La Quantum technology is revolutionizing the way we look at the microscopic worldWhat a few decades ago seemed like science fiction - seeing living cells in extreme detail without damaging them, following the movement of light trapped in a crystal, or photographing atoms one by one - is beginning to become routine in leading laboratories around the world.
Thanks to new quantum microscopes capable of overcoming the classical limits of resolutionScientists are breaking down barriers that have defined the limits of what is possible for over a century. From optical microscopy of living cells based on entangled photons to quantum simulators of ultracold gases and 4D electron microscopes, the common goal is clear: to extract much more information with less light or lower doses of radiation, and to see structures that were previously literally invisible.
The classic resolution limit and why normal light is not enough
In a conventional optical microscope, the The ability to distinguish minute details is limited by the wavelength of light that is used. As a general rule, only structures whose size is at least approximately half that wavelength can be resolved.
This implies that, using standard visible light, there is a point at which You can't keep improving the resolution simply by adding more magnification.We can get closer, yes, but the details begin to blur because the very wave-like nature of light acts as a physical ceiling.
One obvious way to go further is to use light with a shorter wavelengthsuch as violet or even ultraviolet (UV). The shorter the wavelength, the smaller the details the microscope can distinguish. However, this comes with an important drawback: these radiations carry more energy and can damage or kill living cells and delicate molecules, something unacceptable in cell biology, medicine, or in many high-precision experiments.
Researchers have been grappling with this balance for years: If the light intensity is reduced to avoid frying the sample, the image becomes noisy.It loses contrast and critical details. If the intensity is increased too much or very energetic radiation is used, the sample suffers irreversible damage. This is where the ideas of quantum physics come into play.
Traditional optics fall short when trying to juggle low light, high sensitivity, and extreme resolution. In this scenario, the use of carefully prepared quantum light, such as pairs of entangled photonsIt allows us to circumvent some of these limitations and open a completely new window to the micro and nano world.
Between the “creepy” action and the perfect image: quantum entanglement
One of the most striking phenomena in modern physics is the quantum entanglementAccording to quantum mechanics, two particles can become so intimately correlated that the state of one is linked to that of the other, regardless of the distance between them. Albert Einstein described this as "spooky action at a distance" because it clashed with classical intuition and with what his own theory of relativity suggested.
In the context of microscopy, this entanglement translates into pairs of entangled photons, known as biphotonsFrom a quantum point of view, a biphoton behaves almost like a single composite particle whose momentum is approximately twice that of an individual photon.
Quantum mechanics reminds us that Every particle also has a wave-like characterIn this context, wavelength is inversely related to momentum: the greater the momentum, the shorter the wavelength. This means that, since the biphoton has a greater effective momentum, its effective wavelength is approximately half of the loose photons with which it was generated.
This whole interplay of waves and particles is interesting because, if we can get the microscope to work as if it were using a light with a wavelength equivalent to halfWe can see details twice as small without actually resorting to more energetic or more aggressive radiation for the cells.
This clever use of quantum entanglement opens the door to techniques that, by holding photons with soft energies (for example, around 400 nanometers wavelength in the violet range), They achieve a resolution comparable to that of ultraviolet light, but with a much shorter duration., on the order of 200 nanometers, but without destroying the sample.
Quantum coincidence microscopy (QMC): doubling the resolution without frying the cells
A group of researchers from the California Institute of Technology (Caltech) has developed a technique called Quantum Coincidence Microscopy (QMC)This method, described in the journal Nature Communications as “quantum cell microscopy at the Heisenberg limit”, promises to double the resolution obtainable with a conventional optical microscope.
The central idea of QMC is to leverage pairs of photons intertwined to form biphotonsThese biphotons behave as a single entity with twice the momentum and, therefore, a shorter effective wavelength. Thus, a system using 400 nm light (on the edge of violet) can achieve a resolution similar to that of 200 nm light (in the full ultraviolet), while keeping the energy deposited on the sample at a much more manageable level.
Professor Lihong Wang, professor of Medical Engineering and Electrical Engineering at Caltech and lead author of this work, summarizes it very graphically: cells “don’t get along” with ultraviolet light, but if we illuminate with 400 nm and achieve the same resolution effect as with 200 nm, The cells are "happy" and the microscope continues to gain in detail..
This approach resolves the classic dilemma in one fell swoop: It is not necessary to use extremely energetic light to see very small structures.By manipulating quantum entanglement and the way in which matches between paired photons are measured, the QMC system enables the microscope to get more out of each photon without increasing potential damage to living samples.
Unlike traditional microscopes, which only capture details of an object comparable in size to half the wavelength of the light used, QMC It allows you to see much smaller structures by using less harmful lightsAnd, moreover, it does so with an experimental configuration that, according to its creators, is already a viable system and not just a one-off laboratory demonstration.
How QMC works step by step
To bring this idea to life, the Caltech team built a optical device in which a laser shines on a special crystalThis crystal is designed to transform a small fraction of incident photons into entangled pairs, biphotons. For now, the efficiency is very low (on the order of one per million photons), but researchers are already working on improving that rate.
Once generated, these biphotons They separate using mirrors, lenses, and prismsso that the two photons that make them up follow different paths. One of them passes through the sample we want to observe (it is called the signal photon) and the other does not pass through the sample (it is the idle or inactive photon).
Both photons then continue their path through the system's optics until they reach a detector connected to a computer. The trick is that the computer It does not simply count individual photons, but rather coincidences between the two entangled photons.Based on this information, the image of the sample is reconstructed, taking advantage of the intertwined nature of the pair.
What is surprising is that, despite taking separate routes once one has passed through the cell or another type of object, The photons maintain their entanglement and behave like a biphoton. while they are being detected. The system takes advantage of this quantum coherence so that the whole behaves as if it had half the wavelength.
Although other groups had already succeeded in obtaining images with biphotons, Wang's team maintains that this is the first microscopically detailed setup demonstrating a practical and reproducible systemThey have developed a rigorous theory to describe the process, a method for measuring entanglement quickly and accurately, and have demonstrated its usefulness on real biological samples.
View live cells in more detail and with less damage
The Caltech team used its quantum microscope to obtain images of cancer cellsThanks to the improved resolution, they were able to clearly identify various internal structures that a classic optical microscope, with comparable light and dose, could not resolve.
The most striking thing is that The cells were not damaged or destroyed during the processbecause the radiation used was not particularly energetic. The magic lies in how the quantum information carried by the biphotons is harnessed, not in "bombarding" the cell with increasingly aggressive photons.
This technique is perceived as a very promising advance in Medical imaging and biomedical researchBeing able to study living cells, tissues, or even delicate microorganisms with a level of resolution close to the limit imposed by quantum physics (the so-called Heisenberg limit) without destroying them opens the door to early diagnoses, better monitoring of treatments, and a finer understanding of critical biological processes.
Looking ahead, researchers are considering the possibility of use more than two entangled photons to further refine the resolution and optimize the technology to reduce background noise associated with the interaction of photons with the environment. Each improvement would further increase the quality and accuracy of the images obtained.
In parallel, this development lays the groundwork for applications in fields such as quantum computing, cryptography, or the design of new materialswhere the ability to characterize structures at the nanoscale without damaging them is pure gold.
Quantum gas microscopes: freezing atoms and viewing them one by one
Meanwhile, in Europe progress has been made on another complementary front: the quantum microscopes of ultracold gases. An emblematic example is QUIONE, developed by the Institut de Ciències Fotòniques (ICFO) in Castelldefels, which has been presented in the PRX Quantum magazine.
QUIONE functions as a “quantum simulator” that cools strontium atoms to temperatures close to absolute zeroIt organizes them into an optical network and allows them to be observed individually, almost as if they were eggs placed in the holes of a carton, but on an atomic scale.
Traditionally, quantum gas microscopes had been based on alkali atoms such as lithium or potassiumwhich are optically simpler to handle. Bringing strontium—an alkaline earth atom with a more complex spectrum—into the quantum regime opens the door to simulating much more exotic materials and phases of matter.
The scheme is as follows: the temperature of the strontium gas is reduced to extremely low values for a few milliseconds, causing the atoms to slow down almost completely and become trapped in an optical neta kind of "grid" of light generated by lasers. Each site in the grid behaves like a small energy well where, with a high probability, an atom will reside.
Thanks to this configuration, the team has been able to obtain atom-by-atom images and to study phenomena such as superfluidity, in which strontium gas flows without viscosity. Furthermore, the dynamics of the atoms, which "jump" from one site to another in the lattice without needing to overcome classical barriers, directly illustrates the famous quantum tunneling effect.
QUIONE as an analog quantum processor and new materials laboratory
QUIONE is not just a microscope: it is, in essence, a analog quantum processorBy adjusting the shape of the optical lattice, the intensity of the lasers, the interactions between atoms, and other parameters, researchers can “program” the system to mimic the behavior of complex real materialsbut in a highly controlled environment.
This allows us to address difficult questions, for example, Why do certain materials conduct electricity without loss? (superconductivity) at relatively high temperatures, or how electrons are organized into topological phases that are still poorly understood.
The possibility of studying strontium gases with such precision, using a quantum microscope of this type, makes QUIONE a strategic tool for the development of future quantum computers and associated technologies. Strontium is especially attractive for building ultra-precise atomic clocks and robust quantum processors, so having a device that allows it to be manipulated and visualized at the scale of a single atom is a true scientific luxury.
Researchers like Leticia Tarruell and her team point out that This type of quantum simulation will help unravel extremely complex microscopic systems, offering clues on how to design new materials with tailored properties, from improved superconductors to topological insulators.
Thus, we find ourselves with a family of quantum microscopes that not only show the world, but recreate it in miniature to better understand it, something that seemed reserved for theoretical models until very recently.
Very low intensity quantum light: the European project Q-MIC
Another strong bet on the Quantum microscopy comes from the European project Q-MICThis project, also largely led by ICFO and collaborators from Italy and Germany, has been underway since 2018 to develop a microscope capable of using very low-intensity quantum light to obtain images with a wide field of view, high sensitivity, and better resolution than classical microscopes.
The Q-MIC device is distinguished because it has been specifically designed for illuminate the sample with pairs of entangled photonsInstead of conventional light made up of many disordered photons, each pair of photons carries an exquisitely correlated amount of information, allowing more detail to be extracted with less total radiation.
In applications where the sample is extremely sensitive—for example, certain proteins, viruses, molecules, or living tissues—having low-intensity light that won't ruin the experiment It's essential. The problem, as always, is that reducing the intensity increases relative noise in the image, which usually blurs the result.
Q-MIC overcomes this obstacle using interference patterns generated by entangled photonsInstead of simply recording how many photons reach each pixel, the camera detects matching pairs of photons passing through the optical system and samples them, and that information is used to reconstruct the image using advanced mathematical algorithms.
Thanks to this approach, researchers have shown that it is possible reduce noise and increase the sensitivity of measurements by more than 25% compared to classic techniques, maintaining light doses well below the usual levels.
Interference, Savart plates and image reconstruction
The optical heart of Q-MIC includes a set of Savart platesbirefringent crystals capable of splitting a beam of light into two beams with different polarizations (horizontal and vertical) that travel slightly different paths, and guiding elements similar to those used in fiber optic systems.
When pairs of entangled photons pass through this system, the Savart plates They separate their paths and direct them towards the sampleIf the sample is perfectly flat and homogeneous, the photon paths remain almost identical. But if there are variations in thickness, refractive index, or other characteristics, phase differences are generated which, when the beams recombine, give rise to complex interference patterns.
The microscope camera does not measure optical intensity levels in the usual way, but rather records photon arrival coincidences at different points in the field of view. By repeating the process many times, a two-photon interference pattern accumulates, encoding information about the fine structure of the sample.
With the help of reconstruction algorithms, based on mathematical and signal processing techniques, scientists They transform those patterns into detailed imageswithout the need for a point-to-point scanning system. This allows for covering relatively wide fields of view with high sensitivity and good resolution, which is very useful for analyzing surfaces and extended samples.
To verify the improvement, they took a standard sample of protein A The sample was placed on a glass slide with equidistant cells. It was illuminated first with classical light and then with quantum light. Interference patterns were obtained in both cases, and the images were reconstructed. The result was clear: with quantum light, the image was much smoother, with less noise and better-defined edges of the structures.
Q-MIC applications: from flexible materials to viruses
The results of Q-MIC, published in Science AdvancesThey make it clear that this quantum lighting strategy is not just a theoretical curiosity. The anticipated applications include fields as varied as... Materials science, the analysis of transparent surfaces for flexible electronics or the inspection of delicate coatings.
Furthermore, their ability to work with minute light doses This makes it an ideal candidate for studying ultrasensitive microorganisms, such as certain viruses, and molecules that degrade easily under intense light. Its application is also envisioned for areas of quantum cryptography and secure communicationswhere fine control of entangled photons is key.
The Q-MIC microscope demonstrates that, by properly exploiting entanglement, we can improve the quality of the information extracted by each photonreducing noise and increasing accuracy without needing to increase the light dose.
In parallel with Caltech's QMC-type techniques, Q-MIC reinforces the idea that The next great revolution in microscopy lies in quantum opticsnot only by building larger targets or more powerful lasers.
4D quantum electron microscopy: seeing light trapped in photonic crystals
The quantum revolution in imaging is not limited to visible light or ultracold gases. In Israel, researchers from Technion – Israel Institute of Technology have developed a ultrafast 4D electron microscope which allows direct observation of the flow of light trapped inside photonic crystals, something that until now could only be studied through computer simulations.
This system, first described in the journal Nature, is considered one of the world's most advanced near-field optical microscopesalthough its technological core is based on an ultrafast transmission electron microscope with unique capabilities.
The team led by Professor Ido Kaminer has created an experimental platform where Ultrashort light pulses (on the order of less than 100 femtoseconds) excite the sample Pulses of electrons, accelerated to voltages between 40 kV and 200 kV, probe it to capture its transient state. In other words, the sample is "illuminated" and "photographed" with electrons at incredibly short time intervals.
With this configuration, it is possible mapping the interactions between light confined in nanomaterials (such as photonic crystals) and free electrons, accessing information on the dynamics of optical fields with unprecedented spatial and temporal resolution.
The practical result is that, for the first time, scientists can directly observe how light behaves when it is trapped and guided in photonic structuresInstead of having to infer it solely from models and simulations, this opens up a new field for designing quantum materials and photonic devices with optimized properties, for example, to store quantum bits (qubits) with greater stability.
Free electron wave packets and new quantum phenomena
Underlying this advance is the physics of ultrafast interactions between free electrons and lightTraditionally, quantum electrodynamics (QED) has studied how quantum matter—atoms, quantum dots, superconducting circuits, etc.—interacts with light modes confined in cavities. It is the conceptual basis of many current quantum technologies.
However, in those systems the electrons are bound and their energy states, spectral range, and selection rules are highly restricted. Recent advances have focused on another entity: the quantum wave packets of free electronsUnlike bound electrons, these packets can span a wide energy range and explore much more varied interactions.
The problem was that, despite multiple theoretical predictions of fascinating effects in photonic cavities for free electrons, No one had been able to conclusively observe these phenomena, due to fundamental limitations in the strength and duration of the interaction between electrons and confined light.
The Technion microscope overcomes this obstacle, allowing to record near-field optical maps using the quantum nature of electrons directlyA key piece of evidence is the observation of Rabi-type oscillations in the electronic spectrum, a behavior that cannot be explained by purely classical theories.
The more efficient photon-free electron interactions being explored with this system could lead to strong couplings, photon synthesis in special quantum states, and nonlinear phenomena unprecedented. All of this would benefit both electron microscopy (for example, for working with low doses on sensitive materials) and other fields of free electron physics.
Furthermore, the knowledge gained will help to Improve sharpness and color contrast on current screens, such as those based on QLED technology (quantum dots), already designing more uniform nano/quantum materials that allow for even greater image definition.
Taken together, the sum of these lines of research—QMC at Caltech, Q-MIC in Europe, QUIONE, and the Technion's 4D microscope—paints a picture in which the Microscopy becomes a profoundly quantum disciplinecapable of displaying, controlling, and even simulating matter at scales that were previously only a theoretical dream.
This entire ecosystem of new quantum microscopes This marks a turning point: it's no longer simply about seeing smaller, but about seeing differently, harnessing phenomena like entanglement, tunneling, coherence, and multi-particle interference to extract information unimaginable a few decades ago. As these technologies mature and move beyond the laboratory, they are expected to transform medicine, electronics, materials science, and, more broadly, our understanding of the innermost levels of reality.
