ScholarBank@NUShttps://scholarbank.nus.edu.sgThe DSpace digital repository system captures, stores, indexes, preserves, and distributes digital research material.Mon, 22 Jul 2024 10:28:07 GMT2024-07-22T10:28:07Z5081- Long-distance entanglement generation in two-dimensional networkshttps://scholarbank.nus.edu.sg/handle/10635/115172Title: Long-distance entanglement generation in two-dimensional networks
Authors: Broadfoot, S.; Dorner, U.; Jaksch, D.
Abstract: We consider two-dimensional networks composed of nodes initially linked by two-qubit mixed states. In these networks we develop a global error correction scheme that can generate distance-independent entanglement from arbitrary network geometries using rank-2 states. By using this method and combining it with the concept of percolation, we also show that the generation of long-distance entanglement is possible with rank-3 states. Entanglement percolation and global error correction have different advantages depending on the given situation. To reveal the trade-off between them we consider their application to networks containing pure states. In doing so we find a range of pure-state schemes, each of which has applications in particular circumstances: For instance, we can identify a protocol for creating perfect entanglement between two distant nodes. However, this protocol cannot generate a singlet between any two nodes. In contrast, we can also construct schemes for creating entanglement between any nodes, but the corresponding entanglement fidelity is lower. © 2010 The American Physical Society.
Thu, 21 Oct 2010 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1151722010-10-21T00:00:00Z
- Dissipative quantum-light-field engineeringhttps://scholarbank.nus.edu.sg/handle/10635/112418Title: Dissipative quantum-light-field engineering
Authors: Kiffner, M.; Dorner, U.; Jaksch, D.
Abstract: We put forward a dissipative preparation scheme for strongly correlated photon states. Our approach is based on a two-photon loss mechanism that is realized via a single four-level atom inside a bimodal optical cavity. Each elementary two-photon emission event removes one photon out of each of the two modes. The dark states of this loss mechanism are given by NOON states and arbitrary superpositions thereof. We find that the steady state of the two cavity modes exhibits entanglement and, for certain parameters, a mixture of two coherent entangled states is produced. We discuss how the quantum correlations in the cavity modes and the output fields can be measured. © 2012 American Physical Society.
Fri, 10 Feb 2012 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1124182012-02-10T00:00:00Z
- Real-world quantum sensors: Evaluating resources for precision measurementhttps://scholarbank.nus.edu.sg/handle/10635/116564Title: Real-world quantum sensors: Evaluating resources for precision measurement
Authors: Thomas-Peter, N.; Smith, B.J.; Datta, A.; Zhang, L.; Dorner, U.; Walmsley, I.A.
Abstract: Quantum phenomena present in many experiments signify nonclassical behavior, but do not always imply superior performance. Quantifying the enhancement achieved from quantum behavior needs careful analysis of the resources involved. We analyze the case of parameter estimation using an optical interferometer, where increased precision can in principle be achieved using quantum probe states. Common performance measures are examined and some are shown to overestimate the improvement. For the simplest experimental case we compare the different measures and exhibit this overestimation explicitly. We give the preferred analysis of these experiments and calculate benchmark values for experimental parameters necessary to realize a precision enhancement. Our analysis shows that unambiguous real-world enhancements in optical quantum metrology with fixed photon number are yet to be attained. © 2011 American Physical Society.
Thu, 08 Sep 2011 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1165642011-09-08T00:00:00Z
- Quantum frequency estimation with trapped ions and atomshttps://scholarbank.nus.edu.sg/handle/10635/116550Title: Quantum frequency estimation with trapped ions and atoms
Authors: Dorner, U.
Abstract: We discuss strategies for quantum-enhanced estimation of atomic transition frequencies with ions stored in Paul traps or neutral atoms trapped in optical lattices. We show that only marginal quantum improvements can be achieved using standard Ramsey interferometry in the presence of collective dephasing, which is the major source of noise in relevant experimental setups. We therefore analyze methods based on decoherence free subspaces and prove that quantum enhancement can readily be achieved even in the case of significantly imperfect state preparation and faulty detections. © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.
Sun, 01 Apr 2012 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1165502012-04-01T00:00:00Z
- Optical excitation of zigzag carbon nanotubes with photons guided in nanofibershttps://scholarbank.nus.edu.sg/handle/10635/116498Title: Optical excitation of zigzag carbon nanotubes with photons guided in nanofibers
Authors: Broadfoot, S.; Dorner, U.; Jaksch, D.
Abstract: We consider the excitation of electrons in semiconducting carbon nanotubes by photons from the evanescent field created by a subwavelength-diameter optical fiber. The strongly changing evanescent field of such nanofibers requires dropping the dipole approximation. We show that this leads to novel effects, especially a high dependence of the photon absorption on the relative orientation and geometry of the nanotube-nanofiber setup in the optical and near-infrared domain. In particular, we calculate photon absorption probabilities for a straight nanotube and nanofiber depending on their relative angle. Nanotubes orthogonal to the fiber are found to perform much better than parallel nanotubes when they are short. As the nanotube gets longer the absorption of parallel nanotubes is found to exceed the orthogonal nanotubes and approach 100% for extremely long nanotubes. In addition, we show that if the nanotube is wrapped around the fiber in an appropriate way the absorption is enhanced. We find that optical and near-infrared photons could be converted to excitations with efficiencies that may exceed 90%. This may provide opportunities for future photodetectors and we discuss possible setups. © 2012 American Physical Society.
Fri, 25 May 2012 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1164982012-05-25T00:00:00Z
- Noise-assisted Ramsey interferometryhttps://scholarbank.nus.edu.sg/handle/10635/116483Title: Noise-assisted Ramsey interferometry
Authors: Dorner, U.
Abstract: I analyze a metrological strategy for improving the precision of frequency estimation via Ramsey interferometry with strings of atoms in the presence of correlated dephasing. This strategy does not employ entangled states but rather a product state which evolves into a stationary state under the influence of correlated dephasing. It is shown that by using this state an improvement in precision compared to standard Ramsey interferometry can be gained. This improvement is not an improvement in scaling; i.e., the estimation precision has the same scaling with the number of atoms as the standard quantum limit but gains an improvement proportional to the free evolution time in the Ramsey interferometer. Since a stationary state is used, this evolution time can be substantially larger than in standard Ramsey interferometry which is limited by the coherence time of the atoms. © 2013 American Physical Society.
Thu, 26 Dec 2013 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1164832013-12-26T00:00:00Z
- Quantum metrology with imperfect states and detectorshttps://scholarbank.nus.edu.sg/handle/10635/116554Title: Quantum metrology with imperfect states and detectors
Authors: Datta, A.; Zhang, L.; Thomas-Peter, N.; Dorner, U.; Smith, B.J.; Walmsley, I.A.
Abstract: Quantum enhancements of precision in metrology can be compromised by system imperfections. These may be mitigated by appropriate optimization of the input state to render it robust, at the expense of making the state difficult to prepare. In this paper, we identify the major sources of imperfection of an optical sensor: input state preparation inefficiency, sensor losses, and detector inefficiency. The second of these has received much attention; we show that it is the least damaging to surpassing the standard quantum limit in a optical interferometric sensor. Further, we show that photonic states that can be prepared in the laboratory using feasible resources allow a measurement strategy using photon-number-resolving detectors that not only attain the Heisenberg limit for phase estimation in the absence of losses, but also deliver close to the maximum possible precision in realistic scenarios including losses and inefficiencies. In particular, we give bounds for the tradeoff between the three sources of imperfection that will allow true quantum-enhanced optical metrology. © 2011 American Physical Society.
Fri, 24 Jun 2011 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1165542011-06-24T00:00:00Z
- Quantum memory in an optical latticehttps://scholarbank.nus.edu.sg/handle/10635/115895Title: Quantum memory in an optical lattice
Authors: Nunn, J.; Dorner, U.; Michelberger, P.; Reim, K.F.; Lee, K.C.; Langford, N.K.; Walmsley, I.A.; Jaksch, D.
Abstract: Arrays of atoms trapped in optical lattices are appealing as storage media for photons, since motional dephasing of the atoms is eliminated. The regular lattice is also associated with band structure in the dispersion experienced by incident photons. Here we study the influence of this band structure on the efficiency of quantum memories based on electromagnetically induced transparency (EIT) and on Raman absorption. We observe a number of interesting effects, such as both reduced and superluminal group velocities, enhanced atom-photon coupling, and anomalous transmission. These effects are ultimately deleterious to the memory efficiency, but they are easily avoided by tuning the optical fields away from the band edges. © 2010 The American Physical Society.
Mon, 23 Aug 2010 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1158952010-08-23T00:00:00Z