The main scientific achievement of this project is the establishment of space-based quantum memory systems as a solution to the problem of reaching global distances in quantum communication. Until our work, the only method that was thought to provide such long-distance coverage was quantum repeaters (QR) based on active error correction. These systems, however, require extensive hardware overhead: for a distance of 104 km the separation between individual network nodes should be around 1.5 - 2 km which would mean thousands of small scale quantum computers are needed with these scheme. Our work, on the other hand, demonstrated that a handful of satellites equipped with “passive” (i.e. no active error correction needed) QMs in low-earth orbit could provide coverage for such distances, thereby significantly reducing the required hardware. After publishing this proposal, we extended it to include more technical optimizations: such as optimal use of memory storage time by introducing a cut-off time after which the QM is flushed in case there is no heralding signal coming from the other stations. This would make sure that we only work with high quality qubits as we discard the qubits as they diphase in the memories, waiting for a heralding signal. Furthermore, we optimized the geometry of the constellation to maximize the achievable key rates.
Along these lines, we later modified our space-based QR architecture: our new proposal relies on a single satellite equipped with two QMs, one of which has hour-long storage capacity and the other being a shorter lived (~ms) one together with an entangled photon pair source. In this case the satellite moves across the globe in its orbit and the entangled photons are distributed between distant ground stations in a time-delayed quantum repeater fashion. This approach, that combines two different paradigms of physically moving qubits and quantum repeater behaviour, results in at least three orders of magnitude better key rates than the previous single-satellite proposals in that they only relied on a single memory thus did not take advantage of quantum repeater scaling. This architecture further reduces the required hardware overhead as it only needs a single satellite. On the experimental side, we anticipate that the near-term performance of quantum memories based on rare-earth ion doped crystals could reach the required capacity and storage time. These systems have shown to be capable of sustaining coherence for up to 6 hours and optical storage of up to 1 h with bright pulses has recently been demonstrated.
One of our other achievements is to propose a novel quantum memory protocol based on Bose-Einstein condensates (BEC) in microgravity. BEC based QMs suffer from several decoherence mechanisms, some which are magnetic and optical field inhomogeneities; different type of atomic collisions and spin-wave dephasing. Among these, atom-atom collisions have a rate that is proportional to the atomic density and thus cannot be mitigated in trapped configurations. In this work, we brought different paradigms together for the first time: by utilizing atom interferometry techniques in microgravity, our proposed protocol would surpass the performance of ground-based, i.e. trapped, BEC QMs by many orders of magnitude. This is achieved by writing in the input pulse to the internal atomic states when the atom is just released from the trap which was followed by the free expansion of the cloud. Due to microgravity, the expanded cloud would not just fall but rather float in the experimental chamber and would keep expanding. After a while an optical pulse is applied to the cloud to collimate it in time. The expanded cloud would result in vastly decreased atom-atom collision rate thereby reducing the memory dephasing. Another optical pulse is applied to focus the cloud size upon which the stored information is read out. We have shown that this technique is so powerful that the storage time will only be limited by the collisions with the background atoms, i.e. it is limited by the vacuum quality only.
The theory/conceptual work so far analysed QMs in the context of quantum communications and networking. The other motivation for deployment of quantum technologies in space is to utilize these systems to test ideas in “fundamental physics”, which includes search for dark matter and energy with atomic sensors; high precision tests of Lorenz invariance and probing the intersection between gravity and quantum mechanics. Among these, very large atomic or photonic interferometers have been proposed to be useful for the last point. However, macroscopic entanglement has to be created across such distances that gravity should have a significant effect. Photonic interferometers could indeed test these ideas but they require unrealistically tall interferometers. As part of QSPACE, we have shown that the required size of the interferometer goes down to experimentally feasible regimes when QMs are placed in interferometer arms to store the photons to be interfered. By doing so, this work became the first in the literature that proposes the use of QMs for applications other than quantum communications.
This project resulted in 8 articles - 4 published, 1 accepted and 3 submitted. The fellow gave contributed and invited talks in leading conferences and panels to further disseminate the results to the wider scientific community.
