Monday, July 31, 2017

Room-Temperature Photon-Number-Resolving Detection Without Room-Temperature Photon-Number-Resolving Detectors

Elisha S. Matekole, Hwang Lee, Jonathan P. Dowling
We study the average coincidence counts signal at the output of a two-mode squeezing device with |N|α as the two input modes. We show that the input photon-number can be resolved from the average coincidence counts. In particular, we observe jumps in the average coincidence counts signal as a function of input photon number N. Therefore, we propose a photon-number-resolving detector at room temperature with high efficiency.
Comments: 6 pages, 5 figures
Subjects: Quantum Physics (quant-ph)
Cite as: arXiv:1707.02666 [quant-ph]

Fundamental precision limit of a Mach-Zehnder interferometric sensor when one of the inputs is the vacuum

Masahiro Takeoka, Kaushik P. Seshadreesan, Chenglong You, Shuro Izumi, Jonathan P. Dowling
In the lore of quantum metrology, one often hears (or reads) the following no-go theorem: If you put vacuum into one input port of a balanced Mach-Zehnder Interferometer, then no matter what you put into the other input port, and no matter what your detection scheme, the sensitivity can never be better than the shot noise limit (SNL). Often the proof of this theorem is cited to be in Ref. [C. Caves, Phys. Rev. D 23, 1693 (1981)], but upon further inspection, no such claim is made there. A quantum-Fisher-information-based argument suggestive of this no-go theorem appears in Ref. [M. Lang and C. Caves, Phys. Rev. Lett. 111, 173601 (2013)], but is not stated in its full generality. Here we thoroughly explore this no-go theorem and give the rigorous statement: the no-go theorem holds whenever the unknown phase shift is split between both arms of the interferometer, but remarkably does not hold when only one arm has the unknown phase shift. In the latter scenario, we provide an explicit measurement strategy that beats the SNL. We also point out that these two scenarios are physically different and correspond to different types of sensing applications.
Comments: 9 pages, 2 figures
Subjects: Quantum Physics (quant-ph)
Cite as: arXiv:1705.09506 [quant-ph]

Multi-pass configuration for Improved Squeezed Vacuum Generation in Hot Rb Vapor

Mi Zhang, Melissa A. Guidry, R. Nicholas Lanning, Zhihao Xiao, Jonathan P. Dowling, Irina Novikova, Eugeniy E. Mikhailov
We study a squeezed vacuum field generated in hot Rb vapor via the polarization self-rotation effect. Our previous experiments showed that the amount of observed squeezing may be limited by the contamination of the squeezed vacuum output with higher-order spatial modes, also generated inside the cell. Here, we demonstrate that the squeezing can be improved by making the light interact several times with a less dense atomic ensemble. With optimization of some parameters we can achieve up to -2.6 dB of squeezing in the multi-pass case, which is 0.6 dB improvement compared to the single-pass experimental configuration. Our results show that other than the optical depth of the medium, the spatial mode structure and cell configuration also affect the squeezing level.
Comments: 7 pages, 8 figures
Subjects: Atomic Physics (physics.atom-ph); Quantum Physics (quant-ph)
Journal reference: Phys. Rev. A 96, 013835 (2017)
DOI: 10.1103/PhysRevA.96.013835
Cite as: arXiv:1705.02914 [physics.atom-ph]

Lorentz invariant entanglement distribution for the space-based quantum network

Tim Byrnes, Batyr Ilyas, Louis Tessler, Masahiro Takeoka, Segar Jambulingam, Jonathan P. Dowling
In recent years there has been a great deal of focus on a globe-spanning quantum network, including linked satellites for applications ranging from quantum key distribution to distributed sensors and clocks. In many of these schemes, relativistic transformations may have deleterious effects on the purity of the distributed entangled pairs. This becomes particularly important for the application of distributed clocks. In this paper, we have developed a Lorentz invariant entanglement distribution protocol that completely removes the effects due to the relative motions of the satellites.
Subjects: Quantum Physics (quant-ph)
Cite as: arXiv:1704.04774 [quant-ph]

Modeling the atomtronic analog of an optical polarizing beam splitter, a half-wave plate, and a quarter-wave plate for phonons of the motional state of two trapped atom

Naieme Mohseni, Marjan Fani, Jonathan P. Dowling, Shahpoor Saeidian
In this paper we propose a scheme to model the phonon analog of optical elements, including a polarizing beam splitter, a half- wave plate, and a quarter- wave plate, as well as an implementation of CNOT and Pauli gates, by using two atoms confined in a 2D plane. The internal states of the atoms are taken to be Rydberg circular states. Using this model we can manipulate the motional state of the atom, with possible applications in optomechanical integrated circuits for quantum information processing and quantum simulation. Towards this aim, we study the interaction of two trapped atoms with two circularly polarized Laguerre-Gaussian beams, in such a way that the beams illuminate selectively only one of the atoms.
Subjects: Quantum Physics (quant-ph)
Cite as: arXiv:1703.01566 [quant-ph]
 
 Accepted for Publication in PRA.

Optimal digital dynamical decoupling for general decoherence via Walsh modulation

Haoyu Qi, Jonathan P. Dowling, Lorenza Viola
We provide a general framework for constructing digital dynamical decoupling sequences based on Walsh modulation --- applicable to arbitrary qubit decoherence scenarios. By establishing equivalence between decoupling design based on Walsh functions and on concatenated projections, we identify a family of {\em optimal Walsh sequences}, which can be exponentially more efficient, in terms of the required total pulse number, for fixed cancellation order, than known digital sequences based on concatenated design. Optimal sequences for a given cancellation order are highly non-unique --- their performance depending sensitively on the control path. We provide an analytic upper bound to the achievable decoupling error, and show how sequences within the optimal Walsh family can substantially outperform concatenated decoupling, while respecting realistic timing constraints. We validate these conclusions by numerically computing the average fidelity in a toy model capturing the essential feature of hyperfine-induced decoherence in a quantum dot.
Comments: 10 pages, 1 figure
Subjects: Quantum Physics (quant-ph)
Cite as: arXiv:1702.05533 [quant-ph]

Gaussian Beam-Propagation Theory for Nonlinear Optics - Featuring an Exact Treatment of Orbital Angular Momentum Transfer

R. Nicholas Lanning, Zhihao Xiao, Mi Zhang, Irina Novikova, Eugeniy Mikhailov, Jonathan P. Dowling
We present a general, Gaussian spatial mode propagation formalism for describing the generation of higher order multi-spatial-mode beams generated during nonlinear interactions. Furthermore, to implement the theory, we simulate optical angular momentum transfer interactions, and show how one can optimize the interaction to reduce the undesired modes. Past theoretical treatments of this problem have often been phenomenological, at best. Here we present an exact solution for the single-pass no-cavity regime, in which the the nonlinear interaction is not overly strong. We apply our theory to two experiments, with very good agreement, and give examples of several more configurations, easily tested in the laboratory.
Subjects: Optics (physics.optics); Quantum Physics (quant-ph)
Journal reference: Phys. Rev. A 96, 013830 (2017)
DOI: 10.1103/PhysRevA.96.013830