Black-box Hamiltonian simulation and unitary implementation

Details Black-box Hamiltonian simulation and unitary implementation Black-box Hamiltonian simulation and unitary implementation

2009-10-22

arxiv.org/abs/0910.4157v4

Quantum Information and Computation 12, 29 (2012)

Physics

Quantum Physics

19 pages, 3 figures, minor corrections

Scientific paper

We present general methods for simulating black-box Hamiltonians using quantum walks. These techniques have two main applications: simulating sparse Hamiltonians and implementing black-box unitary operations. In particular, we give the best known simulation of sparse Hamiltonians with constant precision. Our method has complexity linear in both the sparseness D (the maximum number of nonzero elements in a column) and the evolution time t, whereas previous methods had complexity scaling as D^4 and were superlinear in t. We also consider the task of implementing an arbitrary unitary operation given a black-box description of its matrix elements. Whereas standard methods for performing an explicitly specified N x N unitary operation use O(N^2) elementary gates, we show that a black-box unitary can be performed with bounded error using O(N^{2/3} (log log N)^{4/3}) queries to its matrix elements. In fact, except for pathological cases, it appears that most unitaries can be performed with only O(sqrt{N}) queries, which is optimal.

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