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The quantization of gravity is widely believed to result in gravitons - particles of discrete energy that form gravitational waves. But their detection has so far been considered impossible. Here we show that signatures of single graviton exchange can be observed in laboratory experiments. We show that stimulated and spontaneous single-graviton processes can become relevant for massive quantum acoustic resonators and that stimulated absorption can be resolved through continuous sensing of quantum jumps. We analyze the feasibility of observing the exchange of single energy quanta between matter and gravitational waves. Our results show that single graviton signatures are within reach of experiments. In analogy to the discovery of the photo-electric effect for photons, such signatures can provide the first experimental clue of the quantization of gravity., (© 2024. The Author(s).)

Since 2015 the gravitational-wave observations of LIGO and Virgo have transformed our understanding of compact-object binaries. In the years to come, ground-based gravitational-wave observatories such as LIGO, Virgo, and their successors will increase in sensitivity, discovering thousands of stellar-mass binaries. In the 2030s, the space-based LISA will provide gravitational-wave observations of massive black holes binaries. Between the ∼ 10 -10 3 Hz band of ground-based observatories and the ∼ 1 0 - 4 -10 - 1 Hz band of LISA lies the uncharted decihertz gravitational-wave band. We propose a Decihertz Observatory to study this frequency range, and to complement observations made by other detectors. Decihertz observatories are well suited to observation of intermediate-mass ( ∼ 1 0 2 -10 4 M ⊙ ) black holes; they will be able to detect stellar-mass binaries days to years before they merge, providing early warning of nearby binary neutron star mergers and measurements of the eccentricity of binary black holes, and they will enable new tests of general relativity and the Standard Model of particle physics. Here we summarise how a Decihertz Observatory could provide unique insights into how black holes form and evolve across cosmic time, improve prospects for both multimessenger astronomy and multiband gravitational-wave astronomy, and enable new probes of gravity, particle physics and cosmology., (© The Author(s) 2021.)

Time has a fundamentally different character in quantum mechanics and in general relativity. In quantum theory events unfold in a fixed order while in general relativity temporal order is influenced by the distribution of matter. When matter requires a quantum description, temporal order is expected to become non-classical-a scenario beyond the scope of current theories. Here we provide a direct description of such a scenario. We consider a thought experiment with a massive body in a spatial superposition and show how it leads to entanglement of temporal orders between time-like events. This entanglement enables accomplishing a task, violation of a Bell inequality, that is impossible under local classical temporal order; it means that temporal order cannot be described by any pre-defined local variables. A classical notion of a causal structure is therefore untenable in any framework compatible with the basic principles of quantum mechanics and classical general relativity.

Current attempts to probe general relativistic effects in quantum mechanics focus on precision measurements of phase shifts in matter-wave interferometry. Yet, phase shifts can always be explained as arising because of an Aharonov-Bohm effect, where a particle in a flat space-time is subject to an effective potential. Here we propose a quantum effect that cannot be explained without the general relativistic notion of proper time. We consider interference of a 'clock'-a particle with evolving internal degrees of freedom-that will not only display a phase shift, but also reduce the visibility of the interference pattern. According to general relativity, proper time flows at different rates in different regions of space-time. Therefore, because of quantum complementarity, the visibility will drop to the extent to which the path information becomes available from reading out the proper time from the 'clock'. Such a gravitationally induced decoherence would provide the first test of the genuine general relativistic notion of proper time in quantum mechanics., (© 2011 Macmillan Publishers Limited. All rights reserved.)

Binding of the platelet GPIb/V/IX (glycoprotein Ib/V/IX) receptor to von Willebrand factor is critical for platelet adhesion and aggregation under conditions of rapid blood flow. The adhesive function of GPIbalpha is regulated by its anchorage to the membrane skeleton through a specific interaction with filamin A. In the present study, we examined the amino acid residues within the cytoplasmic tail of GPIbalpha, which are critical for association with filamin A, using a series of 25-mer synthetic peptides that mimic the cytoplasmic tail sequences of wild-type and mutant forms of GPIbalpha. Peptide binding studies of purified human filamin A have demonstrated a major role for the conserved hydrophobic stretch L567FLWV571 in mediating this interaction. Progressive alanine substitutions of triple, double and single amino acid residues within the Pro561-Arg572 region suggested an important role for Trp570 and Phe568 in promoting GPIbalpha binding to filamin A. The importance of these two residues in promoting filamin A binding to GPIbalpha in vivo was confirmed from the study of Chinese-hamster ovary cells expressing GPIbalpha Trp570-->Ala and Phe568-->Ala substitutions. Phenotypic analysis of these cell lines in flow-based adhesion studies revealed a critical role for these residues in maintaining receptor anchorage to the membrane skeleton and in maintaining cell adhesion to a von Willebrand factor matrix under high-shear conditions. These studies demonstrate a novel filamin A binding motif in the cytoplasmic tail of GPIbalpha, which is critically dependent on both Trp570 and Phe568.

The glycoprotein Ib-V-IX (GPIb-V-IX) complex interacts with subendothelial von Willebrand factor (VWF) to ensure recruitment of platelets at sites of vascular injury, a process that culminates in integrin alpha(IIb)beta(3)-dependent stable adhesion and spreading. Interaction of the 14-3-3zeta adaptor protein with the C-terminal 606-610 phosphoserine motif of the GPIbalpha subunit has been implicated in the control of alpha(IIb)beta(3) activation and cell spreading. In this study, we have examined potentially novel 14-3-3zeta binding sites by expressing mutant forms of GPIbalpha in Chinese-hamster-ovary (CHO) cells. Analysis of a series of neighboring 11-12 residue deletions identified a critical role for the 580-LVAGRRPSALS-590 sequence in promoting GPIbalpha-14-3-3zeta interaction. Development of a phosphospecific antibody demonstrated high levels of phosphorylation of the Ser587 and Ser590 residues in resting platelets (which became dephosphorylated during platelet spreading on VWF), and peptides containing these phosphorylated residues effectively displaced 14-3-3zeta from GPIbalpha. Analysis of single and double alanine substitutions of Ser587 and Ser590 demonstrated a major role for these residues in promoting GPIbalpha-14-3-3zeta binding. Moreover, these cell lines exhibited a defect in cell spreading on immobilized VWF. These studies demonstrate the existence of a second major 14-3-3zeta binding site within the cytoplasmic tail of GPIbalpha that has an important functional role in regulating integrin-dependent cell spreading.

Glycoprotein (GP) Ib/V/IX complex-dependent platelet adhesion to von Willebrand factor (VWF) is supported by the 45-kd N-terminal extracellular domain of the GPIb alpha subunit. Recent results with an adhesion blocking antibody (RAM.1) against GPIb beta, which is disulfide linked to GPIb alpha, have suggested a novel function of this subunit in regulating VWF-mediated platelet adhesion, possibly involving its intracellular face. A putative cooperation between the GPIb alpha and GPIb beta cytoplasmic domains was investigated by measuring the adhesion under flow to immobilized VWF of K562 and Chinese hamster ovary (CHO) cells transfected with GPIb/(V)/IX containing mutations in this region. Adhesion of cells carrying a glycine substitution of the GPIb beta Ser166 phosphorylation site was 50% lower than normal and became insensitive to inhibition by RAM.1. In contrast, forskolin or PGE(1) treatment increased both the phosphorylation of GPIb beta and adhesion of control cells, both effects being reversed by RAM.1, but had no influence on cells expressing the Ser166Gly mutation. A role of the GPIb alpha intracellular domain was also apparent as the VWF-dependent adhesion of cells containing deletions of the entire (Delta 518-610) or portions (Delta 535-568, Delta 569-610) of the GPIb alpha cytoplasmic tail was insensitive to RAM.1 inhibition. Cells carrying progressive 11 amino acid deletions spanning the GPIb alpha 535-590 region were equally unresponsive to RAM.1, with the exception of those containing GPIb alpha Delta 569-579, which behaved like control cells. These findings support a role of the GPIb beta intracellular domain in controlling the adhesive properties of the GPIb/V/IX complex through phosphorylation of GPIb beta Ser166 and point to the existence of cross-talk between the GPIb beta and GPIb alpha intracellular domains.

The interaction of the glycoprotein (GP) Ib-V-IX receptor complex with the membrane skeleton of platelets is dependent on a specific interaction between the cytoplasmic tail of GPIbalpha and filamin-1. This interaction has been proposed to regulate key aspects of platelet function, including the ligand binding of GPIb-V-IX and the ability of the cells to sustain adhesion to von Willebrand factor (vWf) under high shear. In this study we have examined sequences in the GPIbalpha intracellular domain necessary for interaction of the receptor with filamin-1. We have identified two adjacent sequences involving amino acids 557-568 and 569-579 of the GPIbalpha cytoplasmic domain that are critical for normal association between the receptor complex and filamin-1. Under flow conditions, Chinese hamster ovary (CHO) cells expressing these two mutant receptors exhibited an increase in translocation velocity that was associated with increased cell detachment from the vWf matrix at high shear. The shear-dependent acceleration in velocity of mutant Delta557-568 and Delta569-579 CHO cells was associated with a critical defect in receptor anchorage, evident from significant extraction of GPIb-IX from the CHO cell membrane at high shear. These studies define a critical role for amino acids within the 557-579 sequence of GPIbalpha for interaction with filamin-1.