**G J Milburn**

All clocks, classical or quantum, are open non equilibrium irreversible systems subject to the constraints of thermodynamics. Using examples I show that these constraints necessarily limit the performance of clocks and that good clocks require large energy dissipation. For periodic clocks, operating on a limit cycle, this is a consequence of phase diffusion. It is also true for non periodic clocks (for example, radio carbon dating) but due to telegraph noise not to phase diffusion. In this case a key role is played by accurate measurements that decrease entropy, thereby raising the free energy of the clock, and requires access to a low entropy reservoir. In the quantum case, for which thermal noise is replaced by quantum noise (spontaneous emission or tunnelling), measurement plays an essential role for both periodic and non periodic clocks. The paper concludes with a discussion of the Tolman relations and Rovelli’s thermal time hypothesis in terms of clock thermodynamics.

Read more at

# Category Archives: QUANTUM PHYSICS

# The Beginning of the Nuclear Age

**M. Shifman**

The article below is based on lectures delivered to new students remotely in the course of orientation. It presents the quantum theory tree from its inception a century ago till today. The main focus is on the nuclear physics – HEP branch.

Read more at https://arxiv.org/abs/2009.05001

# Teaching gauge theory to first year students

**Nils-Erik Bomark**

One of the biggest revelations of 20th century physics, is virtually unheard of outside the inner circles of particle physics. This is the gauge theory, the foundation for how all physical interactions are described and a guiding principle for almost all work on new physics theories. Is it not our duty as physicists to try and spread this knowledge to a wider audience?

Here, two simple gauge theory models are presented that should be understandable without any advanced mathematics or physics and it is demonstrated how they can be used to show how gauge symmetries are used to construct the standard model of particle physics. This is also used to describe the real reason we need the Higgs field.

Though these concepts are complicated and abstract, it seems possible for at least first year students to understand the main ideas. Since they typically are very interested in cutting edge physics, they do appreciate the effort and enjoy the more detail insight into modern particle physics. These results are certainly encouraging more efforts in this direction.

Read more at https://arxiv.org/abs/2009.02162

# The subtle sound of quantum jumps

**Antoine Tilloy**

Could we hear the pop of a wave-function collapse, and if so, what would it sound like? There exist reconstructions or modifications of quantum mechanics (collapse models) where this archetypal signature of randomness exists and can in principle be witnessed. But, perhaps surprisingly, the resulting sound is disappointingly banal, indistinguishable from any other click. The problem of finding the right description of the world between two completely different classes of models — where wave functions jump and where they do not — is empirically undecidable. Behind this seemingly trivial observation lie deep lessons about the rigidity of quantum mechanics, the difficulty to blame unpredictability on intrinsic randomness, and more generally the physical limitations to our knowledge of reality.

Read more at https://arxiv.org/abs/2007.15420

# Hawking for beginners

**A dimensional analysis activity to perform in the classroom**

**Jorge Pinochet**

In this paper we present a simple dimensional analysis exercise that allows us to derive the equation for the Hawking temperature of a black hole. The exercise is intended for high school students, and it is developed from a chapter of Stephen Hawking’s bestseller A Brief History of Time.

Read more at https://arxiv.org/pdf/2004.11850.pdf

# 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**

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**

**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**“