An On-Demand Coherent Single Electron Source

Abstract: We report on the electron analog of the single photon gun. On demand single electron injection in a quantum conductor was obtained using a quantum dot connected to the conductor via a tunnel barrier. Electron emission is triggered by application of a potential step which compensates the dot charging energy. Depending on the barrier transparency the quantum emission time ranges from 0.1 to 10 nanoseconds. The single electron source should prove useful for the implementation of quantum bits in ballistic conductors. Additionally periodic sequences of single electron emission and absorption generate a quantized AC-current.

• The paper demonstrates a single electron source that precisely emits coherent electron wave packets on demand.
• It uses a quantum dot in a 2DEG with tunnel coupling via a quantum point contact to regulate nanosecond-scale emission timings.
• Experimental results validate theoretical predictions and advance quantum computing applications with robust, controllable electron emission.

Insights into an On-Demand Coherent Single Electron Source

The paper "An On-Demand Coherent Single Electron Source" delineates the realization of a single electron source capable of precise temporal control, suitable for coherent manipulation within ballistic electronic systems. This development represents a significant advancement in the field of quantum electronics by enabling coherent emission from a quantum dot, akin to the single photon guns in quantum optics.

Technical Overview

The implementation involves a quantum dot embedded in a two-dimensional electron gas (2DEG) within a GaAs semiconducting heterostructure. The dot is tunnel-coupled to a conductor via a quantum point contact (QPC). The crucial mechanism involves the triggering of electron emission via a voltage step capable of compensating the dot's charging energy, thereby emitting an electron as a coherent wave packet. The mean energy of the emitted electrons can be regulated above the Fermi energy with fine-tuning of the voltage step amplitude.

This electron source achieves emission timings ranging from 0.1 to 10 nanoseconds, contingent upon barrier transparency. It offers precise control over both the energy and the time profile of the emitted electrons, independent of the source's temperature and primarily governed by the inverse tunneling time—a requisite for on-demand single particle sources.

Experimental Realization

The research utilizes a heterostructure GaAsAl/GaAs to create the 2DEG, with a dot size ensuring discreet energy levels separated by Δ, approximately 2.5 K. The QPC controls the transmission probability $D$, affecting the tunneling rate $\tau^{-1} = D \Delta/h$. The paper reports detailed experimental investigations incorporating both time-domain and frequency-domain measurements. These measurements exhibit coherence in the charge relaxation dynamics and reveal quantized AC-current through electron emission and absorption sequences.

A significant aspect of the experimental procedure involves maintaining an electronic temperature around 200 mK, approximating the quantum Hall regime with negligible spin degeneracy. The source is subjected to periodic high-amplitude voltage excitations that go beyond the linear regime, facilitating single electron emission observable within the constraints posed by the experimental apparatus, particularly the 1 GHz bandwidth acquisition card.

Theoretical and Practical Implications

The detailed theoretical analysis extends existing harmonic linear response theories, supplying a robust framework for understanding the non-linear response observed experimentally. Key equations governing charge ($q$) and escape time ($\tau$) are derived and juxtaposed with experimental results, providing a comprehensive match, with predictions capturing the charge quantization and dynamics authentically.

Practically, this coherent single electron source paves the way for implementing quantum information technologies, particularly in quantum computing and cryptographic applications where control over single particles in mesoscopic systems is paramount. The source's ability to precisely emit electrons at defined energies and times enhances prospects for developing electronic analogs of photonic quantum information processing systems.

Future Developments

In future endeavors, the synchronization of similar on-demand electron sources might facilitate the exploration of advanced quantum phenomena such as electron antibunching and entanglement. Such developments hold promise for elevating ballistic conductor systems into viable contenders for executing quantum logic operations, parallel to current capabilities in photonic systems.

The methodologies and findings of this study form a cornerstone in the field of quantum electronic research, suggesting vast potential for integrating coherent single electron sources in emerging quantum technologies. As the field progresses, advancements in electron detection speed could further augment these capabilities, solidifying the electron-photon analogy and broadening the horizon for the implementation of novel quantum devices.