Experimental Test of the High-Frequency Quantum Shot Noise Theory in a Quantum Point Contact

We report on direct measurements of the electronic shot noise of a quantum point contact at frequencies nu in the range 4-8 GHz. The very small energy scale used ensures energy independent transmissions of the few transmitted electronic modes and their accurate knowledge. Both the thermal energy and the quantum point contact drain-source voltage Vds are comparable to the photon energy hnu leading to observation of the shot noise suppression when $V_{ds}<h\nu/e$. Our measurements provide the first complete test of the finite frequency shot noise scattering theory without adjustable parameters.Comment: Version Published in Phys. Rev. Lett. (Phys. Rev. Lett. 99, 236803 (2007)

Magnetic field asymmetry of mesocopic dc rectification in Aharonov Bohm rings

Fundamental Casimir-Onsager symmetry rules for linear response do not apply to non linear transport. This motivates the investigation of nonlinear dc conductance of mesoscopic GaAs/GaAlAs rings in a 2 wire configuration. The second order current response to a potential bias is of particular interest. It is related to the sensitivity of conductance fluctuations to this bias and contains information on electron interactions not included in the linear response. In contrast with the linear response which is a symmetric function of magnetic field we find that this second order response exhibits a field dependence which contains an antisymmetric part. We analyse the flux periodic and aperiodic components of this asymmetry and find that they only depend on the conductance of the rings which is varied by more than an order of magnitude. These results are in good agreement with recent theoretical predictions relating this asymmetric response to the electron interactions.Comment: 5 pages, 4 figure

Towards universal quantum computation through relativistic motion

We show how to use relativistic motion to generate continuous variable Gaussian cluster states within cavity modes. Our results can be demonstrated experimentally using superconducting circuits where tuneable boundary conditions correspond to mirrors moving with velocities close to the speed of light. In particular, we propose the generation of a quadripartite square cluster state as a first example that can be readily implemented in the laboratory. Since cluster states are universal resources for universal one-way quantum computation, our results pave the way for relativistic quantum computation schemes

Quantum superposition of a single microwave photon in two different "colour" states

The ability to coherently couple arbitrary harmonic oscillators in a fully-controlled way is an important tool to process quantum information. Coupling between quantum harmonic oscillators has previously been demonstrated in several physical systems by use of a two-level system as a mediating element. Direct interaction at the quantum level has only recently been realized by use of resonant coupling between trapped ions. Here we implement a tunable direct coupling between the microwave harmonics of a superconducting resonator by use of parametric frequency conversion. We accomplish this by coupling the mode currents of two harmonics through a superconducting quantum interference device (SQUID) and modulating its flux at the difference (~ 7 GHz) of the harmonic frequencies. We deterministically prepare a single-photon Fock state and coherently manipulate it between multiple modes, effectively controlling it in a superposition of two different "colours". This parametric interaction can be described as a beam-splitter-like operation that couples different frequency modes. As such, it could be used to implement linear optical quantum computing protocols on-chip.Comment: 21 pages, 10 figure