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Quantum Photonics on a Single Chip?

Lithium niobate is becoming the silicon of quantum optics

2 min read
Small coin next to photonic chip to show scale
Photo: Nanjing University/American Physical Society

Researchers from Nanjing University, Beijing Institute of Aerospace Control Devices, and Southeast University, Nanjing, in China have demonstrated the creation of entangled photons and their manipulation on a single chip. The group reported this research last week in Physical Review Letters.

The researchers used lithium niobate (LN) as the material for the chip. LN, widely used in cellphones and modulators in telecommunications, is a material with a highly nonlinear response to light. Because of these optical properties it allows the integration of a number of quantum devices, and it is becoming the material of choice for the fabrication of photonic chips.

To demonstrate the extent to which the integration of optical elements is possible, the researchers created on the chip nine similar units that produce photon pairs of different wavelengths that match the C and L telecom bands. Three of these units contain elements for the manipulation of entangled photons.


Each of these units consist of three sections. In the first section, polarized light enters a Y branch and then passes through an electrooptical modulator which can change the phase difference between the light in the two paths. Then the photons enter two waveguides with periodical structures, created in the second section of the chip. The periodically poled LN (PPLN) is a part of the chip where the electric dipole moment of the NL crystals alternate from up and down in a periodical fashion, changing the waveguides into miniature "wigglers" if you will. This periodic structure is obtained by applying a very short high voltage pulse to the LN surface through a grid-like periodical electrical contact.

One of the properties of PPLN waveguides is that they can split each photon entering the waveguide into two photons with half the energy of the incoming photon. Because these two photons are created from a single photon, they are entangled.

In the third section the entangled photons meet in a waveguide beam splitter and realize quantum interference, then they are transferred to two adjacent waveguides.

The degree of entanglement can be changed by changing the phase difference between the photons in the two channels with the electrooptical modulator in the first section. The type of entanglement that takes place on the chip is called "path entanglement," explains Ping Xu of Nanjing University and a member of the research group. "The photon can be in one path or the other path simultaneously or separately — it is a superposition of the occupation of different paths," she says.

By changing the phase difference between the two photon streams with the electrooptical modulator, the researchers could control the amount of entanglement — the amount of "bunched" electrons. They verified the existence of the entangled photons with two-photon interferometry (Hong-Ou-Mandel interferometer), which determines whether photons arrive exactly at the same time, which they do when they are entangled.

According to Xu, their research is a definite advance: "For the first time a PPLN is used for waveguide circuits, allowing the compact generation and manipulation of entangled photons," she notes.

Illustration: Nanjing University/American Physical Society

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