We study the advantage of pure-state quantum computation without entanglement over classical computation. For the Deutsch-Jozsa algorithm we present the maximal subproblem that can be solved without entanglement, and show that the algorithm still has an advantage over the classical ones. We further show that this subproblem is of greater significance, by proving that it contains all the Boolean functions whose quantum phase-oracle is non-entangling. For Simon's and Grover's algorithms we provide simple proofs that no non-trivial subproblems can be solved by these algorithms without entanglement.
We propose a point to point quantum channel based on a two-color Spontaneous Parametric Down Conversion (SPDC), that may be applied
for a Quantum Key Distribution (QKD) system to gain better
security. We use one arm of the SPDC (770 nm - optimal for Si
detection) and a Si
counter at Alice's side to count the exact
number of photons in each pulse. Whenever the arm
contains exactly
one photon, the correlated photon (1550nm - optimal for fiber
transmission)
in the other arm is sent via a fiber to Bob. In the experiment we used an Ar^{+} laser of
514.5nm wavelength
and a BBO crystal to produce type-I photon pairs. We measured the spectrum of the SPDC and resolved specifically the 770 nm
wavelength. The rate of correlated
pairs (at 890-1050 nm) from our
SPDC source was compared to a non-correlated source. We
further
developed an InGaAs single photon detector based on Geiger mode
APD and achieved 10%
quantum efficiency and 5 * 10^{-3} dark
counts per 20nsec pulse at a temperature of -35
degrees Celsius.
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