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
Colorized micrograph of superconducting circuit used in NIST quantum computing research. The chip combines a quantum bit (pink) for storing quantum information, a quantum bus (green) for transporting information, and a switch (purple) that "tunes" interactions between the other two components. Credit: M.S. Allman/NIST
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.