#quantumdots

Photon number resolving detector

Researchers have made major advances in photon-number-resolving (PNR) detection, a key technology for quantum computing and secure communications. These groups have overcome traditional detectors to count light particles with unprecedented accuracy, providing a robust barrier against modern cyberattacks.

The Light-Counting Challenge

High-performance quantum technologies like quantum imaging and sensing require the detection and characterization of many-photon states. Conventional single-photon detectors can detect light, but they often have problems distinguishing between one, two, or three photons. This restriction is hazardous because an eavesdropper can employ “multi-photon pairs” to steal information unnoticed in Quantum Key Distribution (QKD).

Emitter Cascade: New Architecture

Researchers from the University of Waterloo have devised a PNR detector system using a cascade of waveguide-coupled Λ-type emitters. Instead of using beamsplitters to distribute photons among detectors, this innovative method uses a chain of atoms or quantum dots coupled to a chiral (one-way) waveguide.

This system employs Single-Photon Raman Interaction. This arrangement uses a Λ-type atom as a “photon-activated switch”. The first photon of a light pulse is coherently redirected to a different output port and detected after interacting with the atom. Importantly, after “capturing” one photon, the atom becomes transparent to the pulse's other photons. Researchers can cascade emitters to deterministically “peel off” and count photons.

Guide to Nonlinear Frontier

A major focus at Waterloo was the shift from linear to nonlinear regimes. Photons are far apart in the linear regime, making them independent. As quantum networks accelerate and pulses compress, photons overlap during the “emitter’s lifetime”.

Multiple photons arriving within the atomic response time generate nonlinear interactions. The researchers found that these interactions cause “saturation effects” and photon correlations like bunching and anti-bunching. The researchers used Green's function formalisms and the quantum trajectory technique to generate complex “scattering matrices” to predict how these interactions affect detector accuracy. They found that increasing the number of emitters in the cascade can compensate for nonlinearity issues and outperform beamsplitter-based detectors in practice.

Protection against “Photon Number Splitting”

At the same time, an Indian Institute of Technology Delhi team proved that multi-photon emissions degrade quantum entanglement. Entanglement, or “spooky action at a distance,” underpins safe quantum communication, but it is not perfect.

Single, double, and triple photon pair events produced by spontaneous parametric down-conversion (SPDC) were separated and measured using parallel superconducting nanowire single-photon detectors at IIT Delhi. Increased multi-photon pair generation causes a “marked reduction” in the Bell parameter (S parameter), a standard indicator of entanglement quality.

Security and technical issues arise from this degradation. In a Photon Number Splitting (PNS) attack, Eve, an eavesdropper, intercepts one photon from a multi-photon state while the rest pass to the permitted recipient. Eve can steal the cryptographic key without alarming a typical detector since the whole entanglement appears intact.

The researchers showed that PNR detectors solve this problem. Accurate photon counts in real time allow genuine parties to discover “anomalies in their expected correlations” and eliminate compromised multi-photon states. This allows Alice and Bob, quantum communicators, to detect eavesdropping and maintain their secure connection.

Technical Specifications and Performance

Using different voltage thresholds, the IIT Delhi P-SNSPDs can resolve up to four photons, proving their remarkable PNR capabilities. These detectors have fast recovery times (80 ps) and high efficiency (up to 99.5%, according to studies).

The Waterloo team's emitter-based device offers high performance using quantum dots in photonic crystal waveguides. Experimental platforms have achieved coupling efficiencies of 0.98 and directionality close to unity. When coupling rates reach GHz, photon absorption and re-emission become fast and efficient, making these devices ideal for high-speed quantum processors.

Road Ahead

Both research say effective PNR detection is “essential for achieving robust, secure quantum communication”. Traditional spatial demultiplexing algorithms have been used, but the deterministic emitter cascade allows new quantum tomography forms and higher precision.

Analogue quantum simulation enters a new era with 15,000-Dot Array in Silicon

Researchers have created a massive, painstakingly engineered analogue quantum simulator that can simulate strongly interacting materials' complex behavior, advancing quantum information research. The research describes the creation of a 2D array of 15,000 atom-based quantum dots, which dwarfs prior attempts and provides a new path to low-temperature physics.

The study, led by Silicon Quantum Computing and UNSW Sydney specialists, represents a big step toward quantum advantage. This new physical platform lets scientists "build" the physics they want to study in a controlled environment, unlike numerical simulations on classical computers, which struggle to simulate large-scale fermionic systems, where electronic correlations define material properties.

Engineering at Atomic Limits

The team's sub-nanometer array construction is key to this success. STM hydrogen lithography was used to meticulously remove hydrogen atoms from a silicon surface to create a mask. Next, phosphine dosing embeds phosphorus atoms into silicon to create “atom-based” quantum dots.

This kind of accuracy across large areas has been difficult until now. The team used a Zyvex Labs STM controller to compensate for mechanical issues like piezo creep and hysteresis. This enabled scaling from a ten-dot record to a massive 100 × 150 square lattice.

“This array size eclipses the previous largest reported arrays,” the scientists said, indicating significant advances in atomic-scale fabrication. The absence of confinement electrodes makes atom-based quantum dots easier to scale into the thousands than gate-defined ones.

Observing Metal-Insulator Transition

Replicating a Mott-Hubbard and Anderson-driven metal–insulator (MI) transition was the main goal. Building four arrays (labeled A through D) with varying inter-dot separations from 7.2 nm to 15.5 nm allowed the team to accurately control the “tunnel coupling” (t), or electron jumping between dots.

Researchers discovered that system conductance rapidly dropped as dot distance rose. C and D became insulators, while A and B became metals. For an electron to migrate onto an occupied site, the on-site interaction (U) costs more energy than its kinetic energy.

The researchers found that they could regulate interaction intensity (U/t) from 14 to 203, giving them unprecedented system state control.

Nature of Matter Investigation

Scientists used magneto-transport experiments and bias spectroscopy to advance. Insulating arrays produced a “hard” gap at low temperatures, impeding charge transmission.

Researchers observed that a perpendicular magnetic field amplified the charge gap. This was due to an electron exchange process that separated spin states by a large Zeeman energy, increasing the interaction-enhanced Landé g-factor.

The scientists also found a two-step thermal activation process in insulating array conductance. This transformation from “inelastic” to “elastic” co-tunnelling matched theoretical assumptions for granular metals, showing that electrons behaved as expected.

Future Signings

RH is a remarkable Hall coefficient result. Researchers found a substantial Hall coefficient drop below 2 K in poorly insulated Array C. Researchers were cautious but found “promising signatures” of Fermi-surface reconstruction, a phenomena associated with high-temperature superconductivity and other exotic states of matter.

Flexibility may be the platform's best feature. Researchers printed hexagonal, honeycomb, and complex Lieb lattices. Physicists like the Lieb lattice because it replicates cuprate superconductor copper-oxygen planes.

A New Physics Frontier

The researchers thinks this platform can solve quantum spin liquid, interacting topological matter, and unique superconductivity problems. These simulators can access regions that ultracold atoms and moiré materials cannot due to their strong interactions and extremely low temperatures (approximately 100 mK).

Using a top gate to control electron density, future research might study these complex materials' evolution. The scientists said the results supports using atom-based quantum dots for large-scale analogue quantum simulation.

Researchers found a surprising quantum physics-nanotechnology event that could change how microchips transfer energy. Quantum scars, unique quantum states, propel electrons along predictable, high-conductivity nanostructure routes, defying randomness. Scientists say this discovery may lead to a new area called “scartronics,” which could enable quicker, thinner, and more effective microchips for smartphones and other gadgets.

What Are Quantum Scars?

Quantum scarring, a once-perplexing quantum physics phenomenon, is behind the breakthrough. Scientists have proven that some quantum states can concentrate along certain channels instead of dispersing uniformly in some chaotic microscopic systems, where particles like electrons behave stochastically and unpredictablely. Scars are high-probability channels that create the lingering impression of classical routes in quantum systems.

Classical physics states that electrons in a chaotic environment should be random. The mist contains quantum scars, islands of order. They steer electrons, which seemed counterintuitive until recently.

Imagine rolling marbles in an uneven dish. The marbles should bounce randomly until they settle, says classical physics. In other quantum systems, marbles follow the same constrained paths rather than dispersing, leaving tracks or “scars” that indicate preferred routes. Quantum scars operate as electron pathways, improving conductivity in chaotic environments.

Quantum Scars Drive Electron Transport—New Discovery

Beyond theory, Harvard and Tampere universities in Finland have shown that quantum scars can considerably increase electron flow through quantum dots, small structures. Quantum effects govern these nanometer-wide semiconductor islands, which function like created atoms.

Experimental and theoretical models published in Physical Review B indicated that quantum scars can transport electrons even in chaotic systems. Effective microelectronic components may benefit from better nanoscale electron conductivity.

PhD researcher Fartash Chalangari, the study's principal author, claims they showed that defects can be advantageous. “Scarred states can enhance electron flow.”

Origins of Scartronics

This discovery opens the door to scartronics, a new technique that uses quantum scars to direct electron transport in nanoelectronics devices. Instead of seeing faults and quantum chaos as obstacles, engineers may embrace scars to boost performance.

Scartronics may leverage semiconductor physics like electronics did in the 20th century. It might develop quantum systems whose scar states boost electron mobility to create transistors and interconnects that work quicker and lose less energy at scales approaching the physical limits of conventional manufacturing.

Why Microchips Matter: Limits

Quantum influences replace classical electron behavior ideas; contemporary microchips work. Since transistors are now measured in nanometers, reducing components smaller becomes harder and more expensive to improve performance. Quantum processes like interference and tunneling can degrade performance as diameters decrease. Scar-enhanced transport offers a novel way to regulate electrons that may overcome these limitations.

Modern chips are accurately built using lithography to etch invisible patterns. These sophisticated methods don't use scar states or other quantum properties. Scartronics uses the quantum landscape to improve performance at scales where classical control fails.

Experiments and Theories

The phenomenon has profound origins in quantum physics, even if new research emphasizes how quantum scars may facilitate electron transport. Scar states can persist and affect electron dynamics, according to theoretical study and recent experimental results in graphene quantum dots.

The complex link between quantum activity and classical mechanics affects scarring. In certain geometries, electrons trapped in quantum dots can resonate in ways that approximate classical periodic trajectories even in chaotic systems. Electrons statistically favor recurring resonances' preferred routes, causing scars.

Future Directions: Theory to Devices

Researchers are developing these ideas into useful technologies. Controlled scar states can act as nanoscale electron conduits or switches, hence scartronics is being studied in transistor design. These components may improve speed and energy efficiency for next-generation consumer devices or high-performance computing CPUs.

Scartronics may also affect quantum computer hardware. Quantum processors need precise quantum state control, while classical transistors dominate modern computers. Scar-induced transport may assist build novel qubit interconnects or readout methods, albeit true integration is still far off.

Other than quantum dots, emerging semiconductors and ultra-thin 2D materials like graphene can cause scar phenomena. Prior research has showed quantum scars in graphene systems, suggesting scar-based effects could be exploited on other platforms.

Road Ahead Challenges

Despite the hype, scartronics face considerable challenges before becoming effective. Quantum scars are delicate and depending on system geometry and environment. Nanofabrication, material science, and quantum control advances are needed to create industrial-scale scar state exploiters.

Even if theoretical models seem convincing, large-scale studies, especially in production settings, are very young. Researchers must also consider scar states' interaction with thermal noise and other practical variables that diminish quantum effects.

broader quantum technology context

Functional quantum scars promote quantum physics. More recent breakthroughs like quantum devices with combined electrical and optical control and superconducting qubits with millisecond coherence times are bringing quantum technology closer to real-world applications.

Researchers Increase Quantum Dots with “Epsilon Near Zero” Materials, Breaking Quantum Communication Speed.

A global team of scientists has improved quantum emitter efficiency at telecommunications frequencies, a critical step toward the "quantum internet." The scientists used colloidal quantum dots (QDs) and a customized coating of indium tin oxide (ITO) to boost beam directionality and brightness while shortening emission lifespan by 54 times. In their recent study, Purdue and Heriot-Watt researchers describe how to build high-speed, on-chip quantum devices that interact with fiber-optic infrastructures.

Limitations of Quantum Speed

Quantum technologies like secure communication and quantum computation require single-photon generators with high repetition rates for “photons on demand”. PbS/CdS (core/shell) quantum dots are popular due to their size-tunable near-infrared emission and room temperature operation, but their extended decay periods have proven a drawback.

Native lifetimes of these emitters are one to three microseconds. This long recovery period limits a quantum network's data transmission speed. Researchers tried to modify emitters' environments to release photons faster using the Purcell effect.

“Epsilon-Near-Zero” Method

The researchers used Epsilon-near-zero (ENZ) or near-zero-index (NZI) materials. The fraction of dielectric permittivity in these materials drops at a certain frequency, creating a distinctive optical environment that can suppress electric fields or modify light emission.

The researchers employed transparent conducting oxide indium tin oxide (ITO) for touchscreens, which is widely used in electronics. ITO is attractive because its Epsilon Near Zero features can be changed during fabrication and it is CMOS-compatible, making it easy to incorporate into semiconductor manufacturing.

Findings from experiments

The researchers compared PbS/CdS quantum dots on 240 nm ITO thin films to those on glass slides to measure the enhancement. With a confocal microscope and superconducting nanowire single-photon detectors (SNSPD), they used a custom time-correlated single-photon counting (TCSPC) device to measure results with sub-nanometer accuracy.

The results were revolutionary for 1350 nm quantum dots, which are inside the ITO's Epsilon Near Zero bandwidth:

The ITO substrate's photoluminescence lifespan reduced from 544 nanoseconds on glass to 10 nanoseconds, increasing speed 54-fold.

When saturation intensity raised from 400 to 3000 kcps, brightness rose 7.5-fold.

Laser-like Directionality: The light's “emission cone” was lowered from 17.6° to 10.3° to increase light collecting into optical fibers or on-chip components.

Reduce Epsilon Near Zero Effect

The scientists performed a control experiment to show that the Epsilon Near Zero condition caused these advances, not the ITO material. They used a second batch of quantum dots that glow at 1450 nm, beyond the ENZ zone, to make ITO act like a metal.

In this “outside” case, benefits were less obvious. About 10% of the lifetime was lost, and the emission cone dropped to 12.8°. This confirmed that the emitter-ENZ spectrum overlap is the key cause of performance gains.

Future of Quantum Networking

Epsilon Near Zero settings manage light-matter interactions reliably and scalablely, according to this study. Creating integrated on-chip devices requires correct emission engineering at telecom wavelengths.

The researchers say this study opens the door to more advanced effects like fully optical quantum networks and super-radiance, where several emitters synchronize to create a large light pulse. The study uses CMOS-compatible materials like ITO to examine mass-manufacturable, high-performance quantum light sources.

Researchers Solve Telecom C-Band Single-Photon Indistinguishability 90% Issues

Deterministic Single-photon sources

The University of Stuttgart and Julius-Maximilians-Universität Würzburg have built deterministic single photon sources, advancing quantum communications. For the first time, a solid-state emitter at the essential telecommunications C-band has exceeded a 91.7% photon indistinguishability barrier, closing the performance gap between on-demand sources and their probabilistic equivalents.

The Ideal Photon Search

“Quantum Internet” demands the ability to produce 100% identical, “indistinguishable” photons on demand. Long-distance quantum networking protocols and quantum logic gates use two-photon interference, which requires this characteristic.

Researchers have generally had two source options. Probabilistic spontaneous parametric down-conversion (SPDC) sources emit high-quality, indistinguishable photons at random times. This randomness makes scaling systems with dozens of photons, such photonic quantum computing, nearly hard.

However, semiconductor quantum dots (QDs) are deterministic “artificial atoms” that release one photon every laser pulse. Despite their success at shorter wavelengths (780 nm to 960 nm), the scientific community has struggled to obtain great indistinguishability in the telecom C-band (about 1550 nm).

Starting the Revolution

The Nico Hauser, Matthias Bayerbach, and Stefanie Barz study team used a specific device architecture to overcome these challenges. Indium arsenide (InAs) quantum dot in InAlGaAs cladding is the source. A circular Bragg grating (CBG) resonator with the quantum dot increased photon output and reduced environmental decoherence. In Nature Communications, the researchers noted that optimum growth reduces dephasing time while incorporation into the CBG resonator reduces excitonic durations. Ternary digital alloying improved crystal quality and structural refinement.

Meeting 90% Benchmark

The study's most notable result is 91.7 ± 0.2% two-photon interference visibility. This value, much higher than past estimates of 72% for comparable devices, sets a new bar for C-band spectral spectrum indistinguishability.

To reach this result, the researchers meticulously examined four laser excitation systems:

Excitation over the band gap: 800 nm pumping. Incoherent excitation with LA-phonon assistance: Blue-tuned from resonance.

Pumping 1404.2 nm is resonance #1.

Pumping 1498.2 nm is resonance #2.

The LA-phonon-assisted excitation approach achieved the maximum indistinguishability and lowest multi-photon emission probability with a second-order autocorrelation value (g(2)(τ=0)) of 0.017 ± 0.001. The researchers attributed this success to the shortest excitonic lifespan under this stimulation mode, which decreases noise exposure.

Moving toward real-world implementation

Telecom C-band selection is not random. This wavelength range is the norm for silicon-based integrated photonics and modern fiber-optic networks since it loses the least signal. To enable scalable quantum technologies, the team showed that deterministic sources can match probabilistic SPDC sources in photon quality.

These devices must overcome difficulties before being deployed in commercial networks. Despite good photon quality, the current setup's efficiency is just 0.5% (adjusted to 2.1% with setup losses). The researchers found that blinking—when the quantum dot randomly stops emitting—is the greatest constraint with a blinking-related efficiency of 26.9%.

Significant Turning Point

The study is a “key milestone” that makes solid-state emitters suitable for photonic quantum networking despite efficiency issues. The authors noted that our findings are crucial to scalable quantum dots-based photonic quantum devices that integrate on-demand operation with exceptional photon quality.

Quantum for All: Open Quantum Initiative's Impact on Global Technology.

Quantum science, hitherto the domain of major research institutes and academic labs, is changing. Leading researchers are leveraging the Open Quantum Institute (OQI) at CERN and the US OQI Undergraduate Fellowship to make the “quantum revolution” accessible and inclusive worldwide. These programs are preparing society for quantum technology by democratizing access to cutting-edge hardware in Geneva and diversifying talent in Chicago.

Global Mission at CERN: Open Quantum Institute The projected Open Quantum Institute at CERN in Geneva drives global activities. With Swiss government assistance and founding partners like UBS, the Geneva Science and Diplomacy Anticipator (GESDA) envisioned this institute. The OQI wants to democratize quantum computing so its great potential is not limited to wealthy governments or businesses.

The institution considers quantum technology a “global public good.” At CERN, the OQI provides public access to cloud-based quantum computing capabilities to close the digital divide. The institution's first three-year pilot term focuses on researching and creating quantum use cases that complement the UN's SDGs.

Energy and climate action advances, complex health simulations, and drug discovery are examples. The OQI lowers barriers to high-level hardware, allowing academics worldwide to help solve humanity's biggest issues.

Skills Gap Filling: US Open Quantum Initiative. CERN examines worldwide infrastructure and diplomacy, while OQI investigates humanity in the US. CQE leads this endeavor to build a diverse and sustainable workforce for the quantum industry's rapid growth.

Bridge the Talent Gap: US Open Quantum Initiative Undergraduate Fellowship. It offers summer research to underrepresented scientific students. This comprises first-generation college students, underrepresented minorities, liberal arts, and Hispanic-serving university students. The OQI focuses on these groups to avoid an isolated “echo chamber” and ensure that the future quantum workforce reflects society's diversity.

Fellowship: Labs to Leadership Beyond an internship, the OQI Fellowship offers more. The 10-week residential program takes students to the "Quantum Prairie," including Argonne, Fermilab, Chicago, and Urbana-Champaign.

A $6,000 housing and travel allowance helps participants focus on research and career progress. Fellows engage in various activities to build a “supportive community” outside the lab. These include:

Professional development workshops for technical and communication skills. High-level scientific and commercial mentoring. Site visits to private industry and linked institutions for a complete quantum environment understanding. This controlled environment ensures that students learn how to manage high-tech research as well as laser positioning and quantum programming.

“Fabulous Five”: Argonne Success Stories Participants have the best view of the initiative's effects. At Argonne National Laboratory, an OQI collaborator, the “Fabulous Five” students completed important research. Integrated into multiple research groups, these fellows studied quantum science.

University of Texas at Austin student Sanya Iyer examined nitrogen-vacancy (NV) centers in diamonds over the summer. These atomic-scale defects enable ultrasensitive quantum sensors. Other fellows, like Jayleen Velez, studied quantum dot chemical synthesis, which is essential for quantum communication and displays. These students are more than observers because they contribute to scientific literature and report their findings to the CQE community at summer's end.

Unified Quantum Future Vision The numerous iterations of the “Open Quantum” notion, from Chicago's educational pipelines to CERN's diplomatic channels, agree that inclusive creativity equals greater innovation.

The OQI in Chicago uses a large network of collaborators, including the Fritz Haber Institute and industry leaders, to establish a “quantum ecosystem”. National lab knowledge will reach the classroom and market through this coordinated strategy.

The OQI at CERN uses the reputation of the world's largest particle physics facility to support a “diplomatic anticipator” paradigm, ensuring that quantum computers are used to promote sustainability and peace.

Conclusion: Next Steps The Open Quantum Initiative defends quantum computing's promise as it becomes a tool. These projects emphasize hardware accessibility and human variety to ensure that quantum physics benefits everyone, regardless of region or socioeconomic background.

Trinity researchers unify magnetodielectric cavity dynamics, a quantum nanophotonics breakthrough.

Quantum Nanophotonics A theoretical breakthrough from Trinity College Dublin's School of Physics and CRANN Institute answers a significant quantum nanophotonics challenge. In APL Quantum, Lars Meschede, Daniel D. A. Clarke, and Ortwin Hess presented a unified theoretical framework for the quantization of electromagnetic resonances, or quasinormal modes (QNMs), in spatially inhomogeneous, dissipative, and dispersive magnetodielectric resonators.

Challenge of Lossy Nanostructures Nanoscale cavity quantum electrodynamics (cQED) designs have advanced faster than theoretical models. Traditional semiconductor quantum optics uses high-quality-factor dielectric microcavities to imitate “normal modes” with genuine frequencies and limitless lifetimes. Modern nanostructures, especially plasmonic and magnonic cavities, are fundamentally different.

Plasmonic resonators are valued for confining light to sub-wavelength scales to raise local electromagnetic fields exceeding the diffraction limit. Even single molecules can achieve room-temperature cQED. Spin-photon interfaces are created by collective microwave excitations of spins in ferrimagnetic structures in cavity magnonics, a new field.

They are “lossy” because to radiative leakage and material absorption (Ohmic dissipation). Non-Hermitian systems often fail standard quantization methods or use ad hoc protocols that are not field-theoretically justifiable.

Conquering Modal Divergence Lossy cavities' natural resonances, quasinormal modes (QNMs), have intricate eigenfrequencies, making math difficult. Light leakage from these “open” cavities causes the QNM fields to diverge dramatically at long distances. This discrepancy makes orthogonality, normalization, and completeness difficult to express.

Exterior complex coordinate transformations—equivalent to PMLs—fixed the issue for Trinity. These specialised layers enclosed the computer domain, transforming unlimited space into a confined domain. This method converts unphysical radiative losses into non-radiative material dissipation to strictly regularize modes inside and outside the resonator chamber.

A New Quantum Formalism The researchers used macroscopic quantum electrodynamics to quantify these regularized modes. Their method finds canonical field variables in complex dielectric and magnetic media.

The breakthrough relies on the concept of creation and annihilation operators, which generate modal Fock states. These states describe the excitations of “field-dressed matter”—a blend of material polaritonic excitations and the electromagnetic field.

Researchers found an effective Lindblad master equation. This equation controls the quantum dynamics of modes and their interactions with quantum emitters (QEs) such molecular spins and semiconductor quantum dots. By accounting for coherent and dissipative coupling, the master equation captures complex interference effects like Fano-like features that occur when multiple modes interact with a single emitter.

Results, Numerical Validation The researchers demonstrated their framework's predictive power using two rigorous numerical examples:

Scientists tested a nitrogen-vacancy center QE diamond resonator. Even with low cavity quality factors, the quantum quasinormal modes theory predicted the Purcell factor (the amplification of spontaneous emission) with good agreement with exact semi-classical models. The theory accurately represented the system's temporal Rabi oscillations in the strong coupling region, matching mQED computations. 3D Spherical Cavity: Applying the theory to a silicon sphere in a vacuum eliminated PML layer unphysical gain contributions. Modelling resonance shapes from Lorentzian peaks to Fano-like decay rate suppressions was successful. Making Quantum 2.0 Possible The consequences of this discovery go beyond “Quantum 2.0”. This rigorous and mathematically possible nanophotonic technology evaluation helps build near-field multipartite entanglement creation, ultrafast all-optical switching, and on-demand single-photon sources.

Unprecedented quantum frequency conversion for future quantum networks by scientists

An international research team demonstrated a highly efficient and compact quantum frequency converter that can bridge the gap between high-performance quantum emitters and conventional telecommunications infrastructure, advancing the "quantum internet" concept. The study, led by Mathis Cohen and Université Côte d'Azur, Quandela SAS, and Université Paris-Saclay, describes a system that converts single photons from the near-infrared (NIR) spectrum to the telecommunication C-band with unparalleled end-to-end efficiency and quantum integrity preservation.

Challenge of Wavelength Incompatibility

The main obstacle to long-distance quantum resource distribution is wavelength mismatch. Current high-performance deterministic single-photon emitters, such as those based on semiconductor quantum dots (QDs), atoms, or color centers, operate in the near-infrared range (780–950 nm). However, typical silicon optical fibers lose signal when transmitting these wavelengths long distances.

The telecommunication C-band (about 1550 nm) has the lowest absorption in contemporary fiber networks and is the “natural flying qubit carrier” for long-distance communication. Quantum Frequency Conversion (QFC) adjusts a photonic state's wavelength while preserving its quantum properties, such as unicity and indistinguishability.

An Innovative Efficiency Standard

Researchers use a fiber-pigtailed semiconductor quantum dot single-photon source and a fiber-coupled nonlinear optical Lithium Niobate waveguide in their coherent frequency converter approach. This integrated system converted 925.7-nm photons to 1560-nm.

The team achieved 48.4% end-to-end efficiency. Because quantum information processing is susceptible to photon loss, this measure is crucial. The device maintained an in-fiber single-photon rate of 2.8 MHz (3.7% brightness) at the quantum frequency conversion stage's input, resulting in 1.3 MHz after maximum efficiency processing. A commercial fiber-pigtailed source sets a new photon rate record at the end of a quantum frequency conversion system.

Quantum Fidelity Maintenance

Efficiency is lost if quantum information is confused during conversion. The team prioritized indistinguishability (ensuring photons are the same in all physical modes) and unicity (releasing only one photon at a time).

Using HBT interferometry, researchers assessed the second-order autocorrelation coefficient (g(2)(0)). Single-photon purity was nearly 100%, with values of 0.044 before conversion and 0.051 after.

Hong-Ou-Mandel (HOM) interference tests also assessed indistinguishability. Conversion raised system corrected indistinguishability from 79.3% to 80.0%. These results demonstrate that the interface maintains coherence and that quantum dot photons are not degraded during conversion.

Designed for Real Life

This new system is fiber-integrated, unlike many previous demonstrations that used massive, “free-space” optical structures, which are vibration-sensitive and difficult to scale. For this wavelength shift, the conversion step uses a 4 cm periodically poled lithium niobate waveguide (PPLN/WG).

A 400 by 400 mm breadboard houses the pump preparation, conversion, and filtering stages, making the layout extremely small. The system is portable and can be used in real-world quantum network applications and out-of-lab deployment.

Another significant quality is spectral tunability. A configurable pump laser and crystal temperature allowed the researchers to tailor the conversion process over a 10 nm range. This aligns telecom wavelengths from different quantum dots, which is necessary in actual quantum networks to synchronize many sources.

Noise Reduction and Technical Precision

The conversion technique uses difference-frequency generation (DFG) powered by a strong 2272 nm continuous-wave pump laser. Noise—especially Raman scattering and residual pump light—is a major technical challenge in such systems. A filtering stage was built using four telecom dense-wavelength demultiplexers (DWDM) to address this. The system had excellent noise reduction with an SNR above 400 in the ideal working range.

Future Impact

This work enables several cutting-edge quantum technologies:

Quantum Key Distribution (QKD): Single-photon qubit carriers offer secure communication channels.

Quantum Repeaters: These devices can teleport quantum states over long distances, transcending laboratory limits.

Distributed Quantum Computing: Smoothly connecting quantum information processing units at different wavelengths.