Degradable quantum channels are an important class of completely positive trace-preserving maps. Among other properties, they offer a single-letter formula for the quantum and the private classical capacity and are characterized by the fact that a complementary channel can be obtained from the channel by applying a degrading channel. In this paper, we introduce the concept of approximate degradable channels, which satisfy this condition up to some finite epsilon {\textgreater}= 0. That is, there exists a degrading channel which upon composition with the channel is e-close in the diamond norm to the complementary channel. We show that for any fixed channel the smallest such e can be efficiently determined via a semidefinite program. Moreover, these approximate degradable channels also approximately inherit all other properties of degradable channels. As an application, we derive improved upper bounds to the quantum and private classical capacity for certain channels of interest in quantum communication.

VL - 63 ER - TY - JOUR T1 - Autonomous Quantum Clocks: Does Thermodynamics Limit Our Ability to Measure Time? JF - Physical Review X Y1 - 2017 A1 - Erker, Paul A1 - Mitchison, Mark T. A1 - Silva, Ralph A1 - Woods, Mischa P. A1 - Brunner, Nicolas A1 - Huber, Marcus AB -Time remains one of the least well-understood concepts in physics, most notably in quantum mechanics. A central goal is to find the fundamental limits of measuring time. One of the main obstacles is the fact that time is not an observable and thus has to be measured indirectly. Here, we explore these questions by introducing a model of time measurements that is complete and autonomous. Specifically, our autonomous quantum clock consists of a system out of thermal equilibrium—a prerequisite for any system to function as a clock—powered by minimal resources, namely, two thermal baths at different temperatures. Through a detailed analysis of this specific clock model, we find that the laws of thermodynamics dictate a trade-off between the amount of dissipated heat and the clock’s performance in terms of its accuracy and resolution. Our results furthermore imply that a fundamental entropy production is associated with the operation of any autonomous quantum clock, assuming that quantum machines cannot achieve perfect efficiency at finite power. More generally, autonomous clocks provide a natural framework for the exploration of fundamental questions about time in quantum theory and beyond.

VL - 7 UR - https://link.aps.org/doi/10.1103/PhysRevX.7.031022 ER - TY - JOUR T1 - Achieving sub-shot-noise sensing at finite temperatures JF - Physical Review A Y1 - 2016 A1 - Mehboudi, Mohammad A1 - Correa, Luis A. A1 - Sanpera, Anna VL - 94 IS - 4 JO - Phys. Rev. A ER - TY - JOUR T1 - Autonomous {Quantum} {Refrigerator} in a {Circuit}-{QED} {Architecture} {Based} on a {Josephson} {Junction} JF - Physical Review B Y1 - 2016 A1 - Hofer, Patrick P. A1 - Perarnau-Llobet, Marti A1 - Brask, Jonatan Bohr A1 - Silva, Ralph A1 - Huber, Marcus A1 - Brunner, Nicolas KW - Condensed Matter - Mesoscale and Nanoscale Physics KW - Quantum Physics AB - An implementation of a small quantum absorption refrigerator in a circuit QED architecture is proposed. The setup consists of three harmonic oscillators coupled to a Josephson unction. The refrigerator is autonomous in the sense that it does not require any external control for cooling, but only thermal contact between the oscillators and heat baths at different temperatures. In addition, the setup features a built-in switch, which allows the cooling to be turned on and off. If timing control is available, this enables the possibility for coherence-enhanced cooling. Finally, we show that significant cooling can be achieved with experimentally realistic parameters and that our setup should be within reach of current technology. VL - 94 UR - http://arxiv.org/abs/1607.05218 N1 - arXiv: 1607.05218 ER - TY - JOUR T1 - Autonomous quantum thermal machine for generating steady-state entanglement JF - New Journal of Physics Y1 - 2015 A1 - Bohr Brask, Jonatan A1 - Haack, Géraldine A1 - Nicolas Brunner A1 - Huber, Marcus VL - 17 UR - http://stacks.iop.org/1367-2630/17/i=11/a=113029?key=crossref.0c6a75cb628f113c6cbd2a7b0d092f0d ER - TY - JOUR T1 - All Nonclassical Correlations Can Be Activated into Distillable Entanglement JF - Phys. Rev. Lett. Y1 - 2011 A1 - Piani, Marco A1 - Gharibian, Sevag A1 - Adesso, Gerardo A1 - Calsamiglia, John A1 - Horodecki, Paweł A1 - Winter, Andreas PB - American Physical Society VL - 106 UR - http://prl.aps.org/abstract/PRL/v106/i22/e220403 ER - TY - JOUR T1 - Adiabatic Markovian Dynamics JF - Phys. Rev. Lett. Y1 - 2010 A1 - Oreshkov, Ognyan A1 - Calsamiglia, John KW - Quantum Physics AB - We propose a theory of adiabaticity in quantum Markovian dynamics based on a structural decomposition of the Hilbert space induced by the asymptotic behavior of the Lindblad semigroup. A central idea of our approach is that the natural generalization of the concept of eigenspace of the Hamiltonian in the case of Markovian dynamics is a noiseless subsystem with a minimal noisy cofactor. Unlike previous attempts to define adiabaticity for open systems, our approach deals exclusively with physical entities and provides a simple, intuitive picture at the underlying Hilbert-space level, linking the notion of adiabaticity to the theory of noiseless subsystems. As an application of our theory, we propose a framework for decoherence-assisted computation in noiseless codes under general Markovian noise. We also formulate a dissipation-driven approach to holonomic computation based on adiabatic dragging of subsystems that is generally not achievable by non-dissipative means. VL - 105 UR - http://arxiv.org/abs/1002.2219 ER - TY - JOUR T1 - Aligning spatial frames through quantum channels JF - International Journal of Quantum Information Y1 - 2006 A1 - Bagan, Emili A1 - Muñoz-Tapia, Ramon VL - 4 ER - TY - JOUR T1 - Active and passive quantum erasers for neutral kaons JF - Physical Review A Y1 - 2004 A1 - Bramon, A. A1 - Garbarino, G. A1 - Hiesmayr, B. VL - 69 UR - http://pra.aps.org/abstract/PRA/v69/i6/e062111 ER - TY - JOUR T1 - Atomic Fermi-Bose Mixtures in Inhomogeneous and Random Lattices: From Fermi Glass to Quantum Spin Glass and Quantum Percolation JF - Physical Review Letters Y1 - 2004 A1 - Sanpera, A. A1 - Kantian, A. A1 - Sanchez-Palencia, L. A1 - Zakrzewski, J. A1 - Lewenstein, M. VL - 93 UR - http://prl.aps.org/abstract/PRL/v93/i4/e040401 ER - TY - JOUR T1 - Aligning Reference Frames with Quantum States JF - arXiv Y1 - 2001 A1 - Bagan, Emili A1 - Baig, Marià A1 - Muñoz-Tapia, Ramon UR - http://arxiv.org/abs/quant-ph/0106155 ER - TY - JOUR T1 - Aligning Reference Frames with Quantum States JF - Physical Review Letters Y1 - 2001 A1 - E. Bagan A1 - Baig, M. A1 - Muñoz-Tapia, R. VL - 87 UR - http://prl.aps.org/abstract/PRL/v87/i25/e257903 ER -