Entanglement appears under two different forms in quantum theory, namely as a property of states of joint systems and as a property of measurement eigenstates in joint measurements. By combining ...these two aspects of entanglement, it is possible to generate nonlocality between particles that never interacted, using the protocol of entanglement swapping. We show that even in the more constraining bilocal scenario where distant sources of particles are assumed to be independent, i.e. to share no prior randomness, this process can be simulated classically with bounded communication, using only 9 bits in total. Our result thus provides an upper bound on the nonlocality of the process of entanglement swapping.
J. Phys. A: Math. Gen. 37, 1775 (2004) We computationally investigate the complete polytope of Bell inequalities for
2 particles with small numbers of possible measurements and outcomes. Our
approach ...is limited by Pitowsky's connection of this problem to the
computationally hard NP problem. Despite this, we find that there are very few
relevant inequivalent inequalities for small numbers. For example, in the case
with 3 possible 2-outcome measurements on each particle, there is just one new
inequality. We describe mixed 2-qubit states which violate this inequality but
not the CHSH. The new inequality also illustrates a sharing of bi-partite
non-locality between three qubits: something not seen using the CHSH
inequality. It also inspires us to discover a class of Bell inequalities with m
possible n-outcome measurements on each particle.
Proposed in 1984, quantum key distribution (QKD) allows two users to exchange provably secure keys via a potentially insecure quantum channel. Since then, QKD has attracted much attention and ...significant progress has been made in both theory and practice. On the application front, however, the operating distance of practical fibre-based QKD systems is limited to about 150 km, which is mainly due to the high background noise produced by commonly used semiconductor single-photon detectors (SPDs) and the stringent demand on the minimum classical- post-processing (CPP) block size. Here, we present a compact and autonomous QKD system that is capable of distributing provably-secure cryptographic key over 307 km of ultra-low-loss optical fibre (51.9 dB loss). The system is based on a recently developed standard semiconductor (inGaAs) SPDs with record low background noise and a novel efficient finite-key security analysis for QKD. This demonstrates the feasibility of practical long-distance QKD based on standard fibre optic telecom components.
Quantum communication, and indeed quantum information in general, has changed the way we think about quantum physics. In 1984 and 1991, the first protocol for quantum cryptography and the first ...application of quantum non-locality, respectively, attracted a diverse field of researchers in theoretical and experimental physics, mathematics and computer science. Since then we have seen a fundamental shift in how we understand information when it is encoded in quantum systems. We review the current state of research and future directions in this new field of science with special emphasis on quantum key distribution and quantum networks.
By amplifying photonic qubits it is possible to produce states that contain enough photons to be seen with a human eye, potentially bringing quantum effects to macroscopic scales 1. In this paper we ...theoretically study quantum states obtained by amplifying one side of an entangled photon pair with different types of optical cloning machines for photonic qubits. We propose a detection scheme that involves lossy threshold detectors (such as human eye) on the amplified side and conventional photon detectors on the other side. We show that correlations obtained with such coarse-grained measurements prove the entanglement of the initial photon pair and do not prove the entanglement of the amplified state. We emphasize the importance of the detection loophole in Bell violation experiments by giving a simple preparation technique for separable states that violate a Bell inequality without closing this loophole. Finally we analyze the genuine entanglement of the amplified states and its robustness to losses before, during and after amplification.
In the quantum regime information can be copied with only a finite fidelity. This fidelity gradually increases to 1 as the system becomes classical. In this article we show how this fact can be used ...to directly measure the amount of radiated power. We demonstrate how these principles could be used to build a practical primary standard.
Understanding what can be inferred about a multi-particle quantum system from only the knowledge of its subparts is a highly non-trivial task. Clearly, if the global system doesn't contain any ...information resource, nor do its subparts. However, is the converse also true? We show that the answer to is negative. We provide three two-qubit states that are non-entangled, but such that any three-qubit state compatible with them is entangled. Entanglement can thus be deduced from the mere observation of separable reduced states. We extend this finding to correlations and provide local marginal correlations that are only compatible with global genuinely tripartite non-local correlations.
The Differential-Phase-Shift (DPS) and the Coherent-One-Way (COW) are among the most practical protocols for quantum cryptography, and are therefore the object of fast-paced experimental ...developments. The assessment of their security is also a challenge for theorists: the existing tools, that allow to prove security against the most general attacks, do not apply to these two protocols in any straightforward way. We present new upper bounds for their security in the limit of large distances (\(d \gtrsim 50\)km with typical values in optical fibers) by considering a large class of collective attacks, namely those in which the adversary attaches ancillary quantum systems to each pulse or to each pair of pulses. We introduce also two modified versions of the COW protocol, which may prove more robust than the original one.
The experimental violation of Bell inequalities using spacelike separated measurements precludes the explanation of quantum correlations through causal influences propagating at subluminal speed. ...Yet, any such experimental violation could always be explained in principle through models based on hidden influences propagating at a finite speed v>c, provided v is large enough. Here, we show that for any finite speed v with c<v<infinity, such models predict correlations that can be exploited for faster-than-light communication. This superluminal communication does not require access to any hidden physical quantities, but only the manipulation of measurement devices at the level of our present-day description of quantum experiments. Hence, assuming the impossibility of using nonlocal correlations for superluminal communication, we exclude any possible explanation of quantum correlations in terms of influences propagating at any finite speed. Our result uncovers a new aspect of the complex relationship between multipartite quantum nonlocality and the impossibility of signalling.
We present and analyze a gated single photon avalanche detector using a sine gating scheme with a simple but effective low-pass filtering technique for fast low-noise single photon detection at ...telecom wavelength. The detector is characterized by 130 ps short gates applied with a frequency of 1.25 GHz, yields only 70 ps timing jitter and noise probabilities as low as 7E-7 per gate at 10% detection efficiency. We show that the detector is suitable for high rate quantum key distribution (QKD) and even at room temperature it could allow for QKD over distances larger than 25 km.
