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Colorized micrograph Credit: M.S. Allman/NIST |
Scientists at the
National Institute of Standards and Technology (NIST) have developed the
first “dimmer switch” for a superconducting circuit linking a quantum
bit (qubit) and a quantum bus—promising technologies for storing and
transporting information in future quantum computers. The NIST switch
is a new type of control device that can “tune” interactions between
these components and potentially could speed up the development of a
practical quantum computer.
Quantum computers, if they can be built,
would use the curious rules of quantum mechanics to solve certain
problems that are now intractable, such as breaking today’s most widely
used data encryption codes, or running simulations of quantum systems
that could unlock the secrets of high-temperature superconductors.
Unlike many competing systems that store and transport information using
the quantum properties of individual atoms, superconducting qubits use a
“super flow” of oscillating electrical current to store information in
the form of microwave energy. Superconducting quantum devices are
fabricated like today’s silicon processor chips and may be easy to
manufacture at the large scales needed for computation.
As described in a forthcoming paper in Physical
Review Letters,* the new NIST switch can reliably tune the
interaction strength or rate between the two types of circuits—a qubit
and a bus—from 100 megahertz to nearly zero. The advance could enable
researchers to flexibly control the interactions between many circuit
elements in an intricate network as would be needed in a quantum
computer of a practical size.
Other research groups have demonstrated
switches for two or three superconducting qubits coupled together, but
the NIST switch is the first to produce predictable quantum behavior
over time with the controllable exchange of an individual microwave
photon (particle of light) between a qubit and a resonant cavity. The
resonant cavity serves as what engineers call a “bus”—a channel for
moving information from one section of the computer to another. “We have
three different elements all working together, coherently (in concert
with each other) and without losing a lot of energy,” says the
CU-Boulder graduate student Michael (Shane) Allman who performed the
experiments with NIST physicist Ray Simmonds, the principal
investigator.
All three components (qubit, switch, and
cavity) were made of aluminum in an overlapping pattern on a sapphire
chip (see image). The switch is a radio-frequency SQUID (superconducting
quantum interference device), a magnetic field sensor that acts like a
tunable transformer. The circuit is created with a voltage pulse that
places one unit of energy—a single microwave photon—in the qubit. By
tuning a magnetic field applied to the SQUID, scientists can alter the
coupling energy or transfer rate of the single photon between the qubit
and cavity. The researchers watch this photon slosh back and forth at a
rate they can now adjust with a knob.
The switch research was supported in part by
the Army Research Office. Simmonds’s group previously demonstrated the
first superconducting quantum bus between qubits (see “Digital
Cable Goes Quantum: NIST Debuts Superconducting Quantum Computing Cable,”
www.nist.gov/public_affairs/releases/quantum_cable.html,
which also describes how the superconducting qubits operate).
* M.S. Allman, F. Altomare, J.D. Whittaker,
K. Cicak, D. Li, A. Sirois, J. Strong, J.D. Teufel, R.W. Simmonds. 2010.
rf-SQUID-Mediated Coherent Tunable Coupling Between a Superconducting
Phase Qubit and a Lumped Element Resonator. Physical Review Letters.
Forthcoming.
Media Contact: Laura Ost, laura.ost@nist.gov, (303) 497-4880
