Monday, September 30, 2019
Lazaridis QNC 0101
Tuesday, October 1, 2019
Research Advancement Centre (RAC) 1, Room 2009
Wednesday, October 2, 2019
Lazaridis QNC 0101
9:00 am 
Gretchen Campbell, Joint Quantum Institute, NIST and UMD College Park Plenary talk: A Supersonically expanding BEC: An expanding universe in the lab

10:00 am 
Brynle Barrett, iXblue Hybrid matterwave inertial sensors for mobile sensing applications

10:45 am 
Coffee break 
11:05 am 
Crystal Noel, Joint Quantum Institute, University of Maryland (Monroe group) Demonstration of a large, individually addressable trapped ion quantum information processor and the study of electricfield noise from thermallyactivated fluctuators potentially limiting future performance

11:50 am 
Sara Mouradian, University of California, Berkeley (Haeffner group) Increasing connectivity in complex quantum systems

12:35 pm 
Lunch 
1:35 pm 
Christian Kraglund Anderson, ETH Zürich (Wallraff group) Designing and operating superconducting circuits for quantum error correction

2:20 pm 
ChiaoHsuan Wang, University of Chicago (Jiang group) Autonomous quantum error correction by Hamiltonian and dissipation engineering

3:00 pm 
Coffee break 
3:30 pm 
Christie Chiu, Princeton University (Houck group) Microscopic studies of the doped Hubbard model

4:15 pm 
Adèle Hilico, Laboratory for Photonics, Numbers and Nanosciences  OLGS (Institute of Optics Graduate School) / CRNS / University of Bordeaux Ultracold atoms in a nanostructured optical lattice

5:00 pm 
Free time 
6:15 pm 
Banquet dinner at the University Club 
9:00 PM 
End of day 
Thursday, October 3, 2019
Lazaridis QNC 0101
Abstracts
Lindsay LeBlanc, University of Alberta
Plenary talk: Optical AutlerTownes quantum memory in ultracold atomic ensembles
The ability to store and manipulate quantum information encoded in electromagnetic (often optical) signals represents one of the key tasks for quantum communications and computation schemes. In the pursuit of a practical but efficient and broadband quantum memory, we make use of a threelevel atomic system (in our case, lasercooled rubidium atoms) and realize storage and photonic manipu lations in the regime of AutlerTownes splitting (ATS), where a classicallevel control field controls the absorption of an auxillary, possibly quantum, signal field. We demonstrate ondemand storage and retrieval of both highpower and lessthanonephoton optical signals with total efficiencies up to 30%, using the ground state spinwave as our storage states. Recently, we began storing signals in much colder samples, approaching the transition to BoseEinstein condensation. We also realize a number of photonic manipulations, including temporal beamsplitting, frequency conversion, and pulse shaping. The ATS memory scheme is inherently fast and broadband, and, in contrast to the related schemes, is less demanding in terms of technical resources, making it a leading candidate for practical quantum technologies.
Aurelia Chenu, Donostia International Physics Center
Quantum thermodynamics and superadiabatic control of complex systems
Quantum thermodynamics is an emerging field with potential application to nanoscience. At the quantum level, work becomes a stochastic variable, and the work probability distribution is key to characterize a working medium. Complex quantum systems can boost the performance of quantum machines, but their characterization is challenging due to a complexity exponentially scaling with the system size. I will present a characterization of work in driven chaotic quantum systems, which are paradigmatic complex systems, using theory of random matrix Hamiltonians. Specifically, I will discuss the work statistics associated with a sudden quench for arbitrary temperature and system size [1]. In addition, I shall show how work statistics can generally be related to a dynamical problem: the evolution of quantum correlations of an entangled state [2]. Using this mapping, it is possible to connect work statistics to information scrambling, i.e., the spreading of initially localized quantum information across different degrees of freedom in manybody systems, which is a key quantity in the study of quantum chaos.
In a second part, I shall focus on control schemes to fasten the dynamics of the thermodynamic strokes of a quantum engine. Using shortcuts to adiabaticity, I demonstrate the improvement of the output power in compression and expansion strokes, with experimental implementation in a unitary Fermi gas [3]. This superadiabatic control scheme can be extended to open system [4], making possible the fast thermalization of a quantum system.
References:
[1] A. Chenu, J. MolinaVilaplana, A. del Campo, Quantum 3:127 (2019)
[2] A. Chenu, I. Egusquiza, J. MolinaVilaplana, A. del Campo, Sci. Rep. 8:12634 (2018) [3] S. Deng, A. Chenu, P. Diao, F. Li, S. Yu, I. Coulamy, A. del Campo, and H. Wu, Science Adv. 4:5909 (2018)
[4] S. Alipour, A. Chenu, A. Rezakhani, and A. del Campo, arXiv:1907.07460
Rahul Trivedi, Stanford University (Vuckovic group)
Scattering theory in quantum optics
Scattering matrices have been a key theoretical tool for the analysis of quantum field theories. Quantum optics studies the interaction of optical fields, which can be described by a field theory, with low dimensional systems (such as quantum emitters, optical cavities etc.) which can be described by a finitedimensional or countably infinitedimensional Hilbert space. Such systems serve as building blocks of opticsbased quantum information processing systems. In this talk, I will go over the connection between scattering matrices and the inputoutput formalism for timedependent and timeindependent Markovian quantum optical systems [1, 2], as well as their computation. I will show how the scattering matrix formalism can be used to understand the dynamics of some paradigmatic systems, such as driven twolevel systems for singlephoton generation [1, 2] and broadband linear optical devices [3]. Finally, I will apply this formalism to understanding photonblockade in the TavisCumming system with mesoscopic number of emitters (~50 emitters) [4]. I will show how scattering matrices can allow development of a hierarchy of approximations calculable in polynomial time in the system size for simulating the TavisCumming system. This simulation method makes it possible to study the impact of number of emitters on the light scattered from the TavisCumming system, revealing some previously unknown properties of resonant and detuned photon blockade through this system. Finally, I will provide an outlook on the application of scattering matrix formalism to studying nonMarkovian quantumoptical systems, such as timedelayed feedback systems with possible applications to design of nearly alloptical quantum memories.
References:
[1] Rahul Trivedi, Kevin Fischer, Shanshan Xu, Shanhui Fan, and Jelena Vuckovic. “Fewphoton scattering and emission from lowdimensional quantum systems.” Physical Review B 98, no. 14 (2018): 144112.
[2] Kevin Fischer, Rahul Trivedi, Vinay Ramesh, Irfan Siddiqui and Jelena Vuckovic. “Scattering into onedimensional waveguides from a coherentlydriven quantumoptical system.” Quantum 2, 69 (2018).
[3] Rahul Trivedi, Kevin Fischer, Sattwik Deb Mishra, and Jelena Vuckovic. “Pointcoupling Hamiltonian for broadband linear optical devices.” arXiv:1907.02259 (2019).
[4] Rahul Trivedi, Marina Radulaski, Kevin A. Fischer, Shanhui Fan, and Jelena Vučković. “Photon Blockade in Weakly Driven Cavity Quantum Electrodynamics Systems with Many Emitters.” Physical Review Letters 122, no. 24 (2019): 243602.
Carlos AntónSolanas, Centre de Nanosciences et de Nanotechnologies (Senellart group)
Generation of nonclassical light in a photonnumber superposition
The ability to generate light in a pure quantum superposition is central to the development of quantum enhanced technologies such as distributed quantum computing, long distance quantum communications or quantum sensing. Light offers many degrees of freedom to encode the quantum information including polarization, frequency, time bin, orbital angular momentum among others, allowing for large encoding Hilbert spaces. While the photonnumber basis is natural for discrete variable encoding, the generation of light in coherent superpositions in photonnumbers remains challenging. So far, this has mostly been demonstrated through quantumstate engineering via the interference between heralded singlephoton sources and coherent light [1].
Here we show that a single semiconductor quantum dot in a cavity can directly generate quantum superpositions of zero, one, and two photons. We investigate devices consisting of a single semiconductor quantum dot positioned with nanometre scale accuracy at the centre of a connectedpillar cavity [2,3]. The quantum dot layer is inserted in a pin diode structure and electrical contacts are defined as to control the quantum dot optical resonance through the confined Stark effect. These devices show strong suppression of decoherence processes arising from charge noise or coupling to phonons. They generate highly indistinguishable singlephotons with high extraction efficiency [4]. We coherently drive the quantum dot transition with short laser pulses and observe Rabi oscillations as a function of the laser pulse area. We perform interferometric measurements in a MachZehnder interferometer (MZI) that evidence that the quantum dot emits a coherent superposition of vacuum, and onephoton in a welldefined propagating mode of the electromagnetic field. Below the πpulse, the zero and onephoton populations are controlled through the laser intensity and exhibit near to maximal quantum coherence [5].
Figure 1: The double oscillation of the simultaneous coincidences (bottom, red trace) with respect to the single counts (top, blue trace), detected at the output of a MZI, evidences that the light emitted from our QDcavity device is in a superposition of Fock states 0, 1 and 2. (right) Corresponding Fock state populations.
Driving the quantum dot with a 2πpulse produces coherent superpositions of vacuum, one and two photons where the 2photon Fock state population is larger than the 1photon population. This state shows phase superresolution in interferometric measurements (see left panels of Fig. 1), where p2 ≈ 2p1, (right panel of Fig. 1), and it has a high fidelity to an even Schrödinger kitten state (with average photon number of 0.5) [5]. Our results demonstrate that quantum dot based artificial atoms are now controlled to such a degree that they can perform as textbook idealized systems. They open new paths for optical quantum technologies where generalized multiphoton interferences, including the photonnumber degrees of freedom, can be exploited.
References:
[1] E. Bimbard et al, Nat. Photon. 4, 243 (2010), T. J.
[2] A. Dousse et al., Phys. Rev. Lett. 101, 267404
[3] A. Nowak et al, Nature Commun. 5, 3240 (2014). Bartley et al, Phys. Rev. A 86, 043820 (2012).
[4] N. Somaschi et al., Nature Photon. 10, 340 (2016).
[5] J. C. Loredo∗, C. Anton∗, et. al, arXiv:1810.05170(2008). (2018).
Logan Clark, University of Chicago (Simon group)
Building Laughlin puddles of light
Can strongly correlated materials be built out of light? Ordinary photons, which freely propagate at the speed of light and do not interact with each other at all, cannot form such materials. However, I will explain how we turn photons into stronglyinteracting cavity Rydberg polaritons, quasiparticles which inherit their spatial waveforms from the modes of an optical cavity and gain strong interactions from Rydberg excitations of an atomic gas. These polaritons can indeed form quantum materials. In fact, we have recently observed the formation of photon pairs in the Laughlin state, the paradigmatic example of a topologically ordered state which underlies the fractional quantum Hall effect in electron gases. We characterize these Laughlin “puddles” by measuring photonphoton correlations in both real space and angular momentum space, exemplifying the unique and powerful new perspective that manybody quantum optical systems can provide for understanding quantum matter.
Wenchao Xu, Massachusetts Institute of Technology (Vuletic group)
Strongly interacting photons in a quantum nonlinear medium
Manipulating individual photons is fascinating for building up alloptical quantum devices. In addition, it opens the possibility to realize novel quantum manybody states made with photons. Photons interact weakly in vacuum. However, via the combination of electromagnetically induced transparency and Rydberg atoms, strong mutual interactions between photons are realized. In this talk, I will present our group’s work on the fullcontrol of the effective interactions of individual photons. These interactions range from attraction, characterized by the formation of bound states of photons, to repulsive interactions, which leads to the observation of emergent spatial structure.
Peter McMahon, Cornell University
Plenary talk: A quantum annealer with fully programmable alltoall coupling via Floquet engineering
Quantum annealing is a promising approach to heuristically solving difficult combinatorial optimization problems. However, the connectivity limitations in current devices lead to an exponential degradation of performance on general problems. We propose an architecture for a quantum annealer that achieves full connectivity and full programmability while using a number of physical resources only linear in the number of spins. We do so by application of carefully engineered periodic modulations of oscillatorbased qubits, resulting in a Floquet Hamiltonian in which all the interactions are tunable; this flexibility comes at a cost of the coupling strengths between spins being smaller than they would be had the spins been directly coupled. Our proposal is wellsuited to implementation with superconducting circuits, and we give analytical and numerical evidence that fullyconnected, fullyprogrammable quantum annealers with 1000 qubits could be constructed with Josephson parametric oscillators having cavityphoton lifetimes of 100 microseconds, and other systemparameter values that are routinely achieved with current technology. Our approach could also have impact beyond quantum annealing, since it readily extends to bosonic quantum simulators and would allow the study of models with arbitrary connectivity between lattice sites.
Reference:
[1] T. Onodera*, E. Ng*, P.L. McMahon. arXiv:1907.05483
Adolfo del Campo, Donostia International Physics Center
Probing topological defect formation in a quantum annealer
When a quantum phase transition is crossed in finite time, the breakdown of adiabatic dynamics leads to the formation of topological defects. The average density of defects scales with the quench rate following a universal powerlaw predicted by the Kibble Zurek mechanism. The later provides useful heuristics for adiabatic quantum computation.
Physics beyond the KibbleZurek mechanism can be probed by characterizing the full counting statistics of topological defects. We argue that the distribution of the number of defects generally follows a Poisson binomial distribution with all cumulants exhibiting a universal powerlaw scaling with the quench rate.
As an exampled, we report the exact kink number distribution in the transversefield quantum Ising model. For this system, we test kink statistics in a DWave machine and show that the study of the kink number distribution can be used to benchmark the performance of a quantum processor.
References:
[1] A. del Campo, Phys. Rev. Lett. 121, 200601 (2018)
[2] JinMing Cui, F. J. GómezRuiz, YunFeng Huang, ChuanFeng Li, GuangCan Guo, A. del Campo, arXiv:1903.02145
[3] Y. Bando et al, in preparation
Diana Prado Lopes Aude Craik, Harvard University (Walsworth & Hu groups)
Using microwaves to study charge state in NV diamond
D. P. L. Aude Craik, P. Kehayias, A. S. Greenspon, X. Zhang, M. J. Turner, J. M. Schloss, E. Bauch, C. A. Hart, E. L. Hu, R. L. Walsworth
In its negativelycharged state, the nitrogen vacancy center in diamond (NV−) can be used as an opticallyreadout spin sensor of nanoscale magnetic fields with exciting applications ranging from imaging fields in living cells to extracting information about the formation of our solar system from paleomagnetic earlyEarth rocks. In contrast, the neutral charge state of the defect (NV0) offers no optical spin readout, producing only a spinindependent fluorescence background under the 532nm illumination typically used to read out NV− ensembles. Hence, to maximize sensitivity of ensemblebased magnetometers, we would like to understand how to produce diamond samples in which the NVs are predominantly negatively charged.
I will present a novel, microwavebased technique for determining charge state of nitrogen vacancy (NV) ensembles in diamond. The technique isolates, in situ, the spectral shape of the fluorescence contribution from neutral (NV0) and negativelycharged (NV−) defects, producing samplespecific results which take into account the effects of experimental conditions (such as illu mination intensity and wavelength) and material properties (such as local strain and electric fields). Using this technique, we study the physics of NV ionization from the negative charge state, identi fying previously unobserved ionization trends. Further, I will describe applications of the method to spectroscopy of other solidstate defects and to enhancement of magnetometry sensitivity.
Brendan Shields, University of Basel (Maletinsky group)
Imaging nanoscale antiferromagnetic order with a scanning nitrogenvacancy microscope
The nitrogenvacancy (NV) color center in diamond is an exceptional atomic scale system with a coherent electronic spin degree of freedom that can be initialized and measured optically. These properties make the NV attractive for applications ranging from quantum information to nanoscale metrology. Here, we use the NV electronic spin as a quantum scanning magnetic field probe to quantitatively image antiferromagnetic order in a granular thin film of Cr_{2}O_{3} [1]. By incorporating a single NV into the tip of a monolithic diamond atomic force microscopy probe and monitoring the Zeeman shift of the NV ground state electron spin, we image the stray magnetic field of a 200nm thick film of Cr_{2}O_{3} at nanoscale resolution, observing the formation of antiferromagnetic domains as the film transitions from paramagnetic to antiferromagnetic order. In combination with ZeroOffset Hall Magnetometry, we characterize key material properties of the Cr_{2}O_{3} sample, including local critical temperature and intergranular exchange.
Reference:
[1] Patrick Appel, Brendan J. Shields et al., Nanomagnetism of magnetoelectric granular thinfilm antiferromagnets, Nano Letters 19(3), 1682 1687 (2019).
Sara Campbell, University of California, Berkeley (Muller group)
Laserbased phase contrast transmission electron microscopy
Laserbased preparation, manipulation, and readout of the states of quantum particles has become a powerful research tool that has enabled the most precise measurements of time, fundamental constants, and electromagnetic fields. Laser control of free electrons can improve the detection of electrons' interaction with material objects, thereby advancing the exploration of matter on the atomic scale. For example, temporal modulation of electron waves with light has enabled the study of transient processes with attosecond resolution. In contrast, laserbased spatial shaping of the electron wave function has not yet been realized, even though it could be harnessed to probe radiationsensitive systems, such as biological macromolecules, at the standard quantum limit and beyond. Here, we demonstrate laser control of the spatial phase profile of the electron wave function and apply it to enhance the image contrast in transmission electron microscopy. We first realize an electron interferometer, using continuouswave laserinduced retardation to coherently split the electron beam, and capture TEM images of the light wave. We then demonstrate Zernike phase contrast by using the laser beam to shift the phase of the electron wave scattered by a specimen relative to the unscattered wave. Laserbased Zernike phase contrast will advance TEM studies of protein structure, cell organization, and complex materials. The versatile coherent control of free electrons demonstrated here paves the way towards quantumlimited detection and new imaging modalities.
Hari Nair, Cornell University (Schlom group)
Demystifying the growth of superconducting Sr_{2}RuO_{4} thin films
Sr_{2}RuO_{4} is an unconventional superconductor with potentially a spin triplet, oddparity topologically nontrivial p_{x} ± ip_{y} superconducting ground state. There are many reports of high purity single crystals of Sr_{2}RuO_{4} with a Tc of up to 1.5 K. To date, however, there are only four published reports of superconducting Sr_{2}RuO_{4} thin films. The three others than ours have Tcs significantly below 1.5 K. This relative paucity of superconducting thin films is likely due to the extreme sensitivity of the oddparity superconducting ground state in Sr_{2}RuO_{4} to disorder. Thin films provide a pathway for scalability, which is critical for potential practical applications of spintriplet superconductors such as qubits for topological quantum computing. Here, we outline and demonstrate a thermodynamic growth window to achieve repeatable growth of superconducting Sr_{2}RuO_{4} with higher Tc, up to 1.8 K. This Tc is higher than all prior thin films and even higher than unstrained Sr_{2}RuO_{4} single crystals.
Christie Chiu, Princeton University (Houck group)
Microscopic studies of the doped Hubbard model
Ultracold fermions in optical lattices offer new perspectives for studying the physics of strongly correlated materials. In the group of Markus Greiner, we use this experimental platform to implement the FermiHubbard model, a paradigmatic model thought to capture the physics of hightemperature superconductivity, the pseudogap, and other phenomena containing long standing open questions. The additional tool of quantum gas microscopy enables siteresolved readout and access to projections of the manybody wavefunction in the Fock basis. I report on our recent studies of doped antiferromagnets in two dimensions, where there is no universally agreedupon mechanism describing the interplay between hole motion and antiferromagnetic order.
Adèle Hilico, Laboratory for Photonics, Numbers and Nanosciences  OLGS (Institute of Optics Graduate School) / CRNS / University of Bordeaux
Ultracold atoms in a nanostructured optical lattice
Due to their number of controllable parameters, cold atoms in lattices have been used as quantum simulators. In the current state of the art, the experimental techniques use optical lattices in the farfield, limiting the lattice spacing to a half wavelength. Such large spacing limits the relevant energy scale (tunneling, interaction) which makes it difficult to enter deeply into magnetic quantum correlations regimes or strongly correlated phases. Our project aims at reducing the lattice period to bridge the gap between solid state (0.1 nm) and far field lattice (500 nm). For this we develop a hybrid quantum system of degenerate gaz in close proximity with a nanostructured surface generating subwavelength lattice potentials. In this presentation, I will discuss theoretical results on a novel trapping scheme to compensate the Casimir Polder force at very short distance and experimental evidences of a subwavelength imaging technic for cold atoms.
Gretchen Campbell, Joint Quantum Institute, NIST and UMD College Park
Plenary talk: A Supersonically expanding BEC: An expanding universe in the lab
The massive scale of the universe makes the experimental study of cosmological inflation difficult. This has led to an interest in developing analogous systems using table top experiments. One possible system for such simulations is an expanding atomic quantum gas. In recent experiments, we have modeled the basic features of an expanding universe by drawing parallels with an expanding ringshaped Bose Einstein Condensate (BEC). The Bose Einstein condensate can be thought of as a vacuum for phonons, and used in analogy to the quantum field proposed to have driven the expansion of the early universe. Here, while the ringshaped BEC serves as the background vacuum, the phonons are the analogue to photons in the expanding universe. We have studied the dynamics of a supersonically expanding ring shaped BEC both experimentally and theoretically. I will present our results and discuss prospects for future experiments.
Brynle Barrett, iXblue
Hybrid matterwave inertial sensors for mobile sensing applications
B. Barrett^{1}^{,2,} P. Cheiney^{1}^{,2}, S. Templier^{1}^{,2}, B. Gouraud^{1}^{,2}, B. Battelier^{2}, H. Porte^{1}, F. Napolitano^{1}, and P. Bouyer^{2}
Highsensitivity, lowdrift inertial sensors based on coldatom interferometry are poised to revolutionize the field of inertial guidance and navigation, yet many challenges still remain. For instance, due to the slow data rate of atom interferometers and the large bias drifts of mechanical accelerometers, hybridization schemes will almost certainly be necessary [1,2,3]. We present recent results on the hybridization of classical and quantum accelerometers in a simulated navigation environment exhibiting strong variations in temperature and vibration noise. By correlating the output of each sensor, and utilizing a novel realtime system, we are able to lock the classical accelerometer to the quantum interference fringe [4]. This feedback loop simultaneously rejects motioninduced frequency and phase shifts on the quantum accelerometer, and corrects for bias drifts on the classical one—enabling us to achieve submicrog precision after a few seconds of integration. This system paves the way toward a fullyhybridized multiaxis inertial measurement unit [5] compatible with mobile sensing applications.
Figure 1. Performance of our hybrid quantum accelerometer in a “noisy” environment. (a) Classical accelerometer bias as the temperature is varied over 4.5 hours. (b) Bias error signal from the hybrid accelerometer lock. (c) Allan deviation of the bias error. Hybrid sensor characteristics: interrogation time T = 20 ms, cycle time 1.2 s, bias sensitivity ∼ 1.6 μg/√Hz.
References:
[1] J. Lautier et al, Appl. Phys. Lett. 105, 144102 (2014).
[2] P. Cheiney et al, Phys. Rev. Applied 10, 034030 (2018).
[3] Y. Bidel et al, Nat. Commun. 9, 627 (2018).
[4] P. Cheiney et al, in Proc. of IEEE International Symposium on Inertial Sensors and Systems, Naples, USA (2019).
[5] B. Barrett et al, Phys. Rev. Lett. 122, 043604 (2019).
Crystal Noel, Joint Quantum Institute, University of Maryland (Monroe group)
Demonstration of a large, individually addressable trapped ion quantum information processor and the study of electricfield noise from thermallyactivated fluctuators potentially limiting future performance
C. Noel^{1} , L. Egan^{1} , A. Risinger^{1} , D. Zhu^{1} , M. Goldman^{1} , M. Cetina^{1} , C. Monroe^{1}
^{1}Joint Quantum Institute Department of Physics, University of Maryland, College Park 20742
C. Noel^{2}^{} , M. BerlinUdi^{2} , C. Matthiesen^{2} , J. Yu^{2} , Y. Zhou^{2} , V. Lordi^{3} , and H. Häffner^{2}
^{2}Department of Physics, University of California, Berkeley, California 94720, USA
^{3}Lawrence Livermore National Laboratory, Livermore, California 94551, USA
Under the IARPA LogiQ program, in a collaboration between universities and industrial partners, we have constructed a complex ionbased quantum processor with the goal of realizing a logical quantum bit. I will briefly report on the performance of our firstgeneration integrated system, including fidelities of singlequbit and twoqubit gates, crosstalk, operation with long ion chains, and syndrome readout.
One of the factors limiting performance of a large trapped ion quantum processor is excess electricfield noise that causes ion heating. I will report results from electricfield noise studies performed at high temperatures, in which we observe a nontrivial temperature dependence with the noise amplitude at 1 MHz frequency saturating around 500 K. This behavior can be explained by considering noise from a distribution of thermally activated twolevel fluctuators with activation energies between 0.35 and 0.65 eV. Processes in this energy range may be relevant to understanding electricfield noise in ion traps; for example, defect motion in the solid state and surface adsorbate binding energies. The study of these processes may aid in identification of the origin of excess electricfield noise in ion traps.
*This work is supported by the ARO with funding from the IARPA LogiQ program, the NSF Practical FullyConnected Quantum Computer program, the DOE program on Quantum Computing in Chemical and Material Sciences, the AFOSR MURI on Quantum Measurement and Verification, and the AFOSR MURI on Interactive Quantum Computation and Communication Protocols.
Sara Mouradian, University of California, Berkeley (Haeffner group)
Increasing connectivity in complex quantum systems
Engineered quantum systems are often limited to a handful of nodes with limited  and often immutable  connectivity. Here I will introduce two quantum systems that promise to overcome these limitations  the negatively charged nitrogen vacancy center in diamond and trapped atomic ions. I will compare their unique benefits and drawbacks as building blocks for complex quantum systems and discuss the engineering challenges that must be overcome to realize their full potential.
Christian Kraglund Andersen, ETH Zürich (Wallraff group)
Designing and operating superconducting circuits for quantum error correction
In recent years, quantum computing has seen a surge of progress both theoretically and experimentally. However, the longterm success of quantum computers relies on the ability to perform faulttolerant quantum computations using quantum error correction. In this approach, errors are detected through the repeated measurement of multiqubit parity operators and corrected using feedback operations conditioned on the measurement outcomes. In this talk, I will discuss recent progress towards demonstrating the feasibility of quantum error correction with superconducting qubits. I will show an experiment using of an ancillary qubit to repeatedly measure the ZZ and XX parity operators of two data qubits and to thereby project their joint state into the respective parity subspaces. By applying feedback operations conditioned on the outcomes of individual parity measurements, we demonstrate the realtime stabilization of a Bell state with a fidelity of F≈74% in up to 12 cycles of the feedback loop [1]. The ability to stabilize parity over multiple feedback rounds with no reduction in fidelity provides strong evidence for the feasibility of executing stabilizer codes on timescales much longer than the intrinsic coherence times of the constituent qubits. I finally discuss our current efforts in scaling up to larger errorcorrection schemes.
Reference:
[1] C.K. Andersen, et al., arXiv:1902.06946 (2019)
ChiaoHsuan Wang, University of Chicago (Jiang group)
Autonomous quantum error correction by Hamiltonian and dissipation engineering
Autonomous quantum error correction (AutoQEC) utilizes an engineered coupling between a quantum system and a dissipative ancilla to protect encoded logical quantum information against physical errors. It has been recently shown that if a code space satisfies the KnillLaflamme condition for the Markovian error generators (plus identity operator), there exists a set of engineered dissipative jump operators such that the logical error probability vanishes in the limit of infinitely strong engineered dissipation.
Here we apply the general theory of AutoQEC to bosonic errorcorrecting codes, and propose an explicit Hamiltonian and dissipation engineering method to protect the encoded information against photon loss errors. Specifically, we demonstrate a scheme for autonomously stabilizing cavity cat states in the presence of photon loss, which admits potential experimental implementations in circuit quantum electrodynamics systems.
Maritn Sandberg, IBM
Plenary talk: Superconducting quantum circuits as a path for quantum computing
Quantum computing has received a lot of attention because of its potential of solving computational problems that are considered unsolvable on classical computers. The field has for a long time been driven by academic research but in recent years the interest from industry has gained a lot of momentum. One advantage of a largescale industry research effort is that several very diverse problems can be addressed. In order for quantum computing to become reality a multitude of problems needs to be solved; hardware, software, electronics and theory all needs to come together as one system. Having expertise in all these areas within a single organization can be a great strength. IBM has pioneered the efforts of building quantum computing systems fully accessible to the public through the could. IBM is currently operating 10 quantum processors both for public and commercial use. In this talk I will give an overview of superconducting quantum circuits, with focus on the challenges we are facing as we scale up to larger circuits. In addition, I will discuss aspects of being an industrybased researcher in this fastmoving field.
Zlatko Minev, IBM and Yale University (Devoret group)
To catch and reverse a quantum jump midflight
In quantum physics, measurements can fundamentally yield discrete and random results. Emblematic of this feature is Bohr’s 1913 proposal of quantum jumps between two discrete energy levels of an atom. Experimentally, quantum jumps were first observed in an atomic ion driven by a weak deterministic force while under strong continuous energy measurement. The times at which the discontinuous jump transitions occur are reputed to be fundamentally unpredictable. Despite the nondeterministic character of quantum physics, is it possible to know if a quantum jump is about to occur? Here we answer this question affirmatively: we experimentally demonstrate that the jump from the ground state to an excited state of a superconducting artificial threelevel atom can be tracked as it follows a predictable ‘flight’, by monitoring the population of an auxiliary energy level coupled to the ground state. The experimental results demonstrate that the evolution of each completed jump is continuous, coherent and deterministic. We exploit these features, using realtime monitoring and feedback, to catch and reverse quantum jumps midflight—thus deterministically preventing their completion. Our findings, which agree with theoretical predictions essentially without adjustable parameters, support the modern quantum trajectory theory and should provide new ground for the exploration of realtime intervention techniques in the control of quantum systems, such as the early detection of error syndromes in quantum error correction [1].
Reference:
[1] Nature volume 570, pages 200–204 (2019)
Alexandre CooperRoy, University of Waterloo
Plenary talk: An atomic array optical clock with singleatom readout
Reconfigurable arrays of neutral atoms excited to Rydberg states provide a versatile experimental platform to study manybody quantum dynamics in various geometries with tunable interactions and microscopic control. I will first describe our ongoing effort at the University of Waterloo to deliver such quantum simulators to early adopters. I will then describe our recent work from Caltech on operating atomic array optical clocks using bosonic strontium atoms in tweezer arrays [13].
References:
[1] A. Cooper et al., PRX 8, 041055 (2018)
[2] J. P. Covey et al., PRL 122, 173201 (2019)
[3] I. S. Madjarov et al., arXiv:1908.05619 (2019)
Ahmed Omran, Harvard University (Lukin group)
Controlling entanglement in Rydberg atom arrays
Programmable arrays of neutral atoms provide an exciting avenue for quantum sim ulations and quantum information processing. We employ a 1D array of neutral atoms coupled to Rydberg states to simulate a transversefield Ising model with longrange in teractions. This system can undergo quantum phase transitions breaking different spatial symmetries, which we can study in detail.
I will describe a method we developed to rapidly prepare GreenbergerHorneZeilinger (GHZ) states with up to 20 atoms using site resolved engineering of the manybody spectrum and optimal control of the quantum manybody system. Furthermore, our local addressing enables the demonstration of entanglement distribution to distant sites in the array and highfidelity quantum logic gates. The ability to reliably produce and manipulate entanglement in neutral atom systems opens up a new route towards scalable quantum processors.
Aziza Suleymanzade, University of Chicago (Simon & Schuster groups)
Towards strongly interacting mmwave and optical photons in hybrid cavityQED experiments with Rydberg atoms
The circuit and cavityQED systems are essential tools for exploring quantum phenomena both in the optical and microwave regimes, while millimeterwaves remain relatively unexplored. As a quantum platform, mmwave frequencies offer an abundance of 100GHz resonances in commonly used emitters, single photon resolution at temperatures higher that 1K and unusual length scale for making devices both in the far and near field regimes. In my talk, I will, first, introduce mmwave frequencies as a potential band for quantum computation at high cryogenic temperatures. Then, I will outline our progress towards a hybrid experimental system for creating strong interactions between single optical and mmwave photons with Rydberg atoms as the interface. I will present our recent results, including the realization of the crossed highQ mmwave and optical cavity at 4K and the observation of the Vacuum Rabi splitting. Finally, I will briefly mention our efforts in mmwave devices beyond Rydbergcavity QED systems, such as 2D nonlinear resonators, 100 GHz parametric amplifier and other highQ devices in this frequency band.
Boris Braverman, University of Ottawa (Boyd group)
Nearunitary spin squeezing with Ytterbium
State of the art atomic sensors operate near the standard quantum limit (SQL) of projection noise, where the precision scales as the square root of the particle number. Overcoming this limit by using atomatom entanglement such as spin squeezing is a major goal in quantum metrology.
Spin squeezing can be realized with the techniques of cavity quantum electrodynamics (cQED) by coupling an atomic ensemble to a highfinesse optical resonator. The resulting collective atomlight interaction allows for both measurement and cavity feedback squeezing. These methods for producing spin squeezing are typically nonunitary and generate more antisqueezing than the minimum prescribed by the uncertainty principle, due to a residual entanglement between the atomic ensemble and probing photons. We find that nonunitarity significantly lowers the potential metrological gain from squeezing in atomic clocks and other quantum sensors.
We couple an ensemble of approximately 1000 Yb171 atoms to a high finesse asymmetric micromirror cavity with singleatom cooperativity of 1.8. A laser pulse induces an effective oneaxis twisting Hamiltonian for the atoms, producing the desired squeezed spin state (SSS). We detune the probing light from atomic and cavity resonance by several linewidths to limit the undesirable entanglement between atoms and light.
We characterize the produced SSSs by state tomography, measuring the zquadrature variance after a rotation by a variable angle. For moderate normal ized atomatom interaction strength, we observe states with a nearly equal level of noise reduction and enhancement, confirming the production of a nearunitary spin squeezed state. The observed spin noise suppression and metrological gain are limited by the state readout to 9.4(4) dB and 6.5(4) dB, respectively, while the generated states offer a spin noise suppression of 15.9(6) dB and a metrological gain of 12.9(6) dB over the standard quantum limit (SQL), limited by the curvature of the Bloch sphere. When requiring the squeezing process to be within 30% of unitarity, we demonstrate an interferometer that improves the averaging time over the SQL by a factor of 3.7(2).
This experimental platform will allow for the creation of quantum states with metrologically useful entanglement on the clock transition of Yb171.