Volume 2, Issue 16 November 2005
The Physicist and the SQUID
John Clarke with a photo of a superconducting coil deposited on a SQUID. (courtesy Berkeley Lab)
UC Berkeley scientist John Clarke has spent his career studying SQUIDs. But the SQUIDs that Clarke works with are not the cephalopods of calamari and Jules Verne novels. SQUID is an acronym for Superconducting Quantum Interference Device, a device for detecting incredibly weak magnetic fields. Science's most sensitive energy detecting device, SQUIDs are having a dramatic impact on fields as diverse as medical imaging, cosmology, and computer architecture.
"SQUIDs are the world's most sensitive detector of magnetic flux," says Clarke, a professor of physics and member of Lawrence Berkeley National Laboratory's Materials Sciences Division. "Since flux is generated in so many different ways, SQUIDs have a great number of applications."
Subtle shifts in magnetic fields are difficult to measure, Clarke explains. But SQUIDs convert those fluxes into a voltage that is easily readable. Based on properties of quantum physics, SQUIDs can be billions of times more sensitive to magnetic fluctuations than the needle of a compass.
In lieu of biological tissue, the researchers conduct MRI experiments on bell peppers.
Smaller than the head of a pin, the devices are bulk fabricated in Clarke's own laboratory and in UC Berkeley's Microfabrication Facility with processes similar to those used in the manufacture of silicon computer chips. Each SQUID is a microscopic loop of superconducting metal containing two Josephson junctions, an insulator thin enough for pairs of electrons to tunnel through. When placed near a magnetic field, the voltage across the device changes in response to the strength of that field.
In recent years, Clarke has developed SQUID-based amplifiers to help in the search for axions, hypothetical elementary particles that may be the mysterious "dark matter" that makes up roughly 25 percent of the mass in our universe. Another project focused on using SQUIDs in metallurgy for early detection of failing steel, in bridges or airplane wings, for example.
SQUIDs have also emerged as a possible candidate component for tomorrow's quantum computer, systems that promise to crank out calculations a billion times faster than today's integrated circuits. By exploiting the unusual characteristics of quantum mechanics, the quantum computer might one day quickly determine the physical structure of proteins for drug discovery or generate secure encryption codes for highly sensitive data.
The key ingredient in a quantum computer is the qubit, the equivalent of the binary zero or one of a digital computer. The power of quantum computers lies in a qubit's ability to exist in a zero or one state, or in a superposition somewhere in the middle. This quantum weirdness is what enables quantum computers to process so much data at once. Essentially, each qubit represents an infinite number of values at one time. As more qubits are strung together, the power of the quantum processor grows exponentially.
A variety of physical systems are now being considered as candidates for qubits. Most of these focus on controlling and detecting the state of single photons or electrons. Clarke's approach uses SQUIDS as a key component in a macroscopic system with emergent qubit behavior.
A scanning electron micrograph of two flux qubits.
The flux qubit contains a tiny superconducting loop--in this case containing three Josephson junctions--with current flowing around it. The direction of the current--clockwise, counterclockwise, or a quantum superposition of the two states--can be changed by applying a magnetic field. A SQUID built around the superconducting loop reads out the state of qubit.
"We've had great results making this whole ensemble act like a single atom in terms of its quantum mechanical properties," Clarke says. "Next, we'd like to hook together several so you can have interacting flux qubits. It would take thousands to make a fully-functional quantum computer though."
While quantum computer research is still in its early stages, Clarke has made strides in using the unparalleled sensitivity of SQUIDs to improve medical magnetic resonance imaging (MRI) systems. Conventional MRI systems image soft tissues such as brains and organs by immersing the body within an incredibly strong magnetic field and applying sequences of radio frequency pulses. A computer generates an image based on how the magnetic energy interacts with the protons in the body. Traditional MRI machines require a very large and expensive magnet to produce a good picture.
Clarke's aim is to develop an MRI system based on a single SQUID operating in a magnetic field as much 10,000 times weaker than that of traditional machines. Indeed, the prototype magnets that Clarke's students have constructed are simply copper wire wrapped around wooden forms. Someday, this research could lead to portable and inexpensive MRI machines that generate images as detailed as today's clinical systems but at much lower cost. In addition, low- field MRI may offer a distinct advantage over its high-field counterpart.
"In conventional MRIs, it's very difficult to distinguish between cancer and healthy tissue," Clarke says. "Using SQUIDs, there is a good possibility that we could tell the difference."