Quantum capacity
Debbie
Leung’s
recent
work
explores
the
capacity
of
physical
devices
for
quantum
information
processing
and
quantum
cryptography.
“I’m
looking
at
the
limits
–
how
secure
and
how
much
we
can
communicate
over
a
quantum
channel.”
Quantum channels naturally inflict “noise” on the data transmitted. The noise results from the undesired evolution of qubits while they are transmitted through the channel. “The noise is something we have to deal with,” Debbie explains. “It’s always there. Cryptographically, we worry that an eavesdropper will use the noise as a mask, so we need the capability to see through that mask.”
Debbie and her collaborators have been working to distinguish natural noise from tampering and they have been developing methods for quantum key distribution. “We found that even in a very noisy quantum channel, quantum effects prevent an eavesdropper from using noise as a disguise,” notes Debbie. “The quantum effects are very weak, but we’ve had success in finding out where they kick-in and how we can use them to our advantage. It’s important to find out what gives us the power to do things in quantum that we can’t do classically.”
Debbie has also studied a more futuristic problem: communication of quantum data via a network of quantum channels. In classical networks, there are sophisticated methods for sending information, but there are no better solutions for sending a commodity besides routing. “We wanted to see what happens quantum mechanically, and we discovered that routing is already optimal. Quantum data is incompressible – like water – thus one can be perfectly content with this simple but optimal solution.”
University of Waterloo Mathematics, Annual Report 2006