Entangled photons are the workhorses of quantum technologies: they are needed for quantum computing, secure communication, and ultra-sensitive measurements. Most current sources require a bulky external laser for optical pumping, which hinders scalability. This work sits alongside efforts to integrate sources on chips: neighbors use thin-film lithium niobate, silicon, and silicon nitride, but almost all still rely on external pumping. The first fully chip-based source based on spontaneous four-wave mixing in silicon nitride was dim and narrowband. Our work closes this gap: electrical pumping, polarization entanglement, and record brightness in a single device.
The heart of the device is the hybrid integration of a distributed Bragg reflector (DFB) laser at 780 nm with a photonic circuit on thin-film lithium niobate (TFLN). The laser is butt-coupled to the waveguide on the chip; its pump current of 40 mA gives 7.5 mW of output power. Light is split by a multimode interferometer (MMI) into two channels, each passing through a periodically poled lithium niobate (PPLN) waveguide 6 mm long. There, spontaneous parametric down-conversion (SPDC) generates pairs of photons at 1559 nm. A polarization rotator-combiner (PRC) then converts the horizontally polarized pairs from the lower waveguide into vertically polarized ones and combines them with the upper channel. The result is a state of the form |ψ⟩ = α|$H_sH_i$⟩ + βe^{iφ}|$V_sV_i$⟩, which becomes maximally entangled when the amplitudes are equal. Temperature tuning of the chip up to 67 °C aligns the phase matching with the laser wavelength.
At an on-chip pump power of 18.8 μW, a brightness of 6.2×10⁸ pairs/s/nm/mW was measured, yielding a pair generation rate of 4.5×10¹⁰ pairs/s/mW over a 73 nm bandwidth. This is six orders of magnitude higher than the previous electrically pumped source on silicon nitride, and an order of magnitude broader in spectrum. Quantum state tomography in three pairs of frequency channels (1584.53/1534.25 nm, 1571.24/1546.92 nm, 1567.95/1550.12 nm) showed Bell state fidelities of 0.965, 0.971, and 0.969, respectively. Violation of Bell's inequality was confirmed with visibilities above 0.97 in the horizontal-vertical and diagonal bases. The device, measuring 15×20 mm², operates at room temperature and is powered by a portable battery.
This work stands out from the neighbors by being the first to combine electrical pumping, polarization entanglement without postselection, and record brightness on a thin-film lithium niobate platform. Previous integrated sources either required an external laser, produced only temporal entanglement, or were orders of magnitude weaker. Here, the laser and the nonlinear material live on the same crystal, and the polarization rotator-combiner is fabricated with the same process as the waveguides. This is a step toward fully autonomous quantum systems on a chip.
The next frontier is full monolithic integration: transferring the laser onto the same chip via wafer bonding or micro-transfer to eliminate the butt coupling and improve light injection efficiency. Further, modulators, filters, and detectors can be integrated on the same lithium niobate platform, creating an 'all-in-one' quantum chip. Wavelength multiplexing will pave the way for multi-channel sources for quantum networks.
The technology directly impacts quantum communication: satellite links, fiber-optic networks, and quantum key distribution. It will also accelerate the development of quantum sensors and metrological instruments.
Optimize the geometry of the polarization rotator to reduce losses and increase polarization contrast. Test the source in field conditions with battery power.
In quantum photonics, the problem of scalability remains unsolved: how to scale up the number of entangled photons without exponential growth in losses and instability. Our source solves part of the problem—compactness and brightness—but the integration of detectors and control electronics on the same chip remains an open question.
This section is addressed to the author of the paper and their colleagues. Neighbouring works were found by meaning-based search across our archive, not by word overlap. We do not review: we show the field around the work.
What the work does
The work presents a hybrid on-chip source of entangled photons: a 780 nm DFB laser is integrated with a photonic circuit on thin-film lithium niobate. At a pump current of 40 mA, it achieves a brightness of 4.5×10¹⁰ pairs/s/mW over a 73 nm bandwidth, with Bell-state fidelity exceeding 96% in several frequency channels.
Where its strength is
For the first time, electrical pumping, polarization entanglement without postselection, and record brightness are combined on a thin-film lithium niobate platform, paving the way for fully autonomous quantum systems on a chip.
Where it could go next
Worth trying
Why this may matter
A fully integrated, electrically pumped entangled-photon source could become a key component for scalable quantum communications and computing, eliminating the need for external lasers and reducing system complexity.
Nearby works in our archive
The archive is sparse around this work: the closest review is only 0.57, against a usual 0.64. It looks like few have drilled nearby.
The 73 nm bandwidth is equivalent to about 9 terahertz, which is thousands of times broader than the best single-photon sources based on quantum dots.