A breakthrough in nanoscale characterization enabling single-photon measurements at ultra-low temperatures
Imagine trying to understand the intricate details of a mosaic by touching it with your fingertip while wearing thick gloves. You might feel the general shape, but the fine details would be lost. This is the fundamental challenge scientists face when trying to understand the nanoscale world of quantum materials.
For decades, scanning probe microscopes have been our "fingers" for feeling surfaces at the atomic level, but when it comes to measuring electromagnetic properties at the quantum level, our gloves have been too thick—until now.
Recent breakthroughs have transformed a powerful classical measurement technique into a quantum-scale tool. Near-field scanning microwave microscopy (NSMM), once limited to conventional measurements, has now entered the single-photon regime, operating at temperatures just fractions of a degree above absolute zero. This advancement opens a new window into the quantum world, allowing scientists to probe materials with unprecedented sensitivity—using so few microwave photons that the measurement itself barely disturbs the quantum systems under study 1 .
Operating at just 30 millikelvin to reduce thermal noise
Detection with near-single-photon signals for minimal disturbance
Near-field scanning microwave microscopy combines the precise positioning of scanning probe microscopy (like atomic force microscopy) with microwave frequency electrical measurements. In conventional NSMM, a sharp metal tip is brought extremely close to a sample surface—typically within nanometers. Microwave signals are sent to the tip, and the reflected signals are analyzed to extract information about the sample's electrical properties 6 .
The "near-field" aspect is crucial. Unlike far-field measurements where resolution is limited by the wavelength of the radiation (which can be centimeters for microwaves), near-field techniques bypass this limitation by placing the detector much closer to the sample than the wavelength. This allows NSMM to achieve nanometer-scale resolution despite using microwaves with wavelengths millions of times larger 5 .
The drive to bring NSMM into the quantum regime stems from the unique properties of quantum systems and the limitations of classical measurement approaches.
Quantum systems—such as superconducting qubits, quantum dots, or exotic topological materials—are extremely delicate. Their quantum states can be easily disrupted by thermal energy or measurement disturbances. To study these systems meaningfully, measurements must be performed at ultra-low temperatures (typically below 300 millikelvin) and with minimal energy input 1 .
The advance to quantum-regime NSMM represents a staggering improvement in sensitivity. Where typical NSMMs might use trillions of photons, the new quantum-capable versions operate with near-single-photon signals—up to 10^9 times lower power than conventional instruments 1 3 .
This gentle touch allows researchers to probe quantum materials without destroying the delicate quantum states they hope to study.
In 2019, researchers achieved a major milestone: the first NSMM capable of operating in the single microwave photon regime at just 30 millikelvin 1 . This required overcoming significant challenges across multiple fronts.
The experimental setup was meticulously engineered for quantum measurements. Key to this achievement was the integration of a thin-film niobium superconducting resonator with a resonance frequency of approximately 6 GHz, patterned onto a silicon substrate and mounted onto a single prong of a quartz tuning fork. This combination served as both the microwave probe and the force sensor for maintaining tip-sample distance 1 .
| Requirement | Technical Implementation | Purpose |
|---|---|---|
| Ultra-low temperatures | Dilution refrigerator (30 mK) | Reduce thermal energy below quantum energy scales |
| Extreme low power | Pound-Drever-Hall technique with high-Q resonator | Enable single-photon measurements without saturating quantum systems |
| Nanoscale distance control | Tuning-fork based atomic force microscopy | Maintain precise, stable tip-sample separation |
| High vibration isolation | Copper-beryllium springs and Kevlar thread suspension | Minimize mechanical noise affecting measurements |
Researchers began by creating an exceptionally sharp metallic tip using focused ion beam milling. This tip was positioned at the microwave voltage anti-node of the superconducting resonator to maximize sensitivity 1 .
The entire probe assembly was housed in a custom casing suspended within a dilution refrigerator. This sophisticated suspension system acted as a mechanical low-pass filter, critically reducing vibrations from the refrigerator's pulse tube cooler that could disrupt measurements 1 .
Rather than conventional microwave measurement techniques, the team employed the Pound-Drever-Hall method—a technique borrowed from laser physics for frequency stabilization. This approach provided immunity to variations in electrical length caused by thermal drift and mechanical noise, enabling unprecedented frequency resolution 1 .
The system was carefully characterized to determine the power level corresponding to the single-photon regime. By measuring the resonator's quality factor as a function of input power and applying established formulas, the team confirmed they could operate with an average photon number close to one in the resonator 1 .
The successful demonstration of NSMM operating in the single-photon regime opened multiple possibilities for quantum material characterization.
Most significantly, researchers confirmed that nano-scale dielectric contrast could be maintained down to the single microwave photon level. This means that even with these extraordinarily gentle measurements, different materials could still be distinguished based on their electrical properties—a crucial capability for practical quantum material characterization 1 .
| Microscope Type | Typical Power Level | Operating Temperature | Primary Applications |
|---|---|---|---|
| Conventional NSMM | High (trillions of photons) | Room temperature to cryogenic | Semiconductor analysis, biological samples, material defects |
| Cryogenic NSMM | Intermediate | 4K and above | Superconductors, low-temperature phenomena |
| Quantum-regime NSMM | Single-photon level (10^9× lower) | 30 mK and below | Quantum materials, qubits, topological insulators |
The experimental data revealed how the resonator's internal quality factor changed with photon number, showing characteristic features of two-level system defects at the lowest power levels. This itself provides a valuable method for studying these problematic defects that often limit the performance of quantum devices 1 .
Bringing NSMM into the quantum regime requires a specialized set of tools and techniques. Each component plays a critical role in achieving the necessary sensitivity and stability.
| Component | Function | Quantum Implementation |
|---|---|---|
| Resonator | Enhances sensitivity through quality factor | Thin-film superconducting Nb resonators with high Q-factor |
| Tip | Local field concentration | Sharp metallic tips (often FIB-milled) at microwave anti-node |
| Distance Control | Maintains precise tip-sample separation | Tuning-fork based AFM with self-sensing capability |
| Detection Scheme | Measures minute frequency shifts | Pound-Drever-Hall technique for superior noise immunity |
| Vibration Isolation | Minimizes mechanical noise | Multi-stage suspension (CuBe springs, Kevlar threads) |
| Calibration Standards | Ensures quantitative measurements | Custom impedance standards (resistive, capacitive, inductive) |
The development of specialized calibration standards has been particularly important for transforming NSMM from a qualitative imaging tool to a quantitative measurement instrument. Unlike conventional calibration kits that primarily include capacitive standards, advanced calibration sets now include resistive and inductive circuits with values relevant to nanoscale systems (resistances from 0-960 Ohm, inductances from 0-1.15 nH) 2 .
Similarly, the push toward coaxial probe designs aims to better shield parasitic electromagnetic fields, addressing a key limitation in current NSMM implementations where calibration performed on one sample isn't directly transferable to measurements on different samples 2 .
The entry of NSMM into the quantum regime marks the beginning of a new chapter in nanoscale characterization, not its conclusion.
Current research focuses on extending these capabilities to enable quantum coherent interactions with two-level systems 1 .
This next frontier requires further improvements in resonator quality factors, more refined tip designs, and even better vibration isolation. The ultimate goal is to use the NSMM tip not just as a passive observer but as an active component that can coherently couple to quantum states in materials—potentially enabling the manipulation of quantum information or the precise mapping of quantum wavefunctions 1 .
Meanwhile, related advances continue to push the boundaries of what's possible with microwave microscopy. In 2021, researchers demonstrated that by harnessing nanoscale water menisci, microwave microscopy could achieve remarkable resolution of approximately 1 nanometer—imaging features like the moiré patterns in twisted bilayer graphene with a figure of merit (wavelength-to-resolution ratio) of 10^8 5 .
These parallel advances in resolution and sensitivity are converging to create a powerful new toolkit for quantum material science. As these technologies mature, they may help unlock mysteries in fields ranging from high-temperature superconductivity to topological quantum computing.
The transformation of near-field scanning microwave microscopy from a conventional characterization tool to a quantum-grade instrument represents more than just a technical achievement—it marks a fundamental shift in how we interact with the quantum world.
By learning to probe matter with the lightest possible touch, scientists have opened a new window into the nanoscale realm.
This gentle approach to measurement—listening rather than shouting, as it were—enables the study of quantum phenomena in their most pristine states. As these capabilities continue to develop, they promise to accelerate progress in quantum technologies, from the identification of defect structures that limit qubit performance to the exploration of exotic quantum phases in engineered materials.
The journey of NSMM into the quantum regime demonstrates that sometimes, to see the smallest things most clearly, we need to look with the softest touch.