



02114nas a2200169   4500008004100000245009200041210006900133260001300202490000700215520160400222100001801826700001401844700001701858700001801875700001401893856003701907       2011                        eng d00aNo-go Theorem for One-way Quantum Computing on Naturally Occurring  Two-level Systems
0 aNogo Theorem for Oneway Quantum Computing on Naturally Occurring  c2011/5/90 v833 a  One-way quantum computing achieves the full power of quantum computation by
performing single particle measurements on some many-body entangled state,
known as the resource state. As single particle measurements are relatively
easy to implement, the preparation of the resource state becomes a crucial
task. An appealing approach is simply to cool a strongly correlated quantum
many-body system to its ground state. In addition to requiring the ground state
of the system to be universal for one-way quantum computing, we also want the
Hamiltonian to have non-degenerate ground state protected by a fixed energy
gap, to involve only two-body interactions, and to be frustration-free so that
measurements in the course of the computation leave the remaining particles in
the ground space. Recently, significant efforts have been made to the search of
resource states that appear naturally as ground states in spin lattice systems.
The approach is proved to be successful in spin-5/2 and spin-3/2 systems. Yet,
it remains an open question whether there could be such a natural resource
state in a spin-1/2, i.e., qubit system. Here, we give a negative answer to
this question by proving that it is impossible for a genuinely entangled qubit
states to be a non-degenerate ground state of any two-body frustration-free
Hamiltonian. What is more, we prove that every spin-1/2 frustration-free
Hamiltonian with two-body interaction always has a ground state that is a
product of single- or two-qubit states, a stronger result that is interesting
independent of the context of one-way quantum computing.
1 aChen, Jianxin1 aChen, Xie1 aDuan, Runyao1 aJi, Zhengfeng1 aZeng, Bei  uhttp://arxiv.org/abs/1004.3787v101374nas a2200133   4500008004100000245008300041210006900124260001500193520094600208100001401154700001801168700001701186856003701203       2010                        eng d00aOptimal Perfect Distinguishability between Unitaries and Quantum  Operations
0 aOptimal Perfect Distinguishability between Unitaries and Quantum  c2010/10/123 a  We study optimal perfect distinguishability between a unitary and a general
quantum operation. In 2-dimensional case we provide a simple sufficient and
necessary condition for sequential perfect distinguishability and an analytical
formula of optimal query time. We extend the sequential condition to general
d-dimensional case. Meanwhile, we provide an upper bound and a lower bound for
optimal sequential query time. In the process a new iterative method is given,
the most notable innovation of which is its independence to auxiliary systems
or entanglement. Following the idea, we further obtain an upper bound and a
lower bound of (entanglement-assisted) q-maximal fidelities between a unitary
and a quantum operation. Thus by the recursion in [1] an upper bound and a
lower bound for optimal general perfect discrimination are achieved. Finally
our lower bound result can be extended to the case of arbitrary two quantum
operations.
1 aLu, Cheng1 aChen, Jianxin1 aDuan, Runyao  uhttp://arxiv.org/abs/1010.2298v101032nas a2200181   4500008004100000245003700041210003700078260001500115300001100130490000700141520058000148100001800728700001700746700001800763700002000781700001200801856003700813       2008                        eng d00aExistence of Universal Entangler0 aExistence of Universal Entangler  c2008/01/01  a0121030 v493 a  A gate is called entangler if it transforms some (pure) product states to
entangled states. A universal entangler is a gate which transforms all product
states to entangled states. In practice, a universal entangler is a very
powerful device for generating entanglements, and thus provides important
physical resources for accomplishing many tasks in quantum computing and
quantum information. This Letter demonstrates that a universal entangler always
exists except for a degenerated case. Nevertheless, the problem how to find a
universal entangler remains open.
1 aChen, Jianxin1 aDuan, Runyao1 aJi, Zhengfeng1 aYing, Mingsheng1 aYu, Jun  uhttp://arxiv.org/abs/0704.1473v2