484 research outputs found
Catastrophic failure of quantum annealing owing to non-stoquastic Hamiltonian and its avoidance by decoherence
Quantum annealing (QA) is a promising method for solving combinatorial
optimization problems whose solutions are embedded into a ground state of the
Ising Hamiltonian. This method employs two types of Hamiltonians: a driver
Hamiltonian and a problem Hamiltonian. After a sufficiently slow change from
the driver Hamiltonian to the problem Hamiltonian, we can obtain the target
ground state that corresponds to the solution. The inclusion of non-stoquastic
terms in the driver Hamiltonian is believed to enhance the efficiency of the
QA. Meanwhile, decoherence is regarded as of the main obstacles for QA. Here,
we present examples showing that non-stoaquastic Hamiltonians can lead to
catastrophic failure of QA, whereas a certain decoherence process can be used
to avoid such failure. More specifically, when we include anti-ferromagnetic
interactions (i.e., typical non-stoquastic terms) in the Hamiltonian, we are
unable to prepare the target ground state even with an infinitely long
annealing time for some specific cases. In our example, owing to a symmetry,
the Hamiltonian is block-diagonalized, and a crossing occurs during the QA,
which leads to a complete failure of the ground-state search. Moreover, we show
that, when we add a certain type of decoherence, we can obtain the ground state
after QA for these cases. This is because, even when symmetry exists in
isolated quantum systems, the environment breaks the symmetry. Our counter
intuitive results provide a deep insight into the fundamental mechanism of QA
The peroxisomal membrane protein import receptor Pex3p is directly transported to peroxisomes by a novel Pex19p- and Pex16p-dependent pathway
Two distinct pathways have recently been proposed for the import of peroxisomal membrane proteins (PMPs): a Pex19p- and Pex3p-dependent class I pathway and a Pex19p- and Pex3p-independent class II pathway. We show here that Pex19p plays an essential role as the chaperone for full-length Pex3p in the cytosol. Pex19p forms a soluble complex with newly synthesized Pex3p in the cytosol and directly translocates it to peroxisomes. Knockdown of Pex19p inhibits peroxisomal targeting of newly synthesized full-length Pex3p and results in failure of the peroxisomal localization of Pex3p. Moreover, we demonstrate that Pex16p functions as the Pex3p-docking site and serves as the peroxisomal membrane receptor that is specific to the Pex3p–Pex19p complexes. Based on these novel findings, we suggest a model for the import of PMPs that provides new insights into the molecular mechanisms underlying the biogenesis of peroxisomes and its regulation involving Pex3p, Pex19p, and Pex16p
Quantum annealing with symmetric subspaces
Quantum annealing (QA) is a promising approach for not only solving
combinatorial optimization problems but also simulating quantum many-body
systems such as those in condensed matter physics. However, non-adiabatic
transitions constitute a key challenge in QA. The choice of the drive
Hamiltonian is known to affect the performance of QA because of the possible
suppression of non-adiabatic transitions. Here, we propose the use of a drive
Hamiltonian that preserves the symmetry of the problem Hamiltonian for more
efficient QA. Owing to our choice of the drive Hamiltonian, the solution is
searched in an appropriate symmetric subspace during QA. As non-adiabatic
transitions occur only inside the specific subspace, our approach can
potentially suppress unwanted non-adiabatic transitions. To evaluate the
performance of our scheme, we employ the XY model as the drive Hamiltonian in
order to find the ground state of problem Hamiltonians that commute with the
total magnetization along the axis. We find that our scheme outperforms the
conventional scheme in terms of the fidelity between the target ground state
and the states after QA.Comment: 6 pages, 6 figure
Purely excitonic lasing in ZnO microcrystals: Temperature-induced transition between exciton-exciton and exciton-electron scattering
Since the seminal observation of room-temperature laser emission from ZnO thin films and nanowires, numerous attempts have been carried out for detailed understanding of the lasing mechanism in ZnO. In spite of the extensive efforts performed over the last decades, the origin of optical gain at room temperature is still a matter of considerable discussion. In this work, we show that a ZnO film consisting of well-packed micrometer-sized ZnO crystals exhibits purely excitonic lasing at room temperature without showing any symptoms of electron-hole plasma emission, even under optical excitation more than 25 times above the excitonic lasing threshold. The lasing mechanism is shifted from the exciton-exciton scattering to the exciton-electron scattering with increasing temperature from 3 to 150 K. The exciton-electron scattering process continues to exist with further increasing temperature from 150 to 300 K. Thus, we present distinct experimental evidence that the room-temperature excitonic lasing is achieved not by exciton-exciton scattering, as has been generally believed, but by exciton-electron scattering. We also argue that the long carrier diffusion length and the low optical loss nature of the micrometer-sized ZnO crystals, as compared to those of ZnO nanostructures, plays a key role in showing room-temperature excitonic lasing
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