**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.

Read more at https://arxiv.org/pdf/1811.10085.pdf

# Tag Archives: Quantum Computer

# The universe as quantum computer

**Seth Lloyd**

This article reviews the history of digital computation, and investigates just how far the concept of computation can be taken. In particular, I address the question of whether the universe itself is in fact a giant computer, and if so, just what kind of computer it is. I will show that the universe can be regarded as a giant quantum computer. The quantum computational model of the universe explains a variety of observed phenomena not encompassed by the ordinary laws of physics. In particular, the model shows that the the quantum computational universe automatically gives rise to a mix of randomness and order, and to both simple and complex systems.

Read more at http://arxiv.org/pdf/1312.4455v1.pdf

# Discovery of Majorana Fermions?

## Quest for quirky quantum particles may have struck gold

**Evidence for elusive Majorana fermions raises possibilities for quantum computers.**

**Eugenie Samuel Reich**

Getting into nanoscience pioneer Leo Kouwenhoven’s talk at the American Physical Society’s March meeting in Boston, Massachusetts, today was like trying to board a subway train at rush hour. The buzz in the corridor was that Kouwenhoven’s group, based at the Delft University of Technology in the Netherlands, might have beaten several competing teams in solid-state physics — and the community of high-energy physicists — to a long-sought goal, the detection of Majorana fermions, mysterious quantum-mechanical particles that may have applications in quantum computing.

Kouwenhoven didn’t disappoint. “Have we seen Majorana fermions? I’d say it’s a cautious yes,” he concluded at the end of a data-heavy presentation.

Quantum particles come in two types, fermions and bosons. Whereas bosons can be their own antiparticles, which means that they can annihilate each other in a flash of energy, fermions generally have distinct antiparticles; for example, an electron’s antiparticle is the positively charged positron. But in 1937, Italian physicist Ettore Majorana adapted equations that Englishman Paul Dirac had used to describe the behaviour of fermions and bosons to predict the existence of a type of fermion that was its own antiparticle. Over decades, particle physicists have looked for Majorana fermions in nature, and after 2008, condensed-matter physicists began to think of ways in which they could be formed from the collective behaviour of electrons in solid-state materials, specifically, on surfaces placed in contact with superconductors or in one-dimensional wires.

Kouwenhoven’s apparatus is along the latter lines. In his group’s set-up, indium antimonide nanowires are connected to a circuit with a gold contact at one end and a slice of superconductor at the other, and then exposed to a moderately strong magnetic field. Measurements of the electrical conductance of the nanowires showed a peak at zero voltage that is consistent with the formation of a pair of Majorana particles, one at either end of the region of the nanowire in contact with the superconductor. As a sanity check, the group varied the orientation of the magnetic field and checked that the peak came and went as would be expected for Majorana fermions……….

Read more: www.nature.com

Read also:

**Have we summoned the mysterious Majorana fermion?**

# Physicists move one step closer to quantum computer

**Rice University physicists have created a tiny “electron superhighway” that could one day be useful for building a quantum computer, a new type of computer that will use quantum particles in place of the digital transistors found in today’s microchips.**

n a recent paper in Physical Review Letters, Rice physicists Rui-Rui Du and Ivan Knez describe a new method for making a tiny device called a “quantum spin Hall topological insulator.” The device, which acts as an electron superhighway, is one of the building blocks needed to create quantum particles that store and manipulate data.

Today’s computers use binary bits of data that are either ones or zeros. Quantum computers would use quantum bits, or “qubits,” which can be both ones and zeros at the same time, thanks to the quirks of quantum mechanics….. Continue reading Physicists move one step closer to quantum computer

# First Quantum Computer With Quantum CPU And Separate Quantum RAM

**Computer scientists have built a superconducting number cruncher with a Von Neumann architecture that paves the way for a new era of quantum computation**

Back in 1946, the world’s first general purpose electronic computer was switched on at the University of Pennsylvania. The huge processing power of ENIAC (Electronic Numerical Integrator And Computer) stunned the world, or at least the few dozen people who had any idea what it was for and why it was important.

But ENIAC had an important flaw. It could only be programmed by resetting a myriad switches and dials, a task that could take weeks. And this seriously hindered the computer’s flexibility.

The solution was not hard to find. it had already been outlined by Alan Turing, John Von Neumann and others: have a unit for number crunching and a separate electronic memory that could store instructions and data. That design meant that any reprogramming could be done relatively quickly, easily and electronically.

Today, almost all modern computers use this design, now known as the Von Neumann architecture.

The exception is the quantum computer. These devices use the strange properties of the quantum world to perform huge numbers of calculations in parallel. Consequently they have the potential to vastly outperform conventional number crunchers.

Unfortunately, physicists have only a vague and fleeting power over the quantum world and this means has prevented them the luxury of designing a Von Neumann-type quantum computer.

Until now. Today, Matteo Mariantoni at the UC Santa Barbara and pals reveal the first quantum computer with an information processing unit and a separate random access memory.

Their machine is a superconducting device that stores quantum bits or qubits as counter-rotating currents in a circuit (this allows the qubit to be both a 0 and 1 at the same time). These qubits are manipulated using superconducting quantum logic gates, transferred using a quantum bus and stored in separate microwave resonators.

Let’s say upfront that the result is not a particularly powerful computer. Mariantoni and co show off their device by demonstrating a couple of simple but unspectacular algorithms but ones that were carefully chosen as the building blocks of more impressive tasks such as error correction and factoring large numbers.

Not that they’ve actually done any of those things. What’s impressive, however, is that they soon could since this approach is eminently scalable. “Our results provide optimism for the near-term implementation of a larger-scale quantum processor based on superconducting circuits,” say Mariantoni and co.

There has been no shortage of false dawns for quantum computing in the last 20 years or so. But it could that the Sun is about to rise on a new era of computation. If it does, everything that has gone before will one day seem as primitive as ENIAC seems to us.

Ref: arxiv.org/abs/1109.3743: Implementing the Quantum von Neumann Architecture with Superconducting Circuits