The Quest to Create a Superconducting Quantum Computer

The Quest to Create a Superconducting Quantum Computer

Syracuse University School of Information Studies

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If non-scientists know anything about quantum mechanics, it is probably the thought experiment known as Schrödinger's cat. Imagine that an unfortunate feline is locked in an airtight box, physicist Erwin Schrödinger proposed in 1935, along with a radioactive atom that has a 50-50 chance of decaying within the next hour and a vial of poison that the radioactive emission will shatter. After that hour, is the atom intact and the cat alive, or has it decayed, to the fatal detriment of kitty? Remember, the atom has a 50% probability of decaying in the hour.

The standard answer is that kitty is suspended in a state of both living and dead, what physicists call a "superposition" of equal-probability states. Superpositions are a key consequence of quantum mechanics, which describes the behavior of subatomic particles. The superposition resolves into one or the other of its constituents—a live cat or a dead one—only when an observer opens the box and looks. Not surprisingly, when laypeople think about superpositions of dead and living cats and the notion that reality is created by an observer, their reaction is probably not, aha, I know just how I can apply this to build a 21st-century computer.

But that, oversimplified, is what physicist Britton L.T. Plourde of Syracuse University thinks. Britton is involved with several projects unified by a belief almost as startling as Schrödinger's cat: that the weirdness of quantum mechanics, which for decades had been regarded as more interesting to philosophers than useful to technologists, can be harnessed to produce computers that, when applied to problems like database searching and simulation of quantum systems, would make those existing today look like fancy abacuses.

"A quantum computer would be capable of solving many problems which are intractable on even the most powerful classical computer."

Plourde and his team hope to explore "the strangeness of the quantum world at the scale of circuits on a chip," he says, with the goal of developing the elements of a quantum computer. Such a device The Quest to Create a Superconducting Quantum Computer would execute computations not by manipulating the binary state (on or off) of minuscule spots of electric charge or magnetism, the basis for the 1s and 0s of binary code. Instead, it would carry out computations with superconducting quantum bits, or "qubits."

Each qubit could be placed in an arbitrary superposition of states, not dead cats and living cats but more like, for instance, an ammonia molecule in which the nitrogen atom is above (state 1) or below (state 2) the plane of the three hydrogen atoms. In a quantum computer, "each qubit can be in any arbitrary superposition of states," says Plourde. Partly because the number of possible states is vastly greater than the two used in today's computers, a quantum computer "would be capable of solving many problems which are intractable on even the most powerful classical computer, such as the factorization of large numbers." In a sense, he said, "these entire circuits are able to do two different things at once!" Once the researchers can fabricate a few such circuits on a chip and demonstrate their quantum properties, "it should, in principle, be possible to scale up to many such qubits on a chip using standard microfab techniques," Plourde says.

Another source of a quantum computer's power would come from a property of quantum interactions called entanglement, which seemed so bizarre to Albert Einstein that he used it as part of his decades-long argument that quantum mechanics cannot possibly be correct if it leads to such ridiculous consequences. In a nutshell, entanglement refers to a property of two or more particles that essentially joins them forever. If two particles were created so that they had opposite spins, then before any measurements each particle has a 50-50 chance of being measured in either state. After measuring the state of one particle, however, the state of the other is completely determined; it is as if measuring the spin of one brings into existence the opposite spin of the other. Qubits, too, can be entangled, allowing for computational processes not possible with classical bits.

Since coming to Syracuse in 2005, Plourde has made the university one of the leading players in the development of quantum computing with superconductors. The research is highly collaborative, involving projects with labs from Germany to Wisconsin. A key partnership is with IBM Experimental Quantum Computing Group at the T.J. Watson Research Center in Yorktown Heights, NY, whose proximity allows for frequent exchanges of scientists between Syracuse and IBM. The superconducting circuit technology employed in the IBM/Syracuse effort is also representative of the strength of superconductor research and industry in New York, which boasts a world-leading superconductor device fabrication and digital electronics center at Hypres in Elmsford, superconducting MRI magnets from Philips Medical Systems in Latham and from General Electric Global Research in Schenectady, and superconducting cables from SuperPower in Schenectady. Finally, easy access to the state-of-the­art fabrication capabilities at the Cornell NanoScale Facility and Center for Materials Research has helped Plourde's research.

This is not to say that there are not daunting obstacles to superconducting quantum computing, both scientific and practical. As more superconducting qubits are added to the chips, the circuits become harder to fabricate and control. Also, maintaining a stable supply of liquid helium, which has surged in price in recent years, has made it challenging to operate low-temperature experimental apparatus. Nevertheless, says Plourde, "I am confident that in the coming years, the work we and others are doing will lead to critical advances in the development of novel information processing systems as well as fundamental investigations of quantum mechanics."

Photo: Low-temperature measurement of the oscillatory exchange of a microwave excitation back and forth between a superconducting qubit and a resonant circuit.