Quantum on Quantum
Entangled photons validate
Feynman’s vision for simulating nature
By Charles Petit
Almost three decades ago, Richard Feynman — known popularly as much for his bongo drumming and pranks as for his brilliant insights into phys- ics — told an electrified audience at MIT how to
build a computer so powerful that its simulations “will do
exactly the same as nature.”
Not approximately, as digital computers tend to do
when faced with complex physical problems that must be
addressed via mathematical shortcuts — such as forecasting
orbits of many moons whose gravity constantly readjusts
their trajectories. Computer models of climate and other
processes come close to nature but hardly replicate it.
Feynman meant exactly, as in down to the last jot.
Now, finally, groups at Harvard University and the University of Queensland in Brisbane, Australia, have designed
and built a computer that hews closely to these specs. It is a
quantum computer, as Feynman forecast. And it is the first
quantum computer to simulate and calculate the behavior of
a molecular quantum system.
Much has been written about how such quantum computers would be paragons of calculating power should anybody
learn to build one that is much more than a toy. This latest
one is at the toy stage, too. But it could become just the thing
for solving some of the most vexing problems in science, the
ones Feynman had in mind when he said “nature” — those
problems involving quantum mechanics itself, the system of
physical laws governing the atomic scale. Inherent to quantum mechanics are seeming paradoxes that blur the distinctions between particles and waves, portray all events as
matters of probability rather than deterministic destiny and
place a particle in a state of ambiguity that makes it potentially two or more things, or in two or more places, at once.
Reporting in the February Nature Chemistry, the Harvard
group, led by chemist Alán Aspuru-Guzik, developed the
conceptual algorithm and schematic that defined the computer’s architecture. Aspuru-Guzik has been working on
such things for years, but he didn’t have the hardware to test
his ideas. At the University of Queensland, physicist Andrew
G. White and his team, who have been working on such
sophisticated gadgets, said they thought they could make
one to the Harvard specs and, after some collaboration, did
so. In principle the computer could have been rather small,
“about the size of a fingernail,” White says. But his group
spread the components across a square meter of lab space to
make adjustments and programming easier.
Within the computer’s filters and polarizers and beam
splitters, just two photons at a time travel simultaneously,
their particle-like yet wavelike natures playing peekaboo in
clouds of probability, just as quantum mechanics says they
should.
Quantum computing’s power stems from the curiosity
that a qubit — a bit of quantum information — is not limited
to holding a single discrete binary number, 1 or 0, as is the
bit of standard computing. Qubits exist in a limbo of uncertainty, simultaneously 1 and 0. Until the computation is
done and a detector measures the value, that very ambiguity
allows greater speed and flexibility as a quantum computer
searches multiple permutations at once for a final result.
Not only do the two photons serving as qubits in this device
have this mix of quantum identities, a state formally called
superposition, they are also “entangled.” Entanglement is
another feature of quantum mechanics in which the properties of two or more superposed particles are correlated with
one another. It is a superposition of superpositions, in which
