Quantum Transport in Graphene

people: P. Roulleau, F. D. Parmentier, P. Roche, D. C. Glattli

Electron quantum optics in graphene

Quantum computing is based on the manipulation of quantum bits (qubits) to enhance the efficiency of information processing. In solid-state systems, two approaches have been explored:

  • static qubits, coupled to quantum buses used for manipulation and information transmission,
  • flying qubits which are mobile qubits propagating in quantum circuits for further manipulation.

Flying qubits research led to the recent emergence of the field of electron quantum optics, where electrons play the role of photons in quantum optic like experiments. This has recently led to the development of electronic quantum interferometry as well as single electron sources. As of yet, such experiments have only been successfully implemented in semi-conductor heterostructures cooled at extremely low temperatures. Realizing electron quantum optics experiments in graphene, an inexpensive material showing a high degree of quantum coherence even at moderately low temperatures, would be a strong evidence that quantum computing in graphene is within reach.
One of the most elementary building blocks necessary to perform electron quantum optics experiments is the electron beam splitter, which is the electronic analog of a beam splitter for light. However, the usual scheme for electron beam splitters in semi-conductor heterostructures is not available in graphene because of its gapless band structure. I propose a breakthrough in this direction where pn junction plays the role of electron beam splitter. This will lead to the following achievements considered as important steps towards quantum computing:

  • electronic Mach Zehnder interferometry used to study the quantum coherence properties of graphene,
  • the implementation of on-demand electronic sources in the GHz range for graphene flying qubits.

Van der Waals heterostructures

State-of-the art techniques for the fabrication of ultra-high quality graphene samples rely on van der Waal heterostructures, which are obtained by artificially stacking two-dimensional crystals such as graphene, few-layer graphite, or hexagonal boron nitride (h-BN). In particular, encapsulating a graphene flake between two thin h-BN crystals spectacularly increases the quality and mobility of graphene, which not only sits on a perfectly flat substrate, but is also protected from contamination from air, water, polymer, or any other compound used during sample fabrication.

Thanks to local grants from LabEx PALM, Université Paris-Saclay, and the SIRTEQ project, we have recently set up a van der Waals heterostructures assembly platform, allowing us to fabricate ultra-high quality samples that we will use in our quantum transport experiments.

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Fabrication of high quality encapsulated graphene Hall bars. Left: microscope image of the assembled h-BN/graphene/h-BN stack before processing. Right: Hall bars after processing.

Quantum transport in the Terahertz range

When subjected to electromagnetic radiation, the fluctuation of the electronic current across a quantum conductor increases. This additional noise, called photon-assisted shot noise, arises from the generation and subsequent partition of electron-hole pairs in the conductor. The physics of photon-assisted shot noise has been thoroughly investigated at microwave frequencies up to 20 GHz, and its robustness suggests that it could be extended to the terahertz (THz) range. We have recently preformed present measurements of the quantum shot noise generated in a graphene nanoribbon subjected to a THz radiation. Our results show signatures of photon-assisted shot noise, further demonstrating that hallmark time-dependant quantum transport phenomena can be transposed to the THz range.

  • Photon-Assisted Shot Noise in Graphene in the Terahertz Range, F. D. Parmentier, L. N. Serkovic-Loli, P. Roulleau, and D. C. Glattli, Phys. Rev. Lett. 116, 227401 (2016)(arxiv)
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Scanning electron micrograph of a CVD-grown graphene ribbon connected to THz antennae-shaped electrodes.

We are currently investigating photon-assisted transport in high-mobility graphene samples (see above).

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Optical image of a high-mobility graphene Hall bar connected to a THz antenna for photon-assisted Klein tunneling experiments.

Graphene Plasmonics

The chirality of the collective edge magnetoplasmon (EMP) waves in Graphene is evidenced and the cyclotron orbit drift velocity on Graphene edge determined.

Using an original picosecond pulses method we have measured the edge magnetoplasmon velocity in the QHE regime. This velocity is the sum of the EMP velocity due to Coulomb interaction and of the single particle carrier velocity, the so-called cyclotron orbit dirft velocity.

In the experiments, ~10ps electromagnetic pulses couple to the Graphene sample, propagate along its perimeter and the transmission is measured. The sample is a Graphene mono-layer from exfoliated natural graphite. The set-up can transmit microwaves up to 65 GHz in temperatures down to 2K and in a high magnetic field, up to 19.25 Tesla. The figure below shows different propagation time is observe for positive and negative field showing the chirality. The QHE manifests in plateaus in the propagation time for filling factor 2 and 6. Cyclotron drift velocity is found ~0.7 X Fermi velocity.

  • Edge magnetoplasmons in graphene, I Petkovic, F. I. B. Williams, and D. C. Glattli, Journal of Physics D, 47, 094010 (2014)(arxiv)
  • Carrier Drift Velocity and Edge Magnetoplasmons in Graphene, I. Petković, F. I. B. Williams, K. Bennaceur, F. Portier, P. Roche, and D. C. Glattli, Phys. Rev. Lett. 110, 016801 (2013), (arxiv)

Similar results have been obtained for SiC Graphene by N. Kumada et al at NTT Atsugi (Jpn) http://fr.arxiv.org/abs/1204.5034.

Recently, larger graphene sample from SiC wafer (collaboration NTT Japan) have allowed to measure de dispsersion relation and the decay rate of EMPs.

  • Resonant edge magnetoplasmons and their decay in graphene, N. Kumada, P. Roulleau, Benoît Roche, M. Hashisaka, H. Hibino, I. Petkovic, and D. C. Glattli, submitted **(arxiv)