Takis Kontos

An On-Demand Coherent Single Electron Source

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 was triggered by the application of a potential step that compensated for the dot-charging energy. Depending on the barrier transparency, the quantum emission time ranged from 0.1 to 10 nanoseconds. The single-electron source should prove useful for the use of quantum bits in ballistic conductors. Additionally, periodic sequences of single-electron emission and absorption generate a quantized alternating current.

Coupling a quantum dot, fermionic leads, and a microwave cavity on a chip.

We demonstrate a hybrid architecture consisting of a quantum dot circuit coupled to a single mode of the electromagnetic field. We use single wall carbon nanotube based circuits inserted in superconducting microwave cavities. By probing the nanotube dot using a dispersive readout in the Coulomb blockade and the Kondo regime, we determine an electron-photon coupling strength which should enable circuit QED experiments with more complex quantum dot circuits.

Single carbon nanotube transistor at GHz frequency.

We report on microwave operation of top-gated single carbon nanotube transistors. From transmission measurements in the 0.1-1.6 GHz range, we deduce device transconductance gm and gate-nanotube capacitance Cg of micro- and nanometric devices. A large and frequency-independent gm approximately 20 microS is observed on short devices, which meets the best dc results. The capacitance per unit gate length of 60 aF/microm is typical of top gates on a conventional oxide with epsilon approximately 10. This value is a factor of 3-5 below the nanotube quantum capacitance which, according to recent simulations, favors high transit frequencies fT=gm/2piCg. For our smallest devices, we find a large fT approximately 50 GHz with no evidence of saturation in length dependence.

Nanospintronics with carbon nanotubes

One of the actual challenges of spintronics is the realization of a spin transistor allowing control of spin transport through an electrostatic gate. In this paper, we report on different experiments which demonstrate gate control of spin transport in a carbon nanotube connected to ferromagnetic leads. We also discuss some theoretical approaches which can be used to analyse spin transport in these systems. We emphasize the roles of the gate-tunable quasi-bound states inside the nanotube and the coherent spin-dependent scattering at the interfaces between the nanotube and its ferromagnetic contacts.

Relaxation Time of a Chiral Quantum R-L Circuit

We report on the GHz complex admittance of a chiral one-dimensional ballistic conductor formed by edge states in the quantum Hall regime. The circuit consists of a wide Hall bar (the inductor L) in series with a tunable resistor (R) formed by a quantum point contact. Electron interactions between edges are screened by a pair of side gates. Conductance steps are observed on both real and imaginary parts of the admittance. Remarkably, the phase of the admittance is transmission independent. This shows that the relaxation time of a chiral R -L circuit is resistance independent. A current and charge conserving scattering theory is presented that accounts for this observation with a relaxation time given by the electronic transit time in the circuit.

Noisy Kondo impurities

Sensitive measurements of fluctuations in the current through carbon-nanotube-based quantum dots provide insight into the many-body physics of such systems.

Out-of-equilibrium charge dynamics in a hybrid circuit quantum electrodynamics architecture

(Dated: December 23, 2013)The recent development of hybrid cQED allows one to study how cavity photons interact witha system driven out of equilibrium by fermionic reservoirs. We study here one of the simplestcombination : a double quantum dot coupled to a single mode of the electromagnetic eld. Weare able to couple resonantly the charge levels of a carbon nanotube based double dot to cavityphotons. We perform a microwave read out of the charge states of this system which allows us tounveil features of the out of equilibrium charge dynamics, otherwise invisible in the DC current.We extract relaxation rate, dephasing rate and photon number of the hybrid system using a theorybased on a master equation technique. These ndings open the path for manipulating other degreesof freedom e.g. the spin and/or the valley in nanotube based double dots using microwave light.

Photon-mediated interaction between distant quantum dot circuits.

Engineering the interaction between light and matter is an important goal in the emerging field of quantum opto-electronics. Thanks to the use of cavity quantum electrodynamics architectures, one can envision a fully hybrid multiplexing of quantum conductors. Here we use such an architecture to couple two quantum dot circuits. Our quantum dots are separated by 200 times their own size, with no direct tunnel and electrostatic couplings between them. We demonstrate their interaction, mediated by the cavity photons. This could be used to scale up quantum bit architectures based on quantum dot circuits or simulate on-chip phonon-mediated interactions between strongly correlated electrons.

Spin quantum bit with ferromagnetic contacts for circuit QED.

We theoretically propose a scheme for a spin quantum bit based on a double quantum dot contacted to ferromagnetic elements. Interface exchange effects enable an all electric manipulation of the spin and a switchable strong coupling to a superconducting coplanar waveguide cavity. Our setup does not rely on any specific band structure and can in principle be realized with many different types of nanoconductors. This allows us to envision on-chip single spin manipulation and readout using cavity QED techniques.

Kondo effect in carbon nanotubes at half filling

In a single state of a quantum dot the Kondo effect arises due to the spin-degeneracy, which is present if the dot is occupied with one electron (N=1). The eigenstates of a carbon nanotube quantum dot possess an additional orbital degeneracy leading to a fourfold shell pattern. This additional degeneracy increases the possibility for the Kondo effect to appear. We revisit the Kondo problem in metallic carbon nanotubes by linear and nonlinear transport measurement in this regime, in which the fourfold pattern is present. We have analyzed the ground state of CNTs, which were grown by chemical vapor deposition, at filling N=1, N=2, and N=3. Of particular interest is the half-filled shell, i.e., N=2. In this case, the ground state is either a paired electron state or a state for which the singlet and triplet states are effectively degenerate, allowing in the latter case for the appearance of the Kondo effect. We deduce numbers for the effective missmatch delta of the levels from perfect degeneracy and the exchange energy J. While deltasimilar to0.1-0.2 (in units of level spacing) is in agreement with previous work, the exchange term is found to be surprisingly small: Jless than or similar to0.02. In addition we report on the observation of gaps, which in one case is seen at N=3 and in another is present over an extended sequence of levels.