A UK collaboration has built a quantum device dubbed a gravimeter that uses cold atoms to make ultra-precise measurements of the strength of gravity. It could be used to survey for oil or minerals, and it may be the start of a new commercial sector for quantum devices.

The device is essentially a scaled-down version of the method used by the LIGO collaboration to detect gravitational waves made by colliding black holes. In this case, the gravimeter senses subtle changes in the strength of the gravitational fields generated by any object, using clouds of cold rubidium atoms as sensors. These clouds of atoms are held aloft in a basketball-sized vacuum chamber and cooled down to 80 microkelvin – barely above absolute zero.

The atoms are put into a superposition, where they’re in two states at once – think Schroedinger’s cat, both alive and dead – until a measurement is made. Then the atom clouds are dropped, and while in freefall, zapped with three laser pulses. Those pulses serve as a kind of ruler made of light, measuring the position at those key points in time before the clouds come back together to make what’s called an interference pattern.

That pattern is much like what you’d see if you dropped two stones in a pond and they created separate ripples that cross and interfere with other. Here, it encodes the position of the atom clouds and their paths.

Spot the difference

If two atom clouds fall at different speeds, it would indicate a change in the density of the ground below. This could be due to the presence of oil or certain minerals, for example.

“Essentially it relies on the fact that any mass will generate a gravitational field, which can be detected with a very precise gravity sensor,” says at the University of Birmingham, who helped develop the device.

Quantum effects disappear when exposed to any outside interference or noise, so any quantum system or device must be carefully shielded and cooled to very low temperatures. This has limited their use in many real-world applications. But times are changing.

“We’re starting to see this technology maturing into the commercial domain,” says , founder and CEO of photonics company M Squared in Glasgow, which developed the gravimeter with a team at the University of Birmingham.

That’s why the oil and gas industry could be particularly interested in the gravimeter. It could be a powerful tool to help map out valuable deposits of oil or minerals, because denser materials will have a stronger gravitational pull than open pockets beneath the earth.

Construction companies could use the gravimeter to locate pipes buried deep underground, preventing the costs of accidentally digging up the wrong bits of road. Existing technology for this kind of geophysical mapping is bulky and difficult to use, and not as sensitive as a quantum gravimeter would be.

Shrinkable tech

“It provides them with a better view into the unknowns of the underworld,” says Bongs. It might one day even be used in seismic mapping, helping predict natural disasters like tsunamis or volcanic eruptions.

Because it uses laser cooling rather than bulky cryogenics, the gravimeter prototype is only about one cubic metre. In principle, it should be possible to shrink many of the components like the lasers and the vacuum chamber, to make it more portable, says Malcolm.

M Squared is also developing a quantum accelerometer that could augment GPS navigation to offset interference from bad weather. Other potential quantum devices might make it possible to “see” invisible gases. “I think we’re just at the early stage of commercial adoption of quantum technologies,” says Malcolm.

at the California Institute of Technology also thinks that the future is quantum.

“Quantum physics underlies so many of the technologies we take for granted today, but it is only recently that we have been able to exploit the quantum properties of many-body systems to arrive at insanely precise, low-cost and compact versions of previous tech, like gravimeters,” he says.

“Imagine what will be possible in the near future, when it gives us reliable and scalable quantum computers, cheap materials that levitate at room temperature, and quantum networks that teleport quantum information with unprecedented security guarantees through quantum teleportation.”

Read more: Quantum simulator with 51 bits is the largest ever

Chilly demon

A combination of stomping and whipping explain why your shoelaces seem to come undone all by themselves.

In 2015, MIT researchers came up with an to describe the forces at work – tension, friction, and stiffness – and how they relate to the number of turns that make up the topology of the knot. But although there have been many studies of the durability of various knot configurations, nobody had really focused on the physics of why a knot comes undone on its own.

at the University of California, Berkeley, decided to study spontaneous unknotting after noticing that his young daughter could never keep her shoelaces tied. He and two graduate students ran real-world experiments to investigate further.

“We looked like crazy academics because we were just walking the halls of Berkeley, watching our shoelaces come untied,” says team member Christine Gregg, an avid runner. She ran on a treadmill so her colleagues could film her shoes in slow motion to capture the details of the unravelling.

They found that the culprit is a combination of the inertial forces generated while running. A knot is held together by the friction at its centre. That’s why stronger knots have more turns; each turn contributes to friction.

But the constant downward stomp of the foot while running exerts an acceleration at the base of the knot, while the laces whip back and forth with each stride, tugging on the ends like an invisible hand. Eventually the knot hits a tipping point where the acceleration trumps the internal friction, and it comes undone all at once.

“Once you have a little bit of slip, all the forces are aligned, such that [the slip] gets bigger and bigger,” said Gregg. At that point, it only takes one or two more strides for the entire knot to come undone.

The team also found that acceleration or whipping alone isn’t sufficient. The team sat on tables and swung their legs for half an hour, with little effect. They then stomped on the ground for the same period – also to no avail.

Next, they built a pendulum machine, added weights to the ends of laces, and swung the knots back and forth. As expected, knots failed more often with heavier weights, because the inertial forces generated were greater.

There’s a possibility such work could shed light on the mechanics of other kinds of knotty structures, such as suture knots used in surgery, or the folding of DNA and proteins –and especially how they fail. “We’re not just trying to figure out why shoelaces come untied,” said Gregg. “We think this could be applicable to anything that uses entanglement. A knot is really just an entangled linear structure.”

“Understanding the mechanics of a fairly simply knot, such as the shoelace knot, required high-speed videography – this just shows the challenges towards a rigorous understanding of knots,” says at Carnegie Mellon University in Pennsylvania.

Protein mimicry

We may now have a universal law to describe how pedestrians behave in the wild.

Understanding crowd dynamics could help ease rush hour congestion and prevent tragedies like the stampede at a German music festival in 2010 that killed 21 people.

Individuals within large groups are difficult to track, so crowds are frequently modelled as collections of particles in a fluid, with each pedestrian representing a single particle. But people are more unpredictable than that.

at Eindhoven University of Technology in the Netherlands and his colleagues wanted to build a model capable of taking into account the tiny random variations in how pedestrians move, such as suddenly perfoming a U-turn.

“Most models ignore the possibility of people going back, but in a train station it would happen every few minutes,” said Toschi.

Toschi’s team set up cameras to record the movement of individuals along a single corridor at Eindhoven University connecting the cafeteria to the dining room. Foot traffic wasn’t particularly dense, but it was consistent – allowing individuals to be tracked accurately.

The cameras used a Microsoft Kinect 3D motion sensor – designed for Xbox gaming – with a built-in infrared illuminator to correct for variable lighting conditions, and designed software to track the heads of the people as they walked.

The team then used footage of more than 72,000 pedestrian paths captured over the course of a year to model the average path people took – taking into account random fluctuations.

The model can, for instance, predict how often someone is likely to make a sudden U-turn, which could clog up a more heavily used hallway or lead to traffic jams and dangerous situations in more crowded conditions, like a train station during rush hour.

“That’s the beauty of this sort of experiment – it’s recording from real life,” says Toschi.

He says the model could be expanded to apply to more complicated crowd dynamics. The team has already collected six months’ worth of data from a similar experiment in a train station. They are also collaborating with museums to optimise the flow of people visiting exhibits, and exploring ways to gently steer them along preferred routes, if, say, a particular exhibit is becoming too crowded.

“In general, I like very much the idea of trying to study the level of randomness in human walking patterns from data like this,” says of MIT, who co-authored a 2014 study on how pedestrians avoid collisions in crowded areas. But this model might make some overly generous assumptions, he says.

Journal reference: , DOI: 10.1103/PhysRevE.95.032316

Zigzagging structure

The internet is amazingly robust, but like any complex network is still prone to the occasional failure. A new analysis using network theory explains why the dark net – the hidden underbelly of the regular internet, invisible to search engines – is less vulnerable to attacks. The lessons learned could help inform the design of more robust communications networks in the future.

The regular internet’s design is deliberately decentralised, which makes it very stable under normal circumstances. Think of each site or server as a node, connected to numerous nodes around it, which in turn connect to even more nodes, and so on. Take out a node or two here or there and the network continues to function just fine. But this structure also makes it more vulnerable to a coordinated attack: take out many nodes at once, as happens during a distributed denial of service (DDoS) attack, and the result can be catastrophic failure that cascades through the entire network.

The dark net is much less vulnerable to such directed attacks, thanks to its unique structure. and at Rovira i Virgili University in Tarragona, Spain, used data from the Internet Research Lab at the University of California, Los Angeles, to build their own model of the dark net. They ran simulations to see how it would react to three failure scenarios: random node failures, targeted attacks on specific nodes, and cascading failures throughout the network.

They found that an attack on the dark net would need to hit four times as many nodes to cause a cascading failure as on the regular internet. This stems from its use of “onion routing”, a technique for relaying information that hides data in many layers of encryption. Rather than connecting a user’s computer directly to a host server, onion routing bounces the information through various intermediary nodes before delivering it to the desired location. This stops an attack from spreading so widely.

Powerful connections

Another reason for the dark net’s resilience is its lack of something called the “rich-club effect”. In the regular internet, powerful nodes connect more readily with other powerful nodes, creating what at Carnegie Mellon University in Pittsburgh, Pennsylvania, terms a “smoky back room” of “network elites”. An attack on one such node can trigger the failure of others, which can in turn lead to cascading failure across the network. The dark net doesn’t have this high level of connectivity between powerful nodes.

“This is [another] one of the things that make it more robust to attack,” says DeDeo. “The network elites are more spread out. In fact, the elites appear to be avoiding each other.”

This model of the dark net somewhat resembles a so-called “small-world network”, in which several heavily connected nodes link clusters of smaller local nodes – similar to how major air traffic hubs connect smaller local airports. Both systems exhibit similar resilience to catastrophic failure, although in-depth comparisons have yet to be completed.

Reconfiguring the entire internet to make it as robust as the dark net would be prohibitively expensive, but De Domenico thinks the pair’s work could still offer practical insights. “It is possible to rethink next-generation upgrades and the design of more localised communication networks, like the intranets of large companies,” he says.

Journal reference: Physical Review E, DOI:

Read more: Invisible: A visitors’ guide to the dark web

Material improvement

Dream machine

It’s no longer just a wild theory. Two independent teams of physicists have followed a recipe to build the world’s first versions of an enigmatic form of matter – time crystals.

MIT physicist and Nobel laureate Frank Wilczek first speculated about the existence of time crystals in 2012, while teaching a class on ordinary crystals, such as salt, or snowflakes. In a typical crystal, the atoms or molecules are tightly arranged in regularly repeating patterns in three-dimensional space, resembling a lattice.

Wilczek thought it might be possible to create a similar crystal-like structure in time, which is treated as a fourth dimension under relativity. Instead of regularly repeating rows of atoms, a time crystal would exhibit regularly repeating motion.

Many physicists were sceptical, arguing that a time crystal whose atoms could loop forever, with no need for extra energy, would be tantamount to a perpetual motion machine – forbidden by the laws of physics.

Wilczek countered that a time crystal was more akin to a superconductor, in which electrons flow with no resistance, and in theory could do so forever without the need to add energy to the system. In a time crystal, electrons would travel in a loop rather than a line and occasionally bunch up rather than flow smoothly, repeating in time the way atoms in ordinary crystals repeat in space.

Crystal meth-odology

Now, in a paper published this week, Norman Yao at the University of California, Berkeley, and his colleagues have revealed a blueprint for making a time crystal. The recipe has already been followed by two teams.

For Yao’s time crystal, an external force – like the pulse of a laser – flips the magnetic spin of one ion in a crystal, which then flips the spin of the next, and so forth, setting the system into a repeating pattern of periodic motion.

There are two critical factors. First, after the initial driver, it must be a closed system, unable to interact with and lose energy to the environment. Second, interactions between quantum particles are the driving force behind the time crystal’s stability. “It’s an emergent phenomenon,” says Yao. “It requires many particles and many spins to talk to each other and collectively synchronise.”

Using Yao’s recipe as guidance, two groups have now created time crystals in the lab. Last September, a group headed by Chris Monroe of the University of Maryland in College Park built a time crystal out of a string of trapped ytterbium ions.

One month later, a team led by Harvard University’s Mikhail Lukin built a time crystal by exploiting defects formed in diamond. Both teams have submitted papers for publication.

History repeating

Both approaches yielded the telltale signature of a time crystal:  the repeating pattern should be twice the period of the laser pulse used as the driver. But how could you tell if this was just because you were pushing it periodically with the laser pulse? The evidence is that the period the crystal settles into is different from that of the driving pulse that pushes it.

That means time crystals are more than just a curious oddity: they represent the simplest form of a new state of non-equilibrium matter that physicists have only begun to explore.

Spyridon Michalakis, a physicist at the California Institute of Technology, says Yao’s work “bridges the gap between theory and experiment by making concrete suggestions for experimental platforms”. Those suggestions have now been successfully implemented, and papers accepted for publication next month,

Time crystals could have enormous implications for building stable qubits for quantum computing. These devices rely on maintaining a state of entanglement among qubits to store information, but the slightest outside interference will destroy that entanglement, resulting in errors in the calculations.

One approach to combating this is to carefully isolate the qubits. But Microsoft’s Station Q group, among others, has been exploring the possibility of making building blocks that are inherently robust – braiding qubits into knots, for example. There are topological states analogous to time crystals that may one day prove useful for processing quantum information.