Hi everyone,

We are pleased to inform you that the next Non Permanent Seminar will take place on Wednesday the 15th of May at 11.30 am in room Erwin Bertaut F418 (3rd floor F building) . We will have the pleasure to listen to the talks of :

- Alexis Coissard(Quantum Nano-Electronics and Spectroscopy team, QUEST Department) who will talk about : "High-resolution Landau level spectroscopy in graphene on hBN devices"

Abstract :
Graphene offers an ideal, surface-accessible two-dimensional electron gas for scanning tunnelling spectroscopy. Under intense perpendicular magnetic field, the density of states of graphene splinters into discrete Landau levels that can be directly probed at the local scale with a tunnelling tip. In this talk we present STM spectroscopy performed with a home-made hybrid AFM-STM operating at 4 K and 14 T on high-mobility graphene devices in the quantum Hall regime. We first describe how we approach the tip on the micron scale-graphene flake by means of AFM navigation, then we demonstrate the high quality of the graphene surface with atomic-resolution STM topography. Applying a magnetic field up to 14 T enables us to perform high-resolution Landau level spectroscopy. A systematic study of the Landau level dispersion as a function of magnetic field and gate voltage gives access to the Fermi velocity which exhibits an unusual gate-dependence. Furthermore, tuning the Fermi level with the gate unveils the pinning of Fermi energy in the Landau levels, a key phenomenon in the quantum Hall physics. Depending of the geometry of the tip apex, a quantum dot can form in the graphene beneath the tip and charging peaks appear in the dI/dV spectra, forming well-defined series of diamond structures in the gate maps typical of Coulomb blockade physics.

and

- David Niegemann (Quantum Coherence team, QUEST Department) who will talk about " Gate-Based High Fidelity Spin Readout in a CMOS Device"

Over the last fifty years, the CMOS (Complementary-Metal-Oxide-Semiconductor) electronics industry has been continuously scaling down transistors in size, to increase performance and reduce power consumption. Nowadays, the smallest transistors in industry achieve 5nm features. As a result, those silicon structures tend to exhibit undesirable quantum effects for a classical transistor which appear to be new research opportunities for quantum information processing. In particular, it is nowadays possible to trap single electron spins in silicon quantum dots and perform high fidelity quantum gates [i]. These demonstrations combined with the intrinsic properties of the silicon lattice [ii] (low spin orbit and hyperfine interaction) make CMOS device an excellent candidate for scalable quantum architectures. In this presentation, we will show how we can detect a single spin in a CMOS device thanks to an original approach which combines gate-based dispersive charge sensing and a latched Pauli spin blockade mechanism [iii]. For this purpose, we use a double quantum dot coupled to a single reservoir where one of the dot carries the spin information while the second dot is used as an ancillary dot to perform the readout. This scalable method allows us to read out a single spin with a fidelity above 98% for 0.5 ms integration time [iv]. Moreover, we show that the demonstrated high read-out fidelity is fully preserved up to 0.5 K. This results holds particular relevance for the future co-integration of spin qubits and classical control electronics.
[i] Veldhorst, M. et al. Nat. Nanotechnol. 9, 981 (2014).
[ii] Steger, M. et al. Science 336, 1280 (2012).
[iii] Harvey-Collard, P. et al. Phys. Rev. X 8, 021046 (2018).

We invite you to register for the buffet on this doodle : https://doodle.com/poll/qgwcfx6qrnrgs3xe

We are looking for new speakers for the next seminars of 2019. Don't hesitate to contact us if you wish to give a talk !

Hoping to see you as many of you as possible,

The organizers

We are pleased to inform you that the next Non Permanent Seminar will take place on Wednesday the 3rd of April at 11.30 am in room Erwin Bertaut F418 (3rd floor F building) . We will have the pleasure to listen to the talks of

- Kimon Moratis (Nanophysics and semiconductors team, PLUM Department) who will talk about : "Tailoring the valence band ground state in a semiconductor nanowire quantum dot"

Abstract :

Semiconductor nanowire quantum dots are very promising candidates for applications in the field of quantum information and computation since they can act as sources of single photons [1] or entangled photon pairs [2]. Moreover they can be easily integrated in heterostructures where we can engineer their band structure by tuning their dimensions and taking advantage of induced strain due to lattice mismatch. These are the parameters which govern the switching from the predominantly heavy hole character of the valence band ground state to a light hole one[3][4]. Light holes are characterized by more versatile optical selection rules, through which we can initiate a quantum superposition of spins for optical manipulation of qubits.
In this work we investigateboth experimentally and theoretically the parameters which affect the switching from a heavy to a light hole ground state and their in-between mixing. The heterostructures studied are constituted of a CdTe quantum dot incorporated in a ZnTe nanowire passivated by a ZnMgTe shell. The properties of the valence band are determined by optical measurements. For this we employ a combination of different spectroscopic techniques including micro-photoluminescence, cathodoluminescence and magneto-optical measurements.Experimental observations are further confirmed by numerical calculations on the nanowire quantum dots using6 band k.p theory where we take into account the effect of strain and piezoelectric field.

[1] Science, Vol. 290, Issue 5500, pp. 2282-2285 (2000)
[2] Nature volume 439, pages 179–182 (2006)
[3] Phys. Rev. B 88, 115424 (2013)
[4] Nature Physics volume 10, pages 46–51 (2014)

and

- Marco Guerra (Quantum Nano Electronics and Spectroscopy team, QUEST Department) who will talk about "Unraveling spatial details of transport via scanning gate microscopy "

Abstract :

Among the various esoteric techniques used to investigate quantum transport, there is one called scanning gate microscopy (SGM). It was developed in the late 1990’s in Harvard [1], in an effort to overcome the limitations of scanning-tunneling microscopy (STM) to study devices whose active electron system are too deep below the surface and not accessible to the STM tip. SGM consists of disturbing electrostatically a device thanks to the polarized metallic tip of an atomic force microscopy (AFM), and reading the changes in the device conductance induced by the tip potential. After few years of improvement, this technique has amazed the community, showing both the possibility to image in real-space the wave-function of electrons coming out of a QPC [2], as wells as coherent branched flows of electrons in ballistic 2DEG [3].

In this presentation I will show you some SGM achievements in various electronic systems, with a great attention on the possibility to image optics-like experiments where electrons can experience reflection, refraction and diffraction phenomena.

[1] M.A. Eriksson et al. , Appl. Phys. Lett. 69, 671 (1996).
[2] M.A. Topinka et al. , Science 289, 2323 (2000).
[3] M.A. Topinka et al. , Nature 410, 183 (2001).

We invite you to register for the buffet on this doodle : https://doodle.com/poll/6qie8y97krxtzzt7

We are looking for new speakers for the next seminars of 2019. Don't hesitate to contact us if you wish to give a talk !

Hoping to see you as many of you as possible,

The organizers