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A quantum walk down memory lane

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In elementary and middle school, I felt an affinity for the class three years above mine. Five of my peers had siblings in that year. I carpooled with a student in that class, which partnered with mine in holiday activities and Grandparents’ Day revues. Two students in that class stood out. They won academic-achievement awards, represented our school in science fairs and speech competitions, and enrolled in rigorous high-school programs.

Those students came to mind as I grew to know David Limmer. David is an assistant professor of chemistry at the University of California, Berkeley. He studies statistical mechanics far from equilibrium, using information theory. Though a theorist ardent about mathematics, he partners with experimentalists. He can pass as a physicist and keeps an eye on topics as far afield as black holes. According to his faculty page, I discovered while writing this article, he’s even three years older than I.

I met David in the final year of my PhD. I was looking ahead to postdocking, as his postdoc fellowship was fading into memory. The more we talked, the more I thought, I’d like to be like him.

I had the good fortune to collaborate with David on a paper published by Physical Review A this spring (as an Editors’ Suggestion!). The project has featured in Quantum Frontiers as the inspiration for a rewriting of “I’m a little teapot.”

We studied a molecule prevalent across nature and technologies. Such molecules feature in your eyes, solar-fuel-storage devices, and more. The molecule has two clumps of atoms. One clump may rotate relative to the other if the molecule absorbs light. The rotation switches the molecule from a “closed” configuration to an “open” configuration.

These molecular switches are small, quantum, and far from equilibrium; so modeling them is difficult. Making assumptions offers traction, but many of the assumptions disagreed with David. He wanted general, thermodynamic-style bounds on the probability that one of these molecular switches would switch. Then, he ran into me.

I traffic in mathematical models, developed in quantum information theory, called resource theories. We use resource theories to calculate which states can transform into which in thermodynamics, as a dime can transform into ten pennies at a bank. David and I modeled his molecule in a resource theory, then bounded the molecule’s probability of switching from “closed” to “open.” I accidentally composed a theme song for the molecule; you can sing along with this post.

That post didn’t mention what David and I discovered about quantum clocks. But what better backdrop for a mental trip to elementary school or to three years into the future?

I’ve blogged about autonomous quantum clocks (and ancient Assyria) before. Autonomous quantum clocks differ from quantum clocks of another type—the most precise clocks in the world. Scientists operate the latter clocks with lasers; autonomous quantum clocks need no operators. Autonomy benefits you if you want for a machine, such as a computer or a drone, to operate independently. An autonomous clock in the machine ensures that, say, the computer applies the right logical gate at the right time.

What’s an autonomous quantum clock? First, what’s a clock? A clock has a degree of freedom (e.g., a pair of hands) that represents the time and that moves steadily. When the clock’s hands point to 12 PM, you’re preparing lunch; when the clock’s hands point to 6 PM, you’re reading Quantum Frontiers. An autonomous quantum clock has a degree of freedom that represents the time fairly accurately and moves fairly steadily. (The quantum uncertainty principle prevents a perfect quantum clock from existing.)

Suppose that the autonomous quantum clock constitutes one part of a machine, such as a quantum computer, that the clock guides. When the clock is in one quantum state, the rest of the machine undergoes one operation, such as one quantum logical gate. (Experts: The rest of the machine evolves under one Hamiltonian.) When the clock is in another state, the rest of the machine undergoes another operation (evolves under another Hamiltonian).

Physicists have been modeling quantum clocks using the resource theory with which David and I modeled our molecule. The math with which we represented our molecule, I realized, coincided with the math that represents an autonomous quantum clock.

Think of the molecular switch as a machine that operates (mostly) independently and that contains an autonomous quantum clock. The rotating clump of atoms constitutes the clock hand. As a hand rotates down a clock face, so do the nuclei rotate downward. The hand effectively points to 12 PM when the switch occupies its “closed” position. The hand effectively points to 6 PM when the switch occupies its “open” position.

The nuclei account for most of the molecule’s weight; electrons account for little. They flit about the landscape shaped by the atomic clumps’ positions. The landscape governs the electrons’ behavior. So the electrons form the rest of the quantum machine controlled by the nuclear clock.

Experimentalists can create and manipulate these molecular switches easily. For instance, experimentalists can set the atomic clump moving—can “wind up” the clock—with ultrafast lasers. In contrast, the only other autonomous quantum clocks that I’d read about live in theory land. Can these molecules bridge theory to experiment? Reach out if you have ideas!

And check out David’s theory lab on Berkeley’s website and on Twitter. We all need older siblings to look up to.

The 10 greatest predictions in physics

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Taken from the January 2021 issue of Physics World. Members of the Institute of Physics can enjoy the full issue via the Physics World app.

Over the centuries there have been many theoretical physics predictions that have rocked our understanding of how the world works. David Appell highlights what he thinks are the top 10 of all time

Intertwined entities: sci-fi anthology explores the impact of AI on human relationships

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Taken from the January 2021 issue of Physics World. Members of the Institute of Physics can enjoy the full issue via the Physics World app.

Perovskite sensor sees more like the human eye

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A new type of sensor that closely mimics how the human eye responds to changing visual stimuli could become the foundation for next-generation computer processors used in image recognition, autonomous vehicles and robotics. The so-called “retinomorphic” device is made from a class of semiconducting materials known as perovskites, and unlike a conventional camera, it is sensitive to changes in levels of illumination rather than the intensity of the input light.

The eyes of humans and other mammals are incredibly complex organs. Our retinas, for example, contain roughly 10photoreceptors, yet our optical nerves only transmit about 106 signals to the primary visual cortex – meaning that the retina does a lot of pre-processing before it transmits information.

Part of this pre-processing relates to how the eye treats moving objects. When our field of view is static, our retinal cells are relatively quiet. Expose them to spatially or temporally varying signals, however, and their activity shoots up. This selective response – transmitting signals only in response to change – enables the retina to substantially compress the information it passes on.

Mimicking mammalian visual processing

In recent years, this mammalian optical sensing process has caught the attention of computer scientists. Traditional computer processors – known as von Neumann machines after the mathematical physicist John von Neumann, who pioneered their development in the mid-20th century – deal with input instructions in a sequential fashion. In contrast, the mammalian brain processes inputs via massively parallel networks, and studies have shown that computers that follow suit – neuromorphic computers – should outperform von Neumann machines for certain machine-learning tasks in terms of both speed and power consumption.

Retinomorphic sensors – optical devices that attempt to mimic mammalian visual processing – are a potential building block for such computers, and thin-film semiconductors such as metal halide perovskites are considered good candidates for making them. Materials of this type are attractive because they can be tuned to absorb light over a wide range of wavelengths. They have also already proved themselves in artificial synapses that react to light, albeit in structures that are generally designed for transmitting and processing information rather than for optical sensing. However, while researchers have previously used perovskites to make optical sensors that mimic the geometry of the eye, the fundamental operating mode of these sensors still requires sequential processing.

Spiking sensor

A team led by John Labram at Oregon State University in the US has now shown that a simple photosensitive capacitor can reproduce some characteristics of mammalian retinas. The new device is made from a double-layer dielectric: the bottom layer, silicon dioxide, is highly insulating and hardly responds to light, while the top layer is the light-sensitive perovskite methylammonium lead iodide (MAPbI3).

The team found that the capacitance of this MAPbI3-silicon dioxide bilayer changes dramatically when exposed to light. When Labram and his student Cinthya Trujillo Herrera placed it in series with an external resistor and exposed it to a light source, they observed a large voltage spike across the resistor. Unlike in a normal camera or photodiode, however, this voltage spike quickly decayed away even though the intensity of the light remained constant. The result is a sensor that responds, like the retina, to changes in light levels rather than the intensity of the light.

Filtering out unimportant information

After measuring the light response of several such devices, the team developed a numerical model based on Kirchoff’s laws to show how the devices would behave if they were arranged in arrays. This model enabled them to simulate an array of retinomorphic sensors and predict how a retinomorphic video camera would react to different types of input stimuli. One of their tests involved analysing footage of a bird flying into view. When the bird stopped at a (static, and therefore invisible to the sensor) bird feeder, it all but disappeared. Once the bird took off, it reappeared – and, in the process, revealed the presence of the feeder, which became visible to the sensor only when the bird’s take-off set it swaying.

“The new design thus inherently filters out unimportant information, such as static images, providing a voltage solely in response to movement,” Labram explains. “This behaviour reasonably reflects optical sensing in mammals.”

The researchers, who report their work in Appl. Phys. Lett., say they now plan to better understand the fundamental physics of these devices and how their signals would be interpreted by image-recognition algorithms. They also hope to address some of the challenges associated with scaling these devices up to sensor arrays. “Going from a brand-new device paradigm to a working array is almost certainly going to expose challenges we haven’t yet considered,” Labram tells Physics World. “There are also quite a few operation-related questions we will have to answer — in particular as regards performance limits, stability, predictability and device-to-device variability.”

How to cool ion beams using electron pulses

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Physicists in the US and China have studied how a pulsed beam of electrons can be used to cool a beam of high-energy ions – a task that is normally done by a continuous beam of electrons. Researchers led by Max Bruker at the Thomas Jefferson National Accelerator Facility in the US, alongside a team at the Institute of Modern Physics (IMP) of the Chinese Academy of Sciences, modified a continuous-beam electron cooling system to operate in pulsed mode. Their results suggest that it should be possible to cool much higher energy ion beams using pulsed electron beams – which is good news for physicists designing the next generation of ion storage rings.

Storage-ring facilities that accelerate and store beams of protons and ions at low to medium energies use a technique called “electron cooling” to prevent their beams from degrading. This involves merging the ions with a beam of electrons, with both beams moving at roughly the same speed. Over time, the ions exchange momentum with the electrons until equilibrium is reached. This cools the ions down, preventing them from straying away from the centre of the beam.

Normally, this is done using this continuous electron beams with energies as high as 4.3 MeV. However, technological limitations on using static electric fields to accelerate electrons mean that creating continuous electron beams at higher energies is extremely difficult. This poses a challenge to the designers of future storage rings such as the US’s Electron Ion Collider, which will require electron beams as energetic as 50 MeV or more.

RF fields

To reach higher energies, electron beams are accelerated using radio-frequency (RF) fields, which results in a pulsed beam. Recently, the first pulsed electron cooling system has been installed at the Relativistic Heavy Ion Collider at Brookhaven National Laboratory in the US – operating at a modest electron energies of about 2 MeV.

Studies using computer simulations suggest that the cooling effects of pulsed and continuous electron beams are different – and therefore it is important that pulsed cooling be studied experimentally before it is implemented in higher-energy, next-generation facilities.

Physicists at Jefferson Lab and IMP first teamed up in 2012 to study how pulsed electron beams could be used for cooling. Between 2016-2019, they performed four pulsed-beam cooling experiments at the CSRm storage ring at the IMP’s Heavy Ion Research Facility in Lanzhou. Instead of using an RF system to accelerate cooling electrons, they modified an existing continuous-beam system to deliver pulses of electrons. The researchers then measured how the profile of the cooled ion beam evolved over time, both in transverse and longitudinal directions.

Crucially, the teams’ experiments revealed that ions can be lost through transverse heating caused by uneven electron bunch lengths, highlighting the need for electron bunches with highly stable properties. Yet if bunch timings and lengths can be reliably maintained, the dynamics of the ion beams they interact with will not be adversely affected by their non-continuous nature. The results now pave the way for a new generation of ion ring facilities, capable of cooling ion beams at higher energies than were ever previously possible.

The research is described in Physical Review Accelerators and Beams.