1,721,093 research outputs found
Schrödinger Cats and Quantum Complementarity
No full textComplementarity tells us we cannot know precisely the values of all the properties of a quantum object at the same time: the precise determination of one property implies that the value of some other (complementary) property is undefined. E.g. the precise knowledge of the position of a particle implies that its momentum is undefined. Here we show that a Schrödinger cat has a well defined value of a property that is complementary to its “being dead or alive” property. Then, thanks to complementarity, it has an undefined value of the property “being dead or alive”. In other words, the cat paradox is explained through quantum complementarity: of its many complementary properties, any quantum system, such as a cat, can have a well defined value only of one at a time. Schrödinger’s cat has a definite value of a property which is complementary to “being dead or alive”, so it is neither dead nor alive. Figuratively one can say it is both dead and alive. While this interpretation only uses textbook concepts (the Copenhagen interpretation), apparently it has never explicitly appeared in the literature. We detail how to build an Arduino based simulation of Schrödinger’s experiment based on these concepts for science outreach events
Information-disturbance tradeoff in quantum measurements
No full textWe present a simple information-disturbance tradeoff relation valid for any general measurement apparatus: The disturbance between input and output states is lower bounded by the information the apparatus provides in distinguishing these two states
Il gatto di Schroedinger
No full textEverybody has a precise idea of reality: there exists a world
``out there'' that has objective properties that are independent
from us. This is wrong: modern physics teaches us that many of the
properties of an object are created by the observation act itself.
They did not exist (rather, they did not have a definite value)
before the observation. The cat experiment was provocatively
proposed by Schr\"odinger to emphasize this paradoxical aspect of
quantum mechanics at a time when many still thought that this theory
was incomplete or wrong. Today experimental physicists are close to
realizing Schr\"odinger's proposal in practice. However, theoretical
physicists still have not managed to reconcile quantum mechanics
with common sense. Is this an impossible enterprise?
Tutti noi abbiamo una precisa concezione della realta': esiste
un mondo ``la' fuori'' che ha proprieta' oggettive e indipendenti
da noi. Questo e' errato: la fisica moderna ci insegna che molte
delle propriet\`a di un oggetto sono create dall'atto stesso di
osservazione, e non esistevano (o, meglio, non avevano un valore
definito) prima che l'oggetto fosse osservato. L'esperimento del
gatto fu proposto come provocazione da Schroedinger per evidenziare
questo aspetto paradossale della meccanica quantistica, quando in molti ancora pensavano che tale teoria fosse incompleta oppure
sbagliata. Oggi i fisici sperimentali sono sempre piu' vicini alla
realizzazione pratica della proposta di Schroedinger, ma i fisici
teorici non sono ancora riusciti a conciliare la meccanica
quantistica con il senso comune. Che questa sia un'impresa
impossibile
Intuitive reason for the usefulness of entanglement in quantum metrology
No full textWe give a simple intuition for why and when entanglement is needed for quantum-enhanced precision measurements, and which type of entanglement is useful. We give a simple construction that shows how entanglement transforms parallel estimation strategies into sequential ones of the same precision. We employ this argument to provide some useful results and to rederive in a simpler form some known results
A Fundamental Problem in Quantizing General Relativity
No full textWe point out a fundamental problem that hinders the quantization of general relativity: quantum mechanics is formulated in terms of systems, typically limited in space but infinitely extended in time, while general relativity is formulated in terms of events, limited both in space and in time. Many of the problems faced while connecting the two theories stem from the difficulty in shoe-horning one formulation into the other. A solution is not presented, but a list of desiderata for a quantum theory based on events is laid out
A simple proof of Bell's inequality
No full textBell’s theorem is a fundamental result in quantum mechanics: it discriminates between quantum mechanics and all theories where probabilities in measurement results arise from the ignorance of pre-existing local properties. We give an extremely simple proof of Bell's inequality; a single figure suffices. This simplicity may be useful in the unending debate over what exactly the Bell inequality means, because the hypotheses underlying the proof become transparent. It is also a useful didactic tool, as the Bell inequality can be explained in a single intuitive lecture
Entropic information-disturbance tradeoff
No full textWe show the flaws found in the customary fidelity-based definitions of disturbance in quantum measurements and evolutions. We introduce the "entropic disturbance" D and show that it adequately measures the degree of disturbance, intended essentially as an irreversible change in the state of the system. We also find that it complies with an information-disturbance tradeoff, namely the mutual information between the eigenvalues of the initial state and the measurement results is less than or equal to D
Generating entangled two-photon states with coincident frequencies
No full textIt is shown that parametric down-conversion, with a short-duration pump pulse and a long nonlinear crystal that is appropriately phase matched, can produce a frequency-entangled biphoton state whose individual photons are coincident in frequency. Quantum interference experiments which distinguish this state from the familiar time-coincident biphoton state are described
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