Science News - February 27, 2010

the state of one photon is connected to the state of the other despite the particles’ separation in space. Entanglement enhances the ability of a quantum computer to explore simultaneously all possible solutions to a complex problem.

Whitfield is near completion of his studies to be a theo-
retical chemist. A goal is, eventually, to be able to calculate
the energy levels and reaction levels of complex molecules
bound together by scores or even hundreds of electrons.
Even in problems with just five or so electrons, the challenge
of computation by standard means has grown so exponen-
tially fast that standard computers cannot handle it.
The new work is “great, a proof of principle, more evi-
dence that this stuff is not pie in the sky or
cannot be built,” says chemist Birgitta
Whaley of the University of California,
Berkeley. “It is the first time that a quan-
tum computer has been used to calculate a
molecular energy level.”
And while most of the publicity received
by quantum computers has marveled at the
potential power to break immense numbers
into their prime factors — a key to cracking
secret codes and thus an issue of national
security — “this has major implications for
practical uses with very broad application,”
Whaley says. These uses might include the
ability, with less trial and error, to design complex chemi-
cal systems and advanced materials with properties never
before seen.

But with just two photons as its qubits, the new quantum computer cannot tackle quantum behavior involving more than two objects. So, the researchers asked it to calculate the energy levels of the hydrogen molecule, the simplest one known. Other methods have long revealed the answer, providing a check on the accuracy of doing it with qubits. Corresponding to the two wavelike photons rattling fuzzily along in the computer, the hydrogen molecule has two wavelike electrons binding its two nuclei — each a single proton.

Benjamin Lanyon, who is now at the University of Innsbruck in Austria, and the

Charles Petit is a freelance writer based in Berkeley, Calif.

As Richard Feynman once
envisioned, calcite is a key part
of a quantum computer.

Queensland team programmed the equations that govern how electrons behave near protons into the computer by tweaking its arrangement of filters, wavelength shifters and other optical components. Each piece of optical hardware corresponds to the logic gates that add, subtract, integrate and otherwise manipulate binary data in a standard computer. The researchers then entered initial “data” corresponding to the distance between the molecule’s nuclei — a driver of what energies the electrons might be able to take on when the molecule is excited by an outside influence.

The photons were each given a precise angle of polarization — the orientation of the electric and magnetic components of their fields — providing the researchers with a way to enter data into the computer. On the first run of a calculation, the second photon shared a piece of data via its entanglement with the first and, going at the speed of light, emerged from the machine with the first digit of the answer.

Scaling the machine up to five, 10 or hundreds of qubits will not be easy. In the end, photons are unlikely to serve as qubits because of the difficulty of entangling and monitoring so many of them. Electrons, simulated atoms called quantum dots, ionized atoms or other such particles may eventually form the blurry hearts of quantum computers. How long from now? “I’d say less than 50 years, but more than 10,” White says.

In an iteration process, that digit was then used as data for another run, producing the second digit — a process repeated for 20 rounds.

By following — some would say simulating — the same weird physics as do the electrons of atomic bonds themselves, the computer’s photons got the permitted energies correct to within 6 parts per million.

In a striking bit of symmetry to go with using a quantum computer to solve a quantum problem, the latest work resonates with Feynman’s original idea in another way. At that talk at MIT — published in 1982 in the International Journal of Theoretical Physics — Feynman not only suggested the basis for such a computer, he also drew a little picture of one. It included two little blocks of the semitransparent mineral calcite to control and measure the photons’ polarizations. Looking at the diagram of the device built recently by the University of Queensland team reveals, sure enough, two calcite beam displacers.

“Every time you add an electron or other object to a quantum problem, the complexity of the problem doubles,” says

James Whitfield of Harvard, a coauthor on the paper. “The
great thing,” he adds, “is that every time you add a qubit to
the computer, its power doubles too.” In formal language,
the power of a quantum computer scales exponentially with
its size (as in number of qubits), in step with the size of quan-
tum problems. In fact, says Aspuru-Guzik, a computer of
Whatever shade of Richard Feynman flickers still in the
entanglements of the universe, were it made to collapse into
something corporeal, perhaps it would be smiling. s