research of the group
	In recent years we have seen signs of a new technological revolution in information processing, a revolution caused by a paradigm shift to information processing using the laws of quantum physics. There are now strong indicators that this fundamental research field will lead to a whole new quantum information technology in the near future. There are currently numerous possible routes forward for quantum computation and communication hardware and many new applications being discovered. As there are so many routes to overcome the technological hurdles of implementing quantum information science, numerous projects here at NII hopefully give a hint as to how large scale quantum communication networks and maybe one day, quantum computers, will ultimately be constructed. Our group here at NII is pursing novel research in Quantum Information Science. Particularly, we're interesting in the interdisciplinary areas of Quantum Communication, Quantum Computation and Simulation, Quantum Algorithms, Quantum Metrology and High Precision Measurements. Researchers of our group according to their interests are pursuing theoretical interests and advanced applications that are simple, elegant and practical. The Photonic Module. Entanglement is the enabling resource for a new generation of advanced technology. Photonic entanglement is the most versatile, finding applications in quantum communication, lithography, metrology, cryptography and quantum computation. However there is no convenient, on-demand devices for rapidly generating multi-photon entangled states from single-photon sources. One major area of our work is a new device called the "photonic module" which we propose to address this need. Allowing, for the first time, the rapid, deterministic preparation of a large class of entangled states with an application independent, ``plug and play" device, with the flexibility to prepare entanglement for all major quantum computation and communication applications. The heart of the module is a controllable atom/cavity system, which entangles photons quickly and deterministically. Recent advances in cavity-QED suggest that construction of this device is imminently possible. The module offers great potential for integrating advanced techniques in atom optics, optical computation and photonics providing a new resource for quantum information science. Optical Quantum Gates. In order to implement a two qubit gate between photonic modes a type of non-linearity is required. Hence our group is interested in what types of non-linearities can be used to implement quantum gates, and the optimization of resources for each type of non-linearity. Currently work focuses on three different proposals for photonic computation, namely the quantum bus (qubus) scheme for quantum computation, the optical Zeno gate and linear optics quantum computation (LOQC). In the qubus scheme, weak controlled rotations in conjunction with large quantum coherent bus modes allow the construction of a universal set of gates. In the optical Zeno gate, the Zeno effect is used in the form of two photon absorption to construct a photonic controlled-sign gate. In the LOQC scheme, linear optical elements in the form of beam splitters, phase shifters, wave plates and polarisers are used in conjunction with single photons and number resolving photo-detections to implement a universal set of gates. Topological quantum computation with the Photonic module. The Photonic module allows for the deterministic preparation of stabilized states in photons. It is well known that highly entangled cluster states (which are a universal resource for quantum computation) are stabilizer states. Therefore, the Photonic module allows for the deterministic preparation of large optical cluster states for quantum computation. One of the more exciting research areas here at NII is optical networks (utilizing the Photonic module) that can be used to prepare a continuous and never ending cluster state for optical computation. Additionally, it was shown in 2007 that an appropriate three dimensional cluster could be used to implement topological gates such that high fault-tolerant thresholds can be achieved. Not only can the Photonic module be utilized to deterministically prepare an appropriate 2D cluster for measurement based computation but it can also be used to continuously and deterministically prepare an appropriate 3D cluster for topological measurement based computation. This is extremely exciting work since it leads to an experimentally feasible, large scale, optical quantum computer. Entanglement and noise, quantification and estimation. As with any implementation of quantum information processing in physical system, the theoretical and experimental quantification of noise sources is of enormous importance. One major area of work focuses on the theoretical quantification of noise sources and entanglement generation. Focusing largely on the qubus model of computation, we attempt to not only estimate the largest sources of error when coupling coherent light systems to qubits but also how we are able to quantify (both theoretically and experimentally) the entanglement between qubits and coherent quantum states. Not only does this give us better understanding of the physics underlying the computational system, but is also a vital requirement in estimating possible physical constraints on quantum state control. Characterization protocols for qubit systems. System identification and characterization is instrumental to the entire field of quantum computing and more generally, quantum control. Within the language of quantum information processing, the engineering and characterization of the system Hamiltonian directly controls the resulting interactions required for quantum gate operations and hence large scale quantum algorithms. As large scale quantum information requires extremely low error rates to effectively implement active quantum error correction, detailed knowledge of controllable system dynamics is crucial. One of our current projects is to examine and develop a set of intrinsic experimental protocols to identify and characterize important dynamics related to qubit operation. These techniques differ from standard experimental characterization protocols in that they are designed specifically for use in the context of large scale quantum information and qubit manufacturing. Quantum Gates with Superconductors. In collaboration with the experimental superconducting group at NTT we are also developing pulse designs for two qubit dynamics between flux qubits coupled to a LC resonator. The underlying physics related to this system leads to extremely interesting and non-trivial dynamics and the ability to design appropriate classical control pulses to achieve high fidelity two-qubit gates is a significant challenge. One of the major hurdles to these systems is quantum leakage related to the coupling of individual qubits to a harmonic resonator. While basic pulse designs lead to high fidelity operations, leakage is still significant and prohibits the use of these pulse designs for large scale computation. The solution to this problem is the use of geometric optimization. This is the technique of using specially designed numerical algorithms to "find" high fidelity pulses through the use of basic gradient optimization techniques. In this way, more complicated pulse structures can be discovered which not only eliminate leakage but ultimately lead to much higher fidelity quantum gates. Purification Networks and Error analysis. While quantum computation is arguably the holy grail of quantum information science, quantum communication networks will be a technological reality much sooner. One area of focus for NII is the detailed analysis of quantum purification protocols and repeater networks. We examine theses systems from two reasonably disjoint points of view. The first is large scale network design, where we examine the best possible repeater protocols, qubit node arrangements and entanglement swapping to achieve high quantum data rates. The second avenue of investigation is to examine the quantum protocols for purification. We model quantum gate arrays that are used to purify entanglement from the standpoint of operational errors. Rather that treating all errors as if they were decoherence, we instead assume that implementation of purification circuits are systematically imperfect (due to fabrication of engineering constraints). The analysis of these circuits is extremely interesting, the simple nature of purification circuits allow for analytic analysis of error propagation and exhibits very interesting operating dynamics. This analysis is extremely important for determining optimal purification techniques and faster operational speeds for communication networks. Last updated 29th July 2008.