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New Wavelength-Tunable Silicon Photon-Pair Combined with Pump Rejection Filter in a CMOS chip

Researchers have demonstrated a new wavelength-tunable, silicon photon-pair source integrated with a pump rejection filter in a single CMOS chip. The new device represents an important step toward an entangled photon source that incorporates active photonic devices and feedback control circuits on the same CMOS chip.

Graduate students Josep M. Fargas Cabanillas from Boston University, USA; Danielius Kramnik of University of California, Berkeley, USA; and Anirudh Ramesh of Northwestern University, USA jointly headed the three-university collaboration that led to the work, which will be presented at the Frontiers in Optics + Laser Science Conference (FiO LS) all-virtual meeting, 31 October – 04 November 2021.

Cabanillas' presentation is scheduled for Tuesday, 02 November at 08:00 EDT (UTC – 04:00).

"A tunable, self-contained source of quantum photon states on chip allows such sources to be arrayed in photonic integrated circuits that could enable novel types of low-light sensors and imagers, perhaps even on mobiles, but also components for new quantum optical networks that may be a forthcoming future type of information infrastructure in our society," said Cabanillas.

Using quantum technology for networking or information processing requires a source of indistinguishable correlated photon pairs. However, when spontaneous four-wave mixing is used to generate these photon pairs, it can be very challenging to isolate single photons from the strong pump field.

In previous work, the researchers tackled this challenge by creating the first photon source that integrated a spontaneous four-wave mixing cavity and pump rejection filter on a single chip with no external pump filtering.

"A bright classical light source produces the tiny quantum state on chip, but subsequently completely blinds the rest of the circuit from seeing the latter. Our integrated filter finds those needles in a haystack! It shuttles most of the pristine quantum states to a quiet part of the circuit, while dimming the bright classical mother source by 10 billion times," adds Cabanillas. However, because this design was passive, multiple copies of the source could not be tuned to the same wavelength, which is necessary for some quantum applications.

In the new work, the researchers improved on their previous design by creating a microring-based source and integrated pump rejection filters based on thermally tunable rings. The new design not only cuts the device footprint in half but also will enable quantum interference between multiple identical sources implemented and controlled on a CMOS photonics platform. The source can be made using standard CMOS manufacturing techniques that can also support hundreds of electronically controlled photonic devices operating alongside millions of transistors.

"Scalability is a key requirement for useful quantum systems, and has been a challenge in the field. CMOS electronic-photonic integrated circuits are a promising technology," says Kramnik, who led the integrated electronic circuit design efforts. To this, Ramesh, who led the characterization efforts, adds, "Although the currently demonstrated performance has limitations, being in CMOS allows rapid iteration and improvements, as we have seen with previous, classical optical communication technologies developed in these platforms. So it will be with bringing about quantum electronic-photonic sources. But it will take a team effort… a true quantum superposition of it!"

Tests of the new device revealed a maximum coincidence-to-accidentals ratio (CAR) of 9.1 at -13 dBm power in the input fiber and a maximum coincidence rate of 21 counts per second at 0 dBm power in the input fiber. The CAR and coincidence rate were limited by insertion losses that occurred because of mismatches between the input/output fiber aperture and the spot size of the grating couplers. The researchers anticipate that using fibers matched to the grating coupler apertures will significantly improve the device performance.

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