Rotating packed beds have received considerable attention as a means of process intensification for gas−liquid mass transfer over the past 2 decades. In this work, we take a critical view of the developments in understanding the transport processes in rotating packed beds. The intensification achieved so far falls short of the goal of 2−3 orders of magnitude volume reduction compared to that obtained conventional columns. The directions toward achieving this goal are outlined.

§ These authors contributed equally to this work Building a quantum repeater network for long distance quantum communication requires photons and quantum registers that comprise qubits for interaction with light, good memory capabilities and processing qubits for storage and manipulation of photons. Here we demonstrate a key step, the coherent transfer of a photon in a single solid-state nuclear spin qubit with an average fidelity of 98% and storage over 10 seconds. The storage process is achieved by coherently transferring a photon to an entangled electron-nuclear spin state of a nitrogen vacancy centre in diamond, confirmed by heralding through high fidelity single-shot readout of the electronic spin states. Stored photon states are robust against repetitive optical writing operations, required for repeater nodes. The photon-electron spin interface and the nuclear spin memory demonstrated here constitutes a major step towards practical quantum networks, and surprisingly also paves the way towards a novel entangled photon source for photonic quantum computing.A quantum repeater network is intended to distribute entanglement between distant nodes realizing an elementary quantum network [1]. Building up such a network requires photon sources (single or entangled pairs), processing nodes with the ability to make (i) optical or spin Bell-state measurements, (ii) long coherence times and (iii) ability for entanglement purification or quantumerror correction [2,3] . With such strong requirements it is hard to find physical systems meeting all of the above criteria. In this regard ensembles of atomic gases, trapped ions and solid state systems are intensively studied [1,2,[4][5][6][7]. While e.g., atomic systems provide high interaction efficiency with photons, rare earth systems on the other hand show long coherence times, all are required for processing the absorbed/emitted photons. As opposed to ensembles, single particles though typically have a significantly less interaction efficiency with photons, however are useful for quantum networks due to their ability for in situ information processing [8][9][10][11], like entanglement purification [12,13].For this reason solid state devices with well controllable spins are recently proposed to be promising candidates for quantum repeater networks [15,16]. The nitrogenvacancy (NV) defect centre in diamond does show significant potential in this respect. It provides a hybrid spin system in which electron spins are used for fast [17], high-fidelity control [18] and readout [19,20], and nuclear spins which are well-isolated from their environment yielding ultra-long coherence time [21]. Electron and nuclear spins could form a small-scale quantum register allowing for e.g. necessary high-fidelity quantum error correction. Furthermore, the NV electron spin can be entangled with an emitted optical photon [22,23]. Quantum entanglement and quantum teleportation between two remote NV centres have already been experimentally demonstrated [24,25]. A further and significant step towa...

The liquid-side mass transfer rate in a centrifugal gas-liquid contactor has been reported to be several times higher than that in conventional packed beds. However, no direct measurement of the gas-side mass transfer coefficient has been reported. We present experimental studies on gas-side mass transfer in a rotating packed bed with wire-gauze packing. Contrary to expectations, the gas-side mass transfer coefficient was much lower than that in conventional packed columns. An analysis of the gas flow, based on the equations of motion, revealed that the gas undergoes solid-body-like rotation in the rotor because of the drag offered by the packing. Therefore, the mass transfer coefficient should be in the same range as that in conventional packed columns. The lower value of the coefficient found experimentally is attributed to liquid maldistribution. The possibility of enhancing the mass transfer coefficient by enhancing the slip between the gas and the packing was explored by using a stack of closely spaced disks as the packing. Mass transfer studies with a pair of disks were conducted. Higher throughputs and mass transfer rates than those with the wire-gauze packing were obtained. A stack of disks as packing appears to hold promise.

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