Quantum technology: the second quantum revolution

Time-bin entangled photon pairs from spontaneous parametric down-conversion pumped by a cw multi-mode diode laser

Generation of time-bin entangled photon pairs requires the use of the Franson interferometer which consists of two spatially separated unbalanced Mach-Zehnder interferometers through which the signal and idler photons from spontaneous parametric down-conversion (SPDC) are made to transmit individually. There have been two SPDC pumping regimes where the scheme works: the narrowband regime and the double-pulse regime. In the narrowband regime, the SPDC process is pumped by a narrowband cw laser with the coherence length much longer than the path length difference of the Franson interferometer. In the double-pulse regime, the longitudinal separation between the pulse pair is made equal to the path length difference of the Franson interferometer. In this paper, we propose another regime by which the generation of time-bin entanglement is possible and demonstrate the scheme experimentally. In our scheme, differently from the previous approaches, the SPDC process is pumped by a cw multi-mode (i.e., short coherence length) laser and makes use of the coherence revival property of such a laser. The high-visibility two-photon Franson interference demonstrates clearly that high-quality time-bin entanglement source can be developed using inexpensive cw multi-mode diode lasers for various quantum communication applications.

On structure-preserving transformations of the Ito generator matrix for model reduction of quantum feedback networks

Two standard operations of model reduction for quantum feedback networks, elimination of internal connections under the instantaneous feedback limit and adiabatic elimination of fast degrees of freedom, are cast as structure-preserving transformations of Itō generator matrices. It is shown that the order in which they are applied is inconsequential.

Asymptotic inference in system identification for the atom maser

System identification is closely related to control theory and plays an increasing role in quantum engineering. In the quantum set-up, system identification is usually equated to process tomography, i.e. estimating a channel by probing it repeatedly with different input states. However, for quantum dynamical systems such as quantum Markov processes, it is more natural to consider the estimation based on continuous measurements of the output, with a given input that may be stationary. We address this problem using asymptotic statistics tools, for the specific example of estimating the Rabi frequency of an atom maser. We compute the Fisher information of different measurement processes as well as the quantum Fisher information of the atom maser, and establish the local asymptotic normality of these statistical models. The statistical notions can be expressed in terms of spectral properties of certain deformed Markov generators, and the connection to large deviations is briefly discussed.

Control aspects of quantum computing using pure and mixed states

Steering quantum dynamics such that the target states solve classically hard problems is paramount to quantum simulation and computation. And beyond, quantum control is also essential to pave the way to quantum technologies. Here, important control techniques are reviewed and presented in a unified frame covering quantum computational gate synthesis and spectroscopic state transfer alike. We emphasize that it does not matter whether the quantum states of interest are pure or not. While pure states underly the design of quantum circuits, ensemble mixtures of quantum states can be exploited in a more recent class of algorithms: it is illustrated by characterizing the Jones polynomial in order to distinguish between different (classes of) knots. Further applications include Josephson elements, cavity grids, ion traps and nitrogen vacancy centres in scenarios of closed as well as open quantum systems.

Implementing quantum gates by optimal control with doubly exponential convergence

We introduce a novel algorithm for the task of coherently controlling a quantum mechanical system to implement any chosen unitary dynamics. It performs faster than existing state of the art methods by 1 to 3 orders of magnitude (depending on which one we compare to), particularly for quantum information processing purposes. This substantially enhances the ability to both study the control capabilities of physical systems within their coherence times, and constrain solutions for control tasks to lie within experimentally feasible regions. Natural extensions of the algorithm are also discussed.

Photosynthetic Light-Harvesting Complexes

The light-harvesting antenna systems found in photosynthetic organisms function to collect light and transfer energy in the photon to a reaction center, where electron transfer gives rise to long-term energy storage. The antenna systems found in different types of photosynthetic organisms adapt the organisms to very different photic environments, and almost certainly have been invented multiple times during evolution. The diverse collection of photosynthetic antenna systems is described in terms of their pigment and protein components and their organization in the photosystem. The Förster theory is described as the physical basis of energy transfer in photosynthetic antennas, although in many systems it is not adequate to describe energy transfer in complexes with closely interacting pigments. Regulatory aspects of antennas are described, including the process of non-photochemical quenching.

Coherence properties of spontaneous parametric down-conversion pumped by a multi-mode cw diode laser

Coherence properties of the photon pair generated via spontaneous parametric down-conversion pumped by a multi-mode cw diode laser are studied with a Mach-Zehnder interferometer. Each photon of the pair enters a different input port of the interferometer and the biphoton coherence properties are studied with a two-photon detector placed at one output port. When the photon pair simultaneously enters the interferometer, periodic recurrence of the biphoton de Broglie wave packet is observed, closely resembling the coherence properties of the pump diode laser. With non-zero delays between the photons at the input ports, biphoton interference exhibits the same periodic recurrence but the wave packet shapes are shown to be dependent on both the input delay as well as the interferometer delay. These properties could be useful for building engineered entangled photon sources based on diode laser-pumped spontaneous parametric down-conversion.

Incoherent control of locally controllable quantum systems

An incoherent control scheme for state control of locally controllable quantum systems is proposed. This scheme includes three steps: (1) amplitude amplification of the initial state by a suitable unitary transformation, (2) projective measurement of the amplified state, and (3) final optimization by a unitary controlled transformation. The first step increases the amplitudes of some desired eigenstates and the corresponding probability of observing these eigenstates, the second step projects, with high probability, the amplified state into a desired eigenstate, and the last step steers this eigenstate into the target state. Within this scheme, two control algorithms are presented for two classes of quantum systems. As an example, the incoherent control scheme is applied to the control of a hydrogen atom by an external field. The results support the suggestion that projective measurements can serve as an effective control and local controllability information can be used to design control laws for quantum systems. Thus, this scheme establishes a subtle connection between control design and controllability analysis of quantum systems and provides an effective engineering approach in controlling quantum systems with partial controllability information.

Entangled-photon coincidence fluorescence imaging

We describe fluorescence imaging using the second-order correlation of entangled photon pairs. The proposed method is based on the principle that one photon of the pair carries information on where the other photon has been absorbed and has produced fluorescence in a sample. Because fluorescent molecules serve as "detectors" breaking the entanglement, multiply-scattered fluorescence photons within the sample do not cause image blur. We discuss experimental implementations.

Quantum process tomography of a controlled-NOT gate

We demonstrate complete characterization of a two-qubit entangling process--a linear optics controlled-NOT gate operating with coincident detection--by quantum process tomography. We use a maximum-likelihood estimation to convert the experimental data into a physical process matrix. The process matrix allows an accurate prediction of the operation of the gate for arbitrary input states and a calculation of gate performance measures such as the average gate fidelity, average purity, and entangling capability of our gate, which are 0.90, 0.83, and 0.73, respectively.

Introduction. Information, knowledge and technology

Technology is the sum of the ways in which social groups manipulate order in the world to achieve their ends. It enables our active engagement with the world. Technology is central to our present well-being and vital for our future survival. As such it needs a coherent world view, a conceptual framework which will enable the fundamental problems that it poses for society to be approached in an illuminating way. Furthermore, such an approach, while remaining convincing, must not be overwhelmed by an ever-increasing welter of specialization and diversity of application. It is the purpose of the set of papers presented here to examine some key aspects of such a conceptual framework; not in the sense of offering a fully worked out philosophy of technology--that would be a huge and complex undertaking--but rather by considering some key topics. Subsidiary aims are to survey important relevant areas, to identify key sources that can provide access points for further study, and to consider some possible future developments. Major, coherent domains of activity are characterized by a few, fundamental, extensively used and essentially unifying concepts. Technology is such a domain, and its fundamental concepts are information, knowledge and agency. The following sections give a synoptic overview of the material presented in this theme issue, and set it within a wider context.