ROCHESTER, N.Y. University of Rochester researchers have demonstrated how optical components that are readily available can create a quantum database by harnessing interference among light waves.
Such a quantum database, called an oracle, searches an entire database in a single operation instead of conducting an item-by-item search, as with conventional computers. While others have demonstrated such effects using advanced instrumentation such as nuclear magnetic resonance imaging systems, University of Rochester professor Ian Walmsley was able to compute the same algorithm with optical components.
"One of the earliest algorithms designed for quantum computers was a database search . . . we have found a way to implement it more economically using optical interference," said Walmsley. Other University of Rochester researchers involved in the work were Christophe Dorrer, Sascha Wallentowitz and Konrad Banaszek, as well as graduate student Pablo Londero.
Individual particles such as electrons are represented in quantum mechanics as probability waves that represent both "1" and "0" states simultaneously. It is only when a particle is observed the quantum computing operation of reading a bit that the waveform collapses into a definite value. Before it is observed, however, it happily propagates about in its indeterminate state, only having a probability of being frozen into a "1" or "0."
Quantum interference occurs when those probability waves cross paths before they are observed. Two sets of wave fronts will cross paths, creating an interference pattern that performs useful calculations such as adding the waves together in spots where both are at peaks while still in an indeterminate state. In fact, a quantum "adder" adds all the values in the multiplication table simultaneously, for instance, leaving the programmer the final task of merely picking out the specific case of interest.
Walmsley's quantum oracle accesses those indeterminate states while reasoning to its conclusions in a single flash of insight, rather than via a series of steps, as is the case conventional computers.
The oracle device works by employing a physical transducer to realize the database in a physical waveform. An acousto-optic modulator vibrates a transparent sheet of tellurium dioxide at one end, thereby realizing the database in the form of acoustic waves that propagate down its length. The resulting wave pattern compresses the transparent tellurium dioxide at peaks and expands it at valleys, creating a travelling pattern of slightly different indexes of refraction.
While the database is thus modulating the tellurium dioxide, the query is encoded into a light beam and passed through the transparent oracle. The query beam contains a match for one element of the database say, a name and is supposed to retrieve its corresponding data a telephone number, for example.
Before it is beamed through the tellurium dioxide oracle, it is split into two parts. One part is reserved for later use and the other goes through a prism that separates its frequencies before shining them through the tellurium dioxide oracle.
Each separate light band in the spectrum strikes a different section of the tellurium dioxide, which bends the light differently depending on how compressed or expanded that particular section is at the moment. On the other side of the tellurium dioxide, a second prism recombines the rainbow into a single beam that is then mixed with the original, reserved portion of the beam.
The combining step serves to select the single color of the rainbow that has had its phase shifted by the operation, thereby identifying the desired element of the database.
Walmsley's experimental oracle device encoded 50 different database elements on different frequencies of light, then passed the query beam through it. The element of the database requested was found to belong to the frequency of light emerging from the oracle whose phase was shifted. A conventional computer would take 50 such query steps to guarantee that it found the correct database element.