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Author(s): C. Riek [1]; P. Sulzer [1]; M. Seeger [1]; A. S. Moskalenko [1]; G. Burkard [1]; D. V. Seletskiy [1]; A. Leitenstorfer (corresponding author) [1] Squeezed states [1, 2, [...]

We review our recent work towards quantum communication in a solid-state environment with qubits carried by electron spins. We propose three schemes to produce spin-entangled electrons, where the required separation of the partner electrons is achieved via Coulomb interaction. The non-product spin-states originate either from the Cooper pairs found in a superconductor, or in the ground state of a quantum dot with an even number of electrons. In a second stage, we show how spin-entanglement carried by a singlet can be detected in a beam-splitter geometry by an increased (bunching) or decreased (antibunching) noise signal. We also 'discuss how a local spin-orbit interaction can be used to provide a continuous modulation of the noise as a signature of entanglement. Finally, we review how one can use a quantum dot as a spin-filter, a spin-memory read-out, a probe for single-spin decoherence and, ultimately, a single-spin measurement apparatus. [ABSTRACT FROM AUTHOR]

Monolayer transition metal dichalcogenides (TMDs) hold great promise for future information processing applications utilizing a combination of electron spin and valley pseudospin. This unique spin system has led to observation of the valley Zeeman effect in neutral and charged excitonic resonances under applied magnetic fields. However, reported values of the trion valley Zeeman splitting remain highly inconsistent across studies. Here, we utilize high quality hBN encapsulated monolayer WSe2 to enable simultaneous measurement of both intervalley and intravalley trion photoluminescence. We find the valley Zeeman splitting of each trion state to be describable only by a combination of three distinct g-factors, one arising from the exciton-like valley Zeeman effect, the other two, trion specific, g-factors associated with recoil of the excess electron. This complex picture goes significantly beyond the valley Zeeman effect reported for neutral excitons, and eliminates the ambiguity surrounding the magneto-optical response of trions in tungsten based TMD monolayers. The unique valley and spin texture of atomically thin transition metal dichalcogenides (TMDs) allows the observation of the valley Zeeman effect for neutral and charged excitons. Here, the authors unveil the underlying physics of the magneto-optical response and valley Zeeman splitting of trions in tungsten-based TMDs. [ABSTRACT FROM AUTHOR]

Competing Interests: Competing interests statement:The author declares no competing interest.

Spin-active quantum emitters have emerged as a leading platform for quantum technologies. However, one of their major limitations is the large spread in optical emission frequencies, which typically extends over tens of GHz. Here, we investigate single V 4+ vanadium centres in 4H-SiC, which feature telecom-wavelength emission and a coherent S = 1/2 spin state. We perform spectroscopy on single emitters and report the observation of spin-dependent optical transitions, a key requirement for spin-photon interfaces. By engineering the isotopic composition of the SiC matrix, we reduce the inhomogeneous spectral distribution of different emitters down to 100 MHz, significantly smaller than any other single quantum emitter. Additionally, we tailor the dopant concentration to stabilise the telecom-wavelength V 4+ charge state, thereby extending its lifetime by at least two orders of magnitude. These results bolster the prospects for single V emitters in SiC as material nodes in scalable telecom quantum networks., (© 2023. The Author(s).)

Electron spins in silicon quantum dots are attractive systems for quantum computing owing to their long coherence times and the promise of rapid scaling of the number of dots in a system using semiconductor fabrication techniques. Although nearest-neighbour exchange coupling of two spins has been demonstrated, the interaction of spins via microwave-frequency photons could enable long-distance spin-spin coupling and connections between arbitrary pairs of qubits ('all-to-all' connectivity) in a spin-based quantum processor. Realizing coherent spin-photon coupling is challenging because of the small magnetic-dipole moment of a single spin, which limits magnetic-dipole coupling rates to less than 1 kilohertz. Here we demonstrate strong coupling between a single spin in silicon and a single microwave-frequency photon, with spin-photon coupling rates of more than 10 megahertz. The mechanism that enables the coherent spin-photon interactions is based on spin-charge hybridization in the presence of a magnetic-field gradient. In addition to spin-photon coupling, we demonstrate coherent control and dispersive readout of a single spin. These results open up a direct path to entangling single spins using microwave-frequency photons.

Human African trypanosomiasis (sleeping sickness) is a neglected tropical disease caused by Trypanosoma brucei spp. The parasites are transmitted by tsetse flies and adapt to their different hosts and environments by undergoing a series of developmental changes. During differentiation, the trypanosome alters its protein coat. Bloodstream form trypanosomes in humans have a coat of variant surface glycoprotein (VSG) that shields them from the immune system. The procyclic form, the first life-cycle stage to develop in the tsetse fly, replaces the VSG coat by procyclins; these proteins do not protect the parasite from lysis by serum components. Our study exploits the parasite-specific process of differentiation from bloodstream to procyclic forms to screen for potential drug candidates. Using transgenic trypanosomes with a reporter gene in a procyclin locus, we established a whole-cell assay for differentiation in a medium-throughput format. We screened 7,495 drug-like compounds and identified 28 hits that induced expression of the reporter and loss of VSG at concentrations in the low micromolar range. Small molecules that induce differentiation to procyclic forms could facilitate studies on the regulation of differentiation as well as serving as scaffolds for medicinal chemistry for new treatments for sleeping sickness.

Elucidating the mechanism of action of trypanocidal compounds is an important step in the development of more efficient drugs against Trypanosoma brucei. In a screening approach using an RNAi library in T. brucei bloodstream forms, we identified a member of the mitochondrial carrier family, TbMCP14, as a prime candidate mediating the action of a group of anti-parasitic choline analogs. Depletion of TbMCP14 by inducible RNAi in both bloodstream and procyclic forms increased resistance of parasites towards the compounds by 7-fold and 3-fold, respectively, compared to uninduced cells. In addition, down-regulation of TbMCP14 protected bloodstream form mitochondria from a drug-induced decrease in mitochondrial membrane potential. Conversely, over-expression of the carrier in procyclic forms increased parasite susceptibility more than 13-fold. Metabolomic analyses of parasites over-expressing TbMCP14 showed increased levels of the proline metabolite, pyrroline-5-carboxylate, suggesting a possible involvement of TbMCP14 in energy production. The generation of TbMCP14 knock-out parasites showed that the carrier is not essential for survival of T. brucei bloodstream forms, but reduced parasite proliferation under standard culture conditions. In contrast, depletion of TbMCP14 in procyclic forms resulted in growth arrest, followed by parasite death. The time point at which parasite proliferation stopped was dependent on the major energy source, i.e. glucose versus proline, in the culture medium. Together with our findings that proline-dependent ATP production in crude mitochondria from TbMCP14-depleted trypanosomes was reduced compared to control mitochondria, the study demonstrates that TbMCP14 is involved in energy production in T. brucei. Since TbMCP14 belongs to a trypanosomatid-specific clade of mitochondrial carrier family proteins showing very poor similarity to mitochondrial carriers of mammals, it may represent an interesting target for drug action or targeting.

Normal human serum (NHS) confers human resistance to infection by the parasite Trypanosoma brucei owing to the trypanolytic activity of apolipoprotein L1 (APOL1), present in two serum complexes termed Trypanolytic Factors (TLF-1 and -2). In order to identify parasite components involved in the intracellular trafficking and activity of TLFs, an inducible RNA interference (RNAi) genomic DNA library constructed in bloodstream form T. brucei was subjected to RNAi induction and selection for resistant parasites under NHS conditions favouring either TLF-1 or TLF-2 uptake. While TLF-1 conditions readily selected the haptoglobin-haemoglobin (HP-HB) surface receptor TbHpHbR as expected, given its known ability to bind TLF-1, under TLF-2 conditions no specific receptor for TLF-2 was identified. Instead, the screen allowed the identification of five distinct factors expected to be involved in the assembly of the vacuolar proton pump V-ATPase and consecutive endosomal acidification. These data confirm that lowering the pH during endocytosis is required for APOL1 toxic activity., (© 2014 John Wiley & Sons Ltd.)

The study of individual quantum systems in solids, for use as quantum bits (qubits) and probes of decoherence, requires protocols for their initialization, unitary manipulation, and readout. In many solid-state quantum systems, these operations rely on disparate techniques that can vary widely depending on the particular qubit structure. One such qubit, the nitrogen-vacancy (NV) center spin in diamond, can be initialized and read out through its special spin-selective intersystem crossing, while microwave electron spin resonance techniques provide unitary spin rotations. Instead, we demonstrate an alternative, fully optical approach to these control protocols in an NV center that does not rely on its intersystem crossing. By tuning an NV center to an excited-state spin anticrossing at cryogenic temperatures, we use coherent population trapping and stimulated Raman techniques to realize initialization, readout, and unitary manipulation of a single spin. Each of these techniques can be performed directly along any arbitrarily chosen quantum basis, removing the need for extra control steps to map the spin to and from a preferred basis. Combining these protocols, we perform measurements of the NV center's spin coherence, a demonstration of this full optical control. Consisting solely of optical pulses, these techniques enable control within a smaller footprint and within photonic networks. Likewise, this unified approach obviates the need for both electron spin resonance manipulation and spin addressability through the intersystem crossing. This method could therefore be applied to a wide range of potential solid-state qubits, including those which currently lack a means to be addressed.

Different life-cycle stages of Trypanosoma brucei are characterized by stage-specific glycoprotein coats. GPEET procyclin, the major surface protein of early procyclic (insect midgut) forms, is transcribed in the nucleolus by RNA polymerase I as part of a polycistronic precursor that is processed to monocistronic mRNAs. In culture, when differentiation to late procyclic forms is triggered by removal of glycerol, the precursor is still transcribed, but accumulation of GPEET mRNA is prevented by a glycerol-responsive element in the 3' UTR. A genome-wide RNAi screen for persistent expression of GPEET in glycerol-free medium identified a novel protein, NRG1 (Nucleolar Regulator of GPEET 1), as a negative regulator. NRG1 associates with GPEET mRNA and with several nucleolar proteins. These include two PUF proteins, TbPUF7 and TbPUF10, and BOP1, a protein required for rRNA processing in other organisms. RNAi against each of these components prolonged or even increased GPEET expression in the absence of glycerol as well as causing a significant reduction in 5.8S rRNA and its immediate precursor. These results indicate that components of a complex used for rRNA maturation can have an additional role in regulating mRNAs that originate in the nucleolus., (© 2013 John Wiley & Sons Ltd.)

The coat of Trypanosoma brucei consists mainly of glycosylphosphatidylinositol-anchored proteins that are present in several million copies and are characteristic of defined stages of the life cycle. While these major components of the coats of bloodstream forms and procyclic (insect midgut) forms are well characterised, very little is known about less abundant stage-regulated surface proteins and their roles in infection and transmission. By creating epitope-tagged versions of procyclic-specific surface antigen 2 (PSSA-2) we demonstrated that it is a membrane-spanning protein that is expressed by several different life cycle stages in tsetse flies, but not by parasites in the mammalian bloodstream. In common with other membrane-spanning proteins in T. brucei, PSSA-2 requires its cytoplasmic domain in order to exit the endoplasmic reticulum. Correct localisation of PSSA-2 requires phosphorylation of a cytoplasmic threonine residue (T(305)), a modification that depends on the presence of TbMAPK4. Mutation of T(305) to alanine (T(305)A) has no effect on the localisation of the protein in cells that express wild type PSSA-2. In contrast, this protein is largely intracellular when expressed in a null mutant background. A variant with a T(305)D mutation gives strong surface expression in both the wild type and null mutant, but slows growth of the cells, suggesting that it may function as a dominant negative mutant. The PSSA-2 null mutant exhibits no perceptible phenotype in culture and is fully competent at establishing midgut infections in tsetse, but is defective in colonising the salivary glands and the production of infectious metacyclic forms. Given the protein's structure and the effects of mutation of T(305) on proliferation and localisation, we postulate that PSSA-2 might sense and transmit signals that contribute to the parasite's decision to divide, differentiate or migrate.

WD (tryptophan-aspartic acid dipeptide)-repeat proteins play a central role in signal transduction cascades by co-ordinating the interaction of key signalling molecules. We identified a novel propeller-FYVE [domain identified in Fab1p, YOTB, Vac1p and EEA1 (early endosome antigen 1)] protein, ProF, which is expressed in various cell lines and tissues and consists of seven WD-repeats and a FYVE domain. WD-repeat proteins offer a platform for protein-protein interactions by folding into a seven-bladed propeller-like structure, while the FYVE domain binds to phosphatidylinositol 3-phosphate present mainly on intracellular membranes. The ProF protein partially co-localizes with EEA1 on vesicular structures and binds to the protein kinases Akt and PKCzeta/lambda (protein kinase Czeta/lambda) via its WD-repeat propeller. ProF interacts more strongly with the kinases after hormonal stimulation. Endogenously expressed ProF and the two kinases interact in brain and in the preadipocyte cell line 3T3-L1, suggesting a role in secretory vesicular processes. In summary, we describe a new binding partner for kinases, located on vesicular structures in specialized cells, which may play a role for the spatial organization of signalling cascades.

African trypanosomes undergo differentiation in order to adapt to the mammalian host and the tsetse fly vector. To characterize the role of a mitogen-activated protein (MAP) kinase homologue, TbMAPK5, in the differentiation of Trypanosoma brucei, we constructed a knockout in procyclic (insect) forms from a differentiation-competent (pleomorphic) stock. Two independent knockout clones proliferated normally in culture and were not essential for other life cycle stages in the fly. They were also able to infect immunosuppressed mice, but the peak parasitemia was 16-fold lower than that of the wild type. Differentiation of the proliferating long slender to the nonproliferating short stumpy bloodstream form is triggered by an autocrine factor, stumpy induction factor (SIF). The knockout differentiated prematurely in mice and in culture, suggestive of increased sensitivity to SIF. In contrast, a null mutant of a cell line refractory to SIF was able to proliferate normally. The differentiation phenotype was partially rescued by complementation with wild-type TbMAPK5 but exacerbated by introduction of a nonactivatable mutant form. Our results indicate a regulatory function for TbMAPK5 in the differentiation of bloodstream forms of T. brucei that might be exploitable as a target for chemotherapy against human sleeping sickness.

The role of the two processing sites in the precursor of the small subunit (SS) of ribulose-1,5-bisphosphate carboxylase/oxygenase (pSS) of Chlamydomonas reinhardtii was studied by introducing mutations at the cleavage sites for the stromal processing peptidases SPP-1 and SPP-2, which hydrolyse wild-type pSS (20.6 kDa) to an intermediate-sized product iSS (18.3 kDa) and to the mature SS (16.3 kDa), respectively. The mutations introduced into cDNA resulted in exchange of (a) two amino acids flanking processing site 1, or (b) one or (c) both amino acids flanking processing site 2. Mutation (a) prevented pSS from being processed at site 1 but not from cleavage at site 2. Mutation (c) abolished the action of SPP-2 but not SPP-1. When pSS with mutation (c) was imported into isolated chloroplasts, iSS accumulated while SS formation was abolished. However, mature SS was produced even in the absence of iSS synthesis (mutation a). Import of pSS bearing mutation (b), which only partially inhibited processing at the SPP-2 site, slowed the rate of SS formation down whereas iSS and some slightly smaller derivatives accumulated. These experiments suggested that in Chlamydomonas processing of pSS can occur in two steps, whereby the first step is facultative. The same three mutations were studied in vivo after transformation of SS-deficient C. reinhardtii T60-3 with mutated genomic DNA. Growth and photosynthesis was as in control transformants, except for the slower-growing transformants (mutation c) where no mature SS was immuno-detected. However, pSS fragments with molecular masses between those of iSS and SS were present even in the ribulose-1,5-bisphosphate carboxylase/oxygenase holoenzyme.

When bean plants (Phaseolus vulgaris var. Saxa) are treated with mercuric chloride or infected with alfalfa mosaic virus, they produce pathogenesis-related (PR) proteins. We report here that functional mRNA encoding bean PR4 protein is only present when synthesis of this protein has been induced. Treatment with mercuric chloride results in a rapid induction of functional bean PR4 mRNA (within 2-3 h), whereas in virus-infected plants this mRNA can only be detected the second day following the infection. Bean PR4 protein is synthesized in vitro, using this mRNA in a rabbit reticulocyte lysate system, as a precursor of 35 kDa. This precursor can be processed into a polypeptide having the same molecular mass (33.5 kDa) as the in vivo PR4 protein by the addition to the cell-free translation system of canine pancreatic microsomal membranes.

Construction of a physical map of the chloroplast DNA from Phaseolus vulgaris showed that this circular molecule is segmentally organized into four regions. Unlike other chloroplast DNAs which have analogous organization, two single-copy regions that separate two inverted repeats have been demonstrated to exist in both relative orientations, giving rise to two populations of DNA molecules. Hybridization studies using individual rRNA and tRNA species revealed the location of a set of rRNA genes and at least seven tRNA genes in each inverted repeat region, a minimum of 17 tRNA genes in the large single-copy region and one tRNA gene in the small single-copy region. The tRNA genes code for 24 tRNA species corresponding to 16 amino acids. Comparison of this gene map with those of other chloroplast DNAs suggests that DNA sequence rearrangements, involving some tRNA genes, have occurred.