A new theoretical approach helps connect quantum physics with classical thermodynamics by clarifying what counts as heat and useful work in tiny quantum machines. When a machine is reduced to a single atom and particles of light, even basic concepts such as heat and useful work become difficult to define. This problem sits at the [...]
For the first time, physicists in Japan have carried out an atomic-scale version of the double-slit experiment. Using an ultraprecise electron beam channelled along two adjacent columns of atoms in a crystal of silicon, Naoya Shibata and colleagues at the University of Tokyo observed clear interference fringes in electrons scattered by the atoms: fringes that persisted even at high temperature, thanks to correlations in the atoms’ thermal vibrations.
The double-slit experiment is perhaps the most iconic demonstration in quantum mechanics: showing how a single particle’s wavefunction interferes with itself as it passes through two slits at once, with a tangible impact on where the particle is ultimately observed.
In practical research, this effect is now routinely harnessed in electron, neutron and atom interferometers. By observing the interference fringes that it creates, researchers can extract valuable information about both the quantum matter wave and whatever object is doing the diffracting.
Recently, physicists have attempted to push double-slit interferometry all the way down to the atomic scale, replacing conventional slits with crystal lattices. When a moving electron encounters an atom in a crystal, it scatters across a range of angles – producing an outgoing spherical wave that’s mathematically identical to the diffraction pattern from a classical slit.
Since atoms in a crystal sit at such well-defined separations, any neighbouring pair of them could, in principle, act as a double-slit interferometer. In reality, however, the many repeated unit cells in a crystal mean the resulting interference fringes are washed out, making it difficult to isolate the signal from a single atomic pair.
To address this challenge, Shibata’s team used the ultraprecise probe of a 4D scanning transmission electron microscope: an instrument designed to capture electron diffraction patterns with a pixellated detector. Fired into a pure silicon crystal at just the right orientation, the microscope’s precision let the researchers target two adjacent columns of aligned silicon atoms, separated by just 136 pm. As a result, the electron wave overlapped both columns, and no others.
“Our key idea was to use these two atomic columns as the two slits,” describes team member Takehito Seki. “By channelling the probe along both columns at once, we turned them into two coherent scattering sources – effectively making the crystal itself into an atomic-scale double-slit interferometer.”
At this scale, Shibata and Seki’s team created electron interference fringes some seven orders of magnitude smaller than in Thomas Young’s original double-slit experiment with light, first carried out in 1801.
Just as in a larger-scale experiment, these patterns encoded information about the crystal and the electron beam. But at this scale, the researchers could also detect the signature of individual phonon modes in the columns. Rather than destroying coherence as uncorrelated atomic motion would in a classical picture, these correlated vibrations between the two atomic columns kept them moving in sync, preserving the interference.
“In a which-path picture, relative motion of the two atomic columns makes the two electron pathways more distinguishable and reduces the interference, whereas correlated in-phase motion preserves coherence,” Shibata explains. As a result, distinct interference fringes remained visible across temperatures ranging from 300 to 900 K.
The team’s results already point to how an atomic-scale double-slit experiment could offer new ways to measure phonon correlations directly between single pairs of atoms. But as Shibata explains, the possible applications could stretch much further.
“It also opens a route to probing local lattice dynamics at the scale of individual atomic bonds, including at interfaces and defects, where local lattice dynamics can strongly influence thermal transport,” he says. “In the long run, such atomic-scale insight could help inform better thermal management strategies for future chip technologies.”
The study is published in Nature.
The post Researchers shrink double-slit experiment to atomic scale appeared first on Physics World.
An international research team has announced the first-ever successful observation of gravity’s effect on a falling quantum object, demonstrating a connection between gravity and the quantum world first predicted by Albert Einstein.
The research team said the result does not unify quantum mechanics and gravity, nor does it prove that gravity is a quantum phenomenon.
However, the researchers said their study offers experimental support that Einstein’s equivalence principle, a fundamental concept at the heart of the famed scientist’s theory of gravity, remains consistent with matter in the quantum world.
When physicists want to describe nature, they rely on two distinct sets of rules. Einstein’s theory of gravity explains the mechanics of large-scale phenomena like stars and galaxies. Conversely, quantum mechanics helps scientists characterize the behavior of extremely small objects. Still, finding a scientific bridge uniting the two sets of rules has proven elusive.
“These two pillars of modern physics have so far eluded all attempts at a unified theoretical framework,” explained Professor Ron Folman from Ben-Gurion University of the Negev and the lead author of the new study.
Einstein did, however, identify one mechanism where the two worlds meet. Called the equivalence principle, the concept says that gravity should effectively disappear for an observer in free fall. For example, NASA simulates zero-gravity environments by flying astronauts in a plane, making successive rising and falling parabolic arcs. During the short periods of falling, the astronauts briefly experience ‘weightlessness” due to the equivalence principle.
Curious if the same phenomena would apply to the quantum world, the research team, which included Nobel Prize-winning physicist Professor Sir Roger Penrose, designed experiments. According to a statement announcing the research, the team hoped their setup would allow them to measure a “distinctive change in the quantum properties of atoms as they fell under gravity.”
To search for gravity’s effect in the quantum world, the researchers created clouds of rubidium atoms cooled to just above absolute zero. The researchers used a specially designed atom chip that allowed the team to manipulate the ultracold rubidium atoms.
Next, PhD student Or Dobkowski and colleagues used microwave pulses to place the atoms in quantum superposition. This unique quantum state lets each atom travel down two paths at once.
Using tiny electrical wires embedded on the specialized chip, the researchers generated tunable magnetic fields. The researchers said the field’s precision tunability allowed them to apply an upward force to one half of the atomic wave. For this experiment, the team created a magnetic field at the precise strength needed to counter gravity, effectively holding that half of the wave stationary relative to Earth.
After suspending the first half of the wave, the team used a magnetic pulse to lift the second part. Then, the team switched to a state almost unaffected by the magnetic field, allowing it to fall under gravity.
Once the second half completed its fall, the team used magnetic fields to reunite the two halves. When reunited, the two halves of the atomic wave interfered with each other. The researchers said that interaction allowed them to “measure the tiny difference in quantum phase accumulated while one was falling and the other was held still.”
“The phase measured in the new experiment is the same as the one predicted when Einstein’s principle is applied to such a quantum wave,” they explained.
When discussing the results, the research team said their result should not be construed as proof that gravity is quantum. The results also do not unite gravity and quantum mechanics. However, Folman noted, “this complex experiment gives more hints as to how such a unification may be achieved.”
Study co-author Professor Vlatko Vedral from the University of Oxford’s Department of Physics agreed, while also noting that their experimental results successfully demonstrated Einstein’s equivalence principle’s consistency with quantum mechanics, which alone provides an experimental connection between quantum physics and Einstein’s theory of gravity.
“This experiment pushes quantum mechanics into one of its most intriguing frontiers, gravity, and shows that, once again, its predictions hold,” Professor Vedral explained.
The professor also noted the experiment’s combination of hard experimental data with “a far-reaching theoretical interpretation, about one of the most fundamental questions in physics: How can gravity (described by Einstein’s theory of relativity) and quantum theory be unified into one understanding of the universe?”
The study “Observation of the quantum phase of free fall and the consistency with the equivalence principle” was published in Science Advances.
Christopher Plain has spent the last six years as Associate News Editor and Head Science Reporter at The Debrief. Follow and connect with him on X, learn about his novels at plainfiction.com, or email him at christopher@thedebrief.org.
Physicists at Göttingen University imaged three-dimensional wave functions using a tabletop soft X-ray laser. An electron inside a molecule does not occupy one fixed point. Quantum mechanics instead describes it through a “wavefunction,” a mathematical map that gives the probabilities of properties such as position and momentum. Within molecules, these electron wavefunctions are known as [...]
D-Wave is a bit of an oddity in the quantum computing space, having been founded back in the last century. And its initial offering wasn't a quantum computer like those being developed by IBM or Google. Instead, the company built what's now called a quantum annealer, a machine that isn't general-purpose but can solve a large class of optimization problems. While the hardware shares some similarities with the qubits used in gate-based quantum computers, it operates in a fundamentally different way.
But a few years back, D-Wave started working on gate-based hardware, apparently choosing a somewhat unusual qubit technology called fluxonium. And this year, the company acquired a startup called Quantum Circuits that spun out of Yale University and has been developing what's called a dual-rail qubit (the same technology used by Amazon), which promises to make most errors very easy to detect, simplifying error correction.
On Wednesday, the company is publishing a paper in Nature that describes a key step in validating this dual-rail technology, showing that two of the qubits can be entangled without altering their best feature: Most are a single type that is easy to detect.
© D-Wave
Researchers demonstrated that braiding and fusing particles known as non-Abelian anyons can perform every operation required by a quantum computer. A quantum computer becomes broadly useful only when it can perform any computation rather than a limited set of specialized tasks. Physicists have now demonstrated that unusual quantum objects called non-Abelian anyons can provide that [...]
There are many algorithms for which it has been mathematically proven that a quantum computer can generate results that would take a classical computer an unreasonable amount of time to generate. Unfortunately, today's quantum computers either can't run those algorithms or can only run simplified versions that classical computers can also handle. This has left the field facing a challenging question: Can we demonstrate the promise of quantum computers on today's noisy, limited hardware?
That's a more difficult question than it may first appear. If you generate a result that's out of reach of today's regular computers, it may not be possible to verify that you got the right result. And given that today's quantum computers are somewhat error-prone, getting the wrong result is a distinct possibility. Further, in the absence of a mathematical proof of the capabilities of quantum hardware, it's possible that a better classical algorithm could outperform the quantum hardware.
These issues inspired IBM to launch a quantum advantage tracker. On Thursday, the company announced three new entries that it says clearly show a quantum advantage, each using a different approach to overcoming errors and validating quantum results. "Trusted computing when you can do classical simulations is irrelevant," IBM's Jay Gambetta told Ars. "Trusted computing when you can't do classical simulations is a big deal."
© IBM
If you follow quantum computing news for long enough, it can start to seem like any quantum system that can alternate between two well-separated energy states can be used as a qubit. Atoms, ions, photons, electrons, and manufactured devices all have their backers. One of the key things that attracts backers is the tech's ability to scale. We'll need a lot of high-quality qubits to start doing any complex computations, and the ability of any technology to get us there is the subject of debate.
So even as some technologies can now support thousands of qubits, some competitors are still working on a small handful of qubits—the companies behind them are convinced that they have the potential to scale more effectively.
One of those technologies involves quantum dots that hold a single electron. Their advantage is that we can manufacture them using the same tech we use to build traditional processors, an approach that has proven to be scalable. This week saw two new papers describing different ways of using quantum dots, one of which was appealing enough that IBM bought the company that developed it. Separately, another company has released a processor showing 100 individual electrons being held in diamond defects, technology that wasn't obvious could scale.
© HRL Laboratories
“Strange” metals owe at least some of their peculiar properties to the quantum entanglement of their electrons. This finding comes from physicists at the Vienna University of Technology in Austria, who used a concept from quantum information science to guide their experiments. A similar approach, they say, could also yield a better understanding of certain high-temperature superconductors and other correlated quantum materials.
Electrons travel relatively unimpeded through most metals, but in the 1980s, scientists identified puzzling exceptions in certain cuprate high-temperature superconductors. Further examples of this “strange” metal behaviour subsequently turned up in other classes of materials, including heavy-fermion materials, pnictides and organic compounds. This resistive behaviour cannot be explained by theories that treat electrons as independent, non-interacting particles, nor by theories that include interactions while treating electrons as quasiparticles. And although multiple recent theoretical works suggested a possible role for entanglement, experimental evidence was lacking.
To investigate further, a team led by solid-state physicist Silke Bühler-Paschen used facilities at the Institut Laue-Langevin (ILL) in Grenoble, France to perform inelastic neutron scattering measurements on a heavy-fermion metal with the chemical formula Ce3Pd20Si6. According to team member Federico Mazza, who conducted this part of the study, the scattered neutrons would normally each transfer their energy to individual particles.
However, when the researchers analysed their data using a concept called quantum Fisher information – roughly, a measure of how sensitively a quantum state depends on a given parameter – they found that they could not explain what they were seeing in terms of particles behaving independently. Instead, the data indicated that groups of at least nine quantum-entangled entities were acting collectively. Mazza says this provides direct evidence of highly multipartite quantum entanglement in the material – and, in turn, a new way of understanding why strange metals are so strange.
The strange metal state is of great interest because physicists consider it the “parent” state of high-temperature superconductivity, though it also occurs across other materials platforms. “We had suspected that some of the intriguing properties of this state might be related to entanglement but were not able to pin it down until now,” Bühler-Paschen explains. “The results are a great success for us. They confirm that our unusual approach of using methods from quantum information science for solid-state physics studies of novel materials can reveal fundamentally new insight.”
The experiments, which are detailed in Nature Physics, were not without challenges. For starters, the researchers needed to identify an ideal material to study the effect they were looking for and then grow it as a large, high-quality single crystal. Once they secured inelastic neutron scattering beamtime at the ILL’s powerful high-resolution triple-axis spectrometer – a difficult feat in itself – Bühler-Paschen notes that they had to obtain and analyse the data at the highest standards and support it with simulations. “Last but not least, [we had to] communicate with our peers that multipartite entanglement contains valuable information beyond correlation functions and scaling analyses,” Bühler-Paschen recalls.
Rather than being a “detail” of one particular material, Bühler-Paschen says the team’s result suggests that enhanced multipartite entanglement might be an integral part of the strange metal state. Verifying this, however, will require studies on other strange metals across materials classes, she says.
“Looking further ahead, we can envision that this aspect of strange metals finds application in quantum devices,” she tells Physics World. “It could also help us better understand high-temperature superconductors and other materials in which electrons are so strongly correlated that they lose their (quasi)particle nature.”
The post Quantum entanglement explains why strange metals are so strange appeared first on Physics World.
In the event of a nuclear holocaust, the only life remaining on Earth may be cockroaches, Keith Richards and physicists arguing about philosophical interpretations of quantum mechanics. One especially rich source of arguments is a deterministic alternative to the standard Copenhagen interpretation that dates back to the American (and later British, after Princeton University fired him for alleged communist sympathies) theoretical physicist David Bohm. According to Bohm, the position of a quantum particle is well defined everywhere and guided by a “pilot wave” which, alas, cannot be measured directly.
Despite these philosophical differences, Bohmian mechanics makes the same predictions as the Copenhagen interpretation. Except maybe it doesn’t. Occasionally, a bright theorist hypothesizes that, under very specific circumstances, one could distinguish them. Very occasionally, a bright experimentalist conducts an experiment that claims to actually do so.
This is what happened last year when Jan Klärs and colleagues at the University of Twente in the Netherlands sent photons from a laser down one of two coupled waveguides towards a potential step. When the photons reached the step, they could pass through it by quantum tunnelling. They could also pass into the other waveguide. The researchers interpreted the distance the photons travelled through the barrier before tunnelling into the other waveguide as a measurement of their speed.
The key result was that, when the wave functions on both sides of the barrier were the same, the photons still tunnelled at, ahem, light speed – matching the Copenhagen notion that tunnelling occurred equally in both directions. However, the Twente team calculated that Bohmian mechanics predicted that photons inside the step – where the guiding equation didn’t have a real-valued frequency – would be at rest and get stuck. Interferometric measurements showed that wasn’t happening.
Game, set and match to Copenhagen? Er, no. Proponents of Bohmian mechanics immediately disputed the team’s definition of velocity. “It’s just an operational definition,” says Aurélien Drezet of the CNRS University of Grenoble-Alps in France. “It has the units of velocity…but that doesn’t mean that Bohmian mechanics can interpret the result.”
In a “Matters Arising” article in Nature, Drezet and two colleagues at the Technion–Israel Institute of Technology in Haifa now add an experimental qualm. The fact that the Twente team was able to produce an image shows that radiation must be leaking out of the cavity, Drezet claims: “If you go to higher approximations and include cavity losses, you explain the experiment completely using Bohmian mechanics,” he says.
Klärs, who is preparing a formal response, is unconvinced. On the first point, he sees no reason “why the same speed measurement, which correctly captures the Bohmian velocity in the propagating regime, should cease to be a speed measurement in the evanescent regime”. On the second, he says his group did not ignore radiative leakage from the cavity: they measured it and found it “does not have a significant impact on the scattering physics we investigate or on the interpretation of our results”.
This could go on for some time – regardless of how many seconds it is to midnight.
The post Experiment that may or may not disprove Bohmian mechanics continues to spark debate appeared first on Physics World.
ISTA physicists have confirmed a theory proposed 20 years ago that could advance quantum technology. A future quantum computer may need separate modules to behave as one machine, even when they are physically apart. That requires distributed entanglement, a quantum connection between distant components that has typically depended on active control and repeated measurements. Physicists [...]
A photon is a single particle of light, and under normal circumstances, it can't be divided. But a photon is also not a particle, in the sense that it does not have a specific location. Instead, it is an extended object.
So if a photon is only partway through the process of reflecting from a perfect mirror and you yank the mirror away, what happens? The answer, from a trio of Norwegian physicists, turns out (arxiv.org link) to be more complex than I expected.
Let’s first talk briefly about dividing and combining photons. If this were a common experience in our lives, then shining a single color of light through a piece of glass or reflecting it from a surface might cause photons to divide or combine. This would lead to an amazing array of colors: Our universe would be the most fantastic and legal LSD trip you could imagine. But this doesn't generally happen, hence LSD.
© DrPixel
There are some obvious big picture issues that stand between us and useful quantum computing. Issues like whether we can make enough high-quality hardware qubits to connect into the error-corrected logical qubits we need, and how we generate the states needed to perform universal computation on those logical qubits. But there are also many less prominent challenges that will need to be solved before we can perform calculations.
One of those challenges, which only affects some types of hardware, is calibration. For devices we manufacture, like superconducting qubits, there are always subtle variations among individual qubits. (This is not true when we use something like an atom to hold the qubit, but the lasers that control them can drift.) As a result, this hardware is put through a process called calibration, where we test different frequencies and amplitudes of the microwave pulses that control them to find the combination that produces the lowest error rates, and then save those settings for use in calculations.
However, you can't perform the typical calibration process while you're doing calculations, which means drift becomes an issue for long and complicated algorithms. Google, though, has figured out that it's possible to do calibration using the same data that's used for error correction.
Researchers investigate how quantum time flow can be stretched, blurred, or even reversed. What if the direction of time isn’t as fixed as it seems? While our everyday experience tells us that time moves relentlessly forward, the microscopic laws governing quantum systems are far less restrictive. In fact, many of the equations of quantum mechanics [...]
A quantum “mini universe” made from ultracold atoms revealed that time can emerge from entropy changes inside a system, offering experimental insight into one of physics’ deepest mysteries. A scientist at the University of Birmingham has created a “mini universe” that could help answer one of science’s most fundamental questions: What is time? In a [...]
Quantum computing news usually picks up near the end of the year, as companies try to provide evidence that they are hitting benchmarks on time. However, there have been interesting announcements as the summer starts this year, from incremental progress to attention-grabbing promises. As we did earlier this month, Ars has a rundown of some of the most significant announcements.
These include a promise of useful, error-corrected quantum computing as soon as 2028, details on an updated trapped ion processor, and a case in which claims of quantum supremacy have been cut back a bit thanks to advances in more traditional algorithms.
Many people in the field expect that useful quantum computers are still about five to 10 years away. While there may be a few useful algorithms that can be run on existing error-prone hardware, almost all of the interesting problems that quantum computing can be applied to will require some form of error correction enabled by linking a small collection of hardware qubits together into what's called a logical qubit. Logical qubits include the redundant storage of information along with neighboring qubits that can be measured to determine when errors occur and how to fix them.
© QuEra
Quantum computing news usually picks up near the end of the year, as companies try to provide evidence that they are hitting benchmarks on time. However, there have been interesting announcements as the summer starts this year, from incremental progress to attention-grabbing promises. As we did earlier this month, Ars has a rundown of some of the most significant announcements.
These include a promise of useful, error-corrected quantum computing as soon as 2028, details on an updated trapped ion processor, and a case in which claims of quantum supremacy have been cut back a bit thanks to advances in more traditional algorithms.
Many people in the field expect that useful quantum computers are still about five to 10 years away. While there may be a few useful algorithms that can be run on existing error-prone hardware, almost all of the interesting problems that quantum computing can be applied to will require some form of error correction enabled by linking a small collection of hardware qubits together into what's called a logical qubit. Logical qubits include the redundant storage of information along with neighboring qubits that can be measured to determine when errors occur and how to fix them.
© QuEra
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