one millimeter in length, suspended in a super-cooled copper cavity about 38 millimeters wide.

The IBM work is significant, according to published commentary by Yale University physics professor Steven Girvin, because it is the most recent example of work that has improved the coherence time of superconducting qubits by five orders of magnitude in just over a decade. In his commentary, Girvin contends that “imperfect microwave hygiene,” rather than any intrinsic materials limits, has been the aggravating factor in limiting coherence times. Girvin says continuing experimentation is demonstrating that residual qubit dephasing, or reversion to classical state, is being caused by stray photons entering the cavities surrounding the superconducting qubits.

According to Matthias Steffen, manager of experimental quantum computing at IBM’s Thomas J. Watson Research Center, protection against dephasing was achieved by equalizing the temperature of photons within the cavity with that of the walls of the cavity itself.

“During the qubit operation the cav-
ity should ideally contain no photons
or, strictly speaking, the photon num-
ber in the cavity should not fluctuate,”
Steffen says. “In our case, we found the
cavity population does fluctuate when
the cavity is not properly heat sunk,
and when it is overcoupled to signal
lines which, in turn, are also not heat
sunk well. Our solution then included
a copper cavity so that the cavity is ther-
malized, making sure that no photons
are present in the cavity during the qu-
bit operation.”
The Max Planck researchers also
employed a cavity in which to maxi-
mize their efforts. In this case, the
creation of a single photon capable of
storing the quantum information in a
single atom and transferring it via op-
tical fiber to an identical atom located
20 meters from the first. Max Planck re-
searcher Stephan Ritter says the team
chose a single gaseous rubidium atom
as each node because it was easier to
protect the atoms from the surround-
ing environment and because the en-
ergy levels within each atom are well
defined, thereby making atom manip-
ulation via laser possible.

“We produce the photon by shining
the laser onto the atom from the side,”
Ritter says. “By precisely controlling
the intensity of the light, you can con-
trol the creation of the single photon.
The photon is deposited into the cavity,
then leaks out. If you just shine the laser
beam onto the atom, it can also emit a
photon but that might go off in any di-
rection. You want one directed channel
into which it goes, and basically the cav-
ity provides us with this clearly defined
direction. We have tried to make each
of the nodes the same; you can easily
scale it up by adding more nodes.”
The cavity in which the photon is
deposited is about 500 micrometers
long, Ritter says, while the mirrors on
either side are slightly larger, about
7. 5 millimeters in diameter. The mir-
rors do not share symmetric reflecting
properties, however. One mirror has a
much higher transmission quotient,
so that the resulting photon has what
Ritter says is a 90% probability of leak-
ing out on one side, allowing an opera-
tor to more easily couple the photon
with the networked optical fiber.

the ibm research is
significant because
it has improved the
coherence time of
superconducting
qubits by five orders
of magnitude in
just over a decade,
notes Yale physicist
steven Girvin.

the experiment was 2%. Ritter says this is an improvement of many orders of magnitude over predecessor experiments working with ion entanglement in free space, although the closer one gets to the ultimate goal of 100% the more difficult it gets.

“What’s important is that the process gives us a handle on all the properties that such a single photon can have,” Ritter says. “We can control the frequency, the direction of the photon using the cavity, and the left- or right-hand polarization of the photon. This is really important because the polarization is what we encode the quantum information in.”

Photons from a chip

The third experiment in the recent photonic trifecta, the demonstration of a method to produce entangled photon pairs on an integrated circuit, earns praise from Ritter as yet another key step in advancing quantum research further into the mainstream.

“They’ve done great work on inte-
gration, on making things small, on us-
ing existing microfabrication technol-
ogy to produce something which might
in the future be very useful for doing
quantum information processing,”
Ritter says. “Each of our nodes is only a
single atom with a cavity around, but if
you take into account all the optics and
all the lasers, one of these nodes fills a
whole lab at the moment.”
The Canadian and Austrian re-
search team designed a photonic chan-
nel called a Bragg reflection waveguide
on an aluminum-gallium arsenide
chip that will allow laser light pumped
into the chip to produce lower-energy
entangled photons that exit the other
side of the waveguide. The structure
is likened to a many-layered cake by
University of Toronto professor Amr
Helmy, the chip’s designer. The “cake”
consists of a core, surrounded by re-
flector layers, that will allow a desired
wavelength of light to pass through,
which results in the creation of the en-
tangled photon pair. The wavelength
can be tuned via altering the ratio of
aluminum in each layer of the reflec-
tors, as well as the thickness of each
layer. This tuning allows the energy
levels of the laser light and the photons
to be in sync, or phase-matched, which
enhances efficiency of the process.