Layout of the superconducting flux qubit, which consists of several aluminum circuits. The key elements are the three ‘islands’ of metal in the top half of the image, across which the superconducting current flows. Magnetic pulses applied to the qubit control its quantum states. |
Establishing
a detailed knowledge of the noise properties of superconducting systems
is an important step towards the development of quantum computers,
which will enable new types of computing. However, the signals of these
systems’ tiny electronic components, such as transistors on a chip, are
so small that ambient noise creates interference. This problem is
compounded by the delicate nature of the technology’s quantum physical
states, which are also susceptible to noise. Now, an international
research team has successfully measured the noise spectrum of a
superconducting circuit—called a superconducting flux qubit—that is
widely investigated for its potential in quantum computing applications.
Precise
measurements of the environmental noise affecting superconducting flux
qubits are important, according to team leader Jaw-Shen Tsai from the
RIKEN Advanced Science Institute in Wako. “They may give us crucial
information about the microscopic origin of the noise source, about
which we have no solid understanding at all,” he says.
The
superconducting flux qubit studied by the researchers is a circuit
consisting of several junctions, and is a key technology in quantum
computing because it can be integrated into a chip. The first hurdle
cleared by the researchers was keeping the qubit stable, and therefore
viable, long enough to complete the measurement of the frequency
spectrum of the noise that would occur in a quantum computer. They
achieved this by applying a series of magnetic pulses that effectively
replenished the qubit’s quantum state. The net effect of the magnetic
pulses was to suppress detrimental contributions from low-frequency
noise, as the pulses affect only the quantum states and not the noise.
Suppression
of the low-frequency noise extended the lifetime of the quantum
information in the qubit by almost an order of magnitude, and enabled
the measurement of noise intensity in the system across three orders of
magnitude in frequency from 0.2 to 20 MHz. This new-found knowledge on
the noise spectrum will be valuable in developing strategies to counter
such noise.
“If we understand the nature of the noise, we may be able to reduce it considerably,” Tsai explains.
Moreover,
the team’s strategy of extending qubit lifetimes to measure the noise
spectrum is not limited to the study of quantum computing circuits; it
could also be applied to other systems that operate under similar
conditions. Medical imaging and sensing devices, for example, which
often operate at the limits of signal resolution, could benefit from
this noise reduction strategy.
