Helstrom Theorem by No-Signaling Condition

仅基于贝尔定理的量子密码学

Quantum Cryptography Based Solely on Bell's Theorem

提出一种高效且能容忍噪声的新协议，在仅假设满足非信号传输条件下实现设备无关的量子密钥分发安全性。

Limits on quantum deletion from no signaling principle

One of the fundamental restrictions that quantum mechanics imposes is the "No deletion Theorem" which tells us that given two identical unknown quantum states, it is impossible to delete one of them. But nevertheless if not perfect, people have tried to delete it approximately. In these approximate deleting processes our basic target is to delete one of the two identical copies as much as possible while preserving the other copy. In this brief report, by using the No communication theorem (NCT) (impossibility of sending signal faster than light using a quantum resource) as a guiding principle,…

Coexistence of Quantum Theory and Special Relativity in signaling scenarios

The coexistence between Quantum Mechanics and Special Relativity is usually formulated in terms of the no-signaling condition. Several authors have even suggested that this condition should be included between the basic postulates of Quantum Theory. However, there are several scenarios where signaling is, in principle, possible: based on previous results and the analysis of the relation between unitarity and signaling we present an example of a two-particle interferometric arrangement for which the dynamics is, in principle, compatible with superluminal transmission of information. This type o…

On the Parallel Repetition of Multi-Player Games: The No-Signaling Case

We consider the natural extension of two-player nonlocal games to an arbitrary number of players. An important question for such nonlocal games is their behavior under parallel repetition. For two-player nonlocal games, it is known that both the classical and the non-signaling value of any game converges to zero exponentially fast under parallel repetition, given that the game is non-trivial to start with (i.e., has classical/non-signaling value <1). Very recent results show similar behavior for the quantum value of a two-player game under parallel repetition. For nonlocal games with three or …

A No-go theorem for device-independent security in relativistic causal theories

A crucial task for secure communication networks is to determine the minimum of physical requirements to certify a cryptographic protocol. A widely accepted candidate for certification is the principle of relativistic causality which is equivalent to the disallowance of causal loops. Contrary to expectations, we demonstrate how correlations allowed by relativistic causality could be exploited to break security for a broad class of multi-party protocols (all modern protocols belong to this class). As we show, deep roots of this dramatic lack of security lies in the fact that unlike in previous …

A generalized no-broadcasting theorem

We prove a generalized version of the no-broadcasting theorem, applicable to essentially \emph{any} nonclassical finite-dimensional probabilistic model satisfying a no-signaling criterion, including ones with ``super-quantum'' correlations. A strengthened version of the quantum no-broadcasting theorem follows, and its proof is significantly simpler than existing proofs of the no-broadcasting theorem.

The case against entanglement improved measurement precision

It is widely accepted that quantum entanglement between otherwise independent sensors can yield a measurement precision beyond that achievable when the same resources are employed without entanglement \cite{Helstrom1969, Holevo1973a, Caves1980a, Caves1981, Wootters1981, Yurke1986, Wu1986, Xiao1987, Slusher1987, Shapiro1989, Wineland1992, Polzik1992, Kitagawa1993, Braunstein1994, Wineland1994, Sanders1995, Bollinger1996, Ou1997, Dowling1998,Soerensen1998, Brif1999, Childs2000, Fleischhauer2000, Meyer2001, Geremia2003, Giovannetti2004, Kok2004, Leibfried2004, Leibfried2005, Giovannetti2006, Naga…

Conditions for Lorentz-invariant superluminal information transfer without signaling

We understand emergent quantum mechanics in the sense that quantum mechanics describes processes of physical emergence relating an assumed sub-quantum physics to macroscopic boundary conditions. The latter can be shown to entail top-down causation, in addition to usual bottom-up scenarios. With this example it is demonstrated that definitions of "realism" in the literature are simply too restrictive. A prevailing manner to define realism in quantum mechanics is in terms of pre-determination independent of the measurement. With our counter-example, which actually is ubiquitous in emergent, or s…

Condition for macroscopic realism beyond the Leggett-Garg inequalities

In 1985, Leggett and Garg put forward the concept of macroscopic realism (macrorealism) and, in analogy to Bell's theorem, derived a necessary condition in terms of inequalities, which are now known as the Leggett-Garg inequalities. In this paper, we discuss another necessary condition called no-signaling in time. It solely bases on comparing the probability distribution for a macrovariable at some time for the cases where previously a measurement has or has not been performed. Although the concept is analogous to the no-signaling condition in the case of Bell tests, it can be violated accordi…

Comparing Conditions for Macrorealism: Leggett-Garg Inequalities vs No-Signaling in Time

We consider two different types of conditions which were proposed to test macrorealism in the context of a system described by a single dichotomic variable Q. This is the view that a macroscopic system evolving in time possesses definite properties which can be determined without disturbing the future or past state. The Leggett-Garg (LG) inequalities, the most commonly-studied test, are only necessary conditions for macrorealism, but building on earlier work it is shown that when the four three-time LG inequalities are augmented with a certain set of two-time inequalities also of the LG form, …

Cloning and Broadcasting in Generic Probabilistic Theories

We prove generic versions of the no-cloning and no-broadcasting theorems, applicable to essentially {\em any} non-classical finite-dimensional probabilistic model that satisfies a no-signaling criterion. This includes quantum theory as well as models supporting ``super-quantum'' correlations that violate the Bell inequalities to a larger extent than quantum theory. The proof of our no-broadcasting theorem is significantly more natural and more self-contained than others we have seen: we show that a set of states is broadcastable if, and only if, it is contained in a simplex whose vertices are …

No Fine theorem for macrorealism: Limitations of the Leggett-Garg inequality

Tests of local realism and macrorealism have historically been discussed in very similar terms: Leggett-Garg inequalities follow Bell inequalities as necessary conditions for classical behavior. Here, we compare the probability polytopes spanned by all measurable probability distributions for both scenarios and show that their structure differs strongly between spatially and temporally separated measurements. We arrive at the conclusion that, in contrast to tests of local realism where Bell inequalities form a necessary and sufficient set of conditions, no set of inequalities can ever be neces…