Rigetti Computing Scales Up Quantum Chips
February 5, 2018
A fundamental barrier to scaling quantum
computing machines is "qubit interference."
In new research engineers and physicists
from Rigetti Computing describe a
breakthrough that can expand the size of
practical quantum processors by reducing
interference.
Rigetti's 19Q superconducting quantum
processor.
Matt Reagor, lead author of the paper, says:
"We've developed a technique that enables us
to reduce interference between qubits as we
add more and more qubits to a chip, thus
retaining the ability to perform logical
operations that are independent of the state
of a (large) quantum register."
To explain the concept, the Rigetti team
employs wine glasses as an analogy to qubits:
Clink a wine glass, and you will hear it
ring at its resonant frequency (usually
around 400 Hz). Likewise, soundwaves at that
frequency will cause the same glass to
vibrate. Different shapes or amounts of
liquid in a glass will produce different
clinks, i.e. different resonance
frequencies. A clinked wine glass will cause
identical, nearby glasses to vibrate.
Glasses that are different shapes are
"offresonant glasses," meaning they will
not vibrate much at all.
So, what's the relation between glasses and
qubits?
Reagor explains that each physical qubit on
a superconducting quantum processor stores
energy in the form of an oscillating
electric current. "Think of each qubit as a
wine glass," he says. "The logical state of
a qubit (e.g. "0" or "1") is encoded by the
state of its corresponding electric
currents. In our analogy, this is equivalent
to whether or not a wine glass is
vibrating."
A highly successful class of entangling
gates for superconducting qubits operate by
tuning two or more qubits into resonance
with each other. At this tuning point, the
"wine glasses" pick up on one another's
"vibrations."
This effect can be strong enough to produce
significant, conditional vibration changes
that can be leveraged as conditional logic.
Imagine pouring or siphoning off wine from
one of the glasses to make this tuning
happen. With qubits, there are tunable
circuit elements that fulfill the same
purpose.
"As
we scale up quantum processors, there are
more and more wine glasses to manage when
executing a specific conditional logic
gate," says Reagor. "Imagine lining up a
handful of identical glasses with increasing
amounts of wine. Now we want to tune one
glass into resonance with another, without
disturbing any of the other glasses. To do
that, you could try to equalize the wine
levels of the glasses. But that transfer
needs to be instantaneous to not shake the
rest of the glasses along the way. Let's say
one glass has a resonance at one frequency
(call it 400 Hz) while another, nearby glass
has a different one (e.g. 380 Hz). Now, we
make use of a somewhat subtle musical
effect. We are actually going to fill and
deplete one of the glasses repeatedly."
He continues: "We repeat that filling
operation at the difference frequency
between the glasses (here, 20 times per
second, or 20 Hz). By doing so, we create a
beatnote for this glass that is exactly
resonant with the other. Physicists
sometimes call this a parametric process.
Our beatnote is "pure"  it does not have
frequency content that interferes with the
other glasses. That's what we have
demonstrated in our recent work, where we
navigated a complex eightqubit processor
with parametric twoqubit gates."
Reagor concludes: "While this analogy may
sound somewhat fanciful, its mapping onto
our specific technology, from a mathematical
standpoint, is surprisingly accurate."
