SESSION 9b: Quantum network and communication -- Williams

Chair: (Markus Allgaier (University of Oregon))
3:45pm - 4:15pmSumeet Khatri, Louisiana State University
Practical figures of merit and thresholds for entanglement distribution in quantum networks
Abstract. Before global-scale quantum networks become operational, it is important to consider how to evaluate their performance so that they can be built to achieve the desired performance. We propose two practical figures of merit for the performance of a quantum network: the average connection time and the average largest entanglement cluster size. These quantities are based on the generation of elementary links in a quantum network, which is a crucial initial requirement that must be met before any long-range entanglement distribution can be achieved and is inherently probabilistic with current implementations. We obtain bounds on these figures of merit for a particular class of quantum repeater protocols consisting of repeat-until-success elementary link generation followed by joining measurements at intermediate nodes that extend the entanglement range. Our results lead to requirements on quantum memory coherence times, requirements on repeater chain lengths in order to surpass the repeaterless rate limit, and requirements on other aspects of quantum network implementations. These requirements are based solely on the inherently probabilistic nature of elementary link generation in quantum networks, and they apply to networks with arbitrary topology.
4:15pm - 4:45pmMatthew DiMario, University of New Mexico CQuIC
Coherent state phase estimation based on adaptive photon counting
Abstract. Optical interferometric measurements are an essential tool in many areas in physics, employing a single mode of light to extract information about the properties of a physical system. Coherent states of light have proven to be very convenient states in such measurements, specifically mapping this information into the phase of these states. The difficulty however, is extracting this information with minimal uncertainty, especially in a single-shot measurement. The Cramer-Rao lower bound (CRLB) is the fundamental limit for this uncertainty, which bounds the estimator variance through the quantum Fisher information for coherent states. A physically realizable single-shot measurement strategy that reaches this limit of precision, or even outperforms an ideal heterodyne detection given by twice the CRLB, has yet to be experimentally demonstrated. To this end, we propose and implement a single-shot measurement for phase estimation of coherent states based on coherent displacement operations, single photon counting, and fast feedback. Our demonstration surpasses the limit of an ideal heterodyne measurement without correcting for detection efficiency in our implementation. This superior performance is achieved by real-time optimization of the displacement operation conditioned on the detection history as the measurement progresses. For this optimization, we show that the use of different objective functions yields similar results, which outperform an ideal heterodyne measurement.
4:45pm - 5:15pmEneet Kaur, Louisiana State University
Multipartite entanglement and secret key distribution in quantum networks
Abstract. Distribution and distillation of entanglement over a quantum network is an important task in quantum information theory. A fundamental question is to determine the ultimate performance of entanglement distribution over a given network. Although this question has been extensively explored for bipartite entanglement scenarios, less is known about multipartite entanglement distribution. Here we establish the fundamental limit on distributing multipartite entanglement, in the form of GHZ states, over a quantum network. In particular, we determine the multipartite entanglement/secret key distribution capacity of a quantum network in which the nodes are connected by lossy bosonic quantum channels, which corresponds to a practical quantum network consisting of optical links. Our result is also applicable to the distribution of multipartite secret keys, known as a common key in the quantum network scenario. These results set benchmarks for designing a network topology, as well as for network quantum repeaters for efficient GHZ state/common key distribution. Our result follows from an upper bound on distillable GHZ entanglement introduced here, called the "recursive-cut-and-merge" bound and which constitutes major progress on a longstanding fundamental problem in multipartite entanglement theory. This bound allows us to determine the exact distillable GHZ entanglement for a class of states consisting of products of bipartite pure states.
5:15pm - 5:45pmAshlesha Patil, University of Arizona
Distance-independent rate for entanglement generation in a quantum network
Abstract. Quantum repeaters, built with entangled-photon sources and heralded quantum memories, are connected via lossy links in a network topology. In every time slot, a Bell state is created across each link, among 2 qubits held in memories at either end, with probability p. A node can attempt an n-qubit measurement in a maximally-entangled (e.g., GHZ state) basis, which succeeds with probability q. When n=2, i.e., only Bell-state measurements are used, it was shown: (1) even with local link state (a node knows if its neighbouring links successfully created entanglement in a time slot), end-to-end entanglement rate exceeds routing along the shortest path; (2) but even with global link state information (success-failure outcomes for all links), the rate falls off exponentially with distance, for any. When n >=3, we present protocols for entanglement distribution, and a slightly simpler one for quantum key distribution, that affords a distance-independent rate, using only local link state, in a non-trivial region (i.e., both links and measurements can fail). For the entanglement distribution protocol, the end result is an n-qubit GHZ state between a set of users. When the network topology is a square-grid, for q=1, the threshold for which the above is true, is 0.62. This translates to about 10km of single-mode fiber assuming no other losses. The (p, q) thresholds vary with G and decrease as n increases. Extensions to serving multiple user groups remains ongoing work.
5:45pm - 6:15pmRafael Alexander, University of New Mexico CQuIC
Quantum illumination, illuminated
Abstract. Abstract: Technology based on the use of single-mode squeezed light is now being used to enhance detection of gravitational waves in the LIGO/VIRGO interferometers. A recent key idea in quantum metrology is quantum illumination [S. Lloyd, “Enhanced sensitivity of photodetection via quantum illumination,” Science 321, 1463 (2008)], in which the use of two-mode squeezed (quantum-entangled) light is used to detect the presence of a target with fundamentally better precision than any strategy using unentangled light. Though this fundamental improvement was shown theoretically over a decade ago, its translation into a practical real-world scenario is still an unsolved challenge due to a combination of both the fundamental difficulty of performing a first-principles analysis and the technical limitations set by the requirements of existing technology. We address the first of these issues and shed light on the second. We do this by investigating the role of interferometric symmetries. By framing the problem in terms of the natural SU(1,1) symmetry of two-mode squeezed light, we identify previously unstudied ``symmetry sectors'' for this problem. Within each sector, the output state is entangled and can be modelled as a single (nonlocal) qubit; the entanglement (coherence) within these qubit sectors is responsible for the detection enhancement in quantum illumination.

SQuInT Chief Organizer
Akimasa Miyake, Associate Professor

SQuInT Co-Organizer
Brian Smith, Associate Professor UO

SQuInT Program Committee
Postdoctoral Fellows:
Markus Allgaier (UO OMQ)
Sayonee Ray (UNM CQuIC)
Pablo Poggi (UNM CQuIC)
Valerian Thiel (UO OMQ)

SQuInT Event Co-Organizers (Oregon)
Jorjie Arden
Holly Lynn

SQuInT Event Administrator (Oregon)
Brandy Todd

SQuInT Administrator (CQuIC)
Gloria Cordova
505 277-1850

SQuInT Founder
Ivan Deutsch, Regents' Professor, CQuIC Director

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