The manipulation of the electronic spin degree of freedom is an important goal in the context of the development of quantum information and spin electronics. Coherent conductors connected to ferromagnetic contacts are particularly interesting in this context. Indeed, they are subject to spin-dependent quantum interferences, which can be used to manipulate the spin degree of freedom. In this talk, I will focus on effective fields induced inside a coherent conductor by a ferromagnetic contact. I will first discuss the nature of these fields. Second, I will show that these effective fields can be used to design a spin quantum bit adapted for Circuit Quantum Electrodynamics experiments[1]. This setup is based on a double quantum dot connected to two non-collinear ferromagnetic contacts. This allows to obtain a strong spin/photon coupling, which can be switched on and off with a simple gate voltage. Third, I will show that the contact-induced effective fields can also be used to induce unconventional superconducting correlations inside a normal metal wire[2]. [1] A. Cottet and T. Kontos, Phys. Rev. Lett. 105, 160502 (2010) [2] A. Cottet, unpublished
The manipulation of the electronic spin degree of freedom is an important goal in the context of the development of quantum information and spin electronics. Coherent conductors connected to ferromagnetic contacts are particularly interesting in this context. Indeed, they are subject to spin-dependent quantum interferences, which can be used to manipulate the spin degree of freedom. In this talk, I will focus on effective fields induced inside a coherent conductor by a ferromagnetic contact. I will first discuss the nature of these fields. Second, I will show that these effective fields can be used to design a spin quantum bit adapted for Circuit Quantum Electrodynamics experiments[1]. This setup is based on a double quantum dot connected to two non-collinear ferromagnetic contacts. This allows to obtain a strong spin/photon coupling, which can be switched on and off with a simple gate voltage. Third, I will show that the contact-induced effective fields can also be used to induce unconventional superconducting correlations inside a normal metal wire[2]. [1] A. Cottet and T. Kontos, Phys. Rev. Lett. 105, 160502 (2010) [2] A. Cottet, unpublished
Coupling on-chip microwave cavities to superconducting microcircuits has provided a new platform to study the light/matter interaction at the most elementary level [1]. Coupling such cavities to more complex quantum circuits, like quantum dot circuits, is of high interest since it would allow the use of different degrees of freedom, like for instance the electronic spin, in a cavity quantum electrodynamics context. In this talk, I will present various theoretical proposals which would exploit this possibility. First, I will present a scheme allowing a strong coupling between cavity photons and a single spin trapped in a double quantum dot with ferromagnetic contacts [2]. Such a scheme would enable a scalable spin quantum bit architecture. Second, I will discuss theoretically the coupling of a double-quantum dot based Cooper pair splitter to a cavity [3]. Such a scheme can be used to test the entanglement of split Cooper pairs, due to a subradiance property of the Cooper pairs singlet state. Third, I will present recent experiments on carbon nanotube based quantum dots coupled to microwave cavities. In particular, I will discuss the observation of a charge coupling between two distant quantum dots mediated by cavity photons. [1] A. Wallraff, D. I. Schuster, A. Blais, L. Frunzio, R.- S. Huang, J. Majer, S. Kumar, S.M. Girvin, and R. J. Schoelkopf, Nature (London) 431, 162 (2004). [2] A. Cottet and T. Kontos, Phys. Rev. Lett. 105, 160502 (2010) [3] A. Cottet, T. Kontos, and A. Levy Yeyati, Phys. Rev. Lett. 108, 166803 (2012). [5] M.R. Delbecq, V. Schmitt, F.D. Parmentier, N. Roch, J.J. Viennot, G. Fève, B. Huard, C. Mora, A. Cottet, and T. Kontos, Phys. Rev. Lett. 107,256804 (2011) [6] M.R. Delbecq et al., unpublished
We propose theoretically a scheme for implementing a spin quantum bit in a quantum dot circuit with ferromagnetic leads. This setup does not rely on any specific band structure and can therefore be realized with many different types of nanoconductors. In this talk, we will discuss its implementation with carbon nanotubes. We can reach the strong coupling limit between the electron spin and a superconducting photon cavity, with experimentally realistic parameters compatible with a slow enough decoherence of the spin quantum state. This allows us to envision the use of circuit cavity quantum electrodynamics methods for single spin manipulation and readout.
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.
Coupling on-chip microwave cavities to superconducting microcircuits has provided a new platform to study the light/matter interaction at the most elementary level. Generalizing such a coupling to more complex quantum circuits, like quantum dot circuits, is of high interest since it would allow the use of different degrees of freedom, like for instance the electronic spin, in a cavity quantum electrodynamics context. In this talk, I will present various aspects of the effect of a double quantum dot on cavity photons. I will discuss the case of normal, ferromagnetic and superconducting reservoirs. In particular, the coupling of a Cooper pair splitter with cavity photons will be discussed
This thesis investigates experimentally the coupling mechanism of the charge and spin degrees of freedom in double quantum dots to high finesse superconducting microwave cavities. We use carbon nanotubes as a coherent conductor to host our quantum dots.We conceived an experimental setup and developed new fabrication methods in order to achieve control over these devices. With these methods, we scrutinize the resonant coupling of the dot's charge electronic transitions to a microwave cavity. We drive the system out of equilibrium to characterise its dynamics and extract its relevant intrinsic parameters. We examine a possible coupling of single photons to a single electron spin, using non-collinear exchange fields induced from ferromagnetic interfaces as a key ingredient to engineer this coupling. The preliminary results in this circuit architecture are promising for future spin-based QED experiments with single spins.
Circuit QED techniques have been instrumental in manipulating and probing with exquisite sensitivity the quantum state of superconducting quantum bits coupled to microwave cavities. Recently, it has become possible to fabricate new devices in which the superconducting quantum bits are replaced by hybrid mesoscopic circuits combining nanoconductors and metallic reservoirs. This mesoscopic QED provides a new experimental playground to study the light-matter interaction in electronic circuits. Here, we present the experimental state of the art of mesoscopic QED and its theoretical description. A first class of experiments focuses on the artificial atom limit, where some quasiparticles are trapped in nanocircuit bound states. In this limit, the circuit QED techniques can be used to manipulate and probe electronic degrees of freedom such as confined charges, spins, or Andreev pairs. A second class of experiments uses cavity photons to reveal the dynamics of electron tunneling between a nanoconductor and fermionic reservoirs. For instance, the Kondo effect, the charge relaxation caused by grounded metallic contacts, and the photo-emission caused by voltage-biased reservoirs have been studied. The tunnel coupling between nanoconductors and fermionic reservoirs also enable one to obtain split Cooper pairs, or Majorana bound states. Cavity photons represent a qualitatively new tool to study these exotic condensed matter states.