  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.

Quantum Physics