high-fidelity single-/two-qubit (flying) gates, scalable sources and detectors, and multi-qubit circuits for quantum simulation and computation entirely in the electronic domain (Ding, 14 Nov 2025, Wang et al., 2022).
• Single-shot, quantum-limited detection: Approaching single-electron, single-shot sensitivity in spin-qubit detectors by engineering maximal capacitive coupling, minimizing decoherence, and boosting bandwidth (Thiney et al., 2022, Thalineau et al., 2014).
• Topological state engineering: Exploiting topologically protected flying bound states for robust, dispersionless quantum information channels and synthetic fractionalization phenomena (Pan et al., 1 Jan 2024).
• Maximizing electron–photon coupling: Engineering nanoscale mode volumes, phase-matching, and interaction lengths for deterministic electron-to-photon conversion, Fock-state preparation, and heralded quantum state transfer (Giulio et al., 23 Mar 2024).
• Integration and fabrication: Advanced numerical modeling (time-dependent Schrödinger–Poisson, KWANT/TKWANT, nextnano) guides device design for desired tunnel couplings, potential profiles, and charge dynamics (Edlbauer et al., 2022).
• Mitigating decoherence and noise: Suppressing charge and spin noise by moving to isotopically pure Si/SiGe, optimizing gate geometries, and implementing pulse engineering protocols.
• Direct accelerator applications: All-optical, cavity-free direct laser accelerators using flying-focus pulses for compact sources of high-charge, high-energy electrons and secondary radiation (Meir et al., 29 Oct 2025).

The absence of photonic cavities fundamentally distinguishes this approach, enabling ultrafast, triggerable, and highly scalable quantum platforms, underpinning both modular quantum processors and free-electron quantum optics.