Experiments reveal a hydrodynamic analog of an important effect in quantum optics called superradiance.
Category: quantum physics – Page 385
The University of Tennessee’s physicists have led a scientific team that found silicon—a mainstay of the soon-to-be trillion-dollar electronics industry—can host a novel form of superconductivity that could bring rapidly emerging quantum technologies closer to industrial scale production.
The findings are reported in Nature Physics and involve electron theft, time reversal, and a little electronic ambidexterity.
Superconductors conduct electric current without resistance or energy dissipation. Their uses range from powerful electromagnets for particle accelerators and medical MRI devices to ultrasensitive magnetic sensors to quantum computers. Superconductivity is a spectacular display of quantum mechanics in action on a macroscopic scale. It all comes down to the electrons.
A certain amount of noise is inherent in any quantum system. For instance, when researchers want to read information from a quantum computer, which harnesses quantum mechanical phenomena to solve certain problems too complex for classical computers, the same quantum mechanics also imparts a minimum level of unavoidable error that limits the accuracy of the measurements.
Scientists can effectively get around this limitation by using “parametric” amplification to “squeeze” the noise—a quantum phenomenon that decreases the noise affecting one variable while increasing the noise that affects its conjugate partner. While the total amount of noise remains the same, it is effectively redistributed. Researchers can then make more accurate measurements by looking only at the lower-noise variable.
A team of researchers from MIT and elsewhere has now developed a new superconducting parametric amplifier that operates with the gain of previous narrowband squeezers while achieving quantum squeezing over much larger bandwidths. Their work is the first to demonstrate squeezing over a broad frequency bandwidth of up to 1.75 gigahertz while maintaining a high degree of squeezing (selective noise reduction). In comparison, previous microwave parametric amplifiers generally achieved bandwidths of only 100 megahertz or less.
In the world around us, processes appear to follow a certain time-direction: Dandelions eventually turn into blowballs. However, the quantum realm does not play by the same rules. Physicists from the University of Vienna and IQOQI Vienna have now shown that for certain quantum systems, the time-direction of processes can be reversed. This demonstration of a so-called rewinding protocol has been published in Optica.
Everyday life is full of changes that are well understood, yet practically impossible to reverse; for example, the metamorphosis of a dandelion into a blowball. However, one could imagine undoing this transformation, step by step, if one knew precisely how each molecule in the plant moved in time. In the quantum realm the problem gets even trickier: One of the core principles of quantum physics is that simply observing a system causes it to change.
This makes it impossible, even in principle, to track a system’s change in time and reverse the process. However, at the same time, the laws of quantum mechanics also open up new possibilities such as universal rewinding protocols. These allow for reversing changes in a quantum system without knowing what they were.
O.o! If the universe is some sorta hologram then this could be a clue to our actual reality.
Last December, the Nobel Prize in Physics was awarded for experimental evidence of a quantum phenomenon that has been known for more than 80 years: entanglement. As envisioned by Albert Einstein and his collaborators in 1935, quantum objects can be mysteriously correlated even when separated by great distances. But as strange as the phenomenon may seem, why is such an old idea still worthy of the most prestigious award in physics?
Coincidentally, just weeks before the new Nobel laureates were honored in Stockholm, another team of respected scientists from Harvard, MIT, Caltech, Fermilab and Google reported that they ran a process on Google’s quantum computer that could be interpreted as a wormhole. Wormholes are tunnels through the universe that can function as a shortcut through space and time and are loved by science fiction fans, and although the tunnel realized in this latest experiment only exists in a two-dimensional toy universe, it could be a breakthrough for the future represent research at the forefront of physics.
But why does entanglement have to do with space and time? And how can it be important for future breakthroughs in physics? Properly understood, entanglement means that the universe is what philosophers call “monistic,” that is, at the most fundamental level, everything in the universe is part of a single, unified whole. It is a defining property of quantum mechanics that its underlying reality is described in terms of waves, and a monistic universe would require universal functioning. Decades ago, researchers such as Hugh Everett and Dieter Zeh showed how our everyday reality can emerge from such a universal quantum mechanical description. But it is only now that researchers such as Leonard Susskind and Sean Carroll are developing ideas as to how this hidden quantum reality could explain not only matter but also the structure of space and time.
Researchers have learned much about neutrinos over the past few decades, but some mysteries remain unsolved. For example, the standard model predicts that neutrinos are massless, but experiments say otherwise. One possible solution to this mass mystery involves another group of neutrinos that does not interact directly via the weak nuclear force and is therefore extremely difficult to detect. David Moore of Yale University and his colleagues have proposed a way to search for these so-called sterile neutrinos using a radioactive nanoparticle suspended in a laser beam [1].
Moore and his colleagues suggest levitating a 100-nm-diameter silica sphere in an optical trap and cooling it to its motional ground state. If the nanoparticle is filled with nuclei that decay by emitting neutrinos—such as certain argon or phosphorous isotopes—then electrons and neutrinos zipping from decaying nuclei should give it a momentum kick. By measuring the magnitude of this kick, the team hopes to determine the neutrinos’ momenta. Although most of these neutrinos will be the familiar three neutrino flavors, sterile neutrinos—if they exist—should also occasionally be emitted, producing unexpectedly small momentum kicks. Moore says that monitoring a single nanoparticle for one month would equate to a sterile-neutrino sensitivity 10 times better than that of any experiment tried so far.
Moore and his team are currently working on a proof-of-principle experiment using alpha-emitting by-products of radon, which result in a larger momentum kick. Once the techniques are optimized, they expect that switching to beta-decaying isotopes will let them see heavy sterile neutrinos in the 0.1–1 MeV mass range. Introducing more quantum tricks to manipulate the nanoparticle’s quantum state will make future experiments sensitive to even lighter sterile neutrinos.
Researchers from the University of Sussex and Universal Quantum have demonstrated for the first time that quantum bits (qubits) can directly transfer between quantum computer microchips and demonstrated this with record-breaking speed and accuracy. This breakthrough resolves a major challenge in building quantum computers large and powerful enough to tackle complex problems that are of critical importance to society.
Today, quantum computers operate on the 100-qubit scale. Experts anticipate millions of qubits are required to solve important problems that are out of reach of today’s most powerful supercomputers. There is a global quantum race to develop quantum computers that can help in many important societal challenges from drug discovery to making fertilizer production more energy efficient and solving important problems in nearly every industry, ranging from aeronautics to the financial sector.
In the research paper, published today in Nature Communications, the scientists demonstrate how they have used a new and powerful technique, which they dub “UQ Connect,” to use electric field links to enable qubits to move from one quantum computing microchip module to another with unprecedented speed and precision. This allows chips to slot together like a jigsaw puzzle to make a more powerful quantum computer.
A new design for an optical fiber borrows concepts from topology to protect light from imperfections in the fiber’s light-guiding materials or from distortions in its cross section.
Using concepts from the mathematical field of topology, researchers at the University of Bath, UK, have designed an optical fiber that can robustly propagate light, even if there are variations in the properties of its light-guiding materials or in its overall geometry [1]. The team thinks that this newfound topological protection could enable advances in optical communication and photonic quantum computing.
The concept of topology is often explained using a joke about a donut and a coffee cup. A coffee cup made of rubber can be continuously twisted and stretched—no cuts need to be made—so that it takes on the shape of a donut. Even though the object’s outline changes under this transformation, its essence remains the same—it contains one hole. Thus, the quip goes, a topologist cannot tell the difference between the two things.
Trapped Ions Go the Distance
Posted in quantum physics
Researchers have achieved long-distance entanglement between two calcium ions, each of which lies in a different building, showing that trapped ions could be used to create quantum networks.
Among the many candidate platforms for quantum-information applications, trapped-ion qubits are promising because of their long coherence times and their potential for multiqubit operations (see Viewpoint: Trapped Ions Make Impeccable Qubits). Alone, those properties are insufficient for some quantum applications, however: to build quantum communication networks, for example, requires the qubits’ delicate quantum states be shared over long distances. Demonstrations of this ability have been lacking for trapped-ion systems. Now a team led by Benjamin Lanyon at the Institute for Quantum Optics and Quantum Information, Austria, and Tracy Northup at the University of Innsbruck, Austria, have addressed this shortfall by entangling two trapped-ion qubits residing in different buildings [1].
Lanyon, Northup, and colleagues used trapped-ion qubits inside optical cavities. For each qubit, they excited the ion using a dual-wavelength laser, prompting the ion to emit a single photon. The photon’s polarization depended on which of the two laser wavelengths the ion absorbed, entangling the photon with the ion’s final state. To entangle the two ions, the team then transmitted the photon from one ion through 510 m of optical fiber to a beam splitter near the other ion, where the two photons interacted. The researchers claimed successful entanglement when they subsequently detected a pair of photons with specific individual polarizations.
Quantum sensing represents one of the most promising applications of quantum technologies, with the aim of using quantum resources to improve measurement sensitivity. In particular, sensing of optical phases is one of the most investigated problems, considered key to developing mass-produced technological devices.
Optimal usage of quantum sensors requires regular characterization and calibration. In general, such calibration is an extremely complex and resource-intensive task—especially when considering systems for estimating multiple parameters, due to the sheer volume of required measurements as well as the computational time needed to analyze those measurements. Machine-learning algorithms present a powerful tool to address that complexity. The discovery of suitable protocols for algorithm usage is vital for the development of sensors for precise quantum-enhanced measurements.
A particular type of machine-learning algorithm known as “reinforcement learning” (RL) relies on an intelligent agent guided by rewards: Depending on the rewards it receives, it learns to perform the right actions to achieve the desired optimization. The first experimental realizations using RL algorithms for the optimization of quantum problems have been reported only very recently. Most of them still rely on prior knowledge of the model describing the system. What is desirable is instead a completely model-free approach, which is possible when the agent’s reward does not depend on the explicit system model.