in quantum computers
Two major steps toward putting quantum computers into real practice — sending
a photon signal on demand from a quantum bit (or qubit) onto wires and transmitting
the signal to a second, distant qubit — have been achieved by a team of
scientists at Yale. — By Janet Rettig Emanuel
T H I S
The accomplishments were reported in sequential issues of Nature on Sept. 20
and Sept. 27. In fact, the Yale research is highlighted on the cover of the latter
issue, along with complementary work from a group at the National Institute of
Standards and Technologies.
Over the past several years, the research team of Professor Robert Schoelkopf
in applied physics and Professor Steven Girvin in physics has explored the use
of solid-state devices resembling microchips as the basic building blocks in
the design of a quantum computer.
First step to making quantum computing useful
Previously, information on a quantum computer had only been transferred directly
from qubit to qubit in a superconducting system. Schoelkopf and Girvin’s
team has engineered a superconducting communication “bus” to store
and transfer information between distant quantum bits, or qubits, on a chip.
This work, according to Schoelkopf, is the first step to making the fundamentals
of quantum computing useful.
The first breakthrough reported is the ability to produce on demand — and
to control — single, discrete microwave photons that can carry encoded
quantum information. The sources of microwave energy — which is used in
cell phones and ovens — usually do not produce just one photon. This new
system creates a certainty of producing individual photons, note the researchers.
“It is not very difficult to generate signals with one photon on average,
but, it is quite difficult to generate exactly one photon each time,” explain
postdoctoral associates Andrew Houck and David Schuster, who are lead co-authors
on the first paper. “To encode quantum information on photons, you want
there to be exactly one.”
According to Schoelkopf, “We are reporting the first such source for producing
discrete microwave photons, and the first source to generate and guide photons
entirely within an electrical circuit.”
In order to successfully perform these experiments, the researchers had to control
electrical signals corresponding to one single photon. (In comparison, a
cell phone emits about 1023 — or 100,000,000,000,000,000,000,000 — photons
per second.) Further, because of the extremely low energy of microwave photons,
the Yale scientists had to use highly sensitive detectors and experiment temperatures
just above absolute zero.
“In this work we demonstrate only the first half of quantum communication
on a chip — quantum information efficiently transferred from a stationary
quantum bit to a photon or ‘flying qubit,’” says Schoelkopf. “However,
for on-chip quantum communication to become a reality, we need to be able to
transfer information from the photon back to a qubit.”
This is exactly what the researchers reported in the second breakthrough. Postdoctoral
associate Johannes Majer and graduate student Jerry Chow, lead co-authors of
the second paper, added a second qubit and used the photon to transfer a quantum
state from one qubit to another. This was possible because the microwave photon
could be guided on wires — similarly to the way fiber optics can guide
visible light — and be carried directly to the target qubit.
“A novel feature of this experiment is that the photon used is only virtual,”
say Majer and Chow, “winking into existence for only the briefest instant before disappearing.”
To allow the crucial communication between the many elements of a conventional
computer, engineers wire them all together to form a data “bus,” which
is a key element of any computing scheme. Together the new Yale research constitutes
the first demonstration of a “quantum bus” for a solid-state electronic
system. This approach can in principle be extended to multiple qubits, and to connecting the parts of a future, more complex quantum
computer.
However, Schoelkopf likened the current stage of development of quantum computing
to conventional computing in the 1950s, when individual transistors were first
being built. Standard computer microprocessors are now made up of a billion transistors,
but first it took decades for physicists and engineers to develop integrated
circuits with transistors that could be mass produced.
Schoelkopf and Girvin are members of the newly formed Yale Institute for Nanoscience
and Quantum Engineering, a broad interdisciplinary activity among faculty and
students from across the university. Further information and FAQs about qubits
and quantum computing are available online at www.eng.yale.edu/rslab.
Other Yale authors involved in the research are J.M. Gambetta, J.A. Schreier,
J. Koch, B.R. Johnson, L. Frunzio, A. Wallraff, A. Blais and Michel Devoret.
Funding for the research was from the National Security Agency under the Army
Research Office, the National Science Foundation and Yale University.
W E E K ' SS T O R I E S
Students fan out overseas for architecture studios
University Church in Yale marks 250 years of tradition and reform
NIH honors scientist for innovative work on microscopes
‘Yale at Carnegie’ series to feature performances by students, faculty
Yale makes dramatic changes in research compliance procedures
Web-based system for effort reporting launched
ENDOWED PROFESSORSHIPSOnce a ‘musical theater guy,’ writer is now a ‘gadget freak’
Forum to examine ways that New Haven can become a ‘sustainable city’
The allure of fly fishing is explored in museum exhibit
Workshops to explore global issues . . .
World Fellows share in a night of ‘intercultural understanding’
Beinecke show examines the Italian festival book tradition
Center’s events to feature internationally known architects
Issues of spirituality to be explored in exhibit, poetry reading
Scavenger hunt orients new graduate students to the campus and Elm City
United Way Days of Caring brings out volunteers from the Yale community
Campus Notes
Bulletin Home|Visiting on Campus|Calendar of Events|In the News
Bulletin Board|Classified Ads|Search Archives|Deadlines
Bulletin Staff|Public Affairs|News Releases|
E-Mail Us|Yale Home