9,673 research outputs found
Enhanced No-Go Theorem for Quantum Position Verification
Based on the instantaneous nonlocal quantum computation (INQC), Buhrman et
al. proposed an excellent attack strategy to quantum position verification
(QPV) protocols in 2011, and showed that, if the colluding adversaries are
allowed to previously share unlimited entangled states, it is impossible to
design an unconditionally secure QPV protocol in the previous model. Here,
trying to overcome this no-go theorem, we find some assumptions in the INQC
attack, which are implicit but essential for the success of this attack, and
present three different QPV protocols where these assumptions are not
satisfied. We show that for the general adversaries, who execute the attack
operations at every common time slot or the time when they detect the arrival
of the challenge signals from the verifiers, secure QPV is achievable. This
implies practically secure QPV can be obtained even if the adversaries is
allowed to share unlimited entanglement previously. Here by "practically" we
mean that in a successful attack the adversaries need launch a new round of
attack on the coming qubits with extremely high frequency so that none of the
possible qubits, which may be sent at random time, will be missed. On the other
side, using such Superdense INQC (SINQC) attack, the adversaries can still
attack the proposed protocols successfully in theory. The particular attack
strategies to our protocols are presented respectively. On this basis, we
demonstrate the impossibility of secure QPV with looser assumptions, i.e. the
enhanced no-go theorem for QPV.Comment: 19 pages, single column, 3 tables, 6 figure
Valley depolarization in monolayer WSe2
We have systematically examined the circular polarization of monolayer WSe2
at different temperature, excitation energy and exciton density. The valley
depolarization in WSe2 is experimentally confirmed to be governed by the
intervalley electron-hole exchange interaction. More importantly, a
non-monotonic dependence of valley circular polarization on the excitation
power density has been observed, providing the experimental evidence for the
non-monotonic dependence of exciton intervalley scattering rate on the excited
exciton density. The physical origination of our experimental observations has
been proposed, which is in analogy to the D'yakonov-Perel' mechanism that is
operative in conventional GaAs quantum well systems. Our experimental results
are fundamentally important for well understanding the valley psudospin
relaxation in atomically thin transition metal dichalcogenides
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