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Author (up) Arcizet, O.; Jacques, V.; Siria, A.; Poncharal, P.; Vincent, P.; Seidelin, S. openurl 
  Title A single nitrogen-vacancy defect coupled to a nanomechanical oscillator Type Journal Article
  Year 2011 Publication Nature Physics Abbreviated Journal Nat. Phys.  
  Volume 7 Issue 11 Pages 879-883  
  Keywords fromIPMRAS  
  Abstract We position a single nitrogen-vacancy (NV) centre hosted in a diamond nanocrystal at the extremity of a SiC nanowire. This novel hybrid system couples the degrees of freedom of two radically different systems: a nanomechanical oscillator and a single quantum object. We probe the dynamics of the nano-resonator through time-resolved nanocrystal fluorescence and photon-correlation measurements, conveying the influence of a mechanical degree of freedom on a non-classical photon emitter. Moreover, by immersing the system in a strong magnetic field gradient, we induce a magnetic coupling between the nanomechanical oscillator and the NV electronic spin, providing nanomotion readout through a single electronic spin. Spin-dependent forces inherent to this coupling scheme are essential in a variety of active cooling and entanglement protocols used in atomic physics, and should now be within the reach of nanomechanical hybrid systems.  
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  Call Number RPLAB @ gujma @ Serial 819  
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Author (up) Barreiro, Julio T. openurl 
  Title Quantum physics: Environmental effects controlled Type Journal Article
  Year 2011 Publication Nature Physics Abbreviated Journal Nat. Phys.  
  Volume 7 Issue Pages 927–928  
  Keywords fromIPMRAS  
  Abstract An open quantum system loses its 'quantumness' when information about the state leaks into its surroundings. Researchers now show how this decoherence can be controlled between two incompatible regimes in the case of a single photon.  
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  Call Number RPLAB @ gujma @ Serial 817  
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Author (up) Bason, Mark G.; Viteau, Matthieu; Malossi, Nicola; Huillery, Paul; Arimondo, Ennio; Ciampini, Donatella; Fazio, Rosario; Giovannetti, Vittorio; Mannella, Riccardo; Morsch, Oliver openurl 
  Title High-fidelity quantum driving Type Journal Article
  Year 2012 Publication Nature Physics Abbreviated Journal Nat. Phys.  
  Volume 8 Issue 2 Pages 147-152  
  Keywords fromIPMRAS  
  Abstract Accurately controlling a quantum system is a fundamental requirement in quantum information processing and the coherent manipulation of molecular systems. The ultimate goal in quantum control is to prepare a desired state with the highest fidelity allowed by the available resources and the experimental constraints. Here we experimentally implement two optimal high-fidelity control protocols using a two-level quantum system comprising Bose-Einstein condensates in optical lattices. The first is a short-cut protocol that reaches the maximum quantum-transformation speed compatible with the Heisenberg uncertainty principle. In the opposite limit, we realize the recently proposed transitionless superadiabatic protocols in which the system follows the instantaneous adiabatic ground state nearly perfectly. We demonstrate that superadiabatic protocols are extremely robust against control parameter variations, making them useful for practical applications.  
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  Call Number RPLAB @ gujma @ Serial 816  
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Author (up) Baumert, Thomas openurl 
  Title Quantum technology: Wave packets get a kick Type Journal Article
  Year 2011 Publication Nature Physics Abbreviated Journal Nat. Phys.  
  Volume 7 Issue 5 Pages 373-374  
  Keywords fromIPMRAS  
  Abstract Intense femtosecond pulses of infrared light can manipulate molecules. It is now shown that such control even extends to making different molecular eigenstates interfere with each other in a way never considered before -- a potential tool for optically engineered chemical reactions and for ultrafast information encoding and manipulation.  
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  Call Number RPLAB @ gujma @ Serial 830  
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Author (up) 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. openurl 
  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 (up) 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 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 (up) 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 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 (up) Bozyigit, D.; Lang, C.; Steffen, L.; Fink, J. M.; Eichler, C.; Baur, M.; Bianchetti, R.; Leek, P. J.; Filipp, S.; da Silva, M. P.; Blais, A.; Wallraff, A. openurl 
  Title Antibunching of microwave-frequency photons observed in correlation measurements using linear detectors Type Journal Article
  Year 2011 Publication Nature Physics Abbreviated Journal Nat. Phys.  
  Volume 7 Issue 2 Pages 154-158  
  Keywords fromIPMRAS  
  Abstract At optical frequencies the radiation produced by a source, such as a laser, a black body or a single-photon emitter, is frequently characterized by analysing the temporal correlations of emitted photons using single-photon counters. At microwave frequencies, however, there are no efficient single-photon counters yet. Instead, well-developed linear amplifiers allow for efficient measurement of the amplitude of an electromagnetic field. Here, we demonstrate first- and second-order correlation function measurements of a pulsed microwave-frequency single-photon source integrated on the same chip with a 50/50 beam splitter followed by linear amplifiers and quadrature amplitude detectors. We clearly observe single-photon coherence in first-order and photon antibunching in second-order correlation function measurements of the propagating fields.  
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  Call Number RPLAB @ gujma @ Serial 835  
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Author (up) Buchanan, Mark openurl 
  Title Nothing's impossible Type Manuscript
  Year 2011 Publication Nature Physics Abbreviated Journal Nat. Phys.  
  Volume 7 Issue Pages 5  
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  Call Number RPLAB @ gujma @ Serial 839  
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Author (up) Buchanan, Mark openurl 
  Title Body of evidence Type Manuscript
  Year 2010 Publication Nature Physics Abbreviated Journal Nat. Phys.  
  Volume 6 Issue Pages  
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  Call Number RPLAB @ gujma @ Serial 837  
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Author (up) 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 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 (up) Clerk, Aashish openurl 
  Title Quantum phononics: To see a SAW Type Journal Article
  Year 2012 Publication Nature Physics Abbreviated Journal Nat. Phys.  
  Volume 8 Issue 4 Pages 256-257  
  Keywords fromIPMRAS  
  Abstract Mechanical oscillations of microscopic resonators have recently been observed in the quantum regime. This idea could soon be extended from localized vibrations to travelling waves thanks to a sensitive probe of so-called surface acoustic waves.  
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  Call Number RPLAB @ gujma @ Serial 811  
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Author (up) 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 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 (up) 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 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 (up) 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 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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Author (up) 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 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 (up) Gustafsson, Martin V.; Santos, Paulo V.; Johansson, Göran; Delsing, Per openurl 
  Title Local probing of propagating acoustic waves in a gigahertz echo chamber Type Journal Article
  Year 2012 Publication Nature Physics Abbreviated Journal Nat. Phys.  
  Volume 8 Issue 4 Pages 338-343  
  Keywords fromIPMRAS  
  Abstract In the same way that micro-mechanical resonators resemble guitar strings and drums, surface acoustic waves resemble the sound these instruments produce, but moving over a solid surface rather than through air. In contrast with oscillations in suspended resonators, such propagating mechanical waves have not before been studied near the quantum mechanical limits. Here, we demonstrate local probing of surface acoustic waves with a displacement sensitivity of 30amRMSHz-1/2 and detection sensitivity on the single-phonon level after averaging, at a frequency of 932MHz. Our probe is a piezoelectrically coupled single-electron transistor, which is sufficiently fast, non-destructive and localized to enable us to track pulses echoing back and forth in a long acoustic cavity, self-interfering and ringing the cavity up and down. We project that strong coupling to quantum circuits will enable new experiments, and hybrids using the unique features of surface acoustic waves. Prospects include quantum investigations of phonon-phonon interactions, and acoustic coupling to superconducting qubits for which we present favourable estimates.  
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  Call Number RPLAB @ gujma @ Serial 813  
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Author (up) 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 742  
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  Call Number RPLAB @ gujma @ Serial 818  
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Author (up) Hanneke, D.; Home, J. P.; Jost, J. D.; Amini, J. M.; Leibfried, D.; Wineland, D. J. openurl 
  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 (up) 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 565–566  
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  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 (up) Hollenberg, Lloyd C. L. openurl 
  Title Quantum control: Through the quantum chicane Type Journal Article
  Year 2012 Publication Nature Physics Abbreviated Journal Nat. Phys.  
  Volume 8 Issue 2 Pages 113-114  
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  Abstract In quantum control there is an inherent tension between high fidelity requirements and the need for speed to avoid decoherence. A direct comparison of quantum control protocols at these two extremes indicates where the sweet spot may lie.  
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  Call Number RPLAB @ gujma @ Serial 812  
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Author (up) Home, Jonathan openurl 
  Title Quantum entanglement: Watching correlations disappear Type Journal Article
  Year 2010 Publication Nature Physics Abbreviated Journal Nat. Phys.  
  Volume 6 Issue 12 Pages 938-939  
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  Abstract Engineered decoherence enables tracking of multipartite entanglement as a quantum state decays.  
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  Call Number RPLAB @ gujma @ Serial 832  
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Author (up) Hosseini, M.; Campbell, G.; Sparkes, B. M.; Lam, P. K.; Buchler, B. C. openurl 
  Title Unconditional room-temperature quantum memory Type Journal Article
  Year 2011 Publication Nature Physics Abbreviated Journal Nat. Phys.  
  Volume 7 Issue 10 Pages 794-798  
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  Abstract Just as classical information systems require buffers and memory, the same is true for quantum information systems. The potential that optical quantum information processing holds for revolutionizing computation and communication is therefore driving significant research into developing optical quantum memory. A practical optical quantum memory must be able to store and recall quantum states on demand with high efficiency and low noise. Ideally, the platform for the memory would also be simple and inexpensive. Here, we present a complete tomographic reconstruction of quantum states that have been stored in the ground states of rubidium in a vapour cell operating at around 80 °C. Without conditional measurements, we show recall fidelity up to 98% for coherent pulses containing around one photon. To unambiguously verify that our memory beats the quantum no-cloning limit we employ state-independent verification using conditional variance and signal-transfer coefficients.  
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  Call Number RPLAB @ gujma @ Serial 824  
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Author (up) 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 663-667  
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  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 (up) Kim, Yong-Su; Lee, Jong-Chan; Kwon, Osung; Kim, Yoon-Ho openurl 
  Title Protecting entanglement from decoherence using weak measurement and quantum measurement reversal Type Journal Article
  Year 2012 Publication Nature Physics Abbreviated Journal Nat. Phys.  
  Volume 8 Issue 2 Pages 117-120  
  Keywords fromIPMRAS  
  Abstract Decoherence, often caused by unavoidable coupling with the environment, leads to degradation of quantum coherence. For a multipartite quantum system, decoherence leads to degradation of entanglement and, in certain cases, entanglement sudden death. Tackling decoherence, thus, is a critical issue faced in quantum information, as entanglement is a vital resource for many quantum information applications including quantum computing, quantum cryptography, quantum teleportation and quantum metrology. Here, we propose and demonstrate a scheme to protect entanglement from decoherence. Our entanglement protection scheme makes use of the quantum measurement itself for actively battling against decoherence and it can effectively circumvent even entanglement sudden death.  
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  Call Number RPLAB @ gujma @ Serial 815  
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Author (up) Korotkov, Alexander N. openurl 
  Title Entanglement preservation: The Sleeping Beauty approach Type Journal Article
  Year 2012 Publication Nature Physics Abbreviated Journal Nat. Phys.  
  Volume 8 Issue 2 Pages 107-108  
  Keywords fromIPMRAS  
  Abstract Two-qubit entanglement can be preserved by partially measuring the qubits to leave them in a 'lethargic' state. The original state is restored using quantum measurement reversal after the qubits have travelled through a decoherence channel.  
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  Call Number RPLAB @ gujma @ Serial 814  
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Author (up) Kumar, Sushil; Chan, Chun Wang I.; Hu, Qing; Reno, John L. openurl 
  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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  Notes Approved no  
  Call Number Serial 631  
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Author (up) Kumar, Sushil; Wang I. Chan, Chun; Hu, Qing; Reno, John L. openurl 
  Title A 1.8-THz quantum cascade laser operating significantly above the temperature of ω/kB Type Journal Article
  Year 2011 Publication Nature Physics Abbreviated Journal Nat. Phys.  
  Volume 7 Issue Pages  
  Keywords fromIPMRAS  
  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 ~ω/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 ~1.9ω/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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  Notes Approved no  
  Call Number RPLAB @ gujma @ Serial 836  
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Author (up) Lu, Chao-Yang; Zhou, Xiao-Qi; Gühne, Otfried; Gao, Wei-Bo; Zhang, Jin; Yuan, Zhen-Sheng; Goebel, Alexander; Yang, Tao; Pan, Jian-Wei openurl 
  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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  Notes Approved no  
  Call Number RPLAB @ gujma @ Serial 796  
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Author (up) Lupascu, Adrian openurl 
  Title Nonlinear dynamics: Quantum pendula locked in Type Journal Article
  Year 2011 Publication Nature Physics Abbreviated Journal Nat. Phys.  
  Volume 7 Issue 2 Pages 100-101  
  Keywords fromIPMRAS  
  Abstract A study of the autoresonant behaviour of a superconducting pendulum reveals that quantum fluctuations determine only the initial oscillator motion and not its subsequent dynamics. This could be important in the development of more efficient methods for reading solid-state qubits.  
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  Call Number RPLAB @ gujma @ Serial 840  
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Author (up) Ma, Xiao-Song; Dakic, Borivoje; Naylor, William; Zeilinger, Anton; Walther, Philip openurl 
  Title Quantum simulation of the wavefunction to probe frustrated Heisenberg spin systems Type Journal Article
  Year 2011 Publication Nature Physics Abbreviated Journal Nat. Phys.  
  Volume 7 Issue 5 Pages 399-405  
  Keywords fromIPMRAS  
  Abstract Quantum simulators are controllable quantum systems that can reproduce the dynamics of the system of interest in situations that are not amenable to classical computers. Recent developments in quantum technology enable the precise control of individual quantum particles as required for studying complex quantum systems. In particular, quantum simulators capable of simulating frustrated Heisenberg spin systems provide platforms for understanding exotic matter such as high-temperature superconductors. Here we report the analogue quantum simulation of the ground-state wavefunction to probe arbitrary Heisenberg-type interactions among four spin-1/2 particles. Depending on the interaction strength, frustration within the system emerges such that the ground state evolves from a localized to a resonating-valence-bond state. This spin-1/2 tetramer is created using the polarization states of four photons. The single-particle addressability and tunable measurement-induced interactions provide us with insights into entanglement dynamics among individual particles. We directly extract ground-state energies and pairwise quantum correlations to observe the monogamy of entanglement.  
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  Call Number RPLAB @ gujma @ Serial 842  
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Author (up) Mariantoni, Matteo; Wang, H.; Bialczak, Radoslaw C.; Lenander, M.; Lucero, Erik; Neeley, M.; O'Connell, A. D.; Sank, D.; Weides, M.; Wenner, J.; Yamamoto, T.; Yin, Y.; Zhao, J.; Martinis, John M.; Cleland, A. N. openurl 
  Title Photon shell game in three-resonator circuit quantum electrodynamics Type Journal Article
  Year 2011 Publication Nature Physics Abbreviated Journal Nat. Phys.  
  Volume 7 Issue 4 Pages 287-293  
  Keywords fromIPMRAS  
  Abstract The generation and control of quantum states of light constitute fundamental tasks in cavity quantum electrodynamics (QED). The superconducting realization of cavity QED, circuit QED (refs 11, 12, 13, 14), enables on-chip microwave photonics, where superconducting qubits control and measure individual photon states. A long-standing issue in cavity QED is the coherent transfer of photons between two or more resonators. Here, we use circuit QED to implement a three-resonator architecture on a single chip, where the resonators are interconnected by two superconducting phase qubits. We use this circuit to shuffle one- and two-photon Fock states between the three resonators, and demonstrate qubit-mediated vacuum Rabi swaps between two resonators. By shuffling superposition states we are also able to demonstrate the high-fidelity phase coherence of the transfer. Our results illustrate the potential for using multi-resonator circuits as photon quantum registers and for creating multipartite entanglement between delocalized bosonic modes.  
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  Notes Approved no  
  Call Number RPLAB @ gujma @ Serial 838  
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Author (up) Mineev, Vladimir P. openurl 
  Title Superfluid helium: Order in disorder Type Journal Article
  Year 2012 Publication Nature Physics Abbreviated Journal Nat. Phys.  
  Volume 8 Issue Pages 253–254  
  Keywords fromIPMRAS  
  Abstract Confining liquid 3He in porous silica aerogel prepared with strong anisotropy stabilizes a state of axial superfluidity.  
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  Call Number RPLAB @ gujma @ Serial 810  
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Author (up) 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 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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Author (up) 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 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 (up) 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 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 (up) 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 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 (up) Raussendorf, Robert openurl 
  Title Quantum computing: Shaking up ground states Type Journal Article
  Year 2010 Publication Nature Physics Abbreviated Journal Nat. Phys.  
  Volume 6 Issue 11 Pages 840-841  
  Keywords fromIPMRAS  
  Abstract Measurement-based quantum computation with an Affleck-Kennedy-Lieb-Tasaki state is experimentally realized for the first time.  
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  Call Number RPLAB @ gujma @ Serial 834  
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Author (up) Saffman, Mark openurl 
  Title Quantum computing: A quantum telecom link Type Journal Article
  Year 2010 Publication Nature Physics Abbreviated Journal Nat. Phys.  
  Volume 6 Issue 11 Pages 838-839  
  Keywords fromIPMRAS  
  Abstract Converting data-carrying photons to telecommunication wavelengths enables distribution of quantum information over long distances.  
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  Call Number RPLAB @ gujma @ Serial 833  
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Author (up) Shor, Peter W. openurl 
  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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  Notes Approved no  
  Call Number RPLAB @ gujma @ Serial 800  
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Author (up) Toyabe, Shoichi; Sagawa, Takahiro; Ueda, Masahito; Muneyuki, Eiro; Sano, Masaki openurl 
  Title Experimental demonstration of information-to-energy conversion and validation of the generalized Jarzynski equality Type Journal Article
  Year 2010 Publication Nature Physics Abbreviated Journal Nat. Phys.  
  Volume 6 Issue 12 Pages 988-992  
  Keywords fromIPMRAS  
  Abstract In 1929, Leo Szilard invented a feedback protocol in which a hypothetical intelligence called Maxwell's demon pumps heat from an isothermal environment and transduces it to work. After an intense controversy that lasted over eighty years; it was finally clarified that the demon's role does not contradict the second law of thermodynamics, implying that we can convert information to free energy in principle. Nevertheless, experimental demonstration of this information-to-energy conversion has been elusive. Here, we demonstrate that a nonequilibrium feedback manipulation of a Brownian particle based on information about its location achieves a Szilard-type information-energy conversion. Under real-time feedback control, the particle climbs up a spiral-stairs-like potential exerted by an electric field and obtains free energy larger than the amount of work performed on it. This enables us to verify the generalized Jarzynski equality, or a new fundamental principle of “information-heat engine” which converts information to energy by feedback control.  
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  Call Number RPLAB @ gujma @ Serial 831  
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Author (up) Trabesinger, Andreas openurl 
  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 (up) 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 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 (up) 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 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 (up) 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 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 (up) 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 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 (up) Zhu, J.; Christensen, J.; Jung, J.; Martin-Moreno, L.; Yin, X.; Fok, L.; Zhang, X.; Garcia-Vidal, F. J. openurl 
  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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Author (up) Zurek, Wojciech Hubert openurl 
  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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  Notes Approved no  
  Call Number RPLAB @ gujma @ Serial 799  
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