Category Archives: Quantum Mechanics

Material science, Quantum Mechanics, superhero

Can We Manipulate Matter Like Thanos? (Quantum / Superhero)

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Being able to manipulate matter has been a long-standing goal in material science. Would it not be amazing if we could control matter on the grand scale that Thanos does when in possession of the Infinity Stones in Avengers: Infinity War? Now, Ravensteijn and colleagues evaluated how far mankind has come in the pursuit of Thanos-like matter manipulation powers. Their study appeared in the Journal Superhero Science and Technology.

They have shown that controlling matter, regardless of the length scale, requires control over the forces between objects. To control large (macroscopic) objects, a large amount of energy is needed. One way to control such objects is to acquire the Infinity Gauntlet complete with the six Infinity Stones, just like Thanos in Avengers: Infinity War. But, we can now mimic part of Thanos’ control over matter at the colloidal scale.

“We can now make a wide range of colloidal particles with tunable responsiveness, patchiness, shapes, and sizes. By controlling the interparticle forces, we can manipulate billions (yes billions!) of colloids at the same time by varying triggers such as temperature, pH, and light.”

— authors of the study.

What are colloidals?

The world of colloids lies between atoms and the objects. Colloidal materials consist of a large number of small particles such as solid particles, gas bubbles, or liquid droplets, that are mixed through a medium (such as a liquid, gas or solid). Due to their small dimensions, the earth’s gravity has little to no effect on these particles. This means that colloids dispersed in a medium do not (or barely) sink to the bottom of the container in which you keep them. However, this does not imply that colloids are immobile. Colloids are continuously moving, a phenomenon that scientists refer to as Brownian motion. These movements are the result of constant collisions between molecules of the dispersing medium (such as water molecules) and the colloids.

Figure 1. Schematic representation of the dimensions of objects ranging from the molecular/atomistic world to the macroscopic world. Representative examples of objects from life (real world and the MCU) are depicted to illustrate the specific length scales. The red triangular arrows depict the contribution of thermal motion and gravitational forces for objects with different characteristic dimensions. The colloidal domain ranges from roughly 10¯8 to 10¯6 m and is highlighted on the diagram. © Ravensteijn et al.

What scientists achieved?

Similar to Thanos’ ability to modify matter with by activating or triggering the appropriate Infinity Stone in the Infinity Gauntlet, colloid scientists started to study colloidal systems that can switch between the assembled and disassembled states, or even between assemblies with different internal structures. A popular and successful route for scientists towards these responsive particles is to decorate the surface of the colloids with molecules that can feel and respond to external changes or triggers in the environment, such as changes in pH, temperature, or the level of illumination with particular types of light (Figure 2).

Figure 2. Illustration of responsive colloidal systems. Upon applying Trigger i, e.g., a change (Δ) in pH, temperature, or illumination with UV light, the particles are switched from a non-interacting (left) to an activated state (right). In the active state assembly takes place. The assembly can be disintegrated by applying Trigger ii. Controlling these triggers is analogous to Thanos activating an Infinity Stone to manipulate matter. Switching between the disassembled and assembled state can be followed by microscopy or even macroscopic color changes. Scale bars: 5 μm. Microscopy images were adapted from ref. 50. 2015, Nature Publishing Group. © Ravensteijn et al.

Initially, the particles are not drawn to each other (they are in a non-interactive state). But, applying a trigger creates an attractive force between the particles that eventually leads to the creation of hierarchical structures. Applying a second trigger (or stopping the first one), removes the attraction between the particles and the assembly gradually falls apart again. This is a genuine “activation of the appropriate Infinity Stone” moment to manipulate colloids.

“In contrast to the instantaneous changes Thanos can make with his gauntlet, the assembly and disassembly of colloidal particles generally takes some time. A little patience is required to allow the colloids to find or move away from each other via Brownian motion. The time required for (dis)assembly can vary from seconds to hours and depends on the particle concentration and the strength of the attractive or repulsive forces generated by the applied triggers.”

— authors of the study.

Finally, scientists proved that, to manipulate matter, the Infinity Stones are not strictly necessary. There is no need to roam the universe for the stones just like Thanos did in Avengers: Infinity War. The answer may very well be right in front of us, and at the microscopic scale. The answer is colloids.

Reference: van Ravensteijn, B. G., Magana, J. R., & Voets, I. K. (2020). Manipulating matter with a snap of your fingers: A touch of Thanos in colloid science. Superhero Science and Technology, 2(1), 19–30. https://doi.org/10.24413/sst.2020.1.5329

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Quantum Mechanics

New Approach to Information Transfer Reaches Quantum Speed Limit (Quantum)

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Even though quantum computers are a young technology and aren’t yet ready for routine practical use, researchers have already been investigating the theoretical constraints that will bound quantum technologies. One of the things researchers have discovered is that there are limits to how quickly quantum information can race across any quantum device.

These speed limits are called Lieb-Robinson bounds, and, for several years, some of the bounds have taunted researchers: For certain tasks, there was a gap between the best speeds allowed by theory and the speeds possible with the best algorithms anyone had designed. It’s as though no car manufacturer could figure out how to make a model that reached the local highway limit.

But unlike speed limits on roadways, information speed limits can’t be ignored when you’re in a hurry—they are the inevitable results of the fundamental laws of physics. For any quantum task, there is a limit to how quickly interactions can make their influence felt (and thus transfer information) a certain distance away. The underlying rules define the best performance that is possible. In this way, information speed limits are more like the max score on an old school arcade game(link is external) than traffic laws, and achieving the ultimate score is an alluring prize for scientists.

Now a team of researchers, led by JQI Fellow Alexey Gorshkov, have found a quantum protocol that reaches the theoretical speed limits for certain quantum tasks. Their result provides new insight into designing optimal quantum algorithms and proves that there hasn’t been a lower, undiscovered limit thwarting attempts to make better designs. Gorshkov, who is also a Fellow of the Joint Center for Quantum Information and Computer Science (QuICS) and a physicist at the National Institute of Standards and Technology(link is external), and his colleagues presented their new protocol in a recent article published in the journal Physical Review X(link is external).

“This gap between maximum speeds and achievable speeds had been bugging us, because we didn’t know whether it was the bound that was loose, or if we weren’t smart enough to improve the protocol,” says Minh Tran, a JQI and QuICS graduate student who was the lead author on the article. “We actually weren’t expecting this proposal to be this powerful. And we were trying a lot to improve the bound—turns out that wasn’t possible. So, we’re excited about this result.”

Unsurprisingly, the theoretical speed limit for sending information in a quantum device (such as a quantum computer) depends on the device’s underlying structure. The new protocol is designed for quantum devices where the basic building blocks—qubits—influence each other even when they aren’t right next to each other. In particular, the team designed the protocol for qubits that have interactions that weaken as the distance between them grows. The new protocol works for a range of interactions that don’t weaken too rapidly, which covers the interactions in many practical building blocks of quantum technologies, including nitrogen-vacancy centers, Rydberg atoms, polar molecules and trapped ions.

Crucially, the protocol can transfer information contained in an unknown quantum state to a distant qubit, an essential feature for achieving many of the advantages promised by quantum computers. This limits the way information can be transferred and rules out some direct approaches, like just creating a copy of the information at the new location. (That requires knowing the quantum state you are transferring.)

In the new protocol, data stored on one qubit is shared with its neighbors, using a phenomenon called quantum entanglement. Then, since all those qubits help carry the information, they work together to spread it to other sets of qubits. Because more qubits are involved, they transfer the information even more quickly.

This process can be repeated to keep generating larger blocks of qubits that pass the information faster and faster. So instead of the straightforward method of qubits passing information one by one like a basketball team passing the ball down the court, the qubits are more like snowflakes that combine into a larger and more rapidly rolling snowball at each step. And the bigger the snowball, the more flakes stick with each revolution.

But that’s maybe where the similarities to snowballs end. Unlike a real snowball, the quantum collection can also unroll itself. The information is left on the distant qubit when the process runs in reverse, returning all the other qubits to their original states.

When the researchers analyzed the process, they found that the snowballing qubits speed along the information at the theoretical limits allowed by physics. Since the protocol reaches the previously proven limit, no future protocol should be able to surpass it.

“The new aspect is the way we entangle two blocks of qubits,” Tran says. “Previously, there was a protocol that entangled information into one block and then tried to merge the qubits from the second block into it one by one. But now because we also entangle the qubits in the second block before merging it into the first block, the enhancement will be greater.”

The protocol is the result of the team exploring the possibility of simultaneously moving information stored on multiple qubits. They realized that using blocks of qubits to move information would enhance a protocol’s speed.

“On the practical side, the protocol allows us to not only propagate information, but also entangle particles faster,” Tran says. “And we know that using entangled particles you can do a lot of interesting things like measuring and sensing with a higher accuracy. And moving information fast also means that you can process information faster. There’s a lot of other bottlenecks in building quantum computers, but at least on the fundamental limits side, we know what’s possible and what’s not.”

In addition to the theoretical insights and possible technological applications, the team’s mathematical results also reveal new information about how large a quantum computation needs to be in order to simulate particles with interactions like those of the qubits in the new protocol. The researchers are hoping to explore the limits of other kinds of interactions and to explore additional aspects of the protocol such as how robust it is against noise disrupting the process.

In addition to Gorshkov and Tran, co-authors of the research paper include JQI and QuICS graduate student Abhinav Deshpande, JQI and QuICS graduate student Andrew Y. Guo, and University of Colorado Boulder Professor of Physics Andrew Lucas.

Featured image: In a new quantum protocol, groups of quantum entangled qubits (red dots) recruit more qubits (blue dots) at each step to help rapidly move information from one spot to another. Since more qubits are involved at each step, the protocol creates a snowball effect that achieves the maximum information transfer speed allowed by theory. (Credit: Minh Tran/JQI)

REFERENCE PUBLICATION: “Optimal State Transfer and Entanglement Generation in Power-Law Interacting Systems,” Minh C. Tran, Andrew Y. Guo, Abhinav Deshpande, Andrew Lucas, Alexey V. Gorshkov, Phys. Rev. X, 11, 031016 (2021)

Provided by Joint Quantum Institute

PHYSICS, Quantum Mechanics

Why Strong Interaction at Large Distances And Weaker At Short? (Particle Physics / Quantum)

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Recently, Subhash Kak proposed that space is e-dimensional, rather than the commonly supposed 3-dimensions, where, e is Eulers number that is about 2.71828, and he used this to provide a resolution to the problem of Hubble tension, which is the disagreement between values of the rate at which the universe is expanding obtained using two different methods. Now, Kak has provided further support to this theory by a model that may explain why quarks, constituents of matter, don’t interact with each other when they are up close, but are bound strongly when pulled apart.

“Noninteger dimensionality leads to the anomalous situation of strong interaction at large distances and much weaker interaction at short distances, which is a characteristic of asymptotic freedom,” writes Subhash Kak, Professor in the School of Electrical and Computer Engineering at Oklahoma State University.

His new paper shows that, as the dimensionality falls below the value of critical dimension (dcrit), which lies between 2 and 3, there arises strange behavior where increasing energy reduces the strength of interaction between the particles. 

And when the value is 2 or below, the potential becomes constant (independent of separation) and force between objects or particles completely disappears. This new version of asymptotic freedom, which arises from the squeezing the dimensionality of space, could be of use in studying the anomalous mechanical properties of metamaterials.

“Future investigation will reveal whether this phenomenon based on dimensionality of space has any connection with other models of asymptotic freedom,” he concludes.

Featured image: Potential with respect to dimensionality; blue line p3,B(d), and red line p3,C(d) © S. Kak

Reference: Kak, S. Asymptotic freedom and noninteger dimensionality. Sci Rep 11, 3406 (2021). https://doi.org/10.1038/s41598-021-83002-9

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Reviewed by S. Kak

Quantum Mechanics

New Viable Ways Of Storing Information For Quantum Technologies? (Quantum)

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Quantum information could be the source of the next technological revolution. By analogy with the bit in classical computing, the qubit is the basic element of quantum computing. But demonstrating the existence of this information storage unit and using it is still complex and therefore limited. In a study published on August 3, 2021 in Physical Review X , an international research team, composed by Fabio Pistolesi, CNRS researcher and two foreign researchers, succeeded by theoretical calculations to show that it is possible to realize a new type of qubit where information is stored in the oscillation amplitude of a carbon nanotube. 

Indeed, these nanotubes are able to perform a large number of oscillations without fading, which shows their weak interaction with the environment and makes them excellent potential qubits. This property would allow greater reliability in quantum computation. However, a problem persisted in reading and writing the information stored in the first two energy levels of these oscillators. 

Scientists have succeeded in proving that it is possible to read this information by exploiting the coupling between electrons, a negatively charged particle, and the bending mode of these nanotubes. This makes it possible to sufficiently change the spacing between the first energy levels and thus make them accessible independently of the other levels to read the information they contain. It now remains to experimentally verify these promising theoretical predictions.

To find out more: A computer to be reinvented for quantum computing

Featured image: Representation of the bending mode of a nanotube shown here in turquoise blue, and the locations of electrons in red and brown in the tube. © Fabio Pistolesi

Bibliography

Proposal for a Nanomechanical Qubit . F. Pistolesi, AN Cleland and A. Bachtold. Physical Review X , August 3, 2021. https://doi.org/10.1103/PhysRevX.11.031027

Provided by CNRS

Quantum Mechanics

Researchers Developed Quantum Sensing System To Detect Pipeline Leaks More Quickly (Quantum)

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To minimize potential damage from underground oil and gas leaks, Oak Ridge National Laboratory is co-developing a quantum sensing system to detect pipeline leaks more quickly.

Currently, fiber-optic sensing cables running through or around pipes detect fluid flow and leaks with signals from classical light sources. The new system by the University of Oklahoma, Louisiana State University and ORNL will replace classical light with quantum light originating from entanglement. Quantum-entangled light sources create less background noise than classical light and are sensitive to smaller signals.

“We’ve shown in many other systems that there are signals that are so small that they’re hidden by the classical noise, but you can see them with the quantum noise,” ORNL’s Raphael Pooser said.

OU researchers are building the machine that produces entangled particles. The team will evaluate the system at ORNL and then perform a 5,000-foot deep well test at LSU. – Alexandra DeMarco

Featured image: ORNL’s particle entanglement machine is a precursor to the device that University of Oklahoma researchers are building, which will produce entangled quantum particles for quantum sensing to detect underground pipeline leaks. Credit: ORNL, U.S. Dept. of Energy

This science news has been confirmed by us from ORNL

Provided by ORNL

Cosmology, Maths, Quantum Mechanics

What Leads To Factorization Problem? How Half Wormholes Can Fix It? (Maths / Cosmology / Quantum)

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Wormholes not only play a key role in understanding the nonperturbative physics of quantum black holes, for instance: the eternal traversable wormhole; the long-time behavior of the spectral form factor and correlation functions, the Page curve etc. but also, it leads to puzzles, in particular the factorization problem. Imagine two decoupled boundary systems in the AdS/CFT context, labelled L and R. From the boundary perspective, if one evaluates a partition function in the combined system the result is just the product of the results for the two component systems:

It factorizes. But, if the bulk calculation of ZLR includes a wormhole linking L and R then superficially at least ZLR ≠ ZLZR. It fails to factorize. Some of the phenomena recently explained by wormholes, in particular the spectral form factor and squared matrix elements, are described by decoupled boundary systems and so the wormhole explanation give rise to a factorization puzzle.

But, you can remove this factorization puzzle by averaging the L and R systems over the same ensemble, denoted by (·), with the help of the SYK model. The factorization puzzle solves because (ZLZR) need not to be same as (ZL) (ZR). And infact this link between wormholes and ensembles is not a new one, it dates back to the 1980s. However, it has been recently applied in AdS/CFT context.

We can create a new form of factorization puzzle in such ensembles by asking what happens to the wormholes connecting decoupled systems when we focus on just 1 element of the ensemble. Now, Phil Saad and colleagues addressed this question in the SYK model where instead of averaging the L and R systems they picked a fixed set of couplings between the fermions.

These pictures represent saddle points of the SYK path integral, associated to the sketched bulk topology by the pattern of correlation. As the wormhole contribution is self-averaging, they have depicted it with a small red “x” to indicate the small amount of randomness. The half-wormhole contributions are not self-averaging, so they have depicted them as “half” of a wormhole with a jagged red boundary to indicate the large amount of randomness. They have included a red line linking the pair of half-wormholes on the LHS, to remind them that the LR collective fields are present, but set to zero, distinguishing this contribution from the unlinked pair of half wormholes on the RHS. © Phil Saad et al.

After averaging over fermion couplings, SYK model has a collective fields called G and Σ, that sometimes has “wormhole” solutions. Phil Saad and colleagues studied the fate of these wormholes when the couplings are fixed.

Working mainly in a simple model, they found that the wormhole saddles persist, and the dependence on the couplings is weak. The wormhole is “self-averaging”. But, that new saddles also appear elsewhere in the integration space, which they interpret as “half-wormholes.” The half-wormhole contributions depends sensitively on the particular choice of couplings.

Finally, they showed that, the half-wormholes are crucial for factorization (or restore factorization) of decoupled systems with fixed couplings, but they vanish after averaging, leaving the non-factorizing wormhole behind.

Reference: Phil Saad, Stephen H. Shenker, Douglas Stanford, and Shunyu Yao, “Wormholes without averaging”, Arxiv, pp. 1-34, 2021.
arXiv:2103.16754

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Cosmology, Quantum Mechanics

Can A Traversable Wormhole Exist Only At The Planck Scale? (Quantum / Cosmology)

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Hideki Maeda presented a simple traversable wormhole solution which violate energy conditions only at the planck scale

Wormhole is a configuration of spacetimes containing distinct non-timelike infinities. In particular, a wormhole that contains a casual curve connecting such infinities are referred to as a traversable wormhole. Several examples of traversable wormholes have been already discussed by us in our articles. Now, Hideki Maeda presented a simple static spacetime which describes a spherically symmetric traversable worm-hole characterized by a length parameter, l and reduces to Minkowski in the limit l → 0. His findings recently appeared in Arxiv.

He showed that, this wormhole connects two distinct asymptotically flat regions with vanishing ADM mass and the areal radius of its throat is exactly l. Additionally, all the standard energy conditions i.e. null-energy condition, weak energy condition, dominant energy condition and standard energy condition are violated outside the proper radial distance approximately 1.60 l from the wormhole throat.

Finally, he computed the total amount of negative energy required to support this wormhole and found that, if l is identified as the Planck length lp (≃ 1.616 × 10¯35 m), the total amount of the negative energy supporting this wormhole is only E ≃ −2.65 mpc², which is the rest mass energy of about – 5.77 × 10¯5 g. For a “humanly traversable” wormhole with l = 1m, he obtained mass of −3.57 × 1027 kg, which is about –1.88 times as large as Jupiter’s mass.

“Ofcourse, an important task is to identify a theory of gravity which admits the static spacetime as a solution. Once such theory is identified, a subsequent task is to study the stability. Since the region of the NEC violation is tiny, our wormhole could be a dynamically stable configuration in the semiclassical regime. These important tasks are left for future investigations.”

— he concluded.

Reference: Hideki Maeda, “A simple traversable wormhole violating energy conditions only at the Planck scale”, Arxiv, pp. 1-4, 2021.
arXiv:2107.07052

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Quantum Mechanics

How Would Be Thick Branes In Mimetic Gravity? (Quantum)

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Qun-Ying Xie and colleagues investigated thick branes generated by a scalar field in mimetic gravity theory, which is inspired by considering the conformal symmetry under the conformal transformation of an auxiliary metric. They obtained a series of analytical thick brane solutions and found that, perturbations of the brane system are stable and the effective potentials for the tensor and scalar perturbations are dual to each other. Findings of this study recently appeared in the Journal Symmetry.

Mimetic gravity is a Weyl-symmetric extension of General Relativity, related to the latter by a singular disformal transformation, wherein the appearance of a dust-like perfect fluid can mimic cold dark matter at a cosmological level. Within this framework, it is possible to provide an unified geometrical explanation for dark matter, the late-time acceleration, and inflation, making it a very attractive theory. This theory was also extended to Horava-like theory and applied to galactic rotation curves. It was also applied to other gravity theories such as f(R) gravity, Horndeski gravity and Gauss–Bonnet gravity.

On the other hand, Lisa Randall and Raman Sundrum proposed that our four-dimensional world could be a brane embedded in five-dimensional space-time, in order to solve gauge hierarchy problem and the cosmological constant problem. With the warped extra dimension, it was further found that the size of extra dimension can be infinitely large without conflicting with Newtonian gravitational law. This charming idea has attracted substantial researches in particle physics, cosmology, gravity theory, and other related fields. In the RS model, the brane is geometrically thin, therefore the space-time is singular at the brane. Although in the thin brane approximation many interesting results have been obtained, in some situations the effects of the brane thickness cannot be neglected.

“In five-dimensional problems, the thin brane approximation is valid as long as the brane thickness cannot be resolved, in other words, if the energy scale of the brane thickness is much higher than those in the bulk and on the brane. In contrast, when thickness becomes as large as the scale of interest, its effect is no longer negligible.”

Now, Qun-Ying Xie and colleagues investigated the super-potential method with which the second-order equations can be reduced to the first-order ones for thick brane models in modified gravity with Lagrange multiplier. The main step of this method is to introduce a pair of auxiliary super-potentials, i.e., W(φ) and Q(φ). With these two super-potentials, the field equations are rewritten as Equations (1)–(5).

Then, they used this method to find a series of analytical thick brane solutions via some polynomial super-potentials, period super-potentials, and mixed super-potentials.

Finally, they analyzed the tensor and scalar perturbations of the brane system. It was shown that both equations of motion of the perturbations can be transformed into Schrodinger-like equations. They added that, both perturbations are stable and the effective potentials for the tensor and scalar perturbations are dual to each other. Moreover, the tensor zero mode can be localized on the brane while the scalar zero mode cannot. Thus, the four-dimensional Newtonian potential can be recovered on the brane and there is no additional fifth force contradicting with the experiments.

Featured image: The effective potential VS(z(y)) for solution I. The parameter is set as n = 1 (red solid thick lines), n = 3 (blue dashed lines), and n = 5 (black solid thin lines). © Xie et al.

Reference: Xie, Q.-Y.; Fu, Q.-M.; Sui, T.-T.; Zhao, L.; Zhong, Y. First-Order Formalism and Thick Branes in Mimetic Gravity. Symmetry 2021, 13, https://doi.org/10.3390/sym13081345

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PHYSICS, Quantum Mechanics

How Quantum Fields Could Be Used To Break Low-temperature Records? (Quantum / Physics)

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At first glance, heat and cold do not have much to do with quantum physics. A single atom is neither hot nor cold. Temperature can traditionally only be defined for objects that consist of many particles. But at TU Wien, in collaboration with FU Berlin, Nanyang Technological University in Singapore and the University of Lisbon, it has now been possible to show what possibilities arise when thermodynamics and quantum physics are combined: One can specifically use quantum effects to cool a cloud of ultracold atoms even further.

No matter what sophisticated cooling methods have been used before—with this technique, which has now been presented in the scientific journal Physical Review X-Quantum, it is possible to come a little closer to absolute zero. A lot of work is still needed before this new cooling concept can be turned into an actual quantum refrigerator, but initial experiments already show that the necessary steps are possible in principle.

A new field of research: quantum thermodynamics

“For a long time, thermodynamics has played an important role for classical mechanical machines—think of steam engines or combustion engines, for example. Today, quantum machines are being developed on a tiny scale. And there, thermodynamics has hardly played a role there so far” says Prof. Eisert from the Free University of Berlin.

“If you want to build a quantum heat machine, you have to fulfill two requirements that are fundamentally contradictory,” says Prof. Marcus Huber from TU Wien. “It has to be a system that consists of many particles and in which you cannot control every detail exactly. Otherwise you cannot speak of heat. And at the same time, the system must be simple enough and sufficiently precisely controllable not to destroy quantum effects. Otherwise, you can’t talk about a quantum machine.”

“Back in 2018, we came up with the idea of transferring the basic principles of thermal machines to quantum systems by using quantum field descriptions of many-body quantum systems,” says Prof. Jörg Schmiedmayer (TU Wien). Now the research team from TU Wien and FU Berlin examined in detail how such quantum heat machines can be designed. They were guided by the operating principle of an ordinary refrigerator: initially, everything has the same temperature—the interior of the refrigerator, the environment and the coolant. But when you evaporate the coolant inside the refrigerator, heat is extracted there. The heat is then released outside when the coolant is liquefied again. So by raising and lowering the pressure it is possible to cool the inside and transfer the heat to the environment.

The question was whether there could also be a quantum version of such a process. “Our idea was to use a Bose-Einstein condensate for this, an extremely cold state of matter,” says Prof. Jörg Schmiedmayer. “In recent years, we have gained a lot of experience in controlling and manipulating such condensates very precisely with the help of electromagnetic fields and laser beams, investigating some of the fundamental phenomena at the borderline between quantum physics and thermodynamics. The logical next step was the quantum heat machine.”

How quantum fields could be used to break low-temperature records
Credit: Vienna University of Technology

Energy redistribution at the atomic level

A Bose-Einstein condensate is divided into three parts, which initially have the same temperature. “If you couple these subsystems in exactly the right way and separate them from each other again, you can achieve that the part in the middle acts as a piston, so to speak, and allows heat energy to be transferred from one side to the other,” explains Marcus Huber. “As a result, one of the three subsystems is cooled down.”

Even at the beginning, the Bose-Einstein condensate is in a state of very low energy—but not quite in the lowest possible energy state. Some quanta of energy are still present and can change from one subsystem to another—these are known as “excitations of the quantum field.”

“These excitations take on the role of the coolant in our case,” says Marcus Huber. “However, there are fundamental differences between our system and a classical refrigerator: In a classical refrigerator, heat flow can only occur in one direction—from warm to cold. In a quantum system, it is more complicated; the energy can also change from one subsystem to another and then return again. So you have to control very precisely when which subsystems should be connected and when they should be decoupled.”

So far, this quantum refrigerator is only a theoretical concept—but experiments have already shown that the necessary steps are feasible. “Now that we know that the idea basically works, we will try to implement it in the lab,” says Joao Sabino (TU Wien). “We hope to succeed in the near future.” That would be a spectacular step forward in cryogenic physics—because no matter what other methods you use to reach extremely low temperatures, you could always add the novel ‘quantum refrigerator’ at the end as a final additional cooling stage to make one part of the ultracold system even colder. “If it works with cold atoms, then our ideas can be implemented in many other quantum systems and lead to new quantum technology applications,” says Jörg Schmiedmayer.

Featured image: João Sabino in the lab. Credit: Vienna University of Technology

Reference: Marek Gluza et al, Quantum Field Thermal Machines, PRX Quantum (2021). DOI: 10.1103/PRXQuantum.2.030310

Provided by Vienna University of Technology