Session H1: Quantum Information, Concepts and Computation

Recent progress in trapped-ion quantum information processing

Trapped strings of cold ions provide an ideal system for quantum information processing. The quantum information can be stored in individual ions and these qubits can be individually prepared, the corresponding quantum states can be manipulated and measured with nearly 100{\%} detection efficiency. With a small ion-trap quantum computer based on two and three trapped Ca+ ions as qubits we have generated in a pre-programmed way genuine quantum states. These states are of particular interest for the implementation of an ion quantum register: we have demonstrated selective read-out of single qubits and manipulation of single qubits of the register conditioned on the read-out results. Moreover, entangled states of up to eight particles were generated using an algorithmic procedure and the resulting states were analyzed using state tomography proving genuine multi-partite entanglement. With a new cavity QED setup we create an ion-qubit to photon-qubit interface for interconnecting ion-trap based quantum computers. With this device a source of deterministically generated single photons can be built and atom-photon entanglement can be investigated.   

Quantum optics and quantum information processing with solid state systems

A single semiconductor quantum dot embedded in a monolithic microcavity with a large spontaneous emission enhancement (Purcell) factor is capable of producing identical single photons on demand. Those single photons realize a multi-particle interference effect as quantum mechanically indistinguishable particles. Various quantum phenomena, such as violation of Bell's inequality and quantum teleportation, can be demonstrated with only such single photons and linear optics. An important challenge for quantum information system applications is to replace a man-made quantum dot by a natural substitutional donor impurity with suppressed inhomogeneous linewidth and electron/nuclear spin capability. In this talk we will discuss the recent experimental progress on this new quantum dot system.   

Direct Observation of Atomic Number Squeezing in a Degenerate Bose Gas

I will discuss recent work in my group on many-body quantum control towards the creation of atomic Fock states. In our rubidium experiment, a single Bose-Einstein condensate is optically trapped in crossed TEM01 modes, and we achieve confinement in two dimensions that is comparable to an optical lattice, but with single-atom addressability and detection. Using this system we have directly observed sub-Poissonian atom number statistics for a degenerate Bose gas with numbers as small as 20 atoms. The number squeezing was produced by starting with a Bose-Einstein condensate in an optical box trap and slowly lowering the walls in one dimension, a process we call quantum evaporation. The final atom number is determined by the energy of the N-body state in the box and can be precisely controlled. The observed squeezing is nearly a factor of two below the Poissonian (shot noise) limit. More importantly, known sources of noise can account for the residual fluctuations, so our results are consistent with the production of N-body Fock states. I will discuss the current limit to Fock state creation for even smaller numbers which we believe is due to many-body quantum tunneling. This effect can be controlled by the use of carefully tailored optical potentials and experiments are in progress. I will also discuss applications of our system to quantum computing and to the study of quantum critical phenomena.   

General properties of Nonsignaling Theories

We present a series of properties, usually associated to quantum physics, and show that they are common to all theories that do not allow for superluminal signalling and predict violation of Bell inequalities. These include intrinsic randomness, no cloning, monogamy of correlations, uncertainty relations, privacy of correlations, bounds on the shareability of some states. Finally, we emphasize that correlation data must violate some Bell inequality in order to contain distillable secrecy and introduce a new QKD protocol and prove its security against any individual attack by an adversary only limited by the no-signalling condition.   

Quantum control and robust quantum information

Controlling the time evolution of quantum states and quantum logic operations in an optimal manner gives rise to various desiderata depending on the physical constraints. In addition to fidelity optimization with respect to systematic and environmental errors, there may be requirements to optimize with respect to time or energy. We describe several approaches to control of logic operations on quantum bits that illustrate these different aspects, including both analytic and numerical tools. Applications are made to high fidelity quantum operations on solid state quantum bits.