̽��ֱ�� of Cambridge - Skolkovo Institute of Science and Technology

Giant 'quantum twisters' may form in liquid light

sc604 — Fri, 05 Mar 2021 16:55:21 +0000

Anyone who has drained a bathtub or stirred cream into coffee has seen a vortex, a ubiquitous formation that appears when fluid circulates. But unlike water, fluids governed by the strange rules of quantum mechanics have a special restriction: as was first predicted in 1945 by future Nobel winner Lars Onsager, a vortex in a quantum fluid can only twist by whole-number units.

These rotating structures are predicted to be widely useful for studying everything from quantum systems to black holes. But while the smallest possible quantum vortex, with a single unit of rotation, has been seen in many systems, larger vortices are not stable. While scientists have attempted to force larger vortices to hold themselves together, the results have been mixed: when the vortices have been formed, the severity of the methods used have generally destroyed their usefulness.

Now, Samuel Alperin and Professor Natalia Berloff from the ̽��ֱ�� of Cambridge have discovered a theoretical mechanism through which giant quantum vortices are not only stable but form by themselves in otherwise near-uniform fluids. ̽��ֱ��findings, published in the journal Optica, could pave the way for experiments that might provide insight into the nature of rotating black holes that have similarities with giant quantum vortices.

To do this, the researchers used a quantum hybrid of light and matter, called a polariton. These particles are formed by shining laser light onto specially layered materials. “When the light gets trapped in the layers, the light and the matter become inseparable, and it becomes more practical to look at the resulting substance as something that is distinct from either light or matter, while inheriting properties of both,” said Alperin, a PhD student at Cambridge’s Department of Applied Mathematics and Theoretical Physics.

One of the most significant properties of polaritons comes from the simple fact that light can’t be trapped forever. A fluid of polaritons, which requires a high density of the exotic particles, is constantly expelling light, and needs to be fed with fresh light from the laser to survive. “ ̽��ֱ��result,” said Alperin, “is a fluid which is never allowed to settle, and which doesn’t need to obey what are usually basic restrictions in physics, like the conservation of energy. Here the energy can change as a part of the dynamics of the fluid.”

It was exactly these constant flows of liquid light that the researchers exploited to allow the elusive giant vortex to form. Instead of shining the laser on the polariton fluid itself, the new proposal has the light shaped like a ring, causing a constant inward flow similarly to how water flows to a bathtub drain. According to the theory, this flow is enough to concentrate any rotation into a single giant vortex.

“That the giant vortex really can exist under conditions that are amenable to their study and technical use was quite surprising,” Alperin said, “but really it just goes to show how utterly distinct the hydrodynamics of polaritons are from more well-studied quantum fluids. It’s exciting territory.”

̽��ֱ��researchers say that they are just at the beginning of their work on giant quantum vortices. They were able to simulate the collision of several quantum vortices as they dance around each other with ever increasing speed until they collide to form a single giant vortex analogous to the collision of black holes. They also explained the instabilities that limit the maximum vortex size while exploring intricate physics of the vortex behaviour.

“These structures have some interesting acoustic properties: they have acoustic resonances that depend on their rotation, so they sort of sing information about themselves,” said Alperin. “Mathematically, it’s quite analogous to the way that rotating black holes radiate information about their own properties.”

̽��ֱ��researchers hope that the similarity could lead to new insights into the theory of quantum fluid dynamics, but they also say that polaritons might be a useful tool to study the behaviour of black holes.

Professor Berloff is jointly affiliated with Cambridge and the Skolkovo Institute of Science and Technology in Russia.

Reference:
Samuel N. Alperin and Natalia G. Berloff. ‘Multiply charged vortex states of polariton condensates.’ Optica (2021). DOI: 10.1364/OPTICA.418377

New mechanism found for generating giant vortices in quantum fluids of light.

Stable giant quantum vortices

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Through the looking glass: artificial ‘molecules’ open door to ultrafast devices

sc604 — Wed, 03 Mar 2021 00:00:01 +0000

Polaritons are quantum particles that consist of a photon and an exciton, another quasiparticle, combining light and matter in a curious union that opens up a multitude of possibilities in next-generation devices.

̽��ֱ��researchers have shown that geometrically coupled polariton condensates, which appear in semiconductor devices, are capable of simulating molecules with various properties.

Ordinary molecules are groups of atoms bound together with molecular bonds, and their physical properties differ from those of their constituent atoms quite drastically: consider the water molecule, H₂O, and elemental hydrogen and oxygen.

“In our work, we show that clusters of interacting polaritonic and photonic condensates can form a range of exotic and entirely distinct entities – ‘molecules’ – that can be manipulated artificially,” said first author Alexander Johnston, from Cambridge Department of Applied Mathematics and Theoretical Physics. “These artificial molecules possess new energy states, optical properties, and vibrational modes from those of the condensates comprising them.”

Johnston and his colleagues – Kirill Kalinin from DAMTP and Professor Natalia Berloff, who holds joint positions at Cambridge and Skoltech – were running numerical simulations of two, three, and four interacting polariton condensates, when they noticed some curious asymmetric stationary states in which not all of the condensates have the same density in their ground state.

“Upon further investigation, we found that such states came in a wide variety of different forms, which could be controlled by manipulating certain physical parameters of the system,” said Johnston. “This led us to propose such phenomena as artificial polariton molecules and to investigate their potential uses in quantum information systems.”

In particular, the team focused on an ‘asymmetric dyad’, which consists of two interacting condensates with unequal occupations. When two of those dyads are combined into a tetrad structure, the latter is, in some sense, analogous to a homonuclear molecule – for instance, to molecular hydrogen H₂. Furthermore, artificial polariton molecules can also form more elaborate structures, which could be thought of as artificial polariton compounds.

“There is nothing preventing more complex structures from being created,” said Johnston. “We’ve found that there is a wide range of exotic, asymmetric states possible in tetrad configurations. In some of these, all condensates have different densities, despite all of the couplings being of equal strength, inviting an analogy with chemical compounds.”

In specific tetrad structures, each asymmetric dyad can be viewed as an individual ‘spin,’ defined by the orientation of the density asymmetry. This has interesting consequences for the system’s degrees of freedom, or the independent physical parameters required to define states. ̽��ֱ��spins introduce a separate degree of freedom, in addition to the continuous degrees of freedom given by the condensate phases.

̽��ֱ��relative orientation of each of the dyads can be controlled by varying the coupling strength between them. Since quantum information sem.

“In addition, we have discovered a plethora of exotic asymmetric states in triad and tetrad systems,” said Berloff. “It is possible to seamlessly transition between such states simply by varying the pumping strength used to form the condensates. This property suggests that such states could form the basis of a polaritonic multi-valued logic system, which could enable the development of polaritonic devices that dissipate significantly less power than traditional methods and, potentially, operate orders of magnitude faster.”

Reference:
Alexander Johnston, Kirill P. Kalinin, and Natalia G. Berloff. ‘Artificial polariton molecules.’ Physical Review Letters B (2021). DOI: 10.1103/PhysRevB.103.L060507

Adapted from a Skoltech press release.

Researchers from the ̽��ֱ�� of Cambridge and Skoltech in Russia have shown that polaritons, the quirky particles that may end up running the quantum supercomputers of the future, can form structures that behave like molecules – and these ‘artificial molecules’ can potentially be engineered on demand. Their results are published in the journal Physical Review B Letters.

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‘Multiplying’ light could be key to ultra-powerful optical computers

sc604 — Mon, 08 Feb 2021 10:26:02 +0000

An important class of challenging computational problems, with applications in graph theory, neural networks, artificial intelligence and error-correcting codes can be solved by multiplying light signals, according to researchers from the ̽��ֱ�� of Cambridge and Skolkovo Institute of Science and Technology in Russia.

In a paper published in the journal Physical Review Letters, they propose a new type of computation that could revolutionise analogue computing by dramatically reducing the number of light signals needed while simplifying the search for the best mathematical solutions, allowing for ultra-fast optical computers.

Optical or photonic computing uses photons produced by lasers or diodes for computation, as opposed to classical computers which use electrons. Since photons are essentially without mass and can travel faster than electrons, an optical computer would be superfast, energy-efficient and able to process information simultaneously through multiple temporal or spatial optical channels.

̽��ֱ��computing element in an optical computer – an alternative to the ones and zeroes of a digital computer – is represented by the continuous phase of the light signal, and the computation is normally achieved by adding two light waves coming from two different sources and then projecting the result onto ‘0’ or ‘1’ states.

However, real life presents highly nonlinear problems, where multiple unknowns simultaneously change the values of other unknowns while interacting multiplicatively. In this case, the traditional approach to optical computing that combines light waves in a linear manner fails.

Now, Professor Natalia Berloff from Cambridge’s Department of Applied Mathematics and Theoretical Physics and PhD student Nikita Stroev from Skolkovo Institute of Science and Technology have found that optical systems can combine light by multiplying the wave functions describing the light waves instead of adding them and may represent a different type of connections between the light waves.

They illustrated this phenomenon with quasi-particles called polaritons – which are half-light and half-matter – while extending the idea to a larger class of optical systems such as light pulses in a fibre. Tiny pulses or blobs of coherent, superfast-moving polaritons can be created in space and overlap with one another in a nonlinear way, due to the matter component of polaritons.

“We found the key ingredient is how you couple the pulses with each other,” said Stroev. “If you get the coupling and light intensity right, the light multiplies, affecting the phases of the individual pulses, giving away the answer to the problem. This makes it possible to use light to solve nonlinear problems.”

̽��ֱ��multiplication of the wave functions to determine the phase of the light signal in each element of these optical systems comes from the nonlinearity that occurs naturally or is externally introduced into the system.

“What came as a surprise is that there is no need to project the continuous light phases onto ‘0’ and ‘1’ states necessary for solving problems in binary variables,” said Stroev. “Instead, the system tends to bring about these states at the end of its search for the minimum energy configuration. This is the property that comes from multiplying the light signals. On the contrary, previous optical machines require resonant excitation that fixes the phases to binary values externally.”

̽��ֱ��authors have also suggested and implemented a way to guide the system trajectories towards the solution by temporarily changing the coupling strengths of the signals.

“We should start identifying different classes of problems that can be solved directly by a dedicated physical processor,” said Berloff, who also holds a position at Skolkovo Institute of Science and Technology. “Higher-order binary optimisation problems are one such class, and optical systems can be made very efficient in solving them.”

There are still many challenges to be met before optical computing can demonstrate its superiority in solving hard problems in comparison with modern electronic computers: noise reduction, error correction, improved scalability, guiding the system to the true best solution are among them.

“Changing our framework to directly address different types of problems may bring optical computing machines closer to solving real-world problems that cannot be solved by classical computers,” said Berloff.

Reference:
Nikita Stroev and Natalia G. Berloff. ‘Discrete Polynomial Optimization with Coherent Networks of Condensates and Complex Coupling Switching.’ Physical Review Letters (2021). DOI: 10.1103/PhysRevLett.126.050504

New type of optical computing could solve highly complex problems that are out of reach for even the most powerful supercomputers.

Gleb Berloff, Hills Road Sixth Form College

Artist's impression of light pulses inside an optical computer

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Hunt for an unidentified electron object

lw355 — Tue, 25 Mar 2014 10:44:15 +0000

Researchers have developed a new mathematical framework capable of describing motions in superfluids – low temperature fluids that exhibit classical as well as quantum behaviour. ̽��ֱ��framework was used to lift the veil of mystery surrounding strange objects in superfluid helium (detected ten years ago at Brown ̽��ֱ��). ̽��ֱ��study, conducted by an international collaboration of researchers from the UK, Russia and France is published today in the journal Proceedings of National Academy of Sciences (PNAS).

̽��ֱ��quantum nature of superfluids manifests itself in the form of quantized vortices, tiny twisters, with the core sizes of the order of an Angstrom (0.1nm – approximately the diameter of an atom) that move through fluid severing and coalescing, forming bundles and tangles. To make these processes even more intricate and distinct from motions in usual classical fluids, these tiny twisters live on the background consisting of a mixture of viscous and inviscid fluid components that constitute superfluid. ̽��ֱ��mathematical modelling of such complex systems that involve a range of scales is a notoriously difficult problem.

̽��ֱ��international team of researchers – Natalia Berloff of the ̽��ֱ�� of Cambridge and Skolkovo Institute of Science and Technology, Marc Brachet of Université Pierre-et-Marie-Curie and Nick Proukakis of Joint Quantum Centre Durham-Newcastle – came up with a novel framework for achieving this task. ̽��ֱ��team applied their method to elucidate an intriguing phenomenon in liquid helium research.

Electrons immersed in superfluid helium are useful experimental probes. As they move through superfluid they form soft bubbles of about 2 nm in diameter that get trapped by quantized vortices quite similar to how houses and cars become trapped and transported by a tornado.

A research team from Brown ̽��ֱ�� led by Professor Humphrey Maris has studied the effect of oscillating pressures on electron bubbles. As pressure decreases below the criticality, the bubble expands and explodes, reaching micron sizes, with the bubble trapped by a vortex exploding at a pressure larger than that for the free bubble. Maris’ team also discovered another class of object that existed at very low temperatures only and exploded at even larger pressures. They termed these “unidentified electron objects”.

̽��ֱ��new approach published in PNAS today allowed the researchers to look at the processes as oscillating pressure was applied to a quantum fluid containing a vortex ring at a range of temperatures. ̽��ֱ��researchers discovered a novel mechanism of vortex multiplication: the vortex core expands and then contracts, forming a dense array of new vortex rings during the contraction stage. They conjectured that it becomes quite likely that the electron bubble becomes trapped by more than one vortex line, furthermore reducing the pressure change needed for consequent explosions. They have also shown that the mechanism of vortex multiplication is suppressed at higher temperatures, explaining why such objects were found experimentally only at lower temperatures.

Professor Berloff who led the team commented: “It is fascinating to have a tool to look at the dynamics of processes that occur on the Angstrom lengthscales and at ultra-low temperatures in quantum fluids. ̽��ֱ��mystery of an unidentified electron object is just a teaser problem; we are ready for other challenges.”

“Understanding the intricate features of behaviour of quantized vortices is one of the grand unsolved problems that can be tackled with this framework,” added Professor Proukakis.

̽��ֱ��research was funded by the Engineering and Physical Sciences Research Council, the EU and the Skolkovo Foundation.

New research sheds light on the nature of ‘unidentified electron objects’ - a mysterious class of objects that exists in superfluid helium at low temperature.

̽��ֱ��mystery of an unidentified electron object is just a teaser problem; we are ready for other challenges.

Natalia Berloff

Gleb and Sofia Berloff, ISM

Vortex rings as the result of vortex multiplication in a quantum fluid; some electrons are free, and some got trapped by one or more vortices

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