Computing with Quantum Cats (27 page)

Anderson returned to Bell Labs, where he and John Rowell made the first working Josephson junction and confirmed the reality of the Josephson effect. Meanwhile, in August 1962 Josephson wrote up his work as a “fellowship thesis” in support of his (successful) application for a research fellowship at Trinity College; this may be a unique example of such a thesis being worthy of a Nobel Prize! There were originally just two copies of this masterpiece, one submitted to Trinity and one kept by Josephson; a third (a photocopy) turned up in
Chicago, but Anderson does not know how it got there. More formal publication came in the journal
Physics Letters
later in the year (volume 1, page 251). But in accordance with the regulations of Cambridge University, Josephson still had to remain “in residence” for another two years before he could be awarded his doctorate.

Josephson's paper was as comprehensive as it could possibly have been. It was clear from the outset that the Josephson effect had many practical applications, some of which I will describe. At Bell, Anderson and Rowell consulted their resident patent lawyer about the possibilities: “In his opinion, Josephson's paper was so complete that no one else was ever going to be very successful in patenting any substantial aspect of the Josephson effect.” Nevertheless, they took out patents, but these were never enforced so have never been challenged.

Many of the practical applications of Josephson effect devices depend on their extreme sensitivity to magnetic fields—so extreme that in some cases the Earth's magnetic field has to be compensated for. In one of the early experiments by Anderson and Rowling, they found a “supercurrent” of 0.30 milliAmps in the Earth's magnetic field, which increased to 0.65 mA when the field was compensated for. (
Less
field means
more
current.) Probably the most widespread application is in calibrating and controlling voltage. A Josephson junction exhibits a precise relationship between frequency and voltage. Since frequency is defined in terms of standards such as the emission from atoms of cesium, in effect this leads to a definition of the volt; turning this around, the effect is used to ensure the accuracy of DC voltage standards. Another application, the superconducting single-electron transistor, is a
charge amplifying device with widespread potential uses; Josephson devices can also be used as fast, ultra-sensitive switches which can be turned on and off using light, increasing the processing speed of classical computers, for example, a hundredfold. A group at the University of Sussex, headed by Terry Clark, has developed a device based on this technology which is so sensitive that it can monitor a person's heartbeat from a meter away, without the need for any electrical connections; the technique has been extended to monitoring brain activity. This list is by no means exhaustive; but what we are interested in here is the application of the Josephson effect to quantum computing, where the key (as with some of the other applications) is the superconducting quantum interference device, or SQUID. A SQUID is a ring of superconducting material incorporating a single Josephson junction, so that electric current can flow endlessly around the ring without any voltage being applied. But, in an echo of Schrödinger's famous cat “experiment,” the
same
electric current can flow both ways around the ring at once. Quantum superposition can be demonstrated in a macroscopic object, one big enough to see and feel—I've held one in my own hand, courtesy of Terry Clark.

LEGGETT AND THE SQUID

My own introduction to SQUIDs came in the 1970s, through my contacts at the University of Sussex, where Tony Leggett, a leading low-temperature physicist and later Nobel Prize winner, was then working. Leggett's unusual route to a Nobel Prize in physics is worth reporting. He was born into a Catholic family in Camberwell, South London, in 1938, the eldest of five children (he had two sisters and two brothers).
The family soon moved to Upper Norwood, where he attended a Catholic elementary school before moving on to the College of the Sacred Heart in Wimbledon. Then, at the end of the 1940s, when his father got a job teaching physics and chemistry at a Jesuit school (Beaumont College) in Windsor, all three of the Leggett boys were allowed free tuition there, and the family moved to Staines, near the modern site of Heathrow Airport. In line with normal practice at the time, at the age of thirteen Tony had to choose which academic path to follow—classics, modern languages, or mathematics and science. As was also the practice at the time, the brightest pupils were steered towards classics, science being regarded as somewhat beyond the pale. So, in spite of his father's position, Tony became a classics scholar, studying Latin and Greek languages and literature. Even here, among the elite, he stood out academically, and was placed in classes with boys two years older than himself, eventually winning a scholarship to Balliol College, Oxford, at the end of 1954. The scholarship was, of course, to study classics (the course usually known as Greats); but in the interval between being awarded the scholarship and going up to Oxford in the autumn of 1955, Leggett was introduced to mathematics by a retired university teacher, a priest who was living at Beaumont—more or less as a hobby to pass the time. Although Leggett was fascinated by the subject and found he had an aptitude for it, math slid into the background during a happy undergraduate career reading Greats. But near the end of the third year of the four-year course “it gradually began to dawn on me,” says Leggett, “that I could not go on being a student forever and must start looking for gainful employment.”

The most desirable possible career seemed to be the nearest thing to a continuation of the student life—a PhD and a university lectureship. The natural choice for someone with Leggett's background would have been philosophy; but he was put off by the fact that, as it seemed to him, there was no objective truth in philosophy, no criterion of whether a piece of work is “right” or “wrong.” He wanted, he said, to work in a field where there was “the possibility of being wrong without being stupid.” The subject which fits that criterion par excellence is physics. With no formal training in the subject, but with the confidence instilled by his experience of advanced mathematics, in the summer of 1958 Leggett applied to do a second Oxford degree, this time in physics, following the completion of his Greats course in 1959. Apart from convincing the powers that be at Oxford that he could cope, there was another not inconsiderable hurdle to surmount. The next year, 1959, was to be the last of compulsory military service in the UK, and as a student Leggett would be exempt; so he had to persuade the draft board that his second undergraduate degree was not merely a ruse to escape the call-up. He is convinced that a major factor in gaining the necessary exemption was the fact that in 1957 the Soviet Union had launched the first artificial Earth satellite,
Sputnik 1
, and the authorities had at last woken up to the importance of directing the best brains into science and engineering rather than classics (one wonders how many potential good scientists of Leggett's generation were lost by the streaming of the brightest students into the classics). With the draft board convinced, and a new scholarship from Merton College, Leggett commenced his physics course in 1959, emerging with a first class degree and going on to
postgraduate work which led to a doctorate in 1964, awarded for investigations into the behavior of superfluid liquid helium.

By then, he was supported by a fellowship at Magdalen College, which enabled him to spend a year at the University of Illinois at Urbana-Champaign, and a year at Kyoto University in Japan. In Kyoto, he immersed himself in the culture, living in Japanese accommodation, learning the language and avoiding “foreigners”; this was so unusual in the mid-1960s that, he later learned, his colleagues decided that he must be a trainee CIA agent. After Leggett took up a lectureship at Sussex University (where our paths crossed) in 1967, he continued to travel widely to work at research establishments around the world during the vacations, including another extended visit to Japan after he married a Japanese girl (whom he had actually met at Sussex) in 1972. In 1982, two years after being elected a Fellow of the Royal Society, he was offered and accepted a professorship at Urbana-Champaign, where he was based for the rest of his career. He was awarded the Nobel Prize in 2003 for his contributions to the theory of superconductors and superfluids, and knighted in 2004. “Above all,” he says, “I have worked on the theory of experiments to test whether the formalism of quantum mechanics will continue to describe the physical world as we push it up from the atomic level towards that of everyday life,” and he encapsulates this enterprise in his description of a SQUID-based device that acts like “Schrödinger's cat in the laboratory.”

Common sense would tell us that in a SQUID ring, an electric current could flow one way or the other around the ring, but not both ways at once. Quantum physics says that the
ring could exist in a superposition of states, like Schrödinger's cat, one corresponding to a clockwise current and one corresponding to a counterclockwise flow. This is
not
the same as saying that there are two separate currents, with one stream of electrons going one way and one stream going the other way; the whole ring, a visible, macroscopic object, is in a superposition. Theorists such as Leggett calculated that this situation should produce a measurable effect, described as a “splitting” of energy levels in the system. At the beginning of the twenty-first century, just such an effect was observed in delicate experiments at the State University of New York at Stony Brook, and at the Technical University of Delft. Subsequent experiments have confirmed the reality of these macroscopic superpositions. As Leggett puts it, this is “strong evidence for the existence of quantum superposition of macroscopically distinct states.”

All this has profound implications for our understanding of the nature of quantum reality. It suggests that the measurement problem cannot simply be explained (or wished) away by saying that “collapse of the wave function” occurs just because objects are macroscopic. This might lead to a whole new understanding of quantum reality. But such deep waters are not my concern here. What matters FAPP is that quantum superposition and entanglement involving SQUID rings make them candidates for use in quantum computers.

COMPUTING WITH SQUIDs

The advantages of using SQUIDs are that both the current and the phase
⁵
in the ring are quantum entities which can be in a quantum superposition, making them suitable for use as qubits, while different SQUIDs can be entangled with
one another. So far, experiments have been done entangling both two and three superconducting qubits, involving simple processors that are in effect solid-state quantum processors resembling conventional computer chips. This makes it possible in principle to build CNOT and other gates. Even better, coupling three qubits is particularly important for some quantum error-correction processes which were demonstrated in a three-qubit system of this kind by Yale researchers in 2012. SQUIDs have the huge advantage over atomic-scale systems that they can be “engineered” in a more or less standard, classical way, and manufactured in large numbers on chips using existing technology (they don't
have
to be as big as wedding rings, of course). A team at the University of California, Santa Barbara, has already managed to put nine Josephson-junction-based quantum devices on a single 6 mm by 6 mm chip, although this does not in itself function as a quantum computer.

The disadvantages are that neither the superpositions nor the entanglements last long (nothing unique about that), and the whole thing has to be operated at very low temperatures, close to absolute zero (–273 degrees Celsius) at 0.8 degrees on the Kelvin scale. But researchers have cracked one of the key DiVincenzo requirements of a quantum computer, by using SQUIDs to develop the quantum bus that is needed to interact with each of the qubits in the computer.

The technique involves placing two SQUIDs in a cavity between two layers of conducting material, where microwave photons (just like the photons in a microwave oven) can bounce between the conductors. The SQUIDs can emit and absorb photons, and the way they do this is tuned by adjusting the voltage across the gap. A SQUID that absorbs a photon
becomes “excited,” and if it is already excited it can be triggered to emit a photon into the cavity. This process is a specific example of the phenomenon known as resonance, and is sometimes referred to as “hybridization” of the qubit and photon states. In resonance, the SQUID switches back and forth between excited and non-excited states. In order to transfer a quantum state from one SQUID to the other, the SQUIDs are first both tuned “off resonance” and one is put in some specified quantum state. This SQUID is then tuned in resonance to the cavity, and at the appropriate moment when the quantum rules tell us that the probability of its being excited is zero, it is tuned off resonance. This leaves the photon that “belongs” to the SQUID bouncing around in the cavity. The second SQUID is then tuned into resonance, and interacts with the photon left behind by the first SQUID. At the appropriate moment this SQUID is again tuned off resonance. At that point, the quantum state of the first SQUID has been transferred to the second SQUID. This is known as “quantum optics on a chip.” It's a small step—transferring a state between a single pair of SQUIDs. But it has been done, and it is a step in the right direction, suggesting, perhaps, a specific role for SQUIDs in computers that may also incorporate other kinds of qubits.

Perhaps significantly, IBM is making a major effort to develop superconducting quantum computer technology. Asked how long it will be before we see a practicable quantum computer, Mark Ketchen, manager of the Physics of Information group at IBM's Watson Research Center, said in 2012: “I used to think it was 50 [years away]. Now I'm thinking it's 15 or a little more. It's within reach. It's within our lifetime.”
⁶