Understanding the Schrodinger equation as a kinematic statement: A probability-first approach to quantum

James Daniel Whitfield
Quantum technology is seeing a remarkable explosion in interest due to a wave of successful commercial technology. As a wider array of engineers and scientists are needed, it is time we rethink quantum educational paradigms. Current approaches often start from classical physics, linear algebra, or differential equations. This chapter advocates for beginning with probability theory. In the approach outlined in this chapter, there is less in the way of explicit axioms of quantum mechanics. Instead the historically problematic measurement axiom is inherited from probability theory where many philosophical debates remain. Although not a typical route in introductory material, this route is nonetheless a standard vantage on quantum mechanics. This chapter outlines an elementary route to arrive at the Schrödinger equation by considering allowable transformations of quantum probability functions (density matrices). The central tenet of this chapter is that probability theory provides the best conceptual and mathematical foundations for introducing the quantum sciences.
Read more at https://arxiv.org/pdf/2003.09330.pdf

How sensitive can your quantum detector be?

A new device measures the tiniest energies in superconducting circuits, an essential step for quantum technology

Illustration by Safa Hovinen, Merkitys

Quantum physics is moving out of the laboratory and into our everyday lives. Despite the big headline results about quantum computers solving problems impossible for classical computers, technical challenges are standing in the way of getting quantum physics into the real world. New research published in Nature Communications from teams at Aalto University and Lund University hopes to provide an important tool in this quest.

One of the open questions in quantum research is how heat and thermodynamics coexist with quantum physics. This research field, “quantum thermodynamics”, is one of the areas Professor Jukka Pekola, the leader of the QTF Centre of Excellence of the Academy of Finland, has worked on in his career. ‘This field has up to now been dominated by theory, and only now important experiments are starting to emerge’ says Professor Pekola. His research group has set about creating quantum thermodynamic nano-devices that can solve open questions experimentally.

Quantum states – like the qubits that power quantum computers – interact with their surrounding world, and these interactions are what quantum thermodynamics deals with. Measuring these systems requires detecting energy changes so exceptionally small they are hard to pick out from background fluctuations, like using only a thermometer to try and work out if someone has blown out a candle in the room you’re in. Another problem is that quantum states can change when you measure them, simply because you’ve measured them. This would be like putting a thermometer in a cup of cold water making the water start to boil. The team had to make a thermometer able to measure very small changes without interfering with any of the quantum states they plan to measure.

Doctoral student Bayan Karimi works in QTF and Marie Curie training network QuESTech. Her device is a calorimeter, which measures the heat in a system. It uses a strip of copper about one thousand times thinner than a human hair. ‘Our detector absorbs radiation from the quantum states. It is expected to determine how much energy they have and how they interact with their surroundings. There is a theoretical limit to how accurate a calorimeter can be, and our device is now reaching that limit’, says Karimi.

Read more at https://www.aalto.fi/en/news/how-sensitive-can-your-quantum-detector-be

Spooky Action at a Global Distance

Resource-Rate Analysis of a Space-Based Entanglement-Distribution Network for the Quantum Internet

A hybrid global-quantum-communications network, in which a satellite constellation distributes entangled photon pairs (red wave packets; entanglement depicted by wavy lines) to distant ground stations (observatories) that host multimode quantum memories for storage. These stations act as hubs that connect to local nodes (black dots) via fiber-optic or atmospheric links. Using these nearest-neighbor entangled links, via entanglement swapping, two distant nodes can share entanglement. Note that this architecture can support inter-satellite entanglement links as well, which is useful for exploring fundamental physics , and for forming an international time standard

Sumeet Khatri, Anthony J. Brady, Renée A. Desporte, Manon P. Bart, Jonathan P. Dowling
Recent experimental breakthroughs in satellite quantum communications have opened up the possibility of creating a global quantum internet using satellite links. This approach appears to be particularly viable in the near term, due to the lower attenuation of optical signals from satellite to ground, and due to the currently short coherence times of quantum memories. These drawbacks prevent ground-based entanglement distribution using atmospheric or optical-fiber links at high rates over long distances. In this work, we propose a global-scale quantum internet consisting of a constellation of orbiting satellites that provides a continuous on-demand entanglement distribution service to ground stations. The satellites can also function as untrusted nodes for the purpose of long-distance quantum-key distribution. We determine the optimal resource cost of such a network for obtaining continuous global coverage. We also analyze the performance of the network in terms of achievable entanglement-distribution rates and compare these rates to those that can be obtained using ground-based quantum-repeater networks.

Read more at https://arxiv.org/abs/1912.06678

Read also “Why the quantum internet should be built in space

Top 10 quantum computing experiments of 2019

The last decade has seen quantum computing grow from a niche research endeavour to a large-scale business operation. While it’s exciting that the field is experiencing a surge of private funding and media publicity, it’s worth remembering that nobody yet knows how to build a useful fault-tolerant quantum computer. The path ahead is not “just engineering”, and in the coming decade we have to pay attention to all the “alternative approaches”, “crazy ideas” and “new ways of doing things”.

With this in mind, I created this subjective list of quantum computing research highlights of 2019. It highlights experimental achievements which show new exciting ways of controlling qubits. In such a vast space of literature, I have no doubt I missed some essential works, so I encourage you to get in touch and add your favourites to the list ….

Read more at https://medium.com/@msmalina/top-quantum-computing-experiments-of-2019-1157db177611

Quantum Poker

a pedagogical tool to learn quantum computing that is fun to play

bloch sphere

The state of a qubit can be represented geometrically as any state on the so-called Bloch sphere

Franz G. Fuchs, Vemund Falch, Christian Johnsen
Quantum computers are on the verge of becoming a commercially available reality. They represent a paradigm change to the classical computing paradigm, and the learning curve is considerably long. The creation of games is a way to ease the transition for novices. We present a game similar to the poker variant Texas hold ’em with the intention to serve as an engaging pedagogical tool to learn the basics rules of quantum computing. The difference to the classical variant is that the community cards are replaced by a quantum register that is “randomly” initialized, and the cards for each player are replaced by quantum gates, randomly drawn from a set of available gates. Each player can create a quantum circuit with their cards, with the aim to maximize the number of 1’s that are measured in the computational basis. The basic concepts of superposition, entanglement and quantum gates are employed. We provide a proof-of-concept implementation using Qiskit. A comparison of the results using a simulator and IBM machines is conducted, showing that error rates on contemporary quantum computers are still very high. Improvements on the error rates and error mitigation techniques are necessary, even for simple circuits, for the success of noisy intermediate scale quantum computers.

read more at https://arxiv.org/pdf/1908.00044.pdf

Quantum Computing as a High School Module


Anastasia Perry, Ranbel Sun, Ciaran Hughes, Joshua Isaacson, Jessica Turner
Quantum computing is a growing field at the intersection of physics and computer science. This module introduces three of the key principles that govern how quantum computers work: superposition, quantum measurement, and entanglement. The goal of this module is to bridge the gap between popular science articles and advanced undergraduate texts by making some of the more technical aspects accessible to motivated high school students. Problem sets and simulation based labs of various levels are included to reinforce the conceptual ideas described in the text. This is intended as a one week course for high school students between the ages of 15-18 years. The course begins by introducing basic concepts in quantum mechanics which are needed to understand quantum computing.
Read more at https://arxiv.org/pdf/1905.00282.pdf

Foundations of quantum physics

II. The thermal interpretation

Arnold Neumaier
This paper presents the thermal interpretation of quantum physics. The insight from Part I of this series that Born’s rule has its limitations – hence cannot be the foundation of quantum physics – opens the way for an alternative interpretation – the thermal interpretation of quantum physics. It gives new foundations that connect quantum physics (including quantum mechanics, statistical mechanics, quantum field theory and their applications) to experiment.

Read more at https://arxiv.org/pdf/1902.10779.pdf

Read also: Foundations of quantum physics III. Measurement