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expanding quantum computing using integrated entangled light sources overcomes traditional limitations of quantum photonic systems.
recently, an international research team from leibniz university hannover, the university of twente and the startup quix quantum demonstrated a fully integrated entangled quantum light source on a chip. this breakthrough marks an important step towards the scalability of quantum technology, enabling the integration of quantum light sources into stable, small devices. the scientific research has been published in nature photonics.
on-chip quantum light sources consist of three main components: a nonlinear medium that generates pairs of entangled photons, a laser, and a filter that ensures laser stability within a certain frequency band.
the team used this layout to create a quantum light source with a laser cavity, a highly efficient (>55 db) tunable noise suppression filter using the vernier effect, and a nonlinear microring dielectric for use in telecommunications bandwidths ( naturally mixed photon pairs in four resonance modes with a bandwidth of approximately 1 thz). the source can detect photon pairs at an astonishing rate of ~620 hz and has a high coincidence/accident ratio of ~80.
a novel hybrid technology combines an indium phosphide laser with a silicon nitride filter on a single chip, making it possible to shrink the size of the light source. this technology is suitable forquantum computingand quantum networks, because they can reduce the size of light sources by more than 1,000 times. the researchers claim that until recently, quantum light sources required external and large laser systems, which hindered their use in the field. despite these obstacles, they have overcome them with a new chip architecture and various connectivity platforms.
quantum interferometry with visibility up to 96% and density matrix reconstruction from state tomography both confirm that the source directly generates entangled quantum states (qubits) with high frequency density. this results in fidelity as high as 99%.
photonic qubits: advantages and challenges
superposition, entanglement, and interference are fundamental ideas in quantum theory that are directly relevant to quantum computing. superposition refers to the fact that a particle can exist in several states at the same time; entanglement refers to the phenomenon that particles can be related to each other even at a physical distance; interference refers to the phenomenon that particles can enhance or cancel each other.
quantum light sources produce the fundamental components of quantum computers and quantum networks, known as qubits. photonic qubits offer several advantages over other forms of qubits, including those based on superconducting devices or trapped atoms. for example, photonic qubits are less susceptible to environmental noise (which can disrupt fragile quantum systems) and do not need to be cooled to cryogenic temperatures.
but photonic qubits are more prone to leakage and therefore more difficult to entangle—a necessary step for calculations involving multiple qubits simultaneously. improving light-based quantum computers requires photonic integration—the confinement of photons traveling in micron-wide waveguides etched into circuits.
quantum technology
developing fully integrated quantum processors that can be produced at scale is one of the thorniest hurdles in building quantum computers. trapping ion qubits is typically controlled by individual laser beams, requiring precise alignment, but this approach becomes impractical when the number of qubits increases.
by enabling tens or even millions of qubits, future quantum devices will seek to reduce the complexity of quantum computers and thus increase scalability. ion trap quantum computers use single atoms as qubits through coulomb interactions, which become positively charged after ionization. electromagnetic fields arrange these atoms into lattice patterns, while lasers create quantum gates that change the electron's state.
incorporating chip-level control of these qubits is the biggest difficulty. although they are conventional tools, lasers can cause errors and are difficult to combine.
laser integrated photon quantum light source with frequency entangled photon pairs
design
the design solves many important problems in quantum photonics. the light source is a hybrid integrated iii-v reflective semiconductor optical amplifier (rsoa) with a silicon nitride (si3n4) based feedback circuit. the 700-meter-long quantum well amplifier manufactured by fraunhofer hhi produces a gain of approximately 1,550 nm. using an adhesive bond between the iii-v waveguide and the si3n4 waveguide, the optical system has perfect alignment. for better performance, sloped sides and anti-reflective coatings reduce back reflections.
the integration of waveguide feedback circuitry reduces the intrinsic laser linewidth and eliminates noise, thereby improving the stability and quality of entangled photons. furthermore, the low loss and strong nonlinear refractive index of si3n4 facilitate high-power operation and efficient photon generation. to ensure optimal performance for quantum applications, the device also includes a microring resonator (mrr) to improve signal transmission and photon pair generation.
the si3n4 feedback circuit consists of multiple microring resonators (mrrs) whose design is based on the vernier effect. the mrr is precisely dimensioned to ensure efficient filtering and single-mode laser operation; the ring is selected to reduce losses and maintain a low bend radius. a resistive heater is also included in the circuit for thermal tuning so the feedback mechanism can be precisely controlled.
highly reflective coatings and sagnac loops, combined with a mach-zehnder interferometer (mzi) for balanced feedback, form the mirror of the laser cavity. mode matching is optimized to minimize losses between the gain chip, feedback chip and fiber, ensuring optimal efficiency through the extraction port connected to polarization maintaining fiber. vernier filtering achieves high side mode suppression ratio (smsr) and significantly reduces amplified spontaneous emission (ase) noise, thereby enhancing the noise suppression capability of hybrid quantum sources.
one of the most unique features of this design is the differential extraction efficiency of signal and idler photon pairs, which are generated by spontaneous four-wave mixing (sfwm) in the mrr. the design guarantees almost 100% extraction of non-classical photon pairs while minimizing the presence of pump photons at the output, thereby improving overall signal quality for quantum applications.
microring design and q-factor tuning are important to system performance because they balance coherence length, photon pair generation rate, and system stability. this system is perfect for quantum communication and computing applications because careful tuning of coupling coefficients and thermal effects allows for high coherence times and minimal losses.
this fully integrated approach enables a small and reproducible supply of entangled photons that can be used for practical purposes, marking an important step towards scalable quantum technologies. as a strong contender for next-generation quantum communication and computing systems, the photon pair generation rate and coincidence to accident ratio (car) are comparable to other platforms.
this discovery overcomes traditional limitations of quantum photonic systems and opens the way to more accessible and powerful quantum devices, thus promoting the development of quantum information processing science.
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