## ThinkQ 2015 - Challenges and applications for medium size quantum computers - Abstracts

**Day 1. Chair: Jerry Chow**

**Isaac Chuang (MIT)**

Title: Quantum Computation and Quantum Information: Adolescence

Abstract: Thirty years post-BB84, twenty-years post-Shor's algorithm, and fifteen years post-DiVincenzo criteria, the field of quantum computation and quantum information is growing out of its early days of playful innocence and exuberance. The last decade has seen spurts of experimental realizations, and maturation of system-level thinking. I reflect on these transitions, and contemplate the possible transition to an age of majority.

**Philippe Grangier (Institut d'Optique)**

Title: Quantum computation and simulations using interacting Rydberg atoms

Abstract: In recent years there has been a lot of theoretical interest and experimental developments based of the idea of using the so-called "Rydberg blockade" effect for applications in quantum information processing and communications. Such systems are suitable both for using atomic qubits, or photonic qubits.

**Scott Aaronson (MIT)**

Title: The Largest Possible Quantum Speedups

**Philip Walther (University of Vienna)**

Title: Photonic Quantum Computation

Abstract: The advantages of the photons makes optical quantum system well suited for a variety of applications in quantum information processing. In this talk I will discuss the resource-efficient quantum computation schemes that utilize complex interferometric networks and the superposition of quantum gates as well as secure quantum cloud computing, where quantum information is securely communicated and computed.

As outlook I will discuss the current status of new quantum technology for improving the scalability of photonic quantum systems by using integrated circuits, superconducting single-photon detectors and tailored light-matter interactions.

**Chris Monroe (JQI)**

Title: Modular and Reconfigurable Quantum Computing with Trapped Ions

Abstract: Electromagnetically trapped atomic ions are qubit standards, with unsurpassed levels of quantum coherence and near-perfect measurement. When qubit state-dependent laser or microwave forces are applied to ions in a crystal, their Coulomb interaction is modulated in a way that forms entangling quantum gates or global quantum magnetic interactions. Recent experiments have implemented tunable and reconfigurable long-range interacting spin models with up to 25 qubits. Scaling to even larger numbers can be accomplished by coupling the qubits to optical photons, where entanglement can be formed over remote distances for applications in quantum communication and distributed quantum computing. The engineering of this modular and reconfigurable quantum architecture will likely accompany the development of applications that match the connectivity of the system, in what could be termed "quantum co-design.

**Day 2. Chair: Jay Gambetta **

**Rob Schoelkopf (Yale)**

Title: Extending the lifetime of quantum information through error correction

Abstract: In quantum error correction (QEC) one redundantly encodes an arbitrary bit of quantum information into a larger collection of quantum states,

whose symmetry properties allow error syndrome measurements to project

the state into a known error space

without disturbing the qubit, and enable the recovery from errors via

simple operations. Given the considerable overhead inherent in traditional

proposals, realizing a QEC protocol at the "break-even" point, which

extends the lifetime of information beyond the system's highest quality constituent, remains a difficult and outstanding challenge. Here we demonstrate a fully operational quantum error correction system,

based on a logical encoding comprised of superpositions of cat

states in a superconducting cavity. This system uses real-time classical

feedback to encode, track the naturally occurring errors, decode, and correct, all without the need for post-selection. Using this hardware-efficient approach we reach, for the first time, the break-even point for QEC and preserve quantum information through active means.

**Ed Farhi (MIT)**

Title : A Quantum Approximate Optimization Algorithm

Abstract: I will describe the new QAOA and explain how to analyze its performance on all instances of particular combinatorial optimization problems. I will also explain why this algorithm is well suited to be run on small scale quantum computers that will be developed in the near term because of its low circuit depth and simple gate structure and good fault tolerance without error correction.

**David Cory (IQC)**

Title: Tools for Scaling Up: Using small quantum processors as development tools along the path to yet larger processors

Abstract: I will talk about our approach to a quantum processor based on spins in cavities, and in particular the control we believe is necessary to realize a useful, small processor; our experience controlling and benchmarking control in a small NV system, and the tools we have developed for these; some potential approaches to using mesoscopic systems to couple qubits.

**William Oliver (MIT)**

Title: Engineering Superconducting Circuits for Quantum Information Applications

Abstract: We review our continuing efforts to engineer superconducting circuits for quantum information applications. We revisit the design and fabrication of the persistent-current flux qubit [1]. By adding a high-Q capacitor, we dramatically improve its reproducibility and coherence times while retaining moderate anharmonicity in the longest lived devices [2-4]. We discuss in a detail a device with T_{1} = 55 us and T_{2E} > 90 us. We then discuss our progress towards 3D integration of the quantum-to-classical interface, including bump bonding, through-silicon vias, traveling wave parametric amplifiers, and multi-chip modules based on our SFQ foundry process [5,6].

[1] W.D. Oliver and P.B. Welander, MRS Bulletin, 38, 816 (2013)

[2] J.Q. You, X. Hu, S. Ashhab, F. Nori, PRB, 75, 140515 (2007).

[3] M. Steffen et al., PRL 105, 100502 (2010).

[4] F. Yan et al., under review (2015).

[5] C Macklin et al., Science, 350, 307 (2015).

[6] S.K. Tolpygo et al., IEEE Trans. Appl. Supercond. 25, 110312 (2015).

**Jerry Chow (IBM)**

Title: The 100Q challenge, well actually 97.

**Abstract: The road towards fault-tolerant quantum computing is constantly being paved. Showing the viability of quantum error correction techniques is paramount to continuing advances. I will present on behalf of the IBM Quantum Computing team and discuss our visions for constructing a quantum computing system, with emphasis on a recent implementation of a quantum code using 4 lithographically defined superconducting qubits in a square lattice capable of measuring both types of possible quantum errors occurring on a single qubit. The experiment requires highly coherent qubits, high quality quantum operations implementing the detecting circuit, and a high quality independent qubit measurement set-up. Looking beyond this implementation, there remains both theoretical and experimental control hurdles which must be overcome to build verifiably reliable quantum networks of qubits. We present some experiments which point towards these important questions and give proposals for future integration capability, measurement integration, and scalable control architectures. The focus will be on a variety of questions which will increasingly become important as the field moves towards a larger network of order 100 physical qubits.**

**Peter Zoller (Innsbruck)**

Title: Implementing Quantum Information Processing with Atomic and Solid State Quantum Optical Systems

Abstract: In this talk we discuss recent theoretical work from Innsbruck on implementing quantum information processing on various platforms, including atomic and solid state quantum optical systems.

We will focus on two topics:

(1) A Quantum Annealing Architecture with All-to-All Connectivity from Local Interactions [1]

The basic principle of quantum annealing is to encode the optimization problem in Ising interactions between qubits. A fundamental challenge in building a fully programmable quantum annealer is the competing requirements of full controllable all-to-all connectivity and the quasi locality of the interactions between physical qubits. Here we describe a scalable architecture with full connectivity, which can be implemented with local interactions only. The input of the optimization problem is encoded in local fields acting on an extended set of physical qubits. The output is encoded redundantly in the physical qubits. Our model can be understood as a lattice gauge theory with long-range interactions mediated by gauge constraints. The architecture can be realized on various platforms with local controllability, including superconducting qubits, NV-centers, quantum dots, as well as atomic systems.

(2) Photonic Quantum Circuits: Simulation Techniques and Tools Based on Matrix Product States [2]

Wiring up increasingly complex quantum devices from basic modules is central in the effort to build large scale quantum circuits. Quantum optical systems provide a natural framework to implement such a modular approach as a photonic quantum circuit, where communication between the nodes of the network is implemented as photons in waveguides representing quantum channels for 'flying qubits'. We describe a theoretical approach to simulate efficiently such photonic quantum networks based on matrix product states, as developed originally in the context of (open) quantum many-body systems. Our approach accounts for the entanglement between the nodes and the photon states of the quantum channel, and relates the input and output quantum signals of the quantum circuit on the level of quantum states. We illustrate our approach with simple quantum circuits involving transfer of quantum states between qubits including time delays, and quantum feedback in driven-dissipative quantum optical systems.

[1] Lechner, W., Hauke, P., & Zoller, P. (2015). A quantum annealing architecture with all-to-all connectivity from local interactions. Science Advances, 1(9), e1500838.

[2] Pichler, H., & Zoller, P. (2015). Photonic Quantum Circuits with Time Delays. arXiv preprint arXiv:1510.04646.

**Industry Panel **

Mark Gibbons (JP Morgan)

Carl J. Williams Jr. (Deputy Director of the Physical Measurement Laboratory, NIST)

Mark Ritter (IBM)

Andrew Schoen (NEA)

**Day 3. Chair: Matthias Steffen **

**David Poulin (Sherbrooke)**

**Andrea Morello (UNSW)**

Title: Scaling up donor-based silicon quantum processors: challenges and opportunities

Abstract: Phosphorus donors in isotopically pure 28Si are among the most coherent single-qubits in the solid state [1], and the presence of two qubits (the electron and the nuclear spin) within each donor allows interesting pathways for improving operation fidelities and expanding the Hilbert space size [2].

However, scaling up beyond a single donor remains a hard challenge. In this talk I will review the difficulties involved in following “Kane” or “Loss-DiVincenzo” pathways with donor qubits, but also describe what we think is a potential game-changer for this platform. We have recently discovered that we can strongly couple donor qubits to each other using a relatively long-range electric dipole interaction, which has the additional advantage of greatly mitigating the requirements on donor placement accuracy [3]. In addition, inducing electric dipoles on a donor allows strongly coupling it to a microwave resonator, whereby the whole arsenal of circuit-QED methods can be applied to silicon spin qubits. I will conclude by presenting some preliminary ideas for a large-scale donor-based quantum processor with robust long-distance couplings.

[1] J.T. Muhonen et al., Nature Nanotech. 9, 986 (2014)

[2] J.P. Dehollain et al, Nature Nanotech. doi:10.1038/nnano.2015.262 (2015)

[3] G. Tosi et al., arXiv:1509.08538 (2015)

**David DiVincenzo (Aachen)**

Title: Circuit QED design for longitudinal coupling: alternative scheme for 2D qubit layout

Abstract: We present a circuit construction for a new fixed-frequency superconducting qubit and show how it can be scaled up to a grid with strictly local interactions. The circuit QED realization we propose implements qubit-cavity coupling of the *σ**z*-type. The resulting \textit{longitudinal coupling} is inherently different from the usual *σ**x*-type \textit{transverse coupling}even with always-on interactions, coupling is strictly confined to nearest and next-nearest neighbor resonators; there is never any direct qubit-qubit coupling. We note that just four distinct resonator frequencies, and only a single unique qubit frequency, suffice for the scalability of this scheme. Details of this work are found in arxiv:1511.06138.

### Invited Speakers

Rob Schoelkopf (Yale)

David Cory (IQC)

Isaac Chuang (MIT)

William D. Oliver (LL/MIT)

Ed Farhi (MIT)

David DiVincenzo (RWTH Aachen)

Philip Walther (University of Vienna)

Christopher Monroe (JQI)

Scott Aaronson (MIT)

Peter Zoller (Innsbruck)

David Poulin (Sherbrooke)

Philippe Grangier (Institut d’Optique)

Andrea Morello (UNSW)

### Dates

December 2-4, 2015

### Location

IBM TJ Watson Research Center Yorktown Heights, NY

### Organizing Committee

Jay Gambetta, IBM

Matthias Steffen, IBM

Jerry Chow, IBM