Patrick P. Hofer,

Quantum 1, 32 (2017). https://doi.org/10.22331/q-2017-10-12-32

We develop a general framework to investigate fluctuations of non-commuting observables. To this end, we consider the Keldysh quasi-probability distribution (KQPD). This distribution provide…

Jeongwan Haah, Matthew B. Hastings, D. Poulin, and D. Wecker,

Quantum 1, 31 (2017). https://doi.org/10.22331/q-2017-10-03-31

We present an infinite family of protocols to distill magic states for $T$-gates that has a low space overhead and uses an asymptotic number of input magic states to achieve a given target e…

Aniello Lampo, Soon Hoe Lim, Miguel Ángel García-March, and Maciej Lewenstein,

Quantum 1, 30 (2017). https://doi.org/10.22331/q-2017-09-27-30

We study the dynamics of a quantum impurity immersed in a Bose-Einstein condensate as an open quantum system in the framework of the quantum Brownian motion model. We derive a generalized La…

Antoine Tilloy,

Quantum 1, 29 (2017). https://doi.org/10.22331/q-2017-09-19-29

Non-Markovian stochastic Schrödinger equations (NMSSE) are important tools in quantum mechanics, from the theory of open systems to foundations. Yet, in general, they are but formal objects:…

Debbie Leung and John Watrous,

Quantum 1, 28 (2017). https://doi.org/10.22331/q-2017-09-19-28

The qubit depolarizing channel with noise parameter $\eta$ transmits an input qubit perfectly with probability $1-\eta$, and outputs the completely mixed state with probability $\eta$. We sh…

Pavel Sekatski, Michalis Skotiniotis, Janek Kołodyński, and Wolfgang Dür,

Quantum 1, 27 (2017). https://doi.org/10.22331/q-2017-09-06-27

We establish general limits on how precise a parameter, e.g. frequency or the strength of a magnetic field, can be estimated with the aid of full and fast quantum control. We consider uncorr…

Stacey Jeffery and Shelby Kimmel,

Quantum 1, 26 (2017). https://doi.org/10.22331/q-2017-08-17-26

We give a new upper bound on the quantum query complexity of deciding $st$-connectivity on certain classes of planar graphs, and show the bound is sometimes exponentially better than previou…

Whei Yeap Suen, Jayne Thompson, Andrew J. P. Garner, Vlatko Vedral, and Mile Gu,

Quantum 1, 25 (2017). https://doi.org/10.22331/q-2017-08-11-25

The minimal memory required to model a given stochastic process – known as the statistical complexity – is a widely adopted quantifier of structure in complexity science. Here, we ask if qua…

Alastair Kay,

Quantum 1, 24 (2017). https://doi.org/10.22331/q-2017-08-10-24

The production of quantum states required for use in quantum protocols & technologies is studied by developing the tools to re-engineer a perfect state transfer spin chain so that a separabl…

Rafael Chaves, Daniel Cavalcanti, and Leandro Aolita,

Quantum 1, 23 (2017). https://doi.org/10.22331/q-2017-08-04-23

As with entanglement, different forms of Bell nonlocality arise in the multipartite scenario. These can be defined in terms of relaxations of the causal assumptions in local hidden-variable…

Paul Erker, Mario Krenn, and Marcus Huber,

Quantum 1, 22 (2017). https://doi.org/10.22331/q-2017-07-28-22

We derive a framework for quantifying entanglement in multipartite and high dimensional systems using only correlations in two unbiased bases. We furthermore develop such bounds in cases whe…

Mark E. Pearce, Earl T. Campbell, and Pieter Kok,

Quantum 1, 21 (2017). https://doi.org/10.22331/q-2017-07-26-21

Measurements of an object's temperature are important in many disciplines, from astronomy to engineering, as are estimates of an object's spatial configuration. We present the quantum optima…

A. Chia, T. Paterek, and L. C. Kwek,

Quantum 1, 19 (2017). https://doi.org/10.22331/q-2017-07-21-19

We define the hitting time for a model of continuous-time open quantum walks in terms of quantum jumps. Our starting point is a master equation in Lindblad form, which can be taken as the qu…

Yoshifumi Nakata, Christoph Hirche, Ciara Morgan, and Andreas Winter,

Quantum 1, 18 (2017). https://doi.org/10.22331/q-2017-07-21-18

We investigate decoupling, one of the most important primitives in quantum Shannon theory, by replacing the uniformly distributed random unitaries commonly used to achieve the protocol, with…

Manuel Gessner, Luca Pezzè, and Augusto Smerzi,

Quantum 1, 17 (2017). https://doi.org/10.22331/q-2017-07-14-17

We introduce a multi-mode squeezing coefficient to characterize entanglement in $N$-partite continuous-variable systems. The coefficient relates to the squeezing of collective observables in…

Thomas D. Galley and Lluis Masanes,

Quantum 1, 15 (2017). https://doi.org/10.22331/q-2017-07-14-15

The standard postulates of quantum theory can be divided into two groups: the first one characterizes the structure and dynamics of pure states, while the second one specifies the structure…

Marco Tomamichel and Anthony Leverrier,

Quantum 1, 14 (2017). https://doi.org/10.22331/q-2017-07-14-14

In this work we present a security analysis for quantum key distribution, establishing a rigorous tradeoff between various protocol and security parameters for a class of entanglement-based…

John-Mark A. Allen, Owen J. E. Maroney, and Stefano Gogioso,

Quantum 1, 13 (2017). https://doi.org/10.22331/q-2017-07-14-13

Macro-realism is the position that certain macroscopic observables must always possess definite values: e.g. the table is in some definite position, even if we do not know what that is preci…

Baptiste Royer, Arne L. Grimsmo, Nicolas Didier, and Alexandre Blais,

Quantum 1, 11 (2017). https://doi.org/10.22331/q-2017-05-11-11

We investigate an approach to universal quantum computation based on the modulation of longitudinal qubit-oscillator coupling. We show how to realize a controlled-phase gate by simultaneousl…

Mateus Araújo, Adrien Feix, Miguel Navascués, and Časlav Brukner,

Quantum 1, 10 (2017). https://doi.org/10.22331/q-2017-04-26-10

To study which are the most general causal structures which are compatible with local quantum mechanics, Oreshkov et al. introduced the notion of a process: a resource shared between some pa…