



01544nas a2200145   4500008004100000245006100041210006000102260001300162520109800175100002101273700001901294700002501313700002301338856003701361       2020                        eng d00aNearly optimal time-independent reversal of a spin chain0 aNearly optimal timeindependent reversal of a spin chain  c3/5/20203 a<p>We propose a time-independent Hamiltonian protocol for the reversal of qubit ordering in a chain of N spins. Our protocol has an easily implementable nearest-neighbor, transverse-field Ising model Hamiltonian with time-independent, non-uniform couplings. Under appropriate normalization, we implement this state reversal three times faster than a naive approach using SWAP gates, in time comparable to a protocol of Raussendorf [Phys. Rev. A 72, 052301 (2005)] that requires dynamical control. We also prove lower bounds on state reversal by using results on the entanglement capacity of Hamiltonians and show that we are within a factor 1.502(1+1/N) of the shortest time possible. Our lower bound holds for all nearest-neighbor qubit protocols with arbitrary finite ancilla spaces and local operations and classical communication. Finally, we extend our protocol to an infinite family of nearest-neighbor, time-independent Hamiltonian protocols for state reversal. This includes chains with nearly uniform coupling that may be especially feasible for experimental implementation.&nbsp;</p>1 aBapat, Aniruddha1 aSchoute, Eddie1 aGorshkov, Alexey, V.1 aChilds, Andrew, M.  uhttps://arxiv.org/abs/2003.02843