Integrated photonics may light the way to quantum computing
Since the 1980s, researchers have been chasing after the quantum computer. Such a computer, they believe, could transform the task of information processing by handling data in novel, unprecedented ways. Whereas current computers can only process bits that occupy one of two states (0 or 1), one promising implementation of the quantum computer would rely on arrays of atoms in quantum states, called qubits.
Thanks to the strange nature of the quantum realm, qubits can occupy both the 0 and 1 states simultaneously and can also be entangled with, and thus closely influenced by, one another. Researchers are just beginning to explore the potential processing power that these qubits could unlock.
Yet, the quantum computer has always remained just out of reach because of various fabrication difficulties. For example, researchers are able to manipulate only a small number of qubits — on a scale of tens — as opposed to the required thousands or millions.
“Across all the groups in the world that are working on quantum computing, no one has developed a way to control a very large number of qubits such that you can use them to perform an actual computation of interest,” said Jeremy Sage, a member of the technical staff in Lincoln Laboratory’s Quantum Information and Integrated Nanosystems Group. “We can’t yet do anything that is both practical and better than what a classical computer can do.”
Sage and John Chiaverini, a senior staff member in Sage’s group, lead a team that is pursuing scalability by merging photonic integrated circuits (PICs) with a quantum computing method based on charged atoms, or ions, trapped above the surface of a chip. In 2016, in collaboration with MIT, the team demonstrated that PICs could be used to effectively manipulate the quantum states of ions by performing quantum gates. A quantum gate is the quantum version of a logic gate, which processes information by producing output bits (or qubits) based on inputs and a simple set of logical rules. The Laboratory team’s most recent milestone represents a breakthrough in the precise delivery of light from lasers to the trapped ions by significantly extending the range of wavelengths over which the PICs operate.
“We use lasers to rip off electrons, cool the ions down, and perform quantum gates,” Sage said. These changes to the ions that the lasers bring about are what would power the quantum computer. The ions that the team chose to use for their research are strontium and calcium, which react to specific wavelengths of light. “It turns out we need about 12 different laser colors that range from the near-ultraviolet to the near-infrared,” Sage added.
At the moment, most researchers shine lasers through windows in vacuum chambers to
strike the ions, but this approach leaves a lot of room for error. While it’s possible to hit a few individual ions precisely, scaling to the millions introduces a high probability of hitting the wrong one.
“What we’re trying to do is deliver the light in a different way by integrating the required light-delivery optics into the chip itself,” Sage explained.
“Our PICs distribute the light from several input lasers to an array of trapped ions,” said Paul Juodawlkis, assistant leader of the Quantum Information and Integrated Nanosystems Group, who leads the integrated photonics projects at the Laboratory. “At each trapped-ion site, we use devices called vertical grating couplers to redirect the laser light out of the PIC and focus it on an individual trapped ion.”
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