It’s a big day for quantum physics. Two teams of researchers have published papers in Nature detailing how they were able to create unprecedented quantum simulators consisting of over 50 qubits. This means that these experimental quantum computers are on the cusp of being able to model physics that is too complex even for the most powerful conventional supercomputers.
Quantum simulators are a special type of quantum computer that uses qubits to simulate complex interactions between particles. Qubits are the informational medium of quantum computers, analogous to a bit in an ordinary computer. Yet rather than existing as a 1 or 0, as is the case in a conventional bit, a qubit can exist in some superposition of both of these states at the same time.
Prior to this breakthrough, leading research was only able to create quantum simulators consisting of a few dozen qubits. In the case of the University of Maryland and National Institute of Standards and Technology team, who were behind one of the two new papers, the researchers were able to create a quantum simulator with 53 qubits.
These qubits are made from ytterbium ions and are strung together in rows like pearls on a laser necklace. In the quantum simulator, lasers are used to manipulate the qubits in a vacuum chamber to simulate quantum interactions between the particles—in this case, quantum magnetism.
For the Maryland experiment, each of the qubits was a laser cooled ytterbium ion. Each ion had the same electrical charge, so they repelled one another when placed in close proximity. The system created by Monroe and his colleagues used an electric field to force the repelled ions into neat rows.
At this point, lasers are used to manipulate all the ytterbium qubits into the same initial state. Then another set of lasers is used to manipulate the qubits so that they act like atomic magnets, where each ion has a north and south pole. The qubits either orient themselves with their neighboring ions to form a ferromagnet, where their magnetic fields are aligned, or at random. By changing the strength of the laser beams that are manipulating the qubits, the researchers are able to program them to a desired state (in terms of magnetic alignment).
According to Zhexuan Gong, a physicist at the University of Maryland, the 53 qubits can be used to simulate over a quadrillion different magnetic configurations of the qubits, a number that doubles with each additional qubit added to the array. As these types of quantum simulators keep adding more qubits into the mix, they will be able to simulate ever more complex atomic interactions that are far beyond the capabilities of conventional supercomputers and usher in a new era of physics research.
Fortunately the Maryland team isn’t the only group working on the problem of precise qubit control. Another team from Harvard and Maryland also released a paper today in which it demonstrated a quantum simulator using 51 qubits. The experiment was very similar to the Maryland quantum simulator, but instead of ytterbium ions, the Harvard researchers used non-charged rubidium atoms as their qubits. The difference here is that the charged ytterbium atoms can be sustained in the Maryland system for several hours, whereas the Harvard system can sustain their rubidium qubits for a few seconds.
The important thing, according to Monroe, is that both quantum simulators were able to manipulate the states of over 50 individual qubits using lasers. This degree of control is unprecedented and a crucial development on the road to building large-scale quantum computers. Monroe cited improvements in optical technologies as the reason the teams were able to exert this much control.
“Companies like D-Wave claim to be producing two thousand qubits on a chip,” Monroe said, referring to D-Wave Systems, a Canadian company that sells quantum computers. “The problem is that those qubits don’t behave very well and they’re not being controlled at the level of the individual qubit.”
“That’s why we’re excited about our technologies, because they’re much more flexible and reconfigurable than any solid state quantum computing technologies,” Monroe added. “These two experiments at Maryland and Harvard are the largest simulators with control of individual qubits.”
Indeed, Monroe said these quantum simulators may already have surpassed the modeling abilities of conventional computers.
“We don’t know how to calculate what we measured with 53 qubits [using a conventional computer],” Christopher Monroe, a professor of physics at the University of Maryland, told me on the phone.
Still, Monroe said he tries to “steer away from the term ‘quantum supremacy’,” or the point when quantum computers are able to perform calculations beyond the capabilities of any conventional supercomputer.
“It's very hard to prove that anything we do in these simulations is indeed hard,” Monroe said. “So saying something is supreme is pretty strong, but 53 qubits is indeed a milestone.”
Looking to the future, Monroe hopes that the laser configurations specially made for these experimental quantum simulators can be industrially engineered so they are more readily available for large quantum computers.
“We’re very bullish on the idea the same physics in the simulator will turn into a quantum computer,” Monroe told me. “Sooner or later these companies are going to engineer optics, and when that happens—watch out. Then they’ll really be able to start scaling and make big quantum systems.”