A speed limit also applies in the quantum world

Science Daily  February 19, 2021 In two-level systems, the quantum brachistochrone solutions are long known but they are not applicable to larger systems, especially when the target state cannot be reached through a local transformation. An international team of researchers (Germany, USA – MIT, Italy) has demonstrated fast coherent transport of an atomic wave packet over 15 times its size, a case of quantum processes going beyond the two-level system. The measurements of the transport fidelity revealed the existence of a minimum duration—a quantum speed limit—for the coherent splitting and recombination of matter waves. They obtained physical insight into this […]

Quantum Theory May Twist Cause And Effect Into Loops, With Effect Causing The Cause

Science Alert  February 14, 2021 Quantum correlations violating Bell inequalities defy satisfactory causal explanations within the framework of classical causal models. The first challenge has been addressed through the recent development of intrinsically quantum causal models, allowing causal explanations of quantum processes – provided they admit a definite causal order, i.e., have an acyclic causal structure. An international team of researchers addresses causally nonseparable processes and offers a causal perspective on them through extending quantum causal models to cyclic causal structures. Among other applications of the approach, it is shown that all unitarily extendible bipartite processes are causally separable and […]

Adding or subtracting single quanta of sound

Science Daily  January 25, 2021 An international team of researchers (UK, Denmark, Australia) injected laser light into a crystalline microresonator that supports both the light and the high-frequency sound waves. The two types of waves coupled to one another via an electromagnetic interaction creates light at a new frequency. To subtract a single phonon, the team detected a single photon that has been up shifted in frequency. Detecting a single photon indicates that a subtracted single phonon. When the experiment is performed at a finite temperature, the sound field has random fluctuations from thermal noise. Counterintuitively, when you subtract a […]

Mapping quantum structures with light to unlock their capabilities

Nanowerk  December 4, 2020 All-optical band-structure reconstruction could directly connect electronic structure with the coveted quantum phenomena if strong light waves transported localized electrons within preselected bands. An international team of researchers (USA – University of Michigan, Germany) has shown that harmonic sideband (HSB) generation in monolayer tungsten diselenide creates distinct electronic interference combs in momentum space. Locating these momentum combs in spectroscopy enables super-resolution tomography of key band-structure details in situ. They experimentally tuned the optical-driver frequency by a full octave and showed that the predicted super-resolution manifests in a critical intensity and frequency dependence of HSBs. The concept […]

Demonstrating entanglement through a fiber cable with high fidelity

Phys.org  August 13, 2020 Researchers in the UK exploited a property of quantum physics that allows for mapping the medium (fiber cable) onto the quantum state of a particle moving through it to transport entangled particles through a commercial fiber cable with 84.4% fidelity. They sent one of a pair of photons through a complex medium, but not the other. Both were then directed toward spatial light modulators and then on to detectors, and then finally to a device used to correlate coincidence counting. In their setup, light from the photon that did not pass through the complex medium propagated […]

How growth of the scientific enterprise influenced a century of quantum physics

MIT News  April 29, 2020 In a new book, “Quantum Legacies: Dispatches from an Uncertain World,” Professor David Kaiser of MIT describes dramatic shifts in the history of an evolving discipline. Moving between vignettes of key physicists, original research about the growth of the field, and accounts of his own work in cosmology, the author emphasizes the vast changes in the field over time…read more. Book: “Quantum Legacies: Dispatches from an Uncertain World”

New protocol identifies fascinating quantum states

Phys.org  April 10, 2020 An international team of researchers (Austria, USA – NIST, University of Maryland, Germany, France) proposed and analyzed a universal toolbox of measurement protocols to reveal many-body topological invariants of phases with global symmetries, which can be implemented in state-of-the-art experiments with synthetic quantum systems, such as Rydberg atoms, trapped ions, and superconducting circuits. The protocol is based on extracting the many-body topological invariants from statistical correlations of randomized measurements, implemented with local random unitary operations followed by site-resolved projective measurements. They illustrated the technique and its application in the context of the complete classification of bosonic […]

Does relativity lie at the source of quantum exoticism?

Phys.org  April 2, 2020 In quantum mechanics inherent randomness happens without any cause. Randomness that appears in non-relativistic quantum theory tacitly respects relativity, for example, it makes instantaneous signaling impossible. An international team of researchers (Poland, Singapore, UK) argues that this is because the special theory of relativity can itself account for such a random behavior. They show that the full mathematical structure of the Lorentz transformation, the one which includes the superluminal part, implies the emergence of non-deterministic dynamics, together with complex probability amplitudes and multiple trajectories. This indicates that the connections between the two seemingly different theories are […]

Quantum-entangled light from a vibrating membrane

Phys.org  March 31, 2020 Optical quantum states propagate with ultralow attenuation and resilient to ubiquitous thermal noise. Mechanical systems are envisioned as versatile interfaces between photons and a variety of solid-state quantm information processing platforms. Researchers in Denmark generated entanglement between two propagating optical modes by coupling them to the same cryogenic mechanical system. The entanglement persisted at room temperature. They verified the inseparability of the bipartite state and fully characterized its logarithmic negativity by homodyne tomography. Combined with quantum interfaces between mechanical systems and solid-state qubit processors, this paves the way for mechanical systems enabling long-distance quantum information networking […]

Scientists Find Yet Another Way to Get Qubits Working at Room Temperature

Science Alert  March 22, 2020 Defect-based quantum systems are often complicated by charge-state instabilities and interference by phonons, which can diminish spin-initialization fidelities and limit room-temperature operation. An international team of researchers (Hungary, Sweden, USA – Argonne National Laboratory, University of Chicago, IBM, Russia) has identified a pathway around these drawbacks by showing that an engineered quantum well can stabilize the charge state of a qubit. Using density-functional theory and experimental synchrotron X-ray diffraction studies they constructed a model for previously unattributed point defect centers in silicon carbide as a near-stacking fault axial divacancy and showed how this model explains […]