Quantum Innovators 2024

Hilbert space dimension is a key resource for quantum information processing. A large Hilbert space is not only an essential requirement for quantum error correction, but it can also be advantageous for realizing gates and algorithms more efficiently. There has thus been considerable experimental effort in recent years to develop quantum computing platforms using qudits (d-dimensional quantum systems with d>2) as the fundamental unit of quantum information.

Just as with qubits, quantum error correction of these qudits will be necessary in the long run, but to date error correction of logical qudits has not been demonstrated experimentally. Here we report the experimental realization of an error-corrected logical qutrit (d=3) and ququart (d=4) by employing the Gottesman-Kitaev-Preskill (GKP) bosonic code. Using a reinforcement learning agent, we optimize the GKP qutrit (ququart) as a ternary (quaternary) quantum memory and achieve beyond break-even error correction with a gain of 1.82 +/- 0.03 (1.87 +/- 0.03). This work represents a new way of leveraging the large Hilbert space of a harmonic oscillator for hardware-efficient quantum error correction.

Benjamin Brock is a postdoctoral researcher in Prof. Michel Devoret’s lab at Yale University.  His experimental research focuses on new strategies for bosonic quantum error correction with superconducting circuits.  He received his PhD in Physics from Dartmouth College in 2021 for his work on the cavity-embedded Cooper pair transistor: an ultrasensitive electrometer that can operate at the single-photon level.

We will discuss experimental and theoretical progress towards large-scale error-corrected quantum computation. First, we report recent advances in quantum information processing using dynamically reconfigurable arrays of neutral atoms.  Using this logical processor with various types of error-correcting codes, we demonstrate that we can improve logical two-qubit gates by increasing code distance, create logical GHZ states, and perform computationally complex quantum simulation of information scrambling.

In performing such circuits, we observe that the performance can be substantially improved by accounting for error propagation during transversal logical entangling gates and decoding the logical qubits jointly. We find that by using this correlated decoding technique and correctly handling feedforward operations, the number of noisy syndrome extraction rounds in universal quantum computation can be reduced from O(d) to O(1), where d is the code distance. These techniques result in new theories of fault-tolerance and in practical reductions to the cost of large-scale computation by over an order of magnitude.

Maddie Cain is a 6th year PhD student in theoretical physics working in Professor Mikhail Lukin’s group at Harvard University. Her research explores the theory of quantum information processing, including topics spanning quantum algorithms and quantum error correction. She is interested in developing resource-efficient, fault-tolerant compilations of useful quantum algorithms, with a focus on reducing the overhead of error correction in hardware. She closely collaborates with experimentalists in the Lukin group developing neutral atom arrays.

Single spins associated with nitrogen-vacancy (NV) defects in diamond have emerged as a promising and versatile experimental platform for quantum information processing. They can be used as nodes in optically connected quantum networks, as sensors for magnetic imaging with sub-micron resolution, for detecting and engineering quantum states of nano-mechanical oscillators, and even as probes in biological systems. Our group has demonstrated improvements to dynamic range and sensitivity of magnetometry using phase estimation algorithms, and carried out electron paramagnetic resonance detection and spectroscopy of single Cu ions on the diamond surface. I will also discuss a unique system in our lab where we magnetically trap and laser cool diamond microcrystals under high-vacuum room-temperature conditions for the first time, and discuss the path forward to observing quantum superpositions of macroscopically separated motional states.

Gurudev Dutt is an Associate Professor in the Department of Physics and Astronomy at University of Pittsburgh. He received his Ph.D. in Physics and M. S. in Elect. Engr. from University of Michigan in 2004. Subsequently he was a Research Associate at Harvard University till 2009. PI Dutt has won numerous awards including the Alfred P. Sloan Research Fellow, Kavli Fellow of the National Academy of Sciences, NSF CAREER, and DOE Early Career awards. His areas of expertise include quantum information, solid-state quantum optics, and microwave and optical spectroscopy of solid-state materials and semiconductor nanostructures.

A future quantum internet will enable fundamentally new applications such as secure communication, distributed quantum computing, and quantum-enhanced metrology and sensing [1]. Recently, a rudimentary quantum network hosting multiple nodes has been demonstrated [2]. However, challenges in scaling up this network motivate the further development of different qubit platforms, such as trapped ions and atoms, and solid-state emitters.

Qubits based on single rare-earth ions (REIs) are promising candidates; they exhibit good qubit and optical properties due to their internal atomic structure. Auxiliary qubits are offered in the form of nuclear spins present in the host material. In the Faraon lab, we have recently established distributed entanglement of two and subsequently three qubits, across two nodes [3]. 

In this seminar, I will discuss ongoing projects: extending the auxiliary qubit control and understanding the noise processes limiting the REI qubit coherence. Additionally, I will outline the future research directions for my lab in Delft: exploring different host materials, developing robust and scalable nanofabrication methods for more elaborate REI-based devices, and investigating two-REI-qubit interactions. 

[1 ] S. Wehner, D. Elkouss, and R. Hanson, “Quantum internet: A vision for the road ahead,” Science, vol. 362, no. 6412 (2018)
[2] M. Pompili, S. L. N. Hermans, S. Baier, et al., “Realization of a multinode quantum network of remote solid-state qubits,” Science, vol. 372, no. 6539, pp. 259–264 (2021) 
[3] A. Ruskuc, C.-J. Wu, E. Green, S. L. N. Hermans, J. Choi, and A. Faraon, “Scalable Multipartite Entanglement of Remote Rare-earth Ion Qubits,” arXiv:2402.16224 (2024) 

Dr. Sophie Hermans is an incoming Assistant Professor/Group leader at the research institute QuTech, at the Delft University of Technology in the Netherlands. Starting in May 2025, her research team will focus on rare-earth ions doped in host materials for quantum networking applications. 
Hermans is a postdoctoral scholar in the group of Prof. Faraon at the California Institute of Technology (Caltech). She works on utilizing single ytterbium ions doped in host crystals for quantum networks and quantum sensing purposes. 
Before moving to the United States, Hermans received her PhD degree (with distinction) at the Delft University of Technology in the Netherlands. In the team of Prof. Hanson, she developed the world’s first multi-node quantum network, based on nitrogen-vacancy centers in diamond. Additionally, she worked on quantum frequency conversion of single photons from the visible to the telecom regime.
Hermans was elected one of the Faces of Science by the Dutch Royal Academy of Science and serves as an ambassador of science through outreach and science communication. Upon joining Caltech, she was awarded the AWS Quantum Fellowship. In 2024, Hermans received the Delft Technology Fellowship to support the start of her research program.

Dispersive readout - which consists of encoding a qubit’s state in the phase of light interaction with the qubit - is the de facto method for measuring qubits in circuit quantum electrodynamics.  Despite the high measurement fidelity that can now be reached, this technique is plagued by a loss of its quantum nondemolotion (QND) character and a decrease in fidelity with increased measurement strength. Although this is a widely-observed phenomena, there is no clear parameter dependence for the onset of non-QNDness in experiments. Understanding the nature of this phenomena to eventually implement mitigation strategies is a necessary step in the quest to build a superconducting circuit-based fault-tolerant quantum computer.

In this talk, I will elucidate the nature of this dynamical process, which we refer to as transmon ionization. We develop a comprehensive framework which provides a unified physical picture of the origin of transmon ionization. This framework consists of three complementary levels of descriptions: a fully quantized transmon-resonator model, a semiclassical model where the resonator is treated as a classical drive on the transmon, and a fully classical model. Crucially, all three approaches lead to similar predictions. This framework identifies the multiphoton resonances responsible for transmon ionization. It also allows to efficiently compute numerical estimates of the photon number threshold for ionization, which are in remarkable agreement with recent experimental results. 

Alexander is a postdoc at the Université de Sherbrooke's Institut Quantique in the group of Alexandre Blais. His research involves understanding the limitations and improving the performance of readout in superconducting quantum bits. Before returning to his native Canada, he obtained a PhD from the University of Chicago under the supervision of Aash Clerk. There, he studied quantum non-reciprocity and non-Hermitian quantum physics. Alexander also holds a Master's from McGill and a Bachelor's from the University of Ottawa.

Today’s most advanced ion trap quantum computers have at most one qubit per ion, each defined within the ground state manifold. Additional non-qubit ions provide sympathetic cooling to keep the computational ions cold enough to perform many rounds of high-fidelity coherent operations. Typically, the two subsets of ions must be different species to prevent cooling light from disturbing the computation. To bypass this added system complexity, we can instead promote our computational ions to a long-lived excited state that is isolated from the ground-state cooling transitions. This promotion also enables new features including erasure conversion and projective state preparation.

We will discuss two recent efforts to develop this architecture: entangling gates between metastable qubits and sympathetic cooling of a metastable ion by a ground-state ion. Finally, we will take advantage of the larger metastable manifold to explore high-fidelity qudit control.

Jameson is a postdoctoral scholar at the University of Oregon, where he works with David Allcock and Dave Wineland. He received his PhD from Duke University and his MSc from the University of Maryland, College Park, both of which were supervised by Chris Monroe. During this time, he worked on quantum networking with trapped ions, helping to demonstrate the best photon-mediated entanglement rate and fidelity to-date on this platform.

Rydberg atoms can interact strongly with each other, which is the basis for their applications in quantum computing and simulation. The very same property allows them to also act as exceptional sensors. In my talk I will describe our story of developing new microwave and mm-wave sensors based on Rydberg atoms. In particular, I will describe two unconventional approaches we have undertaken.

First, in a hot atom system we employ a complex loop of transitions to achieve sensing via transduction, which has allowed us to receive thermal radiation. In the most recent experiment, with the goal to reach more fundamental limits than available in hot-atom systems, we tried to use a cold-atom system along with the most-standard Ramsey-sequence protocol to combine optimized signal collection with photon-counting detection. Here, we have observed atomic interaction effects which suggest a collective gain can be achieved in the understanding of quantum metrology, even though the interaction caused collective dephasing.

Overall, the exploration of Rydberg sensing stems from the simple observation of exceptional sensitivity to microwaves, but rapidly leads to both fascinating applications for example in space technologies, as well as interesting fundamental metrological questions.

Dr Michał Parniak is a group leader in the Centre for Quantum Optical Technologies QOT and assistant professor at Optics Division, Institute of Experimental Physics, Faculty of Physics. His research interests cover a range of topics in quantum optics, such as single photon detection, optical quantum information processing and communication, atomic ensembles, nonlinear optics and quantum optomechanics. Within QOT he develops experimental implementations of quantum protocols designed by the theory groups and maintains close experimental collaboration with prof. Wojciech Wasilewski (QOT) and prof. Eugene Polzik (Niels Bohr Institute, University of Copenhagen). His most significant scientific achievements include demonstrating superresolution in imaging using two-photon interference, demonstrating the record-breaking quantum memory in terms of its capacity, and demonstration of the first entanglement of macroscopic spin and mechanical systems. In most recent times his team explores fundamentals as well as applications of Rydberg atom sensors and microwave and THz radiation, with applications in space technologies.

Quantum sensors hold promise for improved sensing of time, electromagnetic fields, and forces; however, the inherent probabilistic nature of quantum mechanics introduces uncertainty that can limit sensor precision. We can hope to overcome this uncertainty by engineering entanglement to create correlated behavior in atomic systems. Unfortunately, in practice, introducing and controlling these correlations is limited by the local nature of interactions on many promising sensing platforms, including optical tweezer clocks and solid-state magnetometers.

In this talk, I will discuss how we can use temporal control over local Rydberg interactions to extend interaction coherence times and minimize atomic loss in an array of atomic ensembles. With these improvements, we generate metrologically useful entanglement across several spatially separated ensembles in parallel. This work demonstrates the power of dynamical control to enhance and expand our understanding of entanglement in atomic systems. I will also discuss future prospects in quantum sensing, simulation, and optimization on a new experiment utilizing a Rydberg atom array in an optical cavity.

Shankari Rajagopal is an atomic physics experimentalist interested in driven systems, measurement, and interactive quantum dynamics. Over the past five years, she has been a postdoctoral research scholar and Stanford Science Fellow at Stanford University in the group of Monika Schleier-Smith, working on improved coherence and uses of dressed Rydberg interactions in atomic tweezer arrays. She completed her graduate studies in 2019 at the University of California Santa Barbara in the lab of David Weld, studying non-equilibrium dynamics with Bose-Einstein condensates in lattices. Shankari is particularly interested in applications of driven quantum systems and measurement feedback to quantum simulation, optimization, and metrology. Starting in January 2025, she will be an assistant professor at the University of Michigan Department of Physics.

The negatively charged tin-vacancy (SnV) center in diamond is a desirable spin/photon interface for use in quantum networking. In particular, the SnV has bright emission, reduced sensitivity to electrical noise, and long spin coherence at elevated temperature. Here, we develop the SnV as a spin qubit including demonstrations of both high-fidelity spin control, and, single-shot spin readout. In the process, we gain understanding of the SnV including study of its Hamiltonian, and, noise affecting its spin and optical properties.

Finally, we use an SnV spin as a tool to study weak quantum measurement in solid-state atomic systems. These results pave the way for SnV spins to be used as a building block for future quantum technologies including quantum networking, sensing, and information processing.

Eric I. Rosenthal is an IC Postdoctoral Scholar from the Vuckovic group at Stanford University. Previously, Eric completed his B.A. in physics at the University of Pennsylvania, and his Ph.D. in physics in Konrad Lehnert’s group at JILA and the University of Colorado, Boulder. After undergraduate research spanning from carbon nanotubes to photonic crystals in deep sea fish skin, Eric was passionate about a career in scientific research and became excited about quantum physics. Eric came to Konrad Lehnert’s group at JILA, the University of Colorado, Boulder, where his research focused on technology related to superconducting qubits – a promising platform for building scalable quantum computers. Specifically, Eric was interested in how to better measure superconducting qubits, which lead to developing superconducting integrated circuits including fast, high-bandwidth switches, circulators, and parametric amplifier technology to be used in conjunction with qubit readout. His Ph.D. culminated with a chip-scale detector for superconducting qubit measurement that had favorable performance compared to conventional approaches. For a postdoc, Eric remained passionate about quantum science but wanted to expand his knowledge and especially, to learn optical measurement techniques. This led him to Jelena Vuckovic’s group at Stanford to study semiconductor spin centers. Specifically, Eric’s postdoc research has focused on the tin-vacancy center in diamond, a promising platform for scaling quantum networks. Eric developed understanding of this qubit including novel demonstrations of coherent microwave spin control and single-shot readout, and subsequent scientific exploration of this center’s spin properties. This work enables use of the tin-vacancy qubit for the next generation of quantum networks. In the future, Eric is interested in developing hybrid quantum technology involving both superconducting and solid-state systems. These platforms have complementing strengths: fast operations for superconducting qubits, and long spin lifetimes and an optical interface for solid-state spin qubits. How to best overcome the materials-related challenges of combining these systems is an open and fascinating question. While engaged in research, Eric will continue his deep commitment to teaching and mentorship spanning from outreach activities, working with undergraduate students, and mentoring Ph.D. students and postdocs. Eric hopes to do this as the future P.I. of a research laboratory.

The field of quantum computing has seen an explosion in research efforts and interest in both academia and industry within recent years. While there are several candidate platforms for quantum computing, superconducting circuits are one of the most promising platforms due to their scalability, design flexibility and stableness to the environment. However, single qubit coherence remains a major limiting factor in building scalable processors based on superconducting qubits. Recently, tantalum-based superconducting qubits have been discovered to enable long lifetimes and coherence times because of their chemical robustness and well-behaved surface oxide.

In this talk, I will first talk about our recent work to characterize the dominant sources of loss in state-of-the-art tantalum superconducting circuits. Using systematic measurements of tantalum resonators, we find the dominant source of loss at qubit operating conditions is from two-level systems (TLSs) present at material interfaces and surfaces, including TLSs residing in the amorphous native oxide layer.

In the second part, I will discuss our strategy for avoiding oxide formation by encapsulating the tantalum with noble metals that do not form native oxide. Microwave loss measurements of superconducting resonators reveal that the noble metal is proximitized, with a superconducting gap over 80% of the bare tantalum at thicknesses where the oxide is fully suppressed, suggesting it as a promising strategy for eliminating surface oxide TLS loss in superconducting qubits.

Nana is a PQI postdoctoral research fellow at Princeton University. She graduated with a B.A. in Physics from University of California, Berkeley and M.S. and Ph.D. in Physics from Princeton University. During her Ph.D., she used scanning tunneling microscopy with vector magnetic fields to study various quantum topological material systems with Prof. M. Zahid Hasan. In her current study, she works with Prof. Nathalie de Leon and Prof. Andrew Houck on exploring material systems to improve coherence of superconducting qubits.

Giant atoms are quantum emitters that can couple to light at several discrete points, which may be spaced wavelengths apart. They receive the name ""giant"" because they break the dipole approximation, i.e., the assumption that atoms are ""small"" compared to the wavelength of the light they interact with. Among their many remarkable properties, giant atoms have the ability to interact through a common bath without decohering, which is promising for applications in quantum computing and quantum simulation.

In this talk, I will focus on how giant atoms behave when coupled to 1D and 2D structured environments. These environments have an energy spectrum characterized by finite bands and band gaps, which affect atomic dynamics beyond the Markovian regime. In particular, I will show how giant atoms can avoid decoherence, and how their dynamics compare to that of small atoms and continuous waveguides.

I am a PhD student in Anton Frisk Kockum’s group, at Chalmers University of Technology in Sweden. My research is in quantum optics with giant atoms, and I study how these quantum emitters behave when coupled to unconventional environments. In particular, I focus on how giant atoms interact with other emitters through the environment and how they can avoid decoherence. At a broader level, I am interested in harnessing giant-atom interactions for applications in quantum simulation and quantum computing. Before my PhD, I got a double BSc in physics and mathematics at the Autonomous University of Barcelona (Spain), and a MSc in engineering physics at KTH Royal Institute of Technology (Sweden).

Silicon is notably the most advanced platform for scalable integrated photonics and electronics. In the realm of quantum science, silicon is also known to be a high-quality host material for solid-state spins. However, an efficient spin-photon interface within silicon is still under development. In this talk, I will present an integrated photonics platform with color centers in silicon operating at the telecom O-band. Specifically, we focus on T centers – a type of carbon-hydrogen defect – featuring long electron and nuclear spin coherence times. For an efficient photonic interface, we incorporate T centers into photonic crystal cavities (PCCs) and achieve a tenfold lifetime reduction due to Purcell enhancement.

Furthermore, using a novel architecture of PCC arrays coupled to a bus waveguide, we perform wavelength-multiplexed emission from T centers in different cavities. We also probe signatures of waveguide-mediated interactions between cavities based on T center emissions.

Lastly, I will discuss our progress in coherent microwave spin control and optical readout for waveguide-integrated individual T centers. These advances represent the first steps towards building a scalable quantum network based on silicon photonics.

Xueyue (Sherry) Zhang will be an Assistant Professor in the Department of Applied Physics and Applied Mathematics at Columbia University starting in January 2025. She earned her B.Eng. from Tsinghua University in 2017 and her Ph.D. in Applied Physics from Caltech in 2023. Dr. Zhang then joined UC Berkeley as a Miller Postdoctoral Fellow in EECS and Physics. Dr. Zhang's research interests include superconducting circuits, quantum many-body simulations, and color centers in silicon. Her work has earned her several awards, including the Miller Research Fellowship, the Boeing Quantum Creator Prize, and the Rising Star in Physics Award in 2023.

Talk title, abstract and bio to be updated

Talk title, abstract and bio to be updated