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ABSTRACT: A prominent signature of Majorana bound states is the exotic Josephson
effects they produce, the classic example being a fractional Josephson current
with 4\pi periodicity in the phase difference across the junction. Recent work
established that topological insulator edges support a novel `magneto-Josephson
effect', whereby a dissipationless current exhibits 4\pi-periodic dependence
also on the relative orientation of the Zeeman fields in the two banks of the
junction. Here, we explore the magneto-Josephson effect in junctions based on
spin-orbit coupled quantum wires. In contrast to the topological insulator
case, the periodicities of the magneto-Josephson effect no longer follow from
an exact superconductor-magnetism duality of the Hamiltonian. We employ
numerical calculations as well as analytical arguments to identify the domain
configurations that display exotic Josephson physics for quantum-wire
junctions, and elucidate the characteristic differences with the corresponding
setups for topological insulators edges. To provide guidance to experiments, we
also estimate the magnitude of the magneto-Josephson effects in realistic
parameter regimes, and compare the Majorana-related contribution to the
coexisting 2\pi-periodic effects emerging from non-Majorana states.
04/2013;
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ABSTRACT: We analyze two approaches to quantum state transfer in solid-state spin
systems. First, we consider unpolarized spin-chains and extend previous
analysis to various experimentally relevant imperfections, including quenched
disorder, dynamical decoherence, and uncompensated long range coupling. In
finite-length chains, the interplay between disorder-induced localization and
decoherence yields a natural optimal channel fidelity, which we calculate.
Long-range dipolar couplings induce a finite intrinsic lifetime for the
mediating eigenmode; extensive numerical simulations of dipolar chains of
lengths up to L=12 show remarkably high fidelity despite these decay processes.
We further consider the extension of the protocol to bosonic systems of coupled
oscillators. Second, we introduce a quantum mirror based architecture for
universal quantum computing which exploits all of the spins in the system as
potential qubits. While this dramatically increases the number of qubits
available, the composite operations required to manipulate "dark" spin qubits
significantly raise the error threshold for robust operation. Finally, as an
example, we demonstrate that eigenmode-mediated state transfer can enable
robust long-range logic between spatially separated Nitrogen-Vacancy registers
in diamond; numerical simulations confirm that high fidelity gates are
achievable even in the presence of moderate disorder.
Physical Review A 02/2013; 87(022306). · 2.88 Impact Factor
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ABSTRACT: We demonstrate a novel approach to obtain a resonance linewidth below the transit limit. The cross correlation between the induced intensity modulation of two lasers coupling the target resonance exhibits a narrow spectrum. 1/30 of the transit-limited width is achieved in a proof-of-principle experiment where two ground states are the target resonance levels. Attainable linewidth is only limited by laser shot noise in principle. The experimental results qualitatively agree with an intuitive analytical model and numerical calculations. This technique can be easily implemented and should be applicable to many atomic, molecular, and solid state spin systems for spectroscopy, metrology, and resonance-based sensing and imaging.
Physical Review Letters 12/2012; 109(23):233006. · 7.37 Impact Factor
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ABSTRACT: The realization of devices that harness the laws of quantum mechanics represents an exciting challenge at the interface of modern technology and fundamental science. An exemplary paragon of the power of such quantum primitives is the concept of "quantum money" [Wiesner S (1983) ACM SIGACT News 15:78-88]. A dishonest holder of a quantum bank note will invariably fail in any counterfeiting attempts; indeed, under assumptions of ideal measurements and decoherence-free memories such security is guaranteed by the no-cloning theorem. In any practical situation, however, noise, decoherence, and operational imperfections abound. Thus, the development of secure "quantum money"-type primitives capable of tolerating realistic infidelities is of both practical and fundamental importance. Here, we propose a novel class of such protocols and demonstrate their tolerance to noise; moreover, we prove their rigorous security by determining tight fidelity thresholds. Our proposed protocols require only the ability to prepare, store, and measure single quantum bit memories, making their experimental realization accessible with current technologies.
Proceedings of the National Academy of Sciences 09/2012; 109(40):16079-82. · 9.68 Impact Factor
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ABSTRACT: We demonstrate a novel approach to obtain resonance linewidth below that
limited by coherence lifetime. Cross correlation between induced intensity
modulation of two lasers coupling the target resonance exhibits a narrow
spectrum. 1/30 of the lifetime-limited width was achieved in a
proof-of-principle experiment where two ground states are the target resonance
levels. Attainable linewidth is only limited by laser shot noise in principle.
Experimental results agree with an intuitive analytical model and numerical
calculations qualitatively. This technique can be easily implemented and should
be applicable to many atomic, molecular and solid state spin systems for
spectroscopy, metrology and resonance based sensing and imaging.
08/2012;
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ABSTRACT: We investigate 1D quantum systems that support Majorana bound states at
interfaces between topologically distinct regions. In particular, we show that
there exists a duality between particle-hole and spin degrees of freedom in
certain spin-orbit-coupled 1D platforms such as topological insulator edges.
This duality results in a spin analogue of previously explored `fractional
Josephson effects'---that is, the spin current flowing across a magnetic
junction exhibits 4\pi-periodicity in the relative magnetic field angle across
the junction. Furthermore, the interplay between the particle-hole and spin
degrees of freedom results in unconventional magneto-Josephson effects, such
that the Josephson current is a function of the magnetic field orientation with
periodicity 4\pi.
06/2012;
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ABSTRACT: A junction between two topological superconductors containing a pair of Majorana fermions exhibits a "fractional" Josephson effect, 4π periodic in the superconductors' phase difference. An additional fractional Josephson effect, however, arises when the Majorana fermions are spatially separated by a superconducting barrier. This new term gives rise to a set of Shapiro steps which are essentially absent without Majorana modes and therefore provides a unique signature for these exotic states.
Physical Review Letters 12/2011; 107(23):236401. · 7.37 Impact Factor
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ABSTRACT: We introduce a new approach to create and detect Majorana fermions using optically trapped 1D fermionic atoms. In our proposed setup, two internal states of the atoms couple via an optical Raman transition-simultaneously inducing an effective spin-orbit interaction and magnetic field-while a background molecular BEC cloud generates s-wave pairing for the atoms. The resulting cold-atom quantum wire supports Majorana fermions at phase boundaries between topologically trivial and nontrivial regions, as well as "Floquet Majorana fermions" when the system is periodically driven. We analyze experimental parameters, detection schemes, and various imperfections.
Physical Review Letters 06/2011; 106(22):220402. · 7.37 Impact Factor
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ABSTRACT: We propose and analyze an interface between a topological qubit and a superconducting flux qubit. In our scheme, the interaction between Majorana fermions in a topological insulator is coherently controlled by a superconducting phase that depends on the quantum state of the flux qubit. A controlled-phase gate, achieved by pulsing this interaction on and off, can transfer quantum information between the topological qubit and the superconducting qubit.
Physical Review Letters 04/2011; 106(13):130504. · 7.37 Impact Factor
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ABSTRACT: Magnetic resonance imaging can characterize and discriminate among tissues using their diverse physical and biochemical properties. Unfortunately, submicrometer screening of biological specimens is presently not possible, mainly due to lack of detection sensitivity. Here we analyze the use of a nitrogen-vacancy center in diamond as a magnetic sensor for nanoscale nuclear spin imaging and spectroscopy. We examine the ability of such a sensor to probe the fluctuations of the "classical" dipolar field due to a large number of neighboring nuclear spins in a densely protonated sample. We identify detection protocols that appropriately take into account the quantum character of the sensor and find a signal-to-noise ratio compatible with realistic experimental parameters. Through various example calculations we illustrate different kinds of image contrast. In particular, we show how to exploit the comparatively long nuclear spin correlation times to reconstruct a local, high-resolution sample spectrum.
The Journal of chemical physics 09/2010; 133(12):124105. · 3.09 Impact Factor
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ABSTRACT: Quantum repeaters based on atomic ensemble quantum memories are promising candidates for achieving scalable distribution of entanglement over long distances. Recently, important experimental progress has been made towards their implementation. However, the entanglement rates and scalability of current approaches are limited by relatively low retrieval and single-photon detector efficiencies. We propose a scheme, which makes use of fluorescent detection of stored excitations to significantly increase the efficiency of connection and hence the rate. Practical performance and possible experimental realizations of the new protocol are discussed. Comment: 4 pages, 3 figures
07/2009;
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ABSTRACT: We present a protocol to prepare decoherence free cluster states using ultracold atoms loaded in a two dimensional superlattice. The superlattice geometry leads to an array of 2*2 plaquettes, each of them holding four spin-1/2 particles that can be used for encoding a single logical qubit in the two-fold singlet subspace, insensitive to uniform magnetic field fluctuations in any direction. Dynamical manipulation of the supperlattice yields distinct inter and intra plaquette interactions and permits to realize one qubit and two qubit gates with high fidelity, leading to the generation of universal cluster states for measurement based quantum computation. Our proposal based on inter and intra plaquette interactions also opens the path to study polymerized Hamiltonians which support ground states describing arbitrary quantum circuits.
12/2008;
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ABSTRACT: We propose a new approach to implement quantum repeaters for long distance
quantum communication. Our protocol generates a backbone of encoded Bell pairs
and uses the procedure of classical error correction during simultaneous
entanglement connection. We illustrate that the repeater protocol with simple
Calderbank-Shor-Steane (CSS) encoding can significantly extend the
communication distance, while still maintaining a fast key generation rate.
APS. 09/2008;
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ABSTRACT: We demonstrate a slow light beam splitter using rapid coherence transport in a wall-coated atomic vapor cell. We show that particles undergoing random and undirected classical motion can mediate coherent interactions between two or more optical modes. Coherence, written into atoms via electromagnetically induced transparency using an input optical signal at one transverse position, spreads out via ballistic atomic motion, is preserved by an antirelaxation wall coating, and is then retrieved in outgoing slow light signals in both the input channel and a spatially-separated second channel. The splitting ratio between the two output channels can be tuned by adjusting the laser power. The slow light beam splitter may improve quantum repeater performance and be useful as an all-optical dynamically reconfigurable router.
Physical Review Letters 08/2008; 101(4):043601. · 7.37 Impact Factor
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ABSTRACT: Strongly correlated quantum systems can exhibit exotic behaviour called topological order which is characterized by non-local correlations that depend on the system topology. Such systems can exhibit remarkable phenomena such as quasiparticles with anyonic statistics and have been proposed as candidates for naturally error-free quantum computation. However, anyons have never been observed in nature directly. Here, we describe how to unambiguously detect and characterize such states in recently proposed spin–lattice realizations using ultracold atoms or molecules trapped in an optical lattice. We propose an experimentally feasible technique to access non-local degrees of freedom by carrying out global operations on trapped spins mediated by an optical cavity mode. We show how to reliably read and write topologically protected quantum memory using an atomic or photonic qubit. Furthermore, our technique can be used to probe statistics and dynamics of anyonic excitations.
Nature Physics 04/2008; 4(6):482-488. · 18.97 Impact Factor
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ABSTRACT: We suggest a new method for quantum optical control with nanoscale resolution. Our method allows for coherent far-field manipulation of individual quantum systems with spatial selectivity that is not limited by the wavelength of radiation and can, in principle, approach a few nanometers. The selectivity is enabled by the nonlinear atomic response, under the conditions of electromagnetically induced transparency, to a control beam with intensity vanishing at a certain location. Practical performance of this technique and its potential applications to quantum information science with cold atoms, ions, and solid-state qubits are discussed.
Physical Review Letters 04/2008; 100(9):093005. · 7.37 Impact Factor
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ABSTRACT: Reliable preparation of entanglement between distant systems is an outstanding problem in quantum information science and quantum communication. In practice, this has to be accomplished by noisy channels (such as optical fibers) that generally result in exponential attenuation of quantum signals at large distances. A special class of quantum error correction protocols, quantum repeater protocols, can be used to overcome such losses. In this work, we introduce a method for systematically optimizing existing protocols and developing more efficient protocols. Our approach makes use of a dynamic programming-based searching algorithm, the complexity of which scales only polynomially with the communication distance, letting us efficiently determine near-optimal solutions. We find significant improvements in both the speed and the final-state fidelity for preparing long-distance entangled states.
Proceedings of the National Academy of Sciences 11/2007; 104(44):17291-6. · 9.68 Impact Factor
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ABSTRACT: We describe and analyze an efficient register-based hybrid quantum computation scheme. Our scheme is based on probabilistic, heralded optical connection among local five-qubit quantum registers. We assume high fidelity local unitary operations within each register, but the error probability for initialization, measurement, and entanglement generation can be very high (~5%). We demonstrate that with a reasonable time overhead our scheme can achieve deterministic non-local coupling gates between arbitrary two registers with very high fidelity, limited only by the imperfections from the local unitary operation. We estimate the clock cycle and the effective error probability for implementation of quantum registers with ion-traps or nitrogen-vacancy (NV) centers. Our new scheme capitalizes on a new efficient two-level pumping scheme that in principle can create Bell pairs with arbitrarily high fidelity. We introduce a Markov chain model to study the stochastic process of entanglement pumping and map it to a deterministic process. Finally we discuss requirements for achieving fault-tolerant operation with our register-based hybrid scheme, and also present an alternative approach to fault-tolerant preparation of GHZ states.
10/2007;
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Ronald L. Walsworth,
Yanhong Xiao,
Tun Wang,
Maria Baryakhtar,
Mackenzie Van Camp,
Michael Crescimanno,
Michael Hohensee, Liang Jiang,
David Forrest Phillips,
Mikhail D. Lukin,
Susanne F. Yelin
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ABSTRACT: We demonstrate and characterize two coherent phenomena that can mitigate the effects of laser phase noise for Electromagnetically Induced Transparency (EIT): a laser-power-broadening-resistant resonance in the transmitted intensity cross-correlation between EIT optical fields; and a resonant suppression of the conversion of laser phase noise to intensity noise when one-photon noise dominates over two-photon-detuning noise. Our experimental observations are in good agreement with both an intuitive physical picture and numerical calculations. The results have wide-ranging applications to spectroscopy, atomic clocks and magnetometers. Physics Author's Original
