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Author Kumar, Sushil; Chan, Chun Wang I.; Hu, Qing; Reno, John L.
Title A 1.8-THz quantum cascade laser operating significantly above the temperature of hw/k Type Journal Article
Year 2011 Publication Nature Physics Abbreviated Journal
Volume 7 Issue Pages 166-171
Keywords QCL, 2 mW at 155 K and 1.8 THz
Abstract Several competing technologies continue to advance the field of terahertz science; of particular importance has been the development of a terahertz semiconductor quantum cascade laser (QCL), which is arguably the only solid-state terahertz source with average optical power levels of much greater than a milliwatt. Terahertz QCLs are required to be cryogenically cooled and improvement of their temperature performance is the single most important research goal in the field. Thus far, their maximum operating temperature has been empirically limited to ~planckω/kB, a largely inexplicable trend that has bred speculation that a room-temperature terahertz QCL may not be possible in materials used at present. Here, we argue that this behaviour is an indirect consequence of the resonant-tunnelling injection mechanism employed in all previously reported terahertz QCLs. We demonstrate a new scattering-assisted injection scheme to surpass this limit for a 1.8-THz QCL that operates up to ~1.9planckω/kB (163 K). Peak optical power in excess of 2 mW was detected from the laser at 155 K. This development should make QCL technology attractive for applications below 2 THz, and initiate new design strategies for realizing a room-temperature terahertz semiconductor laser.
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Call Number Serial 631
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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
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 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 Lu, Chao-Yang; Zhou, Xiao-Qi; Gühne, Otfried; Gao, Wei-Bo; Zhang, Jin; Yuan, Zhen-Sheng; Goebel, Alexander; Yang, Tao; Pan, Jian-Wei
Title Experimental entanglement of six photons in graph states Type Journal Article
Year 2007 Publication Nature Physics Abbreviated Journal Nat. Phys.
Volume 3 Issue 2 Pages 91-95
Keywords fromIPMRAS
Abstract Graph states-multipartite entangled states that can be represented by mathematical graphs-are important resources for quantum computation, quantum error correction, studies of multiparticle entanglement and fundamental tests of non-locality and decoherence. Here, we demonstrate the experimental entanglement of six photons and engineering of multiqubit graph states. We have created two important examples of graph states, a six-photon Greenberger-Horne-Zeilinger state, the largest photonic Schrödinger cat so far, and a six-photon cluster state, a state-of-the-art `one-way quantum computer'. With small modifications, our method allows us, in principle, to create various further graph states, and therefore could open the way to experimental tests of, for example, quantum algorithms or loss- and fault-tolerant one-way quantum computation.
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Call Number RPLAB @ gujma @ Serial 796
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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.
Title Entanglement-based quantum communication over 144km Type Journal Article
Year 2007 Publication Nature Physics Abbreviated Journal Nat. Phys.
Volume 3 Issue 7 Pages 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 Pirandola, Stefano; Mancini, Stefano; Lloyd, Seth; Braunstein, Samuel L.
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 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 Zurek, Wojciech Hubert
Title Quantum Darwinism Type Journal Article
Year 2009 Publication Nature Physics Abbreviated Journal Nat. Phys.
Volume 5 Issue 3 Pages 181-188
Keywords fromIPMRAS
Abstract Quantum Darwinism describes the proliferation, in the environment, of multiple records of selected states of a quantum system. It explains how the quantum fragility of a state of a single quantum system can lead to the classical robustness of states in their correlated multitude; shows how effective `wave-packet collapse' arises as a result of the proliferation throughout the environment of imprints of the state of the system; and provides a framework for the derivation of Born's rule, which relates the probabilities of detecting states to their amplitudes. Taken together, these three advances mark considerable progress towards settling the quantum measurement problem.
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Call Number RPLAB @ gujma @ Serial 799
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Author Shor, Peter W.
Title Quantum information theory: The bits don't add up Type Journal Article
Year 2009 Publication Nature Physics Abbreviated Journal Nat. Phys.
Volume 5 Issue Pages 247 - 248
Keywords fromIPMRAS
Abstract A counterexample to the 'additivity question', the most celebrated open problem in the mathematical theory of quantum information, casts doubt on the possibility of finding a simple expression for the information capacity of a quantum channel.
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Call Number RPLAB @ gujma @ Serial 800
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Author Hanneke, D.; Home, J. P.; Jost, J. D.; Amini, J. M.; Leibfried, D.; Wineland, D. J.
Title Realization of a programmable two-qubit quantum processor Type Journal Article
Year 2010 Publication Nature Physics Abbreviated Journal Nat. Phys.
Volume 6 Issue 1 Pages 13-16
Keywords fromIPMRAS
Abstract The universal quantum computer is a device capable of simulating any physical system and represents a major goal for the field of quantum information science. In the context of quantum information, `universal' refers to the ability to carry out arbitrary unitary transformations in the system's computational space. Combining arbitrary single-quantum-bit (qubit) gates with an entangling two-qubit gate provides a set of gates capable of achieving universal control of any number of qubits, provided that these gates can be carried out repeatedly and between arbitrary pairs of qubits. Although gate sets have been demonstrated in several technologies, they have so far been tailored towards specific tasks, forming a small subset of all unitary operators. Here we demonstrate a quantum processor that can be programmed with 15 classical inputs to realize arbitrary unitary transformations on two qubits, which are stored in trapped atomic ions. Using quantum state and process tomography, we characterize the fidelity of our implementation for 160 randomly chosen operations. This universal control is equivalent to simulating any pairwise interaction between spin-1/2 systems. A programmable multiqubit register could form a core component of a large-scale quantum processor, and the methods used here are suitable for such a device.
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Call Number RPLAB @ gujma @ Serial 801
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Author Trabesinger, Andreas
Title Quantum mechanics: Shaken foundations Type Journal Article
Year 2009 Publication Nature Physics Abbreviated Journal Nat. Phys.
Volume 5 Issue 12 Pages 863
Keywords fromIPMRAS
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Call Number RPLAB @ gujma @ Serial 802
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Author Bialczak, R. C.; Ansmann, M.; Hofheinz, M.; Lucero, E.; Neeley, M.; O'Connell, A. D.; Sank, D.; Wang, H.; Wenner, J.; Steffen, M.; Cleland, A. N.; Martinis, J. M.
Title Quantum process tomography of a universal entangling gate implemented with Josephson phase qubits Type Journal Article
Year 2010 Publication Nature Physics Abbreviated Journal Nat. Phys.
Volume 6 Issue 6 Pages 409-413
Keywords fromIPMRAS
Abstract Quantum gates must perform reliably when operating on standard input basis states and on complex superpositions thereof. Experiments using superconducting qubits have validated truth tables for particular implementations of, for example, the controlled-NOT gate, but have not fully characterized gate operation for arbitrary superpositions of input states. Here we demonstrate the use of quantum process tomography (QPT) to fully characterize the performance of a universal entangling gate between two superconducting qubits. Process tomography permits complete gate analysis, but requires precise preparation of arbitrary input states, control over the subsequent qubit interaction and ideally simultaneous single-shot measurement of output states. In recent work, it has been proposed to use QPT to probe noise properties and time dynamics of qubit systems and to apply techniques from control theory to create scalable qubit benchmarking protocols. We use QPT to measure the fidelity and noise properties of an entangling gate. In addition to demonstrating a promising fidelity, our entangling gate has an on-to-off ratio of 300, a level of adjustable coupling that will become a requirement for future high-fidelity devices. This is the first solid-state demonstration of QPT in a two-qubit system, as QPT has previously been demonstrated only with single solid-state qubits.
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Call Number RPLAB @ gujma @ Serial 803
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Author Perseguers, S.; Lewenstein, M.; Acín, A.; Cirac, J. I.
Title Quantum random networks Type Journal Article
Year 2010 Publication Nature Physics Abbreviated Journal Nat. Phys.
Volume 6 Issue 7 Pages 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 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.
Title Implementation of superconductor/ferromagnet/ superconductor Type Journal Article
Year 2010 Publication Nature Physics Abbreviated Journal Nat. Phys.
Volume 6 Issue 8 Pages 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 π-type Josephson junction in the cell is equivalent to applying flux Φ0/2 and thus makes it possible to solve this problem. Moreover, using π-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 π-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 π-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 π-junction.
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Call Number RPLAB @ gujma @ Serial 805
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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.
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 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 Haviland, David
Title Superconducting circuits: Quantum phase slips Type Journal Article
Year 2010 Publication Nature Physics Abbreviated Journal Nat. Phys.
Volume 6 Issue Pages 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 Zhu, J.; Christensen, J.; Jung, J.; Martin-Moreno, L.; Yin, X.; Fok, L.; Zhang, X.; Garcia-Vidal, F. J.
Title A holey-structured metamaterial for acoustic deep-subwavelength imaging Type Journal Article
Year 2011 Publication Nature Physics Abbreviated Journal Nat. Phys.
Volume 7 Issue 1 Pages 52-55
Keywords fromIPMRAS
Abstract For classical waves such as light or sound, diffraction sets a natural limit on how finely the details of an object can be recorded on its image. Recently, various optical superlenses based on the metamaterials concept have shown the possibility of overcoming the diffraction limit. Similar two-dimensional (2D) acoustic hyperlens designs have also been explored. Here we demonstrate a 3D holey-structured metamaterial that achieves acoustic imaging down to a feature size of λ/50. The evanescent field components of a subwavelength object are efficiently transmitted through the structure as a result of their strong coupling with Fabry-Pérot resonances inside the holey plate. This capability of acoustic imaging at a very deep-subwavelength scale may open the door for a broad range of applications, including medical ultrasonography, underwater sonar and ultrasonic non-destructive evaluation.
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Call Number RPLAB @ gujma @ Serial 809
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