This paper describes work I mentioned earlier. We successfully demonstrated quantum key distribution with signals transmitted from a ground station to a receiver on board a flying airplane. Our receiver (which is significantly upgraded in comparison to our prior truck demonstration) was designed and largely custom-built to have a clear path to flight on a satellite. Our demonstration illustrates the viability of such a payload.

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We take our quantum key distribution system out of the laboratory and mount it in the back of a small truck. Integrating a two-axis pointing system at both sites, polarization correction, and time-of-flight compensation, we demonstrate quantum key distribution from a stationary transmitter to a receiver moving at an angular speed (relative to the transmitter) equivalent to the maximum angular speed of a typical low-Earth-orbit satellite.

Bonus: Read the IQC's news release, which covers both this and the previous paper for a general audience.

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Fundamental laws of quantum physics guarantee the security of encryption keys generated through quantum key distribution, in contrast to standard encryption techniques which rely on assumptions about an eavesdropper's computational ability. That said, special technology is necessary to facilitate quantum key distribution transmissions between parties that are more than a couple of hundred kilometers apart.

A near-term solution is to use an orbiting satellite as a trusted quantum receiver. Here we detail specifically chosen algorithms that make up an implementation of quantum key distribution, suitable for a satellite receiver platform. We examine these algorithms' computational requirements while demonstrating them experimentally as we emulate the variable channel losses that would be experienced during a satellite pass (following those we published about previously).

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It's well known that quantum entangled systems can exhibit correlations that go beyond those that could be seen if Nature worked in intuitive “classical” ways. However, as Richard Feynman noted, classical theory can support exhibiting such correlations if we invoke negative probabilities to describe their properties. What he did not do was specify how these negative probabilities ought to be chosen, and without any justification, an infinite number of different combinations could be chosen that will satisfy the relevant equations.

The concept of negative probabilities seems nonsensical because they cannot actually be observed—indeed, they cannot be observed even within the framework of quantum theory due to the effects of measurement back-action. Here, we show how they can instead be inferred through the use of weak measurements, where a meter is only weakly coupled to the property of interest, thereby avoiding the back-action problem. Each individual weak measurement has a high uncertainty, but by measuring many instances of a larger ensemble, an average can be found that implies a specific set of anomalous (i.e. beyond 0–1) probabilities. With an experimental demonstration, we thus give an empirically justified method for choosing the anomalous probabilities that allow the classical model [...]

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Quantum mechanics implies properties of Nature that clash with our intuitive notions of how the universe ought to work. Testing these properties (to see if quantum mechanics is, indeed, true) involves generating entangled quantum states of two or more particles and measuring them under a number of strict conditions. While work is progressing to meet all of these conditions when using only two particles, no one has yet met even one of these conditions for more than two particles, which is considerably more difficult experimentally. Here, we conduct an experiment where we meet two of the most challenging conditions—namely measurement locality and freedom of choice—while generating triplet entangled photon states. We demonstrate that quantum mechanics wins out over intuition, measuring a violation of Mermin's inequality outside the classical bound by nine standard deviations.

Also check out the News and Views article in the same issue, written [...]

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Generating entangled photon states is vital for numerous quantum communications and quantum computation primitives. Here we pioneer a new approach to in-fiber generation of entangled photon pairs. We take inspiration from a technique in bulk-optics, where two nonlinear crystals are sandwiched close together, and splice two pieces of birefringent optical fiber together at 90 degree orientation. With suitable compensation optics, all of which could be implemented in fiber, we show fidelity with a maximally-entangled Bell state of better than 92%.

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