With our attention monopolized by AI and machine learning, recent progress made in the other digital revolution, quantum computing, rarely gathers much notice.
Nevertheless, a fascinating and important reality about quantum computing, besides the ambiguous properties of its qubits (quantum bits) to be in different states at the same time, and the nearly impossible operating environment (-460° F), is the promise of a calculating engine that will solve problems that would take a fast classical (conventional) computer billions of years to perform. One particular quantum calculation that has interested privacy experts is the ability to do factoring of large numbers that could render all historical RSA encryption as crackable.
On July 18, 2019, Kevin Hartnett, senior writer for the online Quanta Magazine, posted an interesting update on the subject, “Quantum Supremacy Is Coming: Here’s What You Should Know.”
Although the phrase “quantum supremacy” sounds like a marketing goal, Hartnett explains, “To achieve quantum supremacy, a computer would have to perform any calculation that, for all practical purposes, a classical computer can’t.” It’s a milestone to measure quantum progress, but it’s also a dramatic threshold that, unlike most things about quantum computing, is relatively easy to understand.
Hartnett does admit that the milestone is artificial, and he quotes Robert Sutor, the executive in charge of IBM’s quantum strategy, who is dismissive of its value: “Quantum supremacy, we don’t use [the term] at all.” Yet it’s worth recalling that as IBM was developing its AI machines, the important AI milestones were marked with events like Big Blue defeating chess grandmaster Garry Kasparov, and, later, Watson routing the human Jeopardy champions.
Hartnett points out that having the goal of building a quantum computer that beats classical computers also allows researchers to “learn things that will allow them to build a more broadly useful quantum computer later on.”
In terms more stark, he quotes theoretical physicist Fernando Brandao of California Institute of Technology, who asserts, “Before supremacy, there is simply zero chance that a quantum computer can do anything interesting. Supremacy is a necessary milestone.”
There are several problems yet to be overcome. Two critical difficulties involve the size of quantum circuits, which are measured in the number of qubits and the number of times you can manipulate them. Both are currently too small to achieve supremacy. That will probably take a computer with 70 to 100 qubits. The Q System 1 Computer displayed by IBM at the Consumer Electronics Show in January 2019 was only a 20-qubit system.
The other problem is that as you work to increase the number of bits of information and speed up how they are managed, the error rate also increases. The current error rate for two-qubit systems is about 0.5%, and that must be reduced to around 0.1%
WILL THERE BE A TOURNAMENT?
To reach the supremacy milestone, Hackett explains, “You have to show two things: that a quantum computer performed a calculation fast, and that a classical computer could not efficiently perform the same calculation.” But the contest isn’t as clear as trapping an opponent on a 64-square game board or more rapidly responding correctly to trivia queries. Even if the quantum computer is faster, there’s still the chance that a different, more efficient algorithm than the one used by the losing classical computer could have matched the quantum computer’s results. A clear win will require evidence that the classical opponent had the best algorithms available at the time.
Many think that Google will be first to reach quantum supremacy, possibly reaching the threshold by the end of 2019. They are joined by IBM, IonQ, Rigetti, and Harvard University, also among the most likely.
Google, IBM, and Rigetti have superconducting circuits made from solid-state material. These require near-absolute zero cooling, and, today, seem to be advancing the most rapidly. The Harvard computers use rubidium atoms, and Microsoft has something called topological qubits that are closer to math than the actual electrons and nuclei of the others’ qubits.
Do you remember Moore’s Law? The one that predicted that the speed and capacity of classical computers would double every two years? Well, quantum computer scientists have Neven’s Law, which calculates how quickly quantum computers are gaining on classical computers. Helmut Neven presented the rule in 2019 at the Google Quantum Spring Symposium.
Hackett reports what the researcher proposed. “Quantum computers are gaining computational power relative to classical ones at a ‘doubly exponential’ rate—a staggeringly fast clip. Instead of increasing by powers of 2, quantities grow by powers of powers of 2. Doubly exponential growth is so singular that it’s hard to find examples in the real world. The rate of progress in quantum computing may be the first.”
In June 2019, Neven commented on quantum supremacy. “We often say we think we will achieve it in 2019.” And he added a note of inevitability, “The writing is on the wall.”
In the future, useful quantum computers will require thousands of qubits, but today we are struggling with reaching 50 in a reliable system. If that sounds like quite a distance to the horizon, consider that in 1984 it was reported that Bill Gates mentioned out loud that “640K ought to be enough for anybody.” And today, a terabyte on the phone in your pocket isn’t impossible.
THE NEXT MILESTONE
There are researchers who believe that practical quantum computing might not possible, but if the supremacy threshold is crossed, the faithful already have a name for the next stage. The next pursuit will concentrate on usefulness, and the goal will be “quantum advantage.” Sutor of IBM explains, “Quantum advantage is the idea of saying: For a real use case—like financial services, AI, chemistry—when will you be able to see, and how will you be able to see, that a quantum computer is doing something significantly better than any known classical benchmark?” Reach that point and you will be on the second plateau, working not on specific calculations but broad-based applications.
The analysts at Gartner have published a Qubit Timeline Estimate that puts us currently in a “transient quantum supremacy” stage and pegs 2024 as the year for reaching “quantum advantage.”