Q-CTRL digest

Filtering through the mode forest

September 5, 2018
Written by
Marcus Strom
Marcus Strom

Now that the alpha release of Black Opal is out, at Q-CTRL we are hard at work ensuring that the public release of the world's first cloud-based quantum control software will be packed full of the features you need to control your quantum future.

At its core, Black Opal is all about helping you improve your quantum hardware by removing barriers to the use of quantum control. We do this by transferring cutting-edge quantum physics insights into intuitive, beautiful software, so you can leverage our developments as soon as possible and with minimum hassle.

Q-CTRL driving new research

This ethos, of building beautiful and useful products to help teams take advantage of quantum control, is why we are excited to announce a new research paper on the Physics ArXiv, published on Monday 3 September 2018. The work is a collaboration between Q-CTRL and the experimentalists in the University of Sydney's Quantum Control Laboratory.

In this project, Q-CTRL built the software helping their experimentalist colleagues determine what controls to apply to produce better entangling gates. This code will appear in the two-qubit workspace for what are known as Molmer-Sorensen gates in the Black Opal public release later this year.

In classical computing, a gate is just a logic operation such as a NOT or an AND operation; a series of them allows you to run an algorithm to solve a problem. Quantum gates, unlike their classical counterparts, allow particular quantum algorithms that take advantage of the superposition and entanglement of quantum bits.

A common feature of many quantum computing architectures is the use of shared "bosonic" oscillator modes in order to enable entangling gates. With trapped ions, these are shared vibrations of the ions (think of Newton's cradle); in superconducting circuits these are electromagnetic oscillations within microwave cavities.

This method of producing entanglement causes a problem where residual energy left in the oscillator effectively creates a channel whereby information is lost from the system. The resulting errors can doom a quantum algorithm to an early death. This is especially true when there are many qubits producing a densely packed "forest" of modes.

One of the lead authors of the paper is Claire Edmunds, a Quantum Control Engineer at Q-CTRL and a doctoral candidate at the University of Sydney. She explains how the shared oscillator modes are essential to producing an entangled state through the two-qubit gate.

"All the ions, even though electronically separate, are sitting in the same potential energy well. But because they are all charged they repel each other and oscillate in this well with shared modes of motion," Ms Edmunds said.

"You can use that shared motion to swap information about the electronic states between them. This forms the basis of the Molmer-Sorenson gate."

Q-CTRL's solutions are targeting the challenges which arise as teams build bigger quantum computers implementing such entangling gates.

With just two qubits you can brute-force tune your experimental set-up to separate the modes of motion so they can be easily "decoupled". But as you build your system with more ions, you encounter frequency or spectral crowding; and so the mode separation decreases.

Ms Edmunds said: "If you try to couple your qubits with a single mode, you still find a lot of cross-talk from all the modes around it. And that interference leads to information loss and decoherence."

"Rather than have to tune your set-up to isolate the mode you are after, you can keep the mode 'forest' and give that information to the Q-CTRL package. It will return you the exact control needed to effectively isolate the required mode from the entire forest," Ms Edmunds said.

Professor Biercuk said: "Effectively what we have come up with is a control solution that allows us to implement the gates with dramatically reduced errors."

The authors of the paper, posted on the preprint arXiv server, report average two-qubit gate fidelities of 99.3 percent (limited by the measurement), improved from about 50 percent when Q-CTRL's control solutions weren't used.

"A key development in this paper is that the theoretical models implemented using Q-CTRL software to define controls and estimate gate performance all match the data quite well."

Professor Biercuk said: "What we are doing in Black Opal is taking this complicated technique and packaging it beautifully to make it accessible and intuitive."

"We give the user total flexibility to choose how to operate their gate in ways that are impossible without this kind of control," Professor Biercuk said.

## Above All Human

.@MJBiercuk closing his talk on quantum computing - a packed house for this one! #aah18 pic.twitter.com/HnxE2YjNc8

— Startup Victoria (@StartupVic) August 29, 2018

Q-CTRL Founder and CEO, Professor Michael Biercuk, was a keynote speaker at the "Above All Human" event in Melbourne last week.

Above All Human is a curated technology sector event in Melbourne put on by Startup Victoria.

Professor Biercuk spoke about 'Starting up your quantum future'. He described the opportunities for start-up companies in building a new quantum economy.

The event featured technologists, academics, venture capitalists, entrepreneurs from diverse backgrounds and a shared commitment to pushing the boundaries of technology.

Latest news and updates