#quantumwalks

News about QCL The ETH Zurich research team created high-power frequency combs using a dual-waveguide quantum cascade laser (QCL), a notable advance in integrated photonics. An efficiency two orders of magnitude higher than before is achieved while extracting light from ring-shaped lasers. The researchers reached 120 mW output from a stable but “dim” laser state, making it appropriate for high-brightness spectroscopy and telecommunications.

The Ring Laser Power Barrier For years, scientists have sought to create compact, bright frequency comb sources in the terahertz and mid-IR regions. Frequency combs provide regularly spaced spectral lines for molecular sensing and high-precision metrology as "optical rulers." Since Fabry–Perot Quantum Cascade Lasers first demonstrated self-starting combs, ring QCLs have garnered attention for their ability to build stable “quantum walk combs.”

Quantum walk combs exploit quantum cascade lasers' fast gain recovery time. This speed suppresses amplitude variations and creates "liquid" qualities, allowing the difficult Ginzburg-Landau equation to effectively forecast light. However, the closed-loop cavity design that stabilizes these rings traps them. Because light is enclosed in the ring to reduce backscattering, outcoupling efficiency is notoriously low, limiting extracted power to hundreds of microwatts.

A Two-Waveguide Method The ETH Zurich team, led by Jerome Faist and Alessio Cargioli, solved this problem with a complicated structural redesign. Instead of using the ring, they homogeneously integrated a “racetrack” (RC) quantum cascade laser with a passive InGaAs waveguide to emit light.

Metal organic vapour phase epitaxy (MOVPE) generates this passive waveguide on the active area plane. With this “top-layer” architecture, the extraction structure can be built independently of the laser geometry. The passive U-shaped waveguide evanescently couples light to an extraction facet as it travels across the active racetrack.

This strategy maintains the ring's clean, stable quantum walk state and generates a dedicated channel for high-power light output. With passive waveguides instead of active bus sections, the researchers reduced electrical complexity and power dissipation, enabling better “lab-on-a-chip” systems.

Ballistic Expansion Watch The researchers proved their high-power gadget was a quantum walk comb using time-resolved spectrum measurements. They observed ballistic expansion, where radio-frequency (RF) modulation rapidly and linearly expands the laser's spectrum.

The experiments showed that the expansion stabilized in 700 nanoseconds and reached a bandwidth of 30 cm⁻¹, meeting theoretical conditions for quantum walk dynamics in synthetic space. The researchers saw “Bloch oscillations” when the modulation was slightly detuned from the cavity's roundtrip frequency, confirming that the light was following quantum walk principles.

Effect of Waveguide Width Experimental passive extraction waveguide widths were essential to the investigation. The researchers found that the size of this top layer directly affected the stability of the comb and light dispersion:

Narrow waveguides (~3 µm) were found to be more reliable for creating stable, adjustable combs. One “TM00” mode was provided, which enhanced mode matching with the active zone and smoothed power output. Wide waveguides (~9 µm) were introduced to reduce group velocity dispersion (GVD) and possibly enhance comb bandwidth, but they also increased complexity. Due to the wide guides supporting several modes, light leaked between modes like TM20 or TM00 depending on the current bias, generating power oscillations. The researchers achieved 120 mW peak output at 253 K heatsink temperature despite these oscillations in the larger designs. They also showed that the double waveguide laser can operate at room temperature, making it competitive with more complex active-outcoupler designs and perhaps more useful for commercial use.

Future uses and prospects The ability to monolithically integrate passive devices like these waveguides on a chip with a high-power laser opens up several mid-infrared spectral range potential. Mid-IR light is called the “fingerprint region” because many substances absorb differently at these wavelengths. News about QCL The ETH Zurich research team created high-power frequency combs using a dual-waveguide quantum cascade laser (QCL), a notable advance in integrated photonics. An efficiency two orders of magnitude higher than before is achieved while extracting light from ring-shaped lasers. The researchers reached 120 mW output from a stable but “dim” laser state, making it appropriate for high-brightness spectroscopy and telecommunications.

The Ring Laser Power Barrier For years, scientists have sought to create compact, bright frequency comb sources in the terahertz and mid-IR regions. Frequency combs provide regularly spaced spectral lines for molecular sensing and high-precision metrology as "optical rulers." Since Fabry–Perot Quantum Cascade Lasers first demonstrated self-starting combs, ring QCLs have garnered attention for their ability to build stable “quantum walk combs.”

Quantum walk combs exploit quantum cascade lasers' fast gain recovery time. This speed suppresses amplitude variations and creates "liquid" qualities, allowing the difficult Ginzburg-Landau equation to effectively forecast light. However, the closed-loop cavity design that stabilizes these rings traps them. Because light is enclosed in the ring to reduce backscattering, outcoupling efficiency is notoriously low, limiting extracted power to hundreds of microwatts.

A Two-Waveguide Method The ETH Zurich team, led by Jerome Faist and Alessio Cargioli, solved this problem with a complicated structural redesign. Instead of using the ring, they homogeneously integrated a “racetrack” (RC) quantum cascade laser with a passive InGaAs waveguide to emit light.

Metal organic vapour phase epitaxy (MOVPE) generates this passive waveguide on the active area plane. With this “top-layer” architecture, the extraction structure can be built independently of the laser geometry. The passive U-shaped waveguide evanescently couples light to an extraction facet as it travels across the active racetrack.

This strategy maintains the ring's clean, stable quantum walk state and generates a dedicated channel for high-power light output. With passive waveguides instead of active bus sections, the researchers reduced electrical complexity and power dissipation, enabling better “lab-on-a-chip” systems.

Ballistic Expansion Watch The researchers proved their high-power gadget was a quantum walk comb using time-resolved spectrum measurements. They observed ballistic expansion, where radio-frequency (RF) modulation rapidly and linearly expands the laser's spectrum.

The experiments showed that the expansion stabilized in 700 nanoseconds and reached a bandwidth of 30 cm⁻¹, meeting theoretical conditions for quantum walk dynamics in synthetic space. The researchers saw “Bloch oscillations” when the modulation was slightly detuned from the cavity's roundtrip frequency, confirming that the light was following quantum walk principles.

Effect of Waveguide Width Experimental passive extraction waveguide widths were essential to the investigation. The researchers found that the size of this top layer directly affected the stability of the comb and light dispersion:

Narrow waveguides (~3 µm) were found to be more reliable for creating stable, adjustable combs. One “TM00” mode was provided, which enhanced mode matching with the active zone and smoothed power output. Wide waveguides (~9 µm) were introduced to reduce group velocity dispersion (GVD) and possibly enhance comb bandwidth, but they also increased complexity. Due to the wide guides supporting several modes, light leaked between modes like TM20 or TM00 depending on the current bias, generating power oscillations. The researchers achieved 120 mW peak output at 253 K heatsink temperature despite these oscillations in the larger designs. They also showed that the double waveguide laser can operate at room temperature, making it competitive with more complex active-outcoupler designs and perhaps more useful for commercial use.

Future uses and prospects The ability to monolithically integrate passive devices like these waveguides on a chip with a high-power laser opens up several mid-infrared spectral range potential. Mid-IR light is called the “fingerprint region” because many substances absorb differently at these wavelengths.

The ETH Zurich team expects their high-power QWCs to have a “significant potential impact” on several sectors:

Environmental Monitoring: Brighter light detects air trace gasses better. Medical Diagnostics: Non-invasive frequency comb spectroscopy testing. Telecommunications and Ranging: Measure distance and send data swiftly using the high modulation bandwidth (which can exceed 10 GHz in comparable systems). The “back-reflection” of light from the extraction facet still hinders stability, however anti-reflection coatings can aid. Scaling this universal waveguide coupling method to higher wavelengths could enable a single integrated platform covering the 3-15 µm spectral range in the future. The ETH Zurich team expects their high-power QWCs to have a “significant potential impact” on several sectors:

Environmental Monitoring: Brighter light detects air trace gasses better. Medical Diagnostics: Non-invasive frequency comb spectroscopy testing. Telecommunications and RangingNews about QCL The ETH Zurich research team created high-power frequency combs using a dual-waveguide quantum cascade laser (QCL), a notable advance in integrated photonics. An efficiency two orders of magnitude higher than before is achieved while extracting light from ring-shaped lasers. The researchers reached 120 mW output from a stable but “dim” laser state, making it appropriate for high-brightness spectroscopy and telecommunications.

The Ring Laser Power Barrier For years, scientists have sought to create compact, bright frequency comb sources in the terahertz and mid-IR regions. Frequency combs provide regularly spaced spectral lines for molecular sensing and high-precision metrology as "optical rulers." Since Fabry–Perot Quantum Cascade Lasers first demonstrated self-starting combs, ring QCLs have garnered attention for their ability to build stable “quantum walk combs.”

Quantum walk combs exploit quantum cascade lasers' fast gain recovery time. This speed suppresses amplitude variations and creates "liquid" qualities, allowing the difficult Ginzburg-Landau equation to effectively forecast light. However, the closed-loop cavity design that stabilizes these rings traps them. Because light is enclosed in the ring to reduce backscattering, outcoupling efficiency is notoriously low, limiting extracted power to hundreds of microwatts.

A Two-Waveguide Method The ETH Zurich team, led by Jerome Faist and Alessio Cargioli, solved this problem with a complicated structural redesign. Instead of using the ring, they homogeneously integrated a “racetrack” (RC) quantum cascade laser with a passive InGaAs waveguide to emit light.

Metal organic vapour phase epitaxy (MOVPE) generates this passive waveguide on the active area plane. With this “top-layer” architecture, the extraction structure can be built independently of the laser geometry. The passive U-shaped waveguide evanescently couples light to an extraction facet as it travels across the active racetrack.

This strategy maintains the ring's clean, stable quantum walk state and generates a dedicated channel for high-power light output. With passive waveguides instead of active bus sections, the researchers reduced electrical complexity and power dissipation, enabling better “lab-on-a-chip” systems.

Ballistic Expansion Watch The researchers proved their high-power gadget was a quantum walk comb using time-resolved spectrum measurements. They observed ballistic expansion, where radio-frequency (RF) modulation rapidly and linearly expands the laser's spectrum.

The experiments showed that the expansion stabilized in 700 nanoseconds and reached a bandwidth of 30 cm⁻¹, meeting theoretical conditions for quantum walk dynamics in synthetic space. The researchers saw “Bloch oscillations” when the modulation was slightly detuned from the cavity's roundtrip frequency, confirming that the light was following quantum walk principles.

Effect of Waveguide Width Experimental passive extraction waveguide widths were essential to the investigation. The researchers found that the size of this top layer directly affected the stability of the comb and light dispersion:

Narrow waveguides (~3 µm) were found to be more reliable for creating stable, adjustable combs. One “TM00” mode was provided, which enhanced mode matching with the active zone and smoothed power output. Wide waveguides (~9 µm) were introduced to reduce group velocity dispersion (GVD) and possibly enhance comb bandwidth, but they also increased complexity. Due to the wide guides supporting several modes, light leaked between modes like TM20 or TM00 depending on the current bias, generating power oscillations. The researchers achieved 120 mW peak output at 253 K heatsink temperature despite these oscillations in the larger designs. They also showed that the double waveguide laser can operate at room temperature, making it competitive with more complex active-outcoupler designs and perhaps more useful for commercial use.

Future uses and prospects The ability to monolithically integrate passive devices like these waveguides on a chip with a high-power laser opens up several mid-infrared spectral range potential. Mid-IR light is called the “fingerprint region” because many substances absorb differently at these wavelengths.

The ETH Zurich team expects their high-power QWCs to have a “significant potential impact” on several sectors:

Chemical Analysis' Quantum Leap: A New Reaction Network Framework Using Quantum Walks

Many biological and chemical processes depend on chemical reaction networks (CRNs), which scientists have struggled to understand. Basic research and real-world applications must forecast system dynamics from tiny alterations. A groundbreaking partnership between Seenivasan Hariharan, Sebastian Zur, Sachin Kinge, and colleagues from Toyota Motor Europe and the University of Amsterdam has discovered a new framework that could alter this field. A recent journal describes their quantum walk-based CRN modelling and analysis, which provides powerful new tools for approximating reaction fluxes, calculating energy consumption, and anticipating system changes.

The paper proposes a method for investigating fixed-structure networks by precisely simulating perturbations, such as adding molecules, and forecasting system behaviour changes. This unique method evaluates energy consumption and approximates process flow, going beyond determining whether molecules can be reached after a disturbance. These capabilities can develop complex chemical and biological systems and deliver unprecedented insights.

The Complexity of Chemical Reaction Networks

Molecular interactions and transformations in large chemical and biological systems are modelled by chemical reaction networks. In biochemistry, atmospheric chemistry, and catalysis, they uncover short-lived intermediates, basic reaction mechanisms, and generic reaction routes. Data mining from vast networks is notoriously challenging.

CRNs usually have a few species and responses. Even though the underlying structure of these networks remains unchanged, perturbations, such as adding molecules, can radically change concentrations, move steady states, and activate other routes. The non-linear differential equations regulating mass action dynamics enhance the system's effective dimensionality and coupling, making standard methods computationally difficult and often impossible. Combinatorial complexity, where routes and intermediates multiply quickly, is a major challenge.

A Chemistry Circuit Board: Modelling CRNs as Electrical Networks

A new computational framework directly compares electrical circuits and CRNs. This new paradigm depicts chemical species as network nodes (vertices) and reactions as weighted edges. Importantly, these edge weights match electrical resistance to turn the chemical network electrical.

On this electrical network, researchers define a “flow” like molecular flows across processes. With source nodes representing species injection and sink nodes representing consumption, this flow distributes data to edges while conserving flow at every vertex. This comparison relies on "effective resistance," the minimum energy needed to move a unit flow between designated and sink nodes. This shows network connectivity and molecular mobility.

Most computer analysis uses bipartite molecule-reaction graphs. This modelling paradigm simplifies and improves compatibility with quantum techniques and network analysis by neatly splitting species and reactions into discrete vertices. For this comparison to be meaningful, Mass Action Systems (MAS), which are CRNs with mass action kinetics, must meet thermodynamic conditions. Being particle-conserving, reversible, and having a positive equilibrium concentration are required. MAS dynamics elegantly dualise with electrical network dynamics under certain conditions.

Quantum Walks: Deeper Understanding

Quantum walk algorithms make this framework strong. Coherence and interference help quantum walks traverse graphs faster than diffusive random walks. This accelerates computing, especially as network size and complexity rise. Quantum algorithms provide additional resources for:

Determine Reachability: Determine if a disturbance can produce or reach target molecules.

Reachable Species: Sample representative species.

This approach estimates steady-state fluxes through reactions to quantify reaction rates and system activity.

Estimate Gibbs Free-Energy Consumption: This thermodynamic parameter impacts chemical process feasibility and efficiency. Estimating total Gibbs free-energy consumption. This helps explain energy dissipation in large molecular networks.

A detailed relationship exists between electrical network parameters and CRN dynamics. Gibbs free-energy consumption immediately connects to the Mass Action System Graph (MASG) flow energy, and a species' external injection/removal rate matches the electrical network's initial probability distribution.

A New Use of "Alternative Neighbourhoods"

This paper introduces “alternative neighborhoods” in multidimensional quantum walks, a subtle but powerful feature. Usually used to improve quantum state creation, this team took the other approach. They develop alternative communities to make the chemically derived MASG flow the only electrical flow that meets generalised Kirchhoff's and Ohm's laws. This unique method allows to sample states that reflect the contributions of distinct species-reaction pairs to this energy and estimate Gibbs free-energy consumption more accurately. This approach excels at “s-M rigid networks”.

Future Impact