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Author Billangeon, P.-M.; Nakamura, Y. openurl 
  Title Superconducting devices: Quantum cups and balls Type Journal Article
  Year 2011 Publication Nature Physics Abbreviated Journal Nat. Phys.  
  Volume 7 Issue 8 Pages (up) 594-595  
  Keywords fromIPMRAS  
  Abstract A single microwave photon in a superposition of two states of different frequency is now demonstrated using a superconducting quantum interference device to mediate the coupling between two harmonics of a resonator. Such quantum circuits bring closer the possibility of controlling photon-photon interactions at the single-photon level.  
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  Call Number RPLAB @ gujma @ Serial 820  
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Author Zakka-Bajjani, Eva; Nguyen, François; Lee, Minhyea; Vale, Leila R.; Simmonds, Raymond W.; Aumentado, José openurl 
  Title Quantum superposition of a single microwave photon in two different 'colour' states Type Journal Article
  Year 2011 Publication Nature Physics Abbreviated Journal Nat. Phys.  
  Volume 7 Issue 8 Pages (up) 599-603  
  Keywords fromIPMRAS  
  Abstract Fully controlled coherent coupling of arbitrary harmonic oscillators is an important tool for processing quantum information. Coupling between quantum harmonic oscillators has previously been demonstrated in several physical systems using a two-level system as a mediating element. Direct interaction at the quantum level has only recently been realized by means of resonant coupling between trapped ions. Here we implement a tunable direct coupling between the microwave harmonics of a superconducting resonator by means 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 (~7GHz) 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 beamsplitter-like operation that couples different frequency modes. As such, it could be used to implement linear optical quantum computing protocols on-chip.  
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  Call Number RPLAB @ gujma @ Serial 822  
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Author Johnson, B. R.; Reed, M. D.; Houck, A. A.; Schuster, D. I.; Bishop, Lev S.; Ginossar, E.; Gambetta, J. M.; Dicarlo, L.; Frunzio, L.; Girvin, S. M.; Schoelkopf, R. J. openurl 
  Title Quantum non-demolition detection of single microwave photons in a circuit Type Journal Article
  Year 2010 Publication Nature Physics Abbreviated Journal Nat. Phys.  
  Volume 6 Issue 9 Pages (up) 663-667  
  Keywords fromIPMRAS  
  Abstract Thorough control of quantum measurement is key to the development of quantum information technologies. Many measurements are destructive, removing more information from the system than they obtain. Quantum non-demolition (QND) measurements allow repeated measurements that give the same eigenvalue. They could be used for several quantum information processing tasks such as error correction, preparation by measurement and one-way quantum computing. Achieving QND measurements of photons is especially challenging because the detector must be completely transparent to the photons while still acquiring information about them. Recent progress in manipulating microwave photons in superconducting circuits has increased demand for a QND detector that operates in the gigahertz frequency range. Here we demonstrate a QND detection scheme that measures the number of photons inside a high-quality-factor microwave cavity on a chip. This scheme maps a photon number, n, onto a qubit state in a single-shot by means of qubit-photon logic gates. We verify the operation of the device for n=0 and 1 by analysing the average correlations of repeated measurements, and show that it is 90% QND. It differs from previously reported detectors because its sensitivity is strongly selective to chosen photon number states. This scheme could be used to monitor the state of a photon-based memory in a quantum computer.  
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  Call Number RPLAB @ gujma @ Serial 806  
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Author Dada, Adetunmise C.; Leach, Jonathan; Buller, Gerald S.; Padgett, Miles J.; Andersson, Erika openurl 
  Title Experimental high-dimensional two-photon entanglement and violations of generalized Bell inequalities Type Journal Article
  Year 2011 Publication Nature Physics Abbreviated Journal Nat. Phys.  
  Volume 7 Issue 9 Pages (up) 677-680  
  Keywords fromIPMRAS  
  Abstract Quantum entanglement plays a vital role in many quantum-information and communication tasks. Entangled states of higher-dimensional systems are of great interest owing to the extended possibilities they provide. For example, they enable the realization of new types of quantum information scheme that can offer higher-information-density coding and greater resilience to errors than can be achieved with entangled two-dimensional systems (see ref. and references therein). Closing the detection loophole in Bell test experiments is also more experimentally feasible when higher-dimensional entangled systems are used. We have measured previously untested correlations between two photons to experimentally demonstrate high-dimensional entangled states. We obtain violations of Bell-type inequalities generalized to d-dimensional systems up to d=12. Furthermore, the violations are strong enough to indicate genuine 11-dimensional entanglement. Our experiments use photons entangled in orbital angular momentum, generated through spontaneous parametric down-conversion, and manipulated using computer-controlled holograms.  
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  Call Number RPLAB @ gujma @ Serial 828  
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Author Zhang, Qiang; Goebel, Alexander; Wagenknecht, Claudia; Chen, Yu-Ao; Zhao, Bo; Yang, Tao; Mair, Alois; Schmiedmayer, Jörg; Pan, Jian-Wei openurl 
  Title Experimental quantum teleportation of a two-qubit composite system Type Journal Article
  Year 2006 Publication Nature Physics Abbreviated Journal Nat. Phys.  
  Volume 2 Issue 10 Pages (up) 678-682  
  Keywords fromIPMRAS; quantum teleportation  
  Abstract Quantum teleportation, a way to transfer the state of a quantum system from one location to another, is central to quantum communication and plays an important role in a number of quantum computation protocols. Previous experimental demonstrations have been implemented with single photonic or ionic qubits. However, teleportation of single qubits is insufficient for a large-scale realization of quantum communication and computation. Here, we present the experimental realization of quantum teleportation of a two-qubit composite system. In the experiment, we develop and exploit a six-photon interferometer to teleport an arbitrary polarization state of two photons. The observed teleportation fidelities for different initial states are all well beyond the state estimation limit of 0.40 for a two-qubit system. Not only does our six-photon interferometer provide an important step towards teleportation of a complex system, it will also enable future experimental investigations on a number of fundamental quantum communication and computation protocols  
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  Call Number RPLAB @ gujma @ Serial 795  
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Author Grinolds, M. S.; Maletinsky, P.; Hong, S.; Lukin, M. D.; Walsworth, R. L.; Yacoby, A. openurl 
  Title Quantum control of proximal spins using nanoscale magnetic resonance imaging Type Journal Article
  Year 2011 Publication Nature Physics Abbreviated Journal Nat. Phys.  
  Volume 7 Issue 9 Pages (up) 687-692  
  Keywords fromIPMRAS  
  Abstract Quantum control of individual spins in condensed-matter systems is an emerging field with wide-ranging applications in spintronics, quantum computation and sensitive magnetometry. Recent experiments have demonstrated the ability to address and manipulate single electron spins through either optical or electrical techniques. However, it is a challenge to extend individual-spin control to nanometre-scale multi-electron systems, as individual spins are often irresolvable with existing methods. Here we demonstrate that coherent individual-spin control can be achieved with few- nanometre resolution for proximal electron spins by carrying out single-spin magnetic resonance imaging (MRI), which is realized using a scanning-magnetic-field gradient that is both strong enough to achieve nanometre spatial resolution and sufficiently stable for coherent spin manipulations. We apply this scanning-field-gradient MRI technique to electronic spins in nitrogen-vacancy (NV) centres in diamond and achieve nanometre resolution in imaging, characterization and manipulation of individual spins. For NV centres, our results in individual-spin control demonstrate an improvement of nearly two orders of magnitude in spatial resolution when compared with conventional optical diffraction-limited techniques. This scanning-field-gradient microscope enables a wide range of applications including materials characterization, spin entanglement and nanoscale magnetometry.  
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  Call Number RPLAB @ gujma @ Serial 827  
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Author Pirandola, Stefano; Mancini, Stefano; Lloyd, Seth; Braunstein, Samuel L. openurl 
  Title Continuous-variable quantum cryptography using two-way quantum communication Type Journal Article
  Year 2008 Publication Nature Physics Abbreviated Journal Nat. Phys.  
  Volume 4 Issue 9 Pages (up) 726-730  
  Keywords fromIPMRAS  
  Abstract Quantum cryptography has recently been extended to continuous-variable systems, such as the bosonic modes of the electromagnetic field possessing continuous degrees of freedom. In particular, several cryptographic protocols have been proposed and experimentally implemented using bosonic modes with Gaussian statistics. These protocols have shown the possibility of reaching very high secret key rates, even in the presence of strong losses in the quantum communication channel. Despite this robustness to loss, their security can be affected by more general attacks where extra Gaussian noise is introduced by the eavesdropper. Here, we show a `hardware solution' for enhancing the security thresholds of these protocols. This is possible by extending them to two-way quantum communication where subsequent uses of the quantum channel are suitably combined. In the resulting two-way schemes, one of the honest parties assists the secret encoding of the other, with the chance of a non-trivial superadditive enhancement of the security thresholds. These results should enable the extension of quantum cryptography to more complex quantum communications.  
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  Call Number RPLAB @ gujma @ Serial 798  
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Author Hannay, Timo openurl 
  Title A new kind of science? Type Journal Article
  Year 2011 Publication Nature Physics Abbreviated Journal Nat. Phys.  
  Volume 7 Issue Pages (up) 742  
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  Call Number RPLAB @ gujma @ Serial 818  
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Author Prevedel, Robert; Hamel, Deny R.; Colbeck, Roger; Fisher, Kent; Resch, Kevin J. openurl 
  Title Experimental investigation of the uncertainty principle in the presence of quantum memory and its application to witnessing entanglement Type Journal Article
  Year 2011 Publication Nature Physics Abbreviated Journal Nat. Phys.  
  Volume 7 Issue 10 Pages (up) 757-761  
  Keywords fromIPMRAS  
  Abstract Heisenberg's uncertainty principle provides a fundamental limitation on the ability of an observer holding classical information to predict the outcome when one of two measurements is performed on a quantum system. However, an observer with access to a particle (stored in a quantum memory) which is entangled with the system generally has a reduced uncertainty: indeed, if the particle and system are maximally entangled, the observer can perfectly predict the outcome of whichever measurement is chosen. This effect has recently been quantified in a new entropic uncertainty relation. Here we experimentally investigate this relation, showing its effectiveness as an efficient entanglement witness. We use entangled photon pairs, an optical delay line serving as a simple quantum memory and fast, active feed-forward. Our results quantitatively agree with the new uncertainty relation. Our technique acts as a witness for almost all entangled states in our experiment as we obtain lower uncertainties than would be possible without the entangled particle.  
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  Call Number RPLAB @ gujma @ Serial 821  
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Author Fuchs, G. D.; Burkard, G.; Klimov, P. V.; Awschalom, D. D. openurl 
  Title A quantum memory intrinsic to single nitrogen–vacancy centres in diamond Type Journal Article
  Year 2011 Publication Nature Physics Abbreviated Journal Nat. Phys.  
  Volume 7 Issue 10 Pages (up) 789-793  
  Keywords fromIPMRAS  
  Abstract A quantum memory, composed of a long-lived qubit coupled to each processing qubit, is important to building a scalable platform for quantum information science. These two qubits should be connected by a fast and high-fidelity operation to store and retrieve coherent quantum states. Here, we demonstrate a room-temperature quantum memory based on the spin of the nitrogen nucleus intrinsic to each nitrogen–vacancy (NV) centre in diamond. We perform coherent storage of a single NV centre electronic spin in a single nitrogen nuclear spin using Landau–Zener transitions across a hyperfine-mediated avoided level crossing. By working outside the asymptotic regime, we demonstrate coherent state transfer in as little as 120 ns with total storage fidelity of 88±6%. This work demonstrates the use of a quantum memory that is compatible with scaling as the nitrogen nucleus is deterministically present in each NV centre defect.  
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  Call Number RPLAB @ gujma @ Serial 823  
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