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Electronic generation and detection in nanoelectronic devices at the picosecond scale

Periodic Reporting for period 1 - UltraFastNano (Electronic generation and detection in nanoelectronic devices at the picosecond scale)

Reporting period: 2020-01-01 to 2020-12-31

Quantum Technology is an emerging new and radically different information technology. Its advocates foresee it as a future large industry, with a potential impact of a similar scale to conventional IT and of key importance for the digital single market and for industry and growth. Presently huge investments are being made world-wide to consolidate this new technology.
Most quantum technologies under development focus on localized quantum states such as superconducting circuits or the electron spin of an electron trapped by electrostatic confinement. In contrast, quantum nanoelectronics studied in this project aims at manipulating propagating quantum states, at increasingly high frequencies beyond the 10 GHz range. In this regime, elementary excitations are either electronic or photon like, with potential applications that span quantum technologies (communications between quantum devices, sensing, quantum computation) and fundamental scientific problems. Addressing frequencies beyond this range, in the THz regime, is extremely difficult for electronic devices. Prior to this project, this kind of time scale could only be accessed by optical methods, and was not possible electrically. The goal of UltraFastNano is to develop the first quantum nanoelectronics platform operating at picosecond time scales.
To give an example, the coherence of quantum devices such as semiconductor qubits is naturally short lived and require ultrafast operation speed in order to increase the number of possible quantum operations. A key quantity used to characterise the performance of a quantum device is the ratio between two characteristic times: the time a qubit can survive its quantum properties and the time it takes to complete its operation. With UltraFastNano we will push the limit of operation speed for quantum nanoelectronic devices to timescales down to 1 picosecond.
The UltraFastNano project will pioneer new concepts at the crossroads between quantum optics and solid-state nanoelectronics. Its aim is to achieve full control of quantum excitations that propagate through the quantum devices on the picosecond scale, about three orders of magnitude faster than other quantum technologies. The main objectives include
• Picosecond on-demand coherent single-particle source
• Single-shot detection of propagating excitations at the discrete charge level
• Quantum interferometry at the single-charge level
• New software for predictive simulation and optimisation of ultrafast quantum devices
In the first year of the project our effort has been concentrated on the generation of ultrafast voltage pulses which will be used to generate electronic wave packets which contain a single electronic charge. At the same time, we have advanced our theoretical understanding of such propagating wave packets, in particular their dynamics during emission.
To produce single-electron wavepackets, we have developed novel voltage pulse generation tools that allow to generate voltage pulses down to a temporal width of 1 picosecond. Two approaches have been pursued. One is based on voltage pulse generation by superposing several frequency harmonics based on radio-frequency electronics. A second approach exploits optoelectronic conversion. A Terahertz laser pulse is used to excite a photoconductive media and subsequently converted into an electrical signal than can be guided towards an electrical circuit. With this technique we have been able to generate ultrafast voltage pulses down to 1 picosecond at ambient temperature.

We are also developing a semiconductor device technology to convert electronic pulses to photonic pulses aiming to reach a single-particle control with picosecond timing accuracy. This will be achieved by injecting electrons emitted by an electron pump into a p-n junction. So far in this project, we have developed a cleanroom fabrication method to create a p-type region on an n-type GaAs/AlGaAs two-dimensional electron gas system. We have demonstrated the stability and two-dimensionality of the induced hole gas by quantum-Hall measurements. We have also designed and fabricated a scanning optical stage and are presently in the process of installing into our bottom-loading probe in our dilution refrigerator.

Both the emission of single electrons into a quantum nanoelectronic conductor as well as their propagation in that device are strongly impacted by Coulomb interaction effects. One of the main tasks of the theoretical analysis within this project is hence to understand these interaction effects and their impact. We are using a master equation approach combined with numerical methods to analyze the spectrum of multi-particle emission, a bosonization method has been employed to pin down interaction effects on the decoherence of propagating pulses of different shapes. Furthermore, exploiting the newly released open-source software TKWANT as well as self-consistent scattering theory, we have identified effects stemming from the interaction of propagating particles with the quantum nanoelectronic conductor potential. These aspects are currently further developed in order to enable more focussed experimental control, which is vital for the planned realization of flying qubits and interferometers, and open up for novel ways of analysing and exploiting single-electron emission.

Achievement: One of the main achievements of the 1st reporting period of UltraFastNano is the recent release of the open-source software TKWANT (https://tkwant.kwant-project.org/) for time-resolved quantum transport.
The long-term vision of UltraFastNano includes the much-needed quantum bus allowing direct interaction between distant quantum bits, hence a drastic reduction of the hardware footprint for quantum error correction protocols. It also includes the possibility of more disruptive quantum technologies such as flying qubit architectures, a radically different route from the mainstream semiconducting or super-conducting approach followed by the major nanoelectronic companies (Google, IBM, Intel, etc.).
UltraFastNano aims to achieve these goals be fighting decoherence with a dramatic increase of operation speed. In the long-term, pushing towards this frequency range will enable quantum technologies that operate without the need of a cryogenic environment (6 THz 300K).
The three technological milestones of UltraFastNano – (i) 1 ps single-electron source, (ii) single-electron detection and (iii) optoelectronic interface – have important potential for triggering disruptive innovation on ultrafast electronics for (cryogenic) THz voltage source or detection. This novel and efficient method for generating ultrafast pulses can find numerous applications for on-chip THz spectroscopy and for THz wireless communications. UltraFastNano will also find immediate applications for metrologically-accurate measurements of the ampere and picosecond optoelectronic devices that convert between electronic and photonic excitations.
In addition, the simulation tools developed within the UltraFastNano project will be directly integrated into nextnano commercial products/software. The implemented features will increase the capabilities of the nextnano software in simulating a large range of nanoelectronic devices such as silicon nanowire transistors or silicon spin qubits.
Single-electron wave packet propagating along an electronic waveguide above the Fermi sea
Newly released T-Kwant- software