PUBLIC LECTURE - Artur Ekert - What have we learned from quantum computation?

Quantum theory of computation gives us not only the key to a new technology but also, more importantly, much deeper understanding of the foundations of quantum physics, computer science and mathematics. I will revisit some conceptual issues in quantum information science and try to outline possible future directions, challenges, and potential impacts of this new field.


Christof Wunderlich - Spin resonance with trapped ions: experiments and new concepts

Recent experimental results relevant for quantum information processing obtained with electrodynamically trapped Yb+ ions are reported. In addition, novel ideas for coherent manipulation of internal and external degrees of freedom of trapped ions will be outlined. In a suitably modified trap a string of ions can be treated like a molecule used for spin resonance experiments: The collection of trapped ions can be viewed as a $N$-qubit molecule with adjustable spin-spin coupling.


Paolo Mataloni- A New High-Brilliant Optical Parametric Source of Polarization Entangled States

Recent results concerning the realization of a high-brilliant source of polarization-entangled photon pairs, based on a original scheme developed in our laboratory, will be presented. By exciting the source with 15 mW of the 363.8 nm Ar+ line, we were able to generate pairs of photons with a very high degree of polarization entanglement over the entire cone of emission of a type I phase matching crystal, with a production rate of 104 couples/(s×mW). A 213-s violation of Bell's inequalities was obtained in these conditions. Possible applications of this source will be presented.

Klaus Moelmer - Quantum computing with ions and atoms

A review will be presented on recent theory proposals for quantum computing with trapped ions and atoms. This will be supplemented with examples of the current achievements and near-future plans for experiments. It is suggested that we may soon spot landsight and find prosperity on flourishing islands, although the working general purpose quantum computer is still far below the horizon.


Vladimir Buzek - Programmable quantum processors: Implementation of quantum maps

To follow.

Geza Giedke - The characterization of Gaussian operations and distillation of Gaussian States

We characterize the class of all physical operations that transform Gaussian states to Gaussian states. We show that this class coincides with that of all operations which can be performed on Gaussian states using linear optical elements and homodyne measurements. This characterization provides a powerful formalism to describe general Gaussian operations, which we apply to the question of entanglement distillation and entanglement generation by Gaussian operations. First, we prove that Gaussian states cannot be distilled by local Gaussian operations and classical communication. Then we derive the optimal way to generate entanglement or squeezing in a Gaussian two-mode system given one interaction Hamiltonian and one-mode passive linear optics. This directly relates to recent experiments using atomic ensembles for quantum information tasks.

Rainer Blatt - Quantum information processing experiments with trapped Ca+ ions

Single trapped Ca+ ions, stored in a linear Paul trap and laser–cooled to the ground state of their harmonic quantum motion are used for quantum information processing. For a demonstration, we used composite laser pulse sequences to implement a Deutsch-Josza algorithm. For this, Stark shifts on the qubit transitions had to be precisely measured and compensated.



Ronald de Wolf - Quantum Communication Complexity

In th seetting of communication complexity, Alice receives an input x, Bob receives an input y, and together they want to compute some function f(x,y), while minimizing the amount of communication between them. In recent years, it has been found that quantum communication is much more efficient than classical communication for computing various functions. In this talk we survey some recent results in this area: (1) quantum fingerprinting, which gives rise to an exponential quantum-classical improvement in a constrained communication model, due to Buhrman, Cleve, Watrous, and de Wolf; (2) a precise algebraic characterization of a "non-deterministic" variant of quantum communication complexity, due to Hoyer and de Wolf; (3) a slightly improved upper bound for the complexity of the disjointness problem, due to Hoyer and de Wolf, and Razborov's recent lower bound for disjointness (which was a major breakthrough).

Serge Massar - Quantum Non-Locality and Bell Inequalities

Recent results concerning tests of quantum non locality generally (often called Bell inequalities) will be presented. Applications of Bell inequalities will be discussed and a connexion between Bell inequalities and the field of computer science called "communication complexity" will be presented. Recently discovered Bell inequalities resistant to noise and to detector inefficiency will be presented, both for the case of small systems and in the asymptotic case of large dimensionality or of many parties.

Markus Grassl - Quantum Error-Correcting Codes

Methods for both active and passive stabilisation of quantum systems play a central role on the way towards quantum information processing. Since the presentation of the first scheme for quantum error-correction by Peter Shor in 1995, a general theory of quantum error-correcting has been established, showing that fault tolerant quantum computation is possible. This talk will give a survey on different methods of quantum error-correction, concentrating on algorithmic aspects.


Chiara Macchiavello - Communication and detection of entanglement over noisy channels

The ability of exploiting entanglement to achieve more efficient communication protocols and of detecting entanglement are central issues in quantum information. In this talk the role of entanglement in transmission of classical information over noisy channels is analysed. It is shown in particular that entanglement is a useful resource to increase the mutual information when the noisy transmission channel is not memoryless. Moreover, the problem of experimental detection of entanglement is addressed and some cases where entanglement can be detected by performing a few local measurements are presented.

Eugene Polzik - Light-Atoms Quantum Interface for Continuous Variables

Off-resonant interaction of light with an atomic ensemble followed by a suitable quantum projection measurement is sufficient to form a continuous variable quantum interface. Phase-amplitude or polarization state of light can be recorded in atomic ground state coherences. Two protocols are feasible: teleportation-like transfer, which requires two entangled atomic ensembles and another protocol, which requires one atomic ensemble only. In the first experiment we have demonstrated recording of light in a long-lived atomic state with quantum sensitivity.


Dieter Suter - A scalable architecture for spin-based quantum computers

Endohedral fullerenes like N@C60 can be considered as nanometer-sized atom traps. The long decoherence times of the electronic and nuclear spins qualify these molecules as interesting candidates for a solid-state spin-based quantum computer. One- and two-qubit gates may be implemented by sequences of radio-frequency and microwave pulses in an inhomogeneous magnetic field.


Seigo Tarucha - Spin effects in quantum dots and technical approach for implementing spin qu-bits

Electron spin is a natural candidate for a spin-qubit since every spin 1/2 encodes exactly one qubit. It is theoretically predicted that the spin-qubit located in quantum-confined systems satisfies all requirements necessary for a scalable quantum computer. There are, however, quite a few issues left for understanding the spin physics in quantum dots. I will first review experimental studies on spin-related properties of single and double quantum dot structures such as spin lifetime, exchange coupling, and Pauli effect. Then I will discuss technical approaches for spin-rotation, manipulation of exchange coupling, initialization and read-out.


Daniel Esteve - Operation of a solid state quantum bit circuit

We have designed and operated a quantum bit circuit based on the Cooper pair box, a superconducting device for which quantum coherence has already been demonstrated. In this new circuit, the box Josephson junction is replaced by two junctions in parallel forming a loop. The advantage of this design is to provide separate ports for qubit manipulation and readout, and to efficiently decouple the qubit from its external environment when readout is off. The qubit manipulation is performed by applying microwave pulses to the box gate, and the readout by measuring the current in the loop, which is different for both qubit states. This measurement is performed by monitoring the switching of an extra large Josephson junction inserted in the loop when a bias current pulse is applied to it. We show that this readout strategy approaches single-shot resolution, and we demonstrate that all qubit manipulations can be performed using Rabi precession of the qubit state when resonant microwave pulses are applied to the gate. Using a two-pulse sequence analogous to the Ramsey sequence in atomic physics, we have determined the coherence time of the qubit. This coherence time, which corresponds to about 8000 periods of the qubit transition, is sufficiently long to envision coupled qubit circuits.

Gennady Berman - Perturbation theory for scalable solid-state quantum computation and single-spin measurement using magnetic resonance force microscopy

We developed a perturbation theory for solid-state quantum computation with many qubits. Using this theory, we discuss how to simulate simple quantum logic operations with a large number of qubits (more than 1000) in a solid-state quantum computer based on a nuclear spin chain. These simulations are needed for experimental testing of scalable solid-state quantum computer devices. Quantum logic for remote qubits is simulated. Analytical estimates are presented for possible correlated errors caused by non-resonant transitions. A range of parameters is given in which non-resonant effects can be minimized.

Daniel Loss - Electron spins in quantum dots as qubits for quantum information processing

Coherent manipulation, filtering, and measurement of electronic spin in quantum dots and other nanostructures are new technologies which have promising applications both in conventional and in quantum information processing and transmission. I review the spintronics proposal for quantum computing, in which electron spins in quantum confined structures play the role of the quantum bits (qubits), and discuss the essential requirements for such an implementation. I describe several realizations of one- and two-qubit quantum gates and of state preparation and measurement, based on an all-electrical scheme to control the dynamics of spin, including spin-orbit effects. I discuss recently proposed schemes for using a single quantum dot as a spin filter and spin read-out device, and show how the decoherence time can be measured in a transport set-up. I address the issue of spin decoherence due to non-uniform hyperfine interactions with nuclei and show that for electrons confined to dots the spin decay is non-exponential. Finally, I discuss methods for producing and detecting the spin-entanglement of electronic EPR pairs, being an important resource for quantum communication.

Martin Plenio - The entanglement cost under operations respecting the partial transpose

We study the entanglement cost under quantum operations preserving the positivity of the partial transpose (PPT-operations). We demonstrate that this cost is directly related to the logarithmic negativity, thereby providing the operational interpretation for this easily computable entanglement measure. As examples we discuss general Werner states and arbitrary bi-partite Gaussian states. Equipped with this result we then prove that for the anti-symmetric Werner state PPT-cost and PPT-entanglement of distillation coincide giving the first example of a truly mixed state for which entanglement manipulation is asymptotically reversible.

Pranab Sen - Relative entropy and a substate theorem about quantum states

The von Neumann relative entropy of two quantum states is an important function in quantum information theory. The relative entropy satisfies some powerful properties, which makes it a useful tool to work with. In this talk, we discuss a new theorem about relative entropy. It roughly states that if the relative entropy of two quantum states is small, then the first state "sits inside" the second state as a "substate". We also discuss two applications of this theorem to privacy and interaction in quantum communication.

Mauro D'Ariano - Quantum information encoded on CP-maps

Encoding quantum information on completely positive (CP) maps---instead of quantum states---is a powerful point of view, which allows also to include cryptographic issues in the analysis of a communication protocol. For example, the optimal discrimination between maps automatically includes the possibility of EPR cheating in either concealing or binding issues in a quantum commitment. In this talk a review of recent work of the Quantum Optics & Information Group in Pavia will be presented, discussing discrimination schemes for CP-maps, the use of entanglement for reconstructing the unknown CP map of a device, the definition of "disturbance" and tradeoff with information, a complete classification of protocols and cheating strategies for quantum bit commitment, and a classification of unitary transformations corresponding to a CP maps.

Steffen Glaser - Time-Optimal Control of Quantum Dynamics and NMR Quantum Computing

"What is the minimum time to realize a desired unitary transformation in a given quantum system?" This simple question is not only of fundamental theoretical interest, but also is of great relevance for practical realizations of quantum computing algorithms. Based on principles of optimal control theory, this question will be addressed and examples will be given where time-optimal pulse sequences yield significant gains compared to conventional implementations of quantum gates. Furthermore, a scalable NMR quantum computing algorithm will be discussed.