We revisit the problem of using a mechanical resonator to perform the transfer of a quantum state between two electromagnetic cavities (e.g., optical and microwave). We show that this system possesses an effective mechanically dark mode which is immune to mechanical dissipation; utilizing this feature allows highly efficient transfer of intracavity states, as well as of itinerant photon states. We provide simple analytic expressions for the fidelity for transferring both gaussian and non-gaussian states.

We show how strong steady-state entanglement can be achieved in a three-mode optomechanical system (or other parametrically-coupled bosonic system) by effectively laser-cooling a delocalized Bogoliubov mode. This approach allows one to surpass the bound on the maximum stationary intracavity entanglement possible with a coherent two-mode squeezing interaction. In particular, we find that optimizing the relative ratio of optomechanical couplings, rather than simply increasing their magnitudes, is essential for achieving strong entanglement. Unlike typical dissipative entanglement schemes, our results cannot be described by treating the effects of the entangling reservoir via a Linblad master equation. Introduction-The study of highly entangled quantum states is of interest both for fundamental reasons and for a myriad of applications to quantum information processing and quantum communication. Of particular fundamental interest is the possibility to entangle distinct macroscopic objects, a task made difficult by the unavoidable decoherence and dissipation associated with such systems. Equally interesting would be the ability to entangle photons of very different frequencies, e.g. microwave and optical photons.

In a recent publication (Wang and Clerk 2012 Phys. Rev. Lett. 108 153603), we demonstrated that one can use interference to significantly increase the fidelity of state transfer between two electromagnetic cavities coupled to a common mechanical resonator over a naive sequential-transfer scheme based on two swap operations. This involved making use of a delocalized electromagnetic mode which is decoupled from the mechanical resonator, a so-called 'mechanically dark' mode. Here, we demonstrate the existence of a new 'hybrid' state transfer scheme that incorporates the best elements of the dark-mode scheme (protection against mechanical dissipation) and the doubleswap scheme (fast operation time). Importantly, this new scheme also does not require the mechanical resonator to be prepared initially in its ground state. We also provide additional details of the previously described interferenceenhanced transfer schemes, and provide an enhanced discussion of how the interference physics here is intimately related to the optomechanical analogue of electromagnetically induced transparency. We also compare the various transfer schemes over a wide range of relevant experimental parameters, producing a 'phase diagram' showing the optimal transfer scheme for different points in parameter space.

We present an ultrafast feasible scheme for ground state cooling of a mechanical resonator via repeated random time-interval measurements on an auxiliary flux qubit. We find that the ground state cooling can be achieved with several such measurements. The cooling efficiency hardly depends on the time-intervals between any two consecutive measurements. The scheme is also robust against environmental noises.

We propose a quantum description of the cooling of a micromechanical flexural oscillator by a one-dimensional transmission line resonator via a force that resembles cavity radiation pressure. The mechanical oscillator is capacitively coupled to the central conductor of the transmission line resonator. At the optimal point, the micromechanical oscillator can be cooled close to the ground state, and the cooling can be measured by homodyne detection of the output microwave signal.

Antibody-dependent enhancement (ADE) has been reported in several virus infections including dengue fever virus, severe acute respiratory syndrome (SARS) and Middle East respiratory syndrome (MERS) coronavirus infection. To study whether ADE is involved in COVID-19 infections, in vitro pseudotyped SARS-CoV-2 entry into Raji cells, K562 cells, and primary B cells mediated by plasma from recovered COVID-19 patients were employed as models. The enhancement of SARS-CoV-2 entry into cells was more commonly detected in plasma from severely-affected elderly patients with high titers of SARS-CoV-2 spike protein-specific antibodies. Cellular entry was mediated via the engagement of FcγRII receptor through virus-cell membrane fusion, but not by endocytosis. Peptide array scanning analyses showed that antibodies which promote SARS-CoV-2 infection targeted the variable regions of the RBD domain. To further characterize the association between the spike-specific antibody and ADE, an RBD-specific monoclonal antibody (7F3) was isolated from a recovered patient, which potently inhibited SARS-Cov-2 infection of ACE-2 expressing cells and also mediated ADE in Raji cells. Site-directed mutagenesis the spike RBD domain reduced the neutralization activity of 7F3, but did not abolish its binding to the RBD domain. Structural analysis using cryo-electron microscopy (Cryo-EM) revealed that 7F3 binds to spike proteins at a shift-angled pattern with one up and two down RBDs, resulting in partial overlapping with the receptor binding motif (RBM), while a neutralizing monoclonal antibody that lacked ADE activity binds to spike proteins with three up RBDs, resulting in complete overlapping with RBM. Our results revealed that ADE mediated by SARS-CoV-2 spike-specific antibodies could result from binding to the receptor in slightly different pattern from antibodies mediating neutralizations. Studies on ADE using antibodies from recovered patients via cell biology and structural biology technology could be of use for developing novel therapeutic and preventive measures for control of COVID-19 infection.

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We provide analytic insight into the generation of stationary itinerant photon entanglement in a 3-mode optomechanical system. We identify the parameter regime of maximal entanglement, and show that strong entanglement is possible even for weak many-photon optomechanical couplings. We also show that strong tripartite entanglement is generated between the photonic and phononic output fields; unlike the bipartite photon-photon entanglement, this tripartite entanglement diverges as one approaches the boundary of system stability.PACS numbers: 42.50. Wk, 42.50.Ex, 07.10.Cm Entanglement is one of the most fascinating and potentially useful aspects of quantum systems. Of particular interest is the generation of entangled itinerant quanta (which can be easily spatially separated), and of true multipartite entanglement (involving irreducible correlations between three or more subsystems). These goals have been the subject of considerable theoretical and experimental work, in a variety of systems spanning quantum optics setups [1, 2], cold atoms [3], superconducting circuits [4-6] and spin qubits [7]. Optomechanical systems [8], where mechanical motion interacts with electromagnetic fields, could be another powerful platform to realize these goals. A key advantage here is the potential to use mechanical motion to entangle disparate subsystems (e.g. microwave and optical photons). A number of schemes to generate entangled photons in optomechanics have been studied theoretically [9][10][11][12][13][14][15]. Recent experiments have also demonstrated mechanically-mediated entanglement between two microwave pulses [16].Here, we analyze theoretically both itinerant and multipartite entanglement in a 3-mode optomechanical system where two cavities are coupled to a single mode of a mechanical resonator (see inset of Fig. 1). This setup has been realized in several recent experiments [17][18][19]. Previous theory work examined bipartite output entanglement in this system largely numerically [11,[13][14][15] , focusing on experimentally-challenging strong-coupling regimes [13,14] or on transient regimes [15]. In contrast, we focus here on generating stationary output entanglement with weak many-photon optomechanical couplings. We provide a complete yet simple analytic understanding of the physics. This allows us to illustrate the trade-off between large entanglement and thermal resilience, as well as to identify the parameter regime of maximum entanglement, a regime which corresponds to a simple matching of optomechanical cooperativities. Surprisingly, this condition coincides with the least favorable regime for the generation of intra-cavity entanglement. We also show that entanglement is optimal be-