In 2019, Google announced that its 53-cubic-meter machine had achieved quantum supremacy – performing a task that cannot be controlled by a conventional computer – but IBM disputed that claim. In the same year, IBM has launched its 53-bit quantum computer. 2020 IonQ introduced a 32-qubit system that the company said was “the most powerful quantum computer in the world.” And just this week, IBM launched its new 127-qubit quantum processor, which was described in a press release as a “little design marvel.” “The great news, from my perspective, is that it works,” said Jay Gambetta, IBM’s vice president of quantum computing.
Now QuEra claims to have made a device with far more qubits than any of these rivals.
The ultimate goal of quantum computing, of course, is not to play Tetris, but to surpass classical computers in solving problems of practical interest. Enthusiasts believe that when these computers become powerful enough, perhaps in a decade or two, they could bring transformative effects in areas such as medicine and finance, neuroscience and AI. Quantum machines will probably need thousands of qubits to solve such complex problems.
The number of qubits, however, is not the only factor that matters.
QuEra also advertises the improved programmability of its device, in which each qubit is one, ultra-cold atom. These atoms are precisely arranged by a series of lasers (physicists call them optical tweezers). Qubit positioning allows the machine to be programmed, tuned to the problem being investigated, and even reconfigured in real time during the calculation process.
“Different problems will require atoms to be placed in different configurations,” says Alex Keesling, CEO of QuEra and co-inventor of the technology. “One of the things that is unique about our machine is that every time we start it, several times a second, we can completely redefine the geometry and connectivity of the qubits.”
The advantage of the atom
The QuEra machine was made according to a design and technology perfected over several years, led by Mihail Lukin and Marcus Greiner of Harvard and Vladan Vuletic and Dirk Englund of MIT (all on the QuEra founding team). In 2017, only the earlier model of the device from the Harvard Group was used 51 cubits; In 2020, they demonstrated a 256-qubit machine. Within two years, the QuEra team expects to reach 1,000 qubits, and then, without much change to the platform, they hope to continue to increase the system over hundreds of thousands of qubits.
The unique QuEra platform — the physical way the system is assembled and the method by which information is encoded and processed — should allow for such volume spikes.
While Google and IBM quantum computer systems use superconducting qubits, and IonQ uses trapped ions, the QuEra platform uses arrays of neutral atoms that produce qubits with impressive coherence (that is, a high degree of “quantum”). The machine uses laser pulses to guide atoms in interaction, exciting them into an energy state – the “Rydberg state”, described in 1888 by the Swedish physicist Johannes Rydberg – in which they can work quantum logic in a robust way with high fidelity. This Rydberg’s approach to quantum computing which had been in the works for decades, but required technological advances – for example, with lasers and photonics – for it to work reliably.
When computer scientist Umesh Vazirani, director of the Berkeley Quantum Computation Center, first learned of Lukin’s research in this regard, he felt “irrationally boisterous” – it seemed like a miraculous approach, although Vazirani questioned whether his intuition was in touch with reality. “We had a variety of well-developed pathways, such as superconductors and ion traps, that have been worked on for a long time,” he says. “Shouldn’t we think of different schemes?” He reported to John Preskill, a physicist at the California Institute of Technology and director of the Institute for Quantum Information and Matter, who convinced Wazirani that his exuberance was justified.
Preskill finds Rydberg’s platforms (not just QuEra’s) interesting because they produce qubits in strong interaction that are very intricate – “and there’s quantum magic,” he says. “I’m pretty excited about the potential to discover unexpected things in a relatively short period of time.”
In addition to simulation and understanding quantum materials and dynamics, QuEra is working on quantum algorithms to solve computational optimization problems that are NP-completely (i.e. very difficult). “These are really the first examples of useful quantum advantages involving scientific applications,” says Lukin.