No abstract

Superconducting nanostrip photon detectors have been used as single-photon detectors, which can discriminate only photons’ presence or absence. It has recently been found that they can discriminate the number of photons by analyzing the output signal waveform, and they are expected to be used in various fields, especially in optical-quantum-information processing. Here, we improve the photon-number-resolving performance for light with a high-average photon number by pattern matching of the output signal waveform. Furthermore, we estimate the positive-operator-valued measure of the detector by a quantum detector tomography. The result shows that the device has photon-number-resolving performance up to five photons without any multiplexing or arraying, indicating that it is useful as a photon-number-resolving detector.

SummaryThe highly specific and complex connectivity between neurons is the hallmark of nervous systems, but techniques for identifying, imaging, and manipulating synaptically-connected networks of neurons are limited. Monosynaptic tracing, or the gated replication and spread of a deletion-mutant rabies virus to label neurons directly connected to a targeted population of starting neurons1, is the most widely-used technique for mapping neural circuitry, but the rapid cytotoxicity of first-generation rabies viral vectors has restricted its use almost entirely to anatomical applications. We recently introduced double-deletion-mutant second-generation rabies viral vectors, showing that they have little or no detectable toxicity and are efficient means of retrogradely targeting neurons projecting to an injection site2, but they have not previously been shown to be capable of gated replication in vivo, the basis of monosynaptic tracing. Here we present a complete second-generation system for labeling direct inputs to genetically-targeted neuronal populations with minimal toxicity, using double-deletion-mutant rabies viruses. Spread of the viruses requires complementation of both of the deleted viral genes in trans in the starting postsynaptic cells; suppressing the expression of these viral genes following an initial period of viral replication, using the Tet-Off system, reduces toxicity to the starting cells without decreasing the efficiency of viral spread. Using longitudinal two- photon imaging of live monosynaptic tracing in visual cortex, we found that 94.4% of all labeled cells, and an estimated 92.3% of starting cells, survived for the full twelve-week course of imaging. Two-photon imaging of calcium responses in labeled networks of neurons in vivo over ten weeks showed that labeled neurons’ visual response properties remained stable for as long as we followed them. This nontoxic labeling of inputs to genetically-targeted neurons in vivo is a long-held goal in neuroscience, with transformative applications including nonperturbative transcriptomic and epigenomic profiling, long-term functional imaging and behavioral studies, and optogenetic and chemogenetic manipulation of synaptically-connected neuronal networks over the lifetimes of experimental animals.

10A recent article in Cell reported a new form of modified rabies virus that was apparently capable 11 of labeling neurons "without adverse effects on neuronal physiology and circuit function". These 12 "self-inactivating" rabies ("SiR") viruses differed from the widely-used first-generation deletion-13 mutant (∆G) rabies viruses only by the addition of a destabilization domain to the viral 14 nucleoprotein. However, we observed that the transsynaptic tracing results from that article were 15 inconsistent with the logic described in it, and we hypothesized that the viruses used were actually 16 mutants that had lost the intended modification to the nucleoprotein. We obtained samples of two 17 SiR viruses from the authors and show here that, in both "SiR-CRE" and "SiR-FLPo", the great 18 majority of viral particles were indeed mutants that had lost the intended modification and were 19 therefore just first-generation, ∆G rabies viruses. We also found that SiR-CRE killed 70% of 20 infected neurons in vivo within two weeks. We have shown elsewhere that a ∆G rabies virus 21 encoding Cre can leave a large percentage of labeled neurons alive; we presume that Ciabatti et 22 al. found such remaining neurons at long survival times and mistakenly concluded that they had 23 developed a nontoxic version of rabies virus. Here we have analyzed only the two samples that 24 were sent to MIT by Ciabatti et al., and these may not be from the same batches that were used 25 for their paper. However, 1) both of the two viruses that we analyzed had independently lost the 26 intended modification, 2) the mutations in the two samples were genetically quite distinct from 27 each other yet in both cases caused the same result: total or near-total loss of the C-terminal 28 modification, and 3) the mutations that we found in these two virus samples perfectly explain the 29 otherwise-paradoxical transsynaptic tracing results from Ciabatti et al.'s paper. We suggest that 30 the SiR strategy, or any other such attempt to attenuate a virus by addition rather than deletion, 31 is an inherently unstable approach that can easily be evaded by mutation, as it was in this case. 33 39Its core principles are 1) the selective infection of the targeted neuron group with a recombinant 40 rabies virus with one deleted gene (which in all work published to date is the "G" gene encoding 41 its envelope glycoprotein), and 2) the in vivo complementation of the deletion by expression of 42 the deleted gene in trans in the targeted starting neurons. With all its gene products present in 43 the starting cells, the virus can fully replicate within them and spreads, as wild-type rabies virus 44 does, to the cells directly presynaptic to the initially infected neurons. Unlike wild-type rabies virus, 45 however, once inside the presynaptic cells, the deletion-mutant ("∆G", denoting the deletion of G) 46 is unable to produce its glycoprotein and is therefore unable to spread beyond these secondarily 47 infected cells, resulting in labeling of just the neurons in the in...