In a windowless basement laboratory glowing with green lasers,
physics Prof. Chris Monroe is taking the first steps toward
cracking every code that protects data on the Internet.

Diana Krankurs
Three cadmium ions fluoresce under laser light. (Courtesy of Chris Monroe

His research on quantum behavior of atoms may help make possible
the first quantum computer, a technology that would theoretically
be able to quickly decrypt schemes that encode information from
credit card numbers to national security matters.

Most encryption today relies on the fact that factoring a number
— or figuring out which prime numbers can be multiplied to
get that number — is extremely difficult. For instance,
although it’s easy to get the number 15 by multiplying 5 and
3, it isn’t so simple to figure out what to multiply to get
5,681.

In fact, large numbers are so difficult to factor that it took a
team of supercomputers six months to factor a 150-digit number,
Monroe said. The computers searched through millions of numbers by
trial and error before producing their result — a task
perfectly suited for a quantum computer, which stores its
information in atoms.

“Quantum computers could factor numbers fast, if we could
ever build one,” he said. “If you could factor
efficiently, you could break codes.”

In a regular computer, each input to the computer produces a
single output. To compute something with many inputs, each input
must be processed individually, Monroe explained.

But in a quantum computer, “you can store lots of numbers
in the same (atom),” he said. “You can have one atom in
lots of different states at the same time, and each state could
represent a number.”

How does this work? Instead of following the normal laws of
physics, small objects like atoms obey the laws of quantum
mechanics, which allow these objects to behave both as particles
and waves. Since a wave can occupy several positions at the same
time, a quantum object can exist in multiple states at once.

This means that each atom in a quantum computer — called a
quantum “bit,” or qubit — could represent
multiple inputs simultaneously. Unlike in a regular computer, where
each bit represents either a 0 or a 1, each qubit can denote 0, 1
or both 0 and 1 at the same time.

To understand how quantum objects like atoms behave, Monroe and
his research group study single atoms trapped in a vacuum. The
atoms must be studied in a vacuum because, through an oddity of
quantum physics, they only exhibit quantum behavior when they are
neither observed nor in contact with other matter.

“We use trapped ions — these are atoms with an
electron stripped off, so it’s a charged atom,” said
Rackham student Patty Lee, one of Monroe’s graduate students.
The charged ions can then be trapped and manipulated using an
electric field, she said.

Lasers are used to observe the atoms after manipulation, and to
change their state if necessary, Lee added.

In March, Monroe’s group made a particularly important
advance. They managed to show “entanglement” between a
photon, or a particle of light, and an atom. Entanglement means
that the atom’s state and the photon’s state are linked
— if the photon is observed in one state, the atom must also
exist in that state at the moment the photon is observed.

Monroe’s group is one of only three groups in the world
that have shown entanglement between particles, Lee said. The work
was published in the March issue of the journal Nature.

Entanglement is important for quantum computing because it
allows simultaneous control of many particles, said Rackham student
Mark Acton, another of Monroe’s students. Chains of entangled
qubits can represent many large numbers at once by denoting many
states of 0 and 1.

“If you had many ions entangled, if you do something to
one of them, it affects all the others,” Acton said.
“You don’t have to go through and (manipulate) all 100
of them. You can just do something to one of them.”

Despite such progress, Monroe remains modest about the future of
his work. “This is all really speculative stuff,” he
said. A real quantum computer may still be decades away, he added,
or eventually be proved impossible.

If theory becomes reality, though, it’s hard to
underestimate its importance.

“Quantum computing offers the opportunity of changing the
rules of the game,” Monroe said. “That has the
potential for revolutionizing computing.”

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