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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 (down) 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 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 (down) 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 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 (down) 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 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 (down) 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 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 (down) 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 Feofanov, A. K.; Oboznov, V. A.; Bol'Ginov, V. V.; Lisenfeld, J.; Poletto, S.; Ryazanov, V. V.; Rossolenko, A. N.; Khabipov, M.; Balashov, D.; Zorin, A. B.; Dmitriev, P. N.; Koshelets, V. P.; Ustinov, A. V. openurl 
  Title Implementation of superconductor/ferromagnet/ superconductor Type Journal Article
  Year 2010 Publication Nature Physics Abbreviated Journal Nat. Phys.  
  Volume 6 Issue 8 Pages (down) 593-597  
  Keywords fromIPMRAS  
  Abstract High operation speed and low energy consumption may allow the superconducting digital single-flux-quantum circuits to outperform traditional complementary metal-oxide-semiconductor logic. The remaining major obstacle towards high element densities on-chip is a relatively large cell size necessary to hold a magnetic flux quantum Φ0. Inserting a Ï€-type Josephson junction in the cell is equivalent to applying flux Φ0/2 and thus makes it possible to solve this problem. Moreover, using Ï€-junctions in superconducting qubits may help to protect them from noise. Here we demonstrate the operation of three superconducting circuits-two of them are classical and one quantum-that all utilize such Ï€-phase shifters realized using superconductor/ferromagnet/superconductor sandwich technology. The classical circuits are based on single-flux-quantum cells, which are shown to be scalable and compatible with conventional niobium-based superconducting electronics. The quantum circuit is a Ï€-biased phase qubit, for which we observe coherent Rabi oscillations. We find no degradation of the measured coherence time compared to that of a reference qubit without a Ï€-junction.  
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  Call Number RPLAB @ gujma @ Serial 805  
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Author Haviland, David openurl 
  Title Superconducting circuits: Quantum phase slips Type Journal Article
  Year 2010 Publication Nature Physics Abbreviated Journal Nat. Phys.  
  Volume 6 Issue Pages (down) 565–566  
  Keywords fromIPMRAS  
  Abstract Coulomb interactions can cause a rapid change in the phase of the wavefunction along a very narrow superconducting system. Such a phase slip at the quantum level is now measured in a chain of Josephson junctions.  
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  Call Number RPLAB @ gujma @ Serial 807  
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Author Bylander, Jonas; Gustavsson, Simon; Yan, Fei; Yoshihara, Fumiki; Harrabi, Khalil; Fitch, George; Cory, David G.; Nakamura, Yasunobu; Tsai, Jaw-Shen; Oliver, William D. openurl 
  Title Noise spectroscopy through dynamical decoupling with a superconducting flux qubit Type Journal Article
  Year 2011 Publication Nature Physics Abbreviated Journal Nat. Phys.  
  Volume 7 Issue 7 Pages (down) 565-570  
  Keywords fromIPMRAS  
  Abstract Quantum coherence in natural and artificial spin systems is fundamental to applications ranging from quantum information science to magnetic-resonance imaging and identification. Several multipulse control sequences targeting generalized noise models have been developed to extend coherence by dynamically decoupling a spin system from its noisy environment. In any particular implementation, however, the efficacy of these methods is sensitive to the specific frequency distribution of the noise, suggesting that these same pulse sequences could also be used to probe the noise spectrum directly. Here we demonstrate noise spectroscopy by means of dynamical decoupling using a superconducting qubit with energy-relaxation time T1=12μs. We first demonstrate that dynamical decoupling improves the coherence time T2 in this system up to the T2=2T1 limit (pure dephasing times exceeding 100μs), and then leverage its filtering properties to probe the environmental noise over a frequency (f) range 0.2-20MHz, observing a 1/fα distribution with α<1. The characterization of environmental noise has broad utility for spin-resonance applications, enabling the design of optimized coherent-control methods, promoting device and materials engineering, and generally improving coherence.  
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  Call Number RPLAB @ gujma @ Serial 829  
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Author Perseguers, S.; Lewenstein, M.; Acín, A.; Cirac, J. I. openurl 
  Title Quantum random networks Type Journal Article
  Year 2010 Publication Nature Physics Abbreviated Journal Nat. Phys.  
  Volume 6 Issue 7 Pages (down) 539-543  
  Keywords fromIPMRAS  
  Abstract Quantum mechanics offers new possibilities to process and transmit information. In recent years, algorithms and cryptographic protocols exploiting the superposition principle and the existence of entangled states have been designed. They should allow us to realize communication and computational tasks that outperform any classical strategy. Here we show that quantum mechanics also provides fresh perspectives in the field of random networks. Already the simplest model of a classical random graph changes markedly when extended to the quantum case, where we obtain a distinct behaviour of the critical probabilities at which different subgraphs appear. In particular, in a network of N nodes, any quantum subgraph can be generated by local operations and classical communication if the entanglement between pairs of nodes scales as N-2. This result also opens up new vistas in the domain of quantum networks and their applications.  
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  Call Number RPLAB @ gujma @ Serial 804  
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Author Biercuk, Michael J. openurl 
  Title A quantum spectrum analyser Type Journal Article
  Year 2011 Publication Nature Physics Abbreviated Journal Nat. Phys.  
  Volume 7 Issue Pages (down) 525–526  
  Keywords fromIPMRAS  
  Abstract Noise filters based on so-called dynamical decoupling pulse sequences can suppress decoherence in quantum systems. Turning this idea on its head now provides a new technique for studying the noise itself.  
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  Call Number RPLAB @ gujma @ Serial 826  
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Author Kok, Pieter openurl 
  Title Quantum optics: Entangled photons report for duty Type Journal Article
  Year 2010 Publication Nature Photonics Abbreviated Journal Nat. Photon.  
  Volume 4 Issue 8 Pages (down) 504-505  
  Keywords fromIPMRAS  
  Abstract Entangled photons are a key ingredient in optical quantum technologies, but researchers have so far been unable to produce a single pair of entangled photons. Now, two groups from China and Austria independently report just that, with a technique that avoids the need to infer entanglement from detection signatures.  
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  Call Number RPLAB @ gujma @ Serial 772  
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Author Ursin, R.; Tiefenbacher, F.; Schmitt-Manderbach, T.; Weier, H.; Scheidl, T.; Lindenthal, M.; Blauensteiner, B.; Jennewein, T.; Perdigues, J.; Trojek, P.; Ömer, B.; Fürst, M.; Meyenburg, M.; Rarity, J.; Sodnik, Z.; Barbieri, C.; Weinfurter, H.; Zeilinger, A. openurl 
  Title Entanglement-based quantum communication over 144km Type Journal Article
  Year 2007 Publication Nature Physics Abbreviated Journal Nat. Phys.  
  Volume 3 Issue 7 Pages (down) 481-486  
  Keywords fromIPMRAS  
  Abstract Quantum entanglement is the main resource to endow the field of quantum information processing with powers that exceed those of classical communication and computation. In view of applications such as quantum cryptography or quantum teleportation, extension of quantum-entanglement-based protocols to global distances is of considerable practical interest. Here we experimentally demonstrate entanglement-based quantum key distribution over 144km. One photon is measured locally at the Canary Island of La Palma, whereas the other is sent over an optical free-space link to Tenerife, where the Optical Ground Station of the European Space Agency acts as the receiver. This exceeds previous free-space experiments by more than an order of magnitude in distance, and is an essential step towards future satellite-based quantum communication and experimental tests on quantum physics in space.  
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  Call Number RPLAB @ gujma @ Serial 797  
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Author Vishveshwara, Smitha openurl 
  Title Topological qubits: A bit of both Type Journal Article
  Year 2011 Publication Nature Physics Abbreviated Journal Nat. Phys.  
  Volume 7 Issue Pages (down) 450–451  
  Keywords fromIPMRAS  
  Abstract 'Standard' qubits have been implemented in diverse physical systems. Now, so-called topological qubits are coming into the limelight, and could potentially be used for decoherence-free quantum computing. Coupling these two types of qubit might enable devices that exploit the virtues of both.  
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  Call Number RPLAB @ gujma @ Serial 825  
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Author Gabay, Marc; Triscone, Jean-Marc openurl 
  Title Superconductors: Terahertz superconducting switch Type Journal Article
  Year 2011 Publication Nature Photonics Abbreviated Journal Nat. Photon.  
  Volume 5 Issue 8 Pages (down) 447-449  
  Keywords fromIPMRAS  
  Abstract The use of terahertz pulses to 'gate' interlayer charge transport in a superconductor could lead to a variety of new and interesting applications.  
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  Call Number RPLAB @ gujma @ Serial 777  
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Author Nevou, L.; Liverini, V.; Friedli, P.; Castellano, F.; Bismuto, A.; Sigg, H.; Gramm, F.; Müller, E.; Faist, J. openurl 
  Title Current quantization in an optically driven electron pump based on self-assembled quantum dots Type Journal Article
  Year 2011 Publication Nature Physics Abbreviated Journal Nat. Phys.  
  Volume 7 Issue Pages (down) 423–427  
  Keywords fromIPMRAS  
  Abstract The electronic structure of self-assembled semiconductor quantum dots consists of discrete atom-like states that can be populated with a well-defined number of electrons. This property can be used to fabricate a d.c. current standard that enables the unit of ampere to be independently defined. Here we report an optically pumped current source based on self-assembled InAs/GaAs quantum dots. The accuracy obtained so far is 10–1 and is limited by the uncertainty in the number of dots. At 10 K the device generates a current difference of 2.39 nA at a frequency of 1 kHz. The accuracy could be improved by site-selective growth techniques where the number of dots is fixed by pre-patterning. The results are promising for applications in electrical metrology, where a current standard is needed to close the so-called quantum metrological triangle.  
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  Call Number RPLAB @ gujma @ Serial 841  
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