889 research outputs found
Small quantum absorption refrigerator in the transient regime: time scales, enhanced cooling and entanglement
A small quantum absorption refrigerator, consisting of three qubits, is
discussed in the transient regime. We discuss time scales for coherent
dynamics, damping, and approach to the steady state, and we study cooling and
entanglement. We observe that cooling can be enhanced in the transient regime,
in the sense that lower temperatures can be achieved compared to the
steady-state regime. This is a consequence of coherent dynamics, but can occur
even when this dynamics is strongly damped by the dissipative thermal
environment, and we note that precise control over couplings or timing is not
needed to achieve enhanced cooling. We also show that the amount of
entanglement present in the refrigerator can be much larger in the transient
regime compared to the steady-state. These results are of relevance to future
implementations of quantum thermal machines.Comment: 8 pages, 5 figure
Bell scenarios with communication
Classical and quantum physics provide fundamentally different predictions
about experiments with separate observers that do not communicate, a phenomenon
known as quantum nonlocality. This insight is a key element of our present
understanding of quantum physics, and also enables a number of information
processing protocols with security beyond what is classically attainable.
Relaxing the pivotal assumption of no communication leads to new insights into
the nature quantum correlations, and may enable new applications where security
can be established under less strict assumptions. Here, we study such
relaxations where different forms of communication are allowed. We consider
communication of inputs, outputs, and of a message between the parties. Using
several measures, we study how much communication is required for classical
models to reproduce quantum or general no-signalling correlations, as well as
how quantum models can be augmented with classical communication to reproduce
no-signalling correlations.Comment: 12 pages, 3 figures. Includes a more detailed explanation of results
appearing in the appendix of arXiv:1411.4648 [quant-ph
Testing nonlocality of a single photon without a shared reference frame
The question of testing the nonlocality of a single photon has raised much
debate over the last years. The controversy is intimately related to the issue
of providing a common reference frame for the observers to perform their local
measurements. Here we address this point by presenting a simple scheme for
demonstrating the nonlocality of a single photon which does not require a
shared reference frame. Specifically, Bell inequality violation can be obtained
with certainty with unaligned devices, even if the relative frame fluctuates
between each experimental run of the Bell test. Our scheme appears feasible
with current technology, and may simplify the realization of quantum
communication protocols based on single-photon entanglement.Comment: 5 pages, 3 figure
Device-Independent Tests of Entropy
We show that the entropy of a message can be tested in a device-independent
way. Specifically, we consider a prepare-and-measure scenario with classical or
quantum communication, and develop two different methods for placing lower
bounds on the communication entropy, given observable data. The first method is
based on the framework of causal inference networks. The second technique,
based on convex optimization, shows that quantum communication provides an
advantage over classical, in the sense of requiring a lower entropy to
reproduce given data. These ideas may serve as a basis for novel applications
in device-independent quantum information processing
The Role of Urban Living Labs in Fostering Sustainable Cities - Insights from Sweden
Cities face growing pressures from increasing populations, giving rise to environmental and social problems. The transition towards more sustainable urban areas has thus become an urgent matter, demanding innovative solutions and means to test and deploy them. Urban Living Labs (ULLs) have emerged as a new approach to experimentation in real-life city settings. This thesis seeks to examine how ULLs can contribute to sustainability transitions by examining their goals, visions, operations, and evaluation. The contribution of ULL to sustainability transitions is assessed via mapping ULLs in Sweden and analysing four in-depth ULL case studies (i.e. UbiGo, HĂ„llbarheten, Shape Your World, and Fabriken) through the lens of Transition Management. This study finds that the goals and visions of ULLs vary greatly, and that their contributions to sustainability can be either direct or indirect, both playing an important role in transitions. Furthermore, the ways in which the ULL evaluation is performed as well as what aspects are evaluated differs between the ULLs. A higher number of actors involved in ULLs can catalyse sustainability transitions due to a broader spread of knowledge reaching further into several domains of society. In addition, this study reflects on the importance of user involvement and their ability to impact ULL operations, how ULLs fit into the Transition Management Cycle,and how requirements set by funding bodies might affect the learning produced in ULLs.HĂ„llbarhetslabb â ett konkret sĂ€tt att skapa hĂ„llbara stĂ€der StĂ€der stĂ„r inför mĂ„nga utmaningar. Redan idag koncentrerar stĂ€der utslĂ€ppsintensiva aktiviteter sĂ„som transporter, boende och konsumtion och lider ocksĂ„ av negativa effekter av utslĂ€ppen i form av ett förĂ€ndrat klimat med bland annat vĂ€rmeböljor eller översvĂ€mningar. Sociala problem som till exempel segregation Ă€r inte heller ovanligt i stĂ€der. Utmaningarna vĂ€ntas bli större i takt med att fler mĂ€nniskor runt om i vĂ€rlden flyttar till stĂ€der vilket sĂ€tter Ă€nnu större tyck pĂ„ de försörjningssystem som finns. För att kunna möta dess utmaningar behöver nya lösningar för att minska utslĂ€pp och skapa hĂ„llbarhet utvecklas och implementeras i stĂ€der. En metod för att utveckla och testa sĂ„dana lösningar Ă€r sĂ„ kallade Urban living labs (ULLs). StĂ€der Ă€r perfekta testplatser för nya hĂ„llbarhetslösningar eftersom lösningar som ska implementeras i stĂ€der kan anpassas till de förutsĂ€ttningar som Ă€r speciella för stĂ€der om de testas dĂ€r. Det Ă€r essensen av ULLs â i ULLs testas lösningar i riktig stadsmiljö med riktiga samhĂ€llsaktörer involverade för att kunna anpassa det som testas till de faktiska förutsĂ€ttningarna i stadsmiljö. Vanliga personer som bor i stĂ€derna Ă€r ocksĂ„ relativt involverade i labben och ger feedback eller Ă€r direkt involverade i utvecklingen av lösningen som testas. FrĂ„gan Ă€r dock pĂ„ vilket sĂ€tt ULLs bidrar till mer hĂ„llbara stĂ€der? Potential finns i och med att labben riktar in sig direkt pĂ„ problem som förekommer i stĂ€der och anpassar lösningarna efter förutsĂ€ttningarna. Intresset och investeringar i ULLs har pĂ„ senare tid ocksĂ„ ökat men hur ULLs bidrar till hĂ„llbarhet Ă€r Ă€nnu oklart. I den hĂ€r studien undersöks just hur ULLs kan bidra till en utveckling mot mer hĂ„llbara stĂ€der. Svenska ULLs kartlades och fyra studerades sedan djupare för att förstĂ„ deras bidrag till hĂ„llbarhet. De fyra labben som undersöktes var UbiGo, ett labb som testade en mobilapplikation som skulle underlĂ€tta resande inom stĂ€der med andra fĂ€rdmedel Ă€n den egna bilen, HĂ„llbarheten, ett testhus dĂ€r lösningar för ett framtida energisnĂ„lt och hĂ„llbart boende undersöks, Formge din vĂ€rld, ett labb som testade metoder för att involvera invĂ„nare i stadsplaneringen samtidigt som projektet bidrog till att invĂ„narna lĂ€rde sig mer om hĂ„llbarhet genom ett stadsodlingsprojekt, och Fabriken, ett labb i form av en öppen verkstad dĂ€r fokus lĂ„g pĂ„ DIY och Ă„terskapande. Labbens mĂ„l och visioner, hur labbaktiviteterna bidrog till att mĂ„len uppfylldes, samt hur labben utvĂ€rderades undersöktes i studien. Studien visar ULLs bidrar till hĂ„llbarhet pĂ„ mĂ„nga olika sĂ€tt - vissa pĂ„ ett direkt och tydlig sĂ€tt och andra mer indirekt. En del ULLs fokuserar pĂ„ att hitta direkta och ofta tekniska lösningar för att minska utslĂ€ppen i stĂ€der (t.ex. UbiGo och HĂ„llbarheten) medan andra fokusera pĂ„ styrning av stĂ€der och att hitta nya metoder för att involvera invĂ„narna i stadsplaneringsprocesser (t.ex. Formge din vĂ€rld) vilket pĂ„ sikt skulle kunna bidra till hĂ„llbarhet. Vissa labb har inte tydliga mĂ„l om att bidra till hĂ„llbarhet men kan Ă€ndĂ„ pĂ„verka beteende och inspirera till hĂ„llbara livsstilar som förbrukar mindre resurser och pĂ„ sĂ„ sĂ€tt skapar hĂ„llbarhet pĂ„ ett mer indirekt sĂ€tt (t.ex. Fabriken). Det Ă€r vanligt att mĂ„nga samhĂ€llsaktörer Ă€r involverade i ULLs vilket ses som en fördel eftersom flera aktörer frĂ„n olika delar av samhĂ€llet dĂ„ tar del av lĂ€rdomarna frĂ„n labben och kan anvĂ€nda dessa i annat arbete. En nackdel Ă€r dock att ULLs i mĂ„nga fall Ă€r tidsbegrĂ€nsade projekt. Det innebĂ€r att labben kan ha svĂ„rt att etablera sig och blir ett permanent inslag i stĂ€der vilket kan minska genomslagskraften hos de lösningar som testas i labben. Vanligtvis Ă€r det universitet eller andra forskningsinstitut som leder utvĂ€rderingarna av labben och vilka aspekter av labben som studeras varierar stort frĂ„n fall till fall. FinansiĂ€rer av labben stĂ€ller vissa krav pĂ„ vilka aspekter av labben de vill ska utvĂ€rderas och att undersöka dessa krav nĂ€rmare hade kunna ge en bĂ€ttre bild av hur de pĂ„verkar vilka lĂ€rdomar labben bidrar med
Fundamental limits on low-temperature quantum thermometry with finite resolution
While the ability to measure low temperatures accurately in quantum systems
is important in a wide range of experiments, the possibilities and the
fundamental limits of quantum thermometry are not yet fully understood
theoretically. Here we develop a general approach to low-temperature quantum
thermometry, taking into account restrictions arising not only from the sample
but also from the measurement process. We derive a fundamental bound on the
minimal uncertainty for any temperature measurement that has a finite
resolution. A similar bound can be obtained from the third law of
thermodynamics. Moreover, we identify a mechanism enabling sub-exponential
scaling, even in the regime of finite resolution. We illustrate this effect in
the case of thermometry on a fermionic tight-binding chain with access to only
two lattice sites, where we find a quadratic divergence of the uncertainty. We
also give illustrative examples of ideal quantum gases and a square-lattice
Ising model, highlighting the role of phase transitions.Comment: Published version. Main text: 12 pages, 5 figures; see also related
work by K. Hovhannisyan and L. A. Correa at arXiv:1712.0308
Hybrid Quantum Repeater Protocol With Fast Local Processing
We propose a hybrid quantum repeater protocol combining the advantages of
continuous and discrete variables. The repeater is based on the previous work
of Brask et al. [Phys. Rev. Lett. 105, 160501 (2010)] but we present two ways
of improving this protocol. In the previous protocol entangled single-photon
states are produced and grown into superpositions of coherent states, known as
two-mode cat states. The entanglement is then distributed using homodyne
detection. To improve the protocol, we replace the time-consuming non-local
growth of cat states with local growth of single-mode cat states, eliminating
the need for classical communication during growth. Entanglement is generated
in subsequent connection processes. Furthermore the growth procedure is
optimized. We review the main elements of the original protocol and present the
two modifications. Finally the two protocols are compared and the modified
protocol is shown to perform significantly better than the original protocol.Comment: 14 pages, 7 figure
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