99 research outputs found
Universal quantum computation by the unitary control of ancilla qubits and using a fixed ancilla-register interaction
We characterise a model of universal quantum computation where the register
(computational) qubits are controlled by ancillary qubits, using only a single
fixed interaction between register and ancillary qubits. No additional access
is required to the computational register and the dynamics of both the register
and ancilla are unitary. This scheme is inspired by the measurement-based
ancilla-driven quantum computation of Anders et al. [PRA 82, 020301(R), 2010],
but does not require measurements of the ancillas, and in this respect is
similar to the original gate based model of quantum computation. We consider
what possible forms this ancilla-register interaction can take, with a proof
that the interaction is necessarily locally equivalent to SWAP combined with an
entangling controlled gate. We further show which Hamiltonians can create such
interactions and discuss two examples; the two-qubit XY Hamiltonian and a
particular case of the XXZ Hamiltonian. We then give an example of a simple,
finite and fault tolerant gate set for universal quantum computation in this
model.Comment: 10 pages, Published versio
Multiparameter estimation in networked quantum sensors
We introduce a general model for a network of quantum sensors, and we use this model to consider the following question: When can entanglement between the sensors, and/or global measurements, enhance the precision with which the network can measure a set of unknown parameters? We rigorously answer this question by presenting precise theorems proving that for a broad class of problems there is, at most, a very limited intrinsic advantage to using entangled states or global measurements. Moreover, for many estimation problems separable states and local measurements are optimal, and can achieve the ultimate quantum limit on the estimation uncertainty. This immediately implies that there are broad conditions under which simultaneous estimation of multiple parameters cannot outperform individual, independent estimations. Our results apply to any situation in which spatially localized sensors are unitarily encoded with independent parameters, such as when estimating multiple linear or nonlinear optical phase shifts in quantum imaging, or when mapping out the spatial profile of an unknown magnetic field. We conclude by showing that entangling the sensors can enhance the estimation precision when the parameters of interest are global properties of the entire network
Examination of Post-Service Health-Related Quality of Life Among Rural and Urban Military Members of the Millennium Cohort Study
Little information exists on the health-related quality of life (HRQL) of U.S. veterans based on rural (versus urban) status, especially those in younger age groups, and whether deployment influences this outcome. We addressed these questions in the Millennium Cohort Study, a prospective investigation of U.S. military personnel assessed first in 2001 and then subsequently every three years via self-administered questionnaires. Participants separated from the military at the time of the most recent survey were eligible (n = 10,738). HRQL was assessed using the SF-36V Physical Component Summary (PCS) and Mental Component Summary (MCS) scores. Rural status was assigned from zip codes using the Rural-Urban Commuting Area classification. The mean age of participants was 35 years (SD = 8.98). Compared with urban dwellers, rural residents reported significantly lower unadjusted mean PCS (49.80 vs. 50.42) and MCS (49.97 vs. 50.81) scores, but differences became nonsignificant after covariate adjustment. No interaction was seen between deployment and rural status. Rural status is not independently associated with HRQL among recent U.S. veterans
Hybrid quantum computing with ancillas
In the quest to build a practical quantum computer, it is important to use
efficient schemes for enacting the elementary quantum operations from which
quantum computer programs are constructed. The opposing requirements of
well-protected quantum data and fast quantum operations must be balanced to
maintain the integrity of the quantum information throughout the computation.
One important approach to quantum operations is to use an extra quantum system
- an ancilla - to interact with the quantum data register. Ancillas can mediate
interactions between separated quantum registers, and by using fresh ancillas
for each quantum operation, data integrity can be preserved for longer. This
review provides an overview of the basic concepts of the gate model quantum
computer architecture, including the different possible forms of information
encodings - from base two up to continuous variables - and a more detailed
description of how the main types of ancilla-mediated quantum operations
provide efficient quantum gates.Comment: Review paper. An introduction to quantum computation with qudits and
continuous variables, and a review of ancilla-based gate method
Quantum sensing networks for the estimation of linear functions
The theoretical framework for networked quantum sensing has been developed to a great extent in the past few years, but there are still a number of open questions. Among these, a problem of great significance, both fundamentally and for constructing efficient sensing networks, is that of the role of inter-sensor correlations in the simultaneous estimation of multiple linear functions, where the latter are taken over a collection local parameters and can thus be seen as global properties. In this work we provide a solution to this when each node is a qubit and the state of the network is sensor-symmetric. First we derive a general expression linking the amount of inter-sensor correlations and the geometry of the vectors associated with the functions, such that the asymptotic error is optimal. Using this we show that if the vectors are clustered around two special subspaces, then the optimum is achieved when the correlation strength approaches its extreme values, while there is a monotonic transition between such extremes for any other geometry. Furthermore, we demonstrate that entanglement can be detrimental for estimating non-trivial global properties, and that sometimes it is in fact irrelevant. Finally, we perform a non-asymptotic analysis of these results using a Bayesian approach, finding that the amount of correlations needed to enhance the precision crucially depends on the number of measurement data. Our results will serve as a basis to investigate how to harness correlations in networks of quantum sensors operating both in and out of the asymptotic regime
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