Quantum key distribution (QKD) is a family of protocols for growing a private encryption key between two parties. Despite much progress, all ground-based QKD approaches have a distance limit due to atmospheric losses or in-fibre attenuation. These limitations make purely ground-based systems impractical for a global distribution network. However, the range of communication may be extended by employing satellites equipped with high-quality optical links. This manuscript summarizes research and development which is beginning to enable QKD with satellites. It includes a discussion of protocols, infrastructure, and the technical challenges involved with implementing such systems, as well as a top level summary of on-going satellite QKD initiatives around the world.

We demonstrate and characterize interference between discrete photons emitted by two separate semiconductor quantum dot states in different samples excited by a pulsed laser. Their energies are tuned into resonance using strain. The photons have a total coalescence probability of 18.1% and the coincidence rate is below the classical limit. Postselection of coincidences within a narrow time window increases the coalescence probability to 47%. The probabilities are reduced from unity because of dephasing and the postselection value is also reduced by the detector time response.

Quantum theory predicts and experiments confirm that nature can produce correlations between distant events that are nonlocal in the sense of violating a Bell inequality 1 . Nevertheless, Bell's strong sentence 'Correlations cry out for explanations' (ref. 2) remains relevant. The maturing of quantum information science and the discovery of the power of non-local correlations, for example for cryptographic key distribution beyond the standard quantum key distribution schemes 3-5 , strengthen Bell's wish and make it even more timely. In 2003, Leggett proposed an alternative model for non-local correlations 6 that he proved to be incompatible with quantum predictions. We present here a new approach to this model, along with new inequalities for testing it. These inequalities can be derived in a very simple way, assuming only the non-negativity of probability distributions; they are also stronger than previously published and experimentally tested Leggett-type inequalities 6-9 . The simplest of the new inequalities is experimentally violated. Then we go beyond Leggett's model, and show that we cannot ascribe even partially defined individual properties to the components of a maximally entangled pair.Formally, a correlation is a conditional probability distribution P(α, β|a, b), where α, β are the outcomes observed by two partners, Alice and Bob, when they make measurements labelled by a and b, respectively. On the abstract level, a and b are merely inputs, freely and independently chosen by Alice and Bob. On a more physical level, Alice and Bob hold two subsystems of a quantum state; in the simple case of qubits, the inputs are naturally characterized by vectors on the Poincaré sphere, hence the notation a,b.How should we understand non-local correlations, in particular those corresponding to entangled quantum states? A natural approach consists in decomposing P(α, β|a, b) into a statistical mixture of hopefully simpler correlations:Bell's locality assumption is P l (α, β|a, b) = P A l (α|a)P B l (β|b), admittedly the simplest choice, but an inadequate one as it turns out: quantum correlations violate Bell's locality 1 . Setting out to explore other choices, it is natural to require first that the P l fulfil the so-called no-signalling condition, that is, that none of the correlations P l results from a communication between Alice and Bob. This can be guaranteed by ensuring spacelike separation between Alice and Bob. Non-signalling correlations happen without any time ordering: there is not a first event, let us say on Alice's side, that causes the second event via some spooky action at a distance. We may phrase it differently: nonsignalling correlations happen from outside space-time, in the sense that there is no story in space-time that tells us how they happen. This is the case in orthodox quantum physics, or in some illuminating toy models such as the non-local box of Popescu and Rohrlich (PR box) 10 . Mathematically, the no-signalling condition reads P l (α|a,b) = P l (α|a) and P l (β|a,b) = P l (β|b): Alice'...