Tandem actions in the particle world

It’s not long into the conversation with Jasmin Meinecke that the magic word comes up: entanglement. Without it, nothing seems possible in the quantum world. It is a mysterious connection between things like photons or atoms, Meinecke explains. It means that these particles cannot be regarded as separate, even if they are far apart. Albert Einstein once called this invisible force “spooky action at a distance.” To this day, it is still not really understood.

Jasmin Meinecke is interested in such strange phenomena. She wants to learn more about a world that existed mainly only in theory for decades. Partly because it is so difficult to observe. Quantum theory is considered one of the most powerful physical theories of the 20th century. Although the finer details of many of its peculiar features remain unknown and still puzzle scientists to this day, there has recently been a great deal of interest around potential applications: from ultra-powerful quantum computers that are superior to today’s computers, to communication encrypted with quantum cryptography making it tap-proof, to super-precise quantum sensors that could enable completely new kinds of measuring instruments to become reality. The current euphoria is driven by the fascinating possibilities offered by the quantum world itself: The theory promises dramatic improvements over the classical equipment we know and use today. “Right now we’re learning more about what you can actually do with entangled particles,” says Meinecke.

The young physicist is one of many scientists to feel torn between the conflicting priorities of ambitious basic research and the great hopes for practical applications in the near future – in areas like quantum computing. Meinecke, who heads a junior research group at the Max Planck Institute of Quantum Optics (MPQ) in Garching, is keen to conduct basic research, and so she is studying photons — light particles — in that context. Yet at the same time, she also wants to keep an eye on possible later applications, for instance by understanding how quantum systems can be used to measure the physical and chemical properties of matter.

Jasmin Meinecke completed her doctorate in Bristol in 2015 before moving to LMU Munich. She is also one of the researchers in the cluster of excellence MCQST (the Munich Center for Quantum Science and Technology), and in January 2020 she was awarded a START Fellowship. This is a program designed to enable excellent postdocs to set up their own project within two years and receive 300,000 euros in funding for it. Meinecke is currently working in LMU physicist Harald Weinfurter’s group, where she is using this funding to study open quantum systems, which are systems that interact with their environment — something that scientists normally try to avoid at all costs. That is because such systems are very sensitive and are quickly disturbed by external influences — all it takes is a change in temperature or a vibration.

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Wave patterns in glass

To conduct her experiments, Meinecke uses photons that she observes under controlled conditions in integrated waveguides. Waveguides are small, unremarkable-looking glass plates with a kind of pattern inscribed inside them; the patterns are predetermined pathways for the light particles to travel along. Various entangled photons move towards and away from each other along these pathways.

Meinecke has sketched such wave patterns in her lab book and on whiteboards in the lab: curved paths, pretty to look at. Located inside the glass, they are not visible from the outside. The aim of these experiments is for the researcher to begin to understand, for example, how the quantum system and its environment exchange information and how that affects the quantum properties within it. “I’m fascinated by how and when quantum properties like coherence or entanglement are lost,” says Meinecke. “Quantum properties don’t just suddenly disappear. The question is, where does the information go?”

For some years now, researchers have had increasing success with comparatively simple experiments like these as a way to better understand some of the concepts of quantum mechanics, such as superposition and entanglement, and be able to use them in technological applications. Entanglement is not just one of the most important properties of quantum particles, it is also the central resource for promising quantum technologies.

Meinecke’s field, experimental quantum optics, is ultimately based on the ability of scientists to control light, matter, and how they interact to an ever better degree. It’s a field in which the Munich cluster of excellence brings together a huge amount of expertise and offers broad-ranging opportunities for cooperation. Meinecke chose photons as an experimental platform partly, she says, “because light particles are easy to control,” easier than solid-state systems with atoms, which are often very sensitive to their environment and in many cases can only serve as a platform for experiments at extremely low temperatures close to absolute zero. Added to that, photons have many properties that will be needed for future applications. For example, they can be readily transmitted in fiber optic cables like those used in telecommunications, and they are also easy to generate.

Several of the properties of photons can in principle be used for entanglement: the polarization of the light particles, the color of the light (the wavelength, in other words), the energy, the spin. Part of the reason why Meinecke is so excited about the possibilities offered by photons is because, unlike her colleagues who work with ultracold atoms, her experiments can be realized in a small space and she is able to take advantage of advances in the miniaturization of optical components.

The secret life of photons

The setups in her lab are, in fact, astonishingly simple compared to other quantum experiments. A laser, a crystal, a small glass plate, and a little bit of electronics to analyze the experiments. All of it fits into a space the size of a kitchen table. The laser beams its light through a fiber optic cable into a nonlinear crystal, in which the laser’s light particles can be used to generate photon pairs that are entangled. Whether an entangled pair is created from a photon coming out of the laser is pure chance.

That’s the mysterious bit about the experiment. It is a process that has only a certain probability of working. Once the particles have become entangled, that’s when things get exciting for the physicist. The particles can then move along different paths through the waveguides. But what do they really do? Which pathway do they choose? Do they remain entangled? These seemingly simple questions transport you deep into the jungle of the quantum world. Meinecke has a more fundamental way of putting all these questions: “What actually happens to the particles when they are not being observed?”

For the LMU physicist, this is not just a philosophical question. Observation is an important thing in the quantum world. Because on the one hand, you don’t know anything about what’s happening in the waveguide until you observe the particles. And on the other hand, the entanglement between the photons is usually lost very suddenly if you only measure one state of a photon, such as its energy.

The peculiarities of the quantum walk

So what can be done? “We’re working on weak measurement too,” says Meinecke. That means measuring without completely destroying the entanglement. Many research groups are interested in these same topics: Fellow scientists at the MPQ recently developed a method for detecting the entanglement of two distant atomic qubits, the quantum stores of information.

The scientists’ findings are not always easy for laypeople to grasp. A conversation with Jasmin Meinecke is like taking a high-speed ride through many exotic topics around quantum physics. She talks about Bell states and the peculiarities of the quantum walk, a kind of random walk that particles do, and of course the integrated waveguides that can be used for a wide variety of applications thanks to the complex structures inside the glass plates. “They can now provide the level of stability and miniaturization we need to build larger experimental arrays,” says Meinecke.

Qubits for quantum computers

Integrated waveguides are an ideal tool for exploring fundamental questions of quantum physics. This is another reason why Meinecke is now keen to test the possibilities of integrated waveguide structures as quantum simulators in several further series of experiments. She says that entangled photons make a good study platform precisely because quantum systems are sensitive to the tiniest of disturbances in the environment. And that is what Meinecke wants to develop measurement techniques for.

She evidently prefers the small experiments to experiments using large-scale lasers, such as those still set up from years past in the Laboratory for Multiphoton Physics in Garching. One of these lasers, a powerful, high-performance machine, has been dubbed “Tsunami”. Fitted with much more complex attachments, it can be used to generate up to six entangled photons; only a few laboratories are capable of generating more entangled pairs. The current record is twelve. The hope here is that these could be used as qubits for quantum computers. But the effort is immense, especially considering that useful applications would require many times more photons. “China still has research groups that keep reporting new records with more and more entangled particles,” she says. This is not for her: The amount of resources and lab time it consumes is enormous. “And it doesn’t tell me anything more about physics,” she says. “A scientist at a university needs to be generating more fundamental insights anyway.”

Despite all the current interest in areas like quantum computing, the researcher tries to keep her focus on the fundamentals. “Quantum simulators — and perhaps, later, quantum computers — are not just faster computers that can solve problems like how to optimize delivery routes in logistics or work out the structure and effect of new molecules,” says Meinecke. “It’s not just a matter of switching over from something like diesel engines to electric powertrains. It’s a long road. It’s quite simply something totally different, a new way of computing and understanding the world.”

Dr. Jasmin Meinecke is a postdoctoral researcher in the Institute of Experimental Quantum Physics at LMU and is a START Fellow in the cluster of excellence Munich Center for Quantum Science and Technology (MCQST).

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