Lifeboat Foundation

There’s a hot new BEC in town that has nothing to do with bacon, egg, and cheese. You won’t find it at your local bodega, but in the coldest place in New York: the lab of Columbia physicist Sebastian Will, whose experimental group specializes in pushing atoms and molecules to temperatures just fractions of a degree above absolute zero.

Scientists at the Cavendish Laboratory have discovered spin coherence in Hexagonal Boron Nitride (hBN) under normal conditions, offering new prospects for quantum technology applications.

Cavendish Laboratory researchers have discovered that a single ‘atomic defect’ in a material known as Hexagonal Boron Nitride (hBN) maintains spin coherence at room temperature and can be manipulated using light.

Spin coherence refers to an electronic spin being capable of retaining quantum information over time. The discovery is significant because materials that can host quantum properties under ambient conditions are quite rare.

To reliably perform complex, large-scale calculations, computing systems rely on so-called error correction schemes, techniques designed to protect information against errors. These techniques are perhaps even more essential when it comes to quantum computers, devices that perform computations leveraging the principles of quantum mechanics.

Industry and academia in Japan and the United States are collaborating on research to pioneer quantum-centric supercomputing.

In the general formulation of quantum information, quantum states are represented by a special class of matrices called density matrices. This lesson describes the basics of how density matrices work and explains how they relate to quantum state vectors. It also introduces the Bloch sphere, which provides a useful geometric representation of qubit states, and discusses different types of correlations that can be described using density matrices.

0:00 — Introduction.
1:46 — Overview.
2:55 — Motivation.
4:40 — Definition of density matrices.
9:55 — Examples.
12:58 — Interpretation.
15:37 — Connection to state vectors.
20:13 — Probabilistic selections.
25:23 — Completely mixed state.
28:41 — Probabilistic states.
32:03 — Spectral theorem.
37:36 — Bloch sphere (introduction)
38:36 — Qubit quantum state vectors.
41:30 — Pure states of a qubit.
43:52 — Bloch sphere.
47:38 — Bloch sphere examples.
51:36 — Bloch ball.
55:40 — Multiple systems.
56:46 — Independence and correlation.
1:00:55 — Reduced states for an e-bit.
1:04:16 — Reduced states in general.
1:08:53 — The partial trace.
1:12:23 — Conclusion.

Continue reading “Lesson 09: Density Matrices | Understanding Quantum Information & Computation” »

Since Nobel-Prize-winning physicist Frank Wilczek first proposed his theory over a decade ago, researchers have been on the search for elusive “time crystals”—many-body systems composed of particles and quasiparticles like excitons, photons, and polaritons that, in their most stable quantum state, vary periodically in time.

Wilczek’s theory centered around a puzzling question: Can the most stable state of a quantum system of many particles be periodic in time? That is, can it display temporal oscillations characterized by a beating with a well-defined rhythm?

It was quite rapidly shown that time crystal behavior cannot occur in isolated systems (systems which do not exchange energy with the surrounding environment). But far from closing the subject, this disturbing question motivated scientists to search for the conditions under which an open system (i.e., one that exchanges energy with the environment) may develop such time crystal behavior.

Transferring information from one location to another without transmitting any particles or energy seems to run counter to everything we’ve learned in the history of physics.

Yet there is some solid reasoning that this ‘counterfactual communication’ might not only be plausible, but depending on how it works could reveal fundamental aspects of reality that have so far been hidden from view.

Counterfactual physics isn’t a new thing in itself, describing a way of deducing activity by an absence of something. In one sense, it’s pretty straight forward. If your dog barks at strangers, and you hear silence when the front door opens, you’ve received information that says a familiar person has entered your house in spite of the absence of sound.

The true nature of time has eluded physicists for centuries, but a new theoretical model suggests it may only exist due to entanglement between quantum objects.

By Karmela Padavic-Callaghan

Proposed set-up might help reconcile gravity with quantum descriptions of the other fundamental forces – a long sought-after goal in physics.