Seminars 2021

High-fidelity single- and two-qubit gates are essential building blocks for a fault-tolerant quantum computer. While there has been much progress in suppressing single-qubit gate errors in superconducting qubit systems, two-qubit gates still suffer from error rates that are orders of magnitude higher. 

One limiting factor is the residual ZZ-interaction, which originates from a coupling between computational states and higher-energy states. While this interaction is usually viewed as a nuisance, here we experimentally demonstrate that it can be exploited to produce a universal set of fast single- and two-qubit entangling gates in a coupled transmon qubit system. 

To implement arbitrary single-qubit rotations, we design a new protocol called the two-axis gate that is based on a three-part composite pulse. It rotates a single qubit independently of the state of the other qubit despite the strong ZZ-coupling. 

We achieve single-qubit gate fidelities as high as 99.1% from randomized benchmarking measurements. We then demonstrate both a CZ gate and a CNOT gate. Because the system has a strong ZZ-interaction, a CZ gate can be achieved by letting the system freely evolve for a gate time tg=53.8ns. To design the CNOT gate, we utilize an analytical microwave pulse shape based on the SWIPHT protocol for realizing fast, low-leakage gates. We obtain fidelities of 94.6% and 97.8% for the CNOT and CZ gates respectively from quantum progress tomography. 

arxiv.org/abs/2103.12305 

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Over the past few years, there has been a growing interest on the national level in the development of “quantum technologies”:  devices that utilize quantum entanglement and superposition, often directed for the use of computation.  Advances in this field depend critically on the ability for such devices —qubits— to maintain coherence across long time scales.  Is there a natural limit to such coherence times?  Why are currently designed devices falling well short of theoretical expectations?  In this talk, I will explore the role that environmental radioactivity may be playing in limiting the coherence of superconducting qubits.  

I will share recent results from a dedicated set of measurements that show the interplay between natural radioactivity and coherence.  I will also discuss what measures can be taken to counteract these effects to enhance the performance of these essential devices.

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I review the the four architectures of quantum computers with their existing implementations, and the three deep learning paradigms respectively. Next, I introduce the concept of quantum artificial intelligence and I show some recent results, including a quantum perceptron and quantum restricted Bolzmann machines. I conclude by summarizing the directions and the perspectives to exploit quantum artificial intelligence in fields ranging from aerospace, to material design, to finance and many others.

Quantum entanglement comes in a variety of different forms and measures with different conceptual and operational interpretations. In this talk I will review some of the most relevant definitions of entanglement, the different physical effects that they characterize, and their use in the study of quantum many-body systems and models of quantum statistical mechanics.

slides - video recording