4 research outputs found
The smallest refrigerators can reach maximal efficiency
We investigate whether size imposes a fundamental constraint on the
efficiency of small thermal machines. We analyse in detail a model of a small
self-contained refrigerator consisting of three qubits. We show analytically
that this system can reach the Carnot efficiency, thus demonstrating that there
exists no complementarity between size and efficiency.Comment: 9 pages, 1 figure. v2: published versio
Information Causality as a Physical Principle
Quantum physics exhibits remarkable distinguishing characteristics. For
example, it gives only probabilistic predictions (non-determinism) and does not
allow copying of unknown state (no-cloning). Quantum correlations may be
stronger than any classical ones, nevertheless information cannot be
transmitted faster than light (no-signaling). However, all these features do
not single out quantum physics. A broad class of theories exist which share
such traits with quantum mechanics, while they allow even stronger than quantum
correlations. Here, we introduce the principle of Information Causality. It
states that information that Bob can gain about a previously completely unknown
to him data set of Alice, by using all his local resources (which may be
correlated with her resources) and a classical communication from her, is
bounded by the information volume of the communication. In other words, if
Alice communicates m bits to Bob, the total information access that Bob gains
to her data is not greater than m. For m=0, Information Causality reduces to
the standard no-signaling principle. We show that this new principle is
respected both in classical and quantum physics, whereas it is violated by all
the no-signaling correlations which are stronger that the strongest quantum
correlations. Maximally strong no-signalling correlations would allow Bob
access to any m bit subset of the whole data set held by Alice. If only one bit
is sent by Alice (m=1), this is tantamount to Bob being able to access the
value of any single bit of Alice's data (but of course not all of them). We
suggest that Information Causality, a generalization of no-signaling, might be
one of the foundational properties of Nature.Comment: This version of the paper is as close to the published one as legally
possibl
Energetic instability of passive states in thermodynamics
Passivity is a fundamental concept in thermodynamics that demands a quantum system’s energy cannot be lowered by any reversible, unitary process acting on the system. In the limit of many such systems, passivity leads in turn to the concept of complete passivity, thermal states and the emergence of a thermodynamic temperature. Here we only consider a single system and show that every passive state except the thermal state is unstable under a weaker form of reversibility. Indeed, we show that given a single copy of any athermal quantum state, an optimal amount of energy can be extracted from it when we utilise a machine that operates in a reversible cycle. This means that for individual systems, the only form of passivity that is stable under general reversible processes is complete passivity, and thus provides a physically motivated identification of thermal states when we are not operating in the thermodynamic limit