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 ….


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.


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.

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 also: Foundations of quantum physics III. Measurement

How does a quantum particle see the world?

Researchers at the University of Vienna study the relevance of quantum reference frames for the symmetries of the world

Quantum features, such as quantum superposition, are only defined relative to an observer. When we look at the train from the point of view of an observer standing on the platform, the train looks in a quantum superposition of different positions. (© Christian Murzek/IQOQI-Vienna)

However, an observer sitting on the train sees the observer on the platform and the ball in a quantum superposition. (© Christian Murzek/IQOQI-Vienna)

According to one of the most fundamental principles in physics, an observer on a moving train uses the same laws to describe a ball on the platform as an observer standing on the platform – physical laws are independent on the choice of a reference frame. Reference frames such as the train and the platform are physical systems and ultimately follow quantum-mechanical rules. They can be, for example, in a quantum state of superposition of different positions at once. So, what would the description of the ball look like for an observer on such a “quantum platform”? Researchers at the University of Vienna and the Austrian Academy of Sciences proved that whether an object (in our example, the ball) shows quantum features depends on the reference frame. The physical laws, however, are still independent of it. The results are published in Nature Communications.

Read more at and

An optical analogue to Schrödinger’s cat

Entanglement between a rubidium-87 atom and a laser pulse leads to the creation of macroscopic superposition states.

A rubidium-87 atom (blue sphere) is entangled with a light pulse that is in two distinct superpositions of phase states. A measurement of spin collapses the atom state and leads to the odd (top) or even (bottom) superposition state of the light.

Erwin Schrödinger’s cat gedanken experiment of 1935 presented the possibility of entanglement between microscopic and macroscopic physical systems and the resulting quantum behavior of macroscopic objects. In the years since, researchers have realized analogous systems with entanglement between microscopic and macroscopic structures—for example, the entanglement between a trapped ion and the vibrational state in the trap described in David Wineland’s Nobel Prize–winning work. However, the resulting macroscopic quantum superposition states typically had limited utility because their creation had a low probability or they had short transmission distances. Now Gerhard Rempe of the Max Planck Institute of Quantum Optics in Germany and his colleagues have deterministically produced a macroscopic light pulse with a controlled superposition state.

Rempe and his colleagues used a single rubidium-87 atom trapped in an optical cavity as the microscopic object and a laser pulse as the macroscopic cat. After putting the atom into a quantum superposition of up- and down-spin states, the researchers reflected the light pulse off the optical cavity. Although the light would not gain a phase shift for an atom in the up-spin state, it would gain one for the down-spin state. Because the atom had both up and down spin, the light experienced both phase shifts and became entangled with it. After rotating the atom’s spin by 90 degrees, the researchers measured the spin state, projecting it into either the up or down state. For a spin-up measurement, the light was in an odd superposition of the phase states; for a spin-down measurement, it was in an even superposition, as shown in the figure. Rempe and his colleagues confirmed the creation of a macroscopic superposition state by measuring the probability distribution in phase space with a homodyne detector.

The experiment performed by Rempe and his colleagues offers more than the realization of a famous thought experiment. Compared with previous work, the results provide some important advantages, such as the creation of atom-light entanglement in every trial. Furthermore, because it is optical, the macroscopic object can propagate without losing its superposition state. Those features make the new technique particularly promising for quantum network applications. Schrödinger’s cat, simultaneously alive and dead, may become a new mode of information transmission. (B. Hacker et al., Nat. Photonics 13, 110, 2019.)


Simulating quantum field theory with a quantum computer

John Preskill
Forthcoming exascale digital computers will further advance our knowledge of quantum chromodynamics, but formidable challenges will remain. In particular, Euclidean Monte Carlo methods are not well suited for studying real-time evolution in hadronic collisions, or the properties of hadronic matter at nonzero temperature and chemical potential. Digital computers may never be able to achieve accurate simulations of such phenomena in QCD and other strongly-coupled field theories; quantum computers will do so eventually, though I’m not sure when. Progress toward quantum simulation of quantum field theory will require the collaborative efforts of quantumists and field theorists, and though the physics payoff may still be far away, it’s worthwhile to get started now. Today’s research can hasten the arrival of a new era in which quantum simulation fuels rapid progress in fundamental physics.


The End of Spacetime

Nima Arkani-Hamed is a theoretical physicist with broad interests in high-energy physics and cosmology. He was educated at Toronto and Berkeley, held a postdoctoral fellowship at SLAC National Accelerator Laboratory, and was a professor of physics at Berkeley and Harvard before joining the Institute for Advanced Study in 2008. He was an inaugural recipient of the Fundamental Physics Prize in 2012, and was one of six physicists featured in the documentary “Particle Fever” in 2014.