# SQuInT 2021 Program

#### SESSION 1: Talks at Zoom

10:00am-10:30amPaola Cappellaro, Massachusetts Institute of Technology
Quantum simulation with coherent engineering of synthetic dimensions
Abstract The high controllability of engineered qubit systems can be leveraged to explore exotic condensed matter systems by simulating synthetic topological phases of matters. Observation of novel effects can be achieved even in small quantum systems by exploiting their periodic driving, which can mimic the properties of spatially periodic materials and elucidate their symmetry and topological features. Two challenges have so-far prevented such exploration, the lack of an experimentally accessible characterization protocol and of strong-enough driving fields. Here I'll show how to overcome both challenges to achieve the first experimental study of dynamical symmetries and the observation of symmetry-protected selection rules -- and their breaking. I will further show how these methods can be used to synthesize and characterize a tensor monopole in the 4D parameter space described by the spin degrees of freedom of a single solid-state defect in diamond. These results demonstrate the power of coherent control and Floquet engineering for quantum simulation.
10:30am-10:50amAleksander Kubica, Amazon Web Services Center for Quantum Computing
Single-shot error correction and universal fault-tolerant computation with the 3D subsystem toric code
Abstract We introduce a new topological quantum code, the three-dimensional subsystem toric code (3D STC), which is a generalization of the stabilizer toric code. The 3D STC can be realized by measuring geometrically-local parity checks of weight at most three on the cubic lattice with open boundary conditions. We prove that single-shot quantum error correction (QEC) is possible with the 3D STC, i.e., one round of local parity-check measurements suffices to perform reliable QEC even in the presence of measurement errors. We also explain how to fault-tolerantly implement a universal gate set in the 3D STC without state distillation. Lastly, we propose an efficient single-shot QEC strategy for the 3D STC and investigate its performance. In particular, we numerically estimate the resulting storage threshold against independent bit-flip, phase-flip and measurement errors to be above 1%. Such a high threshold together with local parity-check measurements of small weight make the 3D STC particularly appealing for realizing fault-tolerant quantum computing.
10:50am-11:10amBharath Hebbe Madhusudhana, Max-Planck-Institute for Quantum Optics
Benchmarking an efficient approximate method for localized 1D Fermi-Hubbard systems on a quantum simulator
Abstract Identifying and understanding the applications of NISQ-era quantum simulators and quantum computers is a topical problem. Quantum many-body physics embodies a unique set of problems that are both computationally hard and physically pertinent and are therefore apt for applications of NISQ devices. While state-of-the art neutral atom quantum simulators have made remarkable progress in studying many-body dynamics, they are noisy and limited in the variability of initial state and the observables that can be measured. Here we show that despite these limitations, quantum simulators can be used to develop new numerical techniques to solve for the dynamics of many-body systems in regimes that are practically inaccessible to established numerical techniques [1]. Considering localized 1D Fermi-Hubbard systems, we use an approximation ansatz to develop a new numerical method that facilitates efficient classical simulations in such regimes. Since this new method does not have an error estimate and is not valid in general, we use a neutral-atom quantum simulator with L_exp = 290 lattice sites to benchmark its performance in terms of accuracy and convergence for evolution times up to 700 tunnelling times. We then use this method to make a prediction of the behaviour of interacting dynamics for spin-imbalanced Fermi-Hubbard systems, which we show to be in quantitative agreement with experimental results. [1.] Bharath Hebbe Madhusudhana et. al. arXiv:2105.06372
11:10am-11:30amChristopher Pattison, California Institute of Technology
Improved quantum error correction using soft information
Abstract The typical model for measurement noise in quantum error correction is to randomly flip the binary measurement outcome. In experiments, measurements yield much richer information - e.g., continuous current values, discrete photon counts - which is then mapped into binary outcomes by discarding some of this information. In this work, we consider methods to incorporate all of this richer information, typically called soft information, into the decoding of quantum error correcting codes, and in particular the surface code. We describe how to modify both the Minimum Weight Perfect Matching and Union-Find decoders to leverage soft information, and demonstrate these soft decoders outperform the standard (hard) decoders that can only access the binary measurement outcomes. We also introduce a soft measurement error model with amplitude damping, in which measurement time leads to a trade-off between measurement resolution and additional disturbance of the qubits. Under this model we observe that the performance of the surface code is very sensitive to the choice of the measurement time - for a distance-19 surface code, a five-fold increase in measurement time can lead to a thousand-fold increase in logical error rate. Moreover, the measurement time that minimizes the physical error rate is distinct from the one that minimizes the logical performance, pointing to the benefits of jointly optimizing the physical and quantum error correction layers.
11:30am-11:50amCiaran Ryan-Anderson, Honeywell
Realization of real-time fault-tolerant quantum error correction
Abstract Correcting errors in real-time is essential for reliable large-scale quantum computations. Realizing this high-level function requires a system capable of several low-level primitives, including single-qubit and two-qubit operations, mid-circuit measurements of subsets of qubits, real-time processing of measurement outcomes, and the ability to condition subsequent gate operations on those measurements. In this work, we use a ten qubit QCCD (quantum charge-coupled device) trapped-ion quantum computer to encode a single logical qubit using the $$[[7,1,3]]$$ color code, first proposed by Steane. The logical qubit is initialized into the eigenstates of three mutually unbiased bases using an encoding circuit, and we measure an average logical SPAM error of $$1.7(2) \times 10^{-3}$$ compared to the average physical SPAM error $$2.4(4) \times 10^{-3}$$ of our qubits. We then perform multiple syndrome measurements on the encoded qubit, using a real-time decoder to determine any necessary corrections that are done either as software updates to the Pauli frame or as physically applied gates. Moreover, these procedures are done repeatedly while maintaining coherence, demonstrating a dynamically protected logical qubit memory. Additionally, we demonstrate non-Clifford qubit operations by encoding a magic state with an error rate below the threshold required for magic state distillation. Finally, we present system-level simulations that allow us to identify key hardware upgrades.

#### SESSION 3: Talks at Zoom

2:00pm-2:30pmShruti Puri , Yale University
Scalable Fault-tolerant Quantum Computation with Kerr-Cats
Abstract With fault-tolerant QEC it is possible to solve a computational problem accurately by transforming a many-body quantum state with a sequence of faulty operations on imperfect quantum hardware. However, the enormous overhead cost of fault-tolerant protocols is the key bottleneck in practical realization of quantum computers. In this talk I will present the superconducting Kerr-cat qubit architecture. This qubit is realized in a parametrically driven non-linear oscillator and has some in-built protection against noise. Finally, I will discuss how this protected cat-qubit can significantly reduce the overheads for fault-tolerant QEC.
2:30pm-2:50pmZoe Holmes, Los Alamos National Laboratory
Barren plateaus preclude learning scramblers
Abstract Scrambling, the rapid spread of information through many-body quantum systems, is fundamental to a wide range of fields, from quantum chaos to thermalisation and black holes. However, given the complexity of many body quantum systems, scrambling can be hard to study using standard techniques. Recently, quantum machine learning (QML) has emerged as a promising paradigm for the study of complex physical processes. It is therefore natural to ask whether QML could be used to study scrambling. In this talk, we present a no-go theorem which restricts this possible use of QML. Specifically, we show that any QML approach used to learn the unitary dynamics implemented by a typical scrambler will exhibit a barren plateau, i.e. the cost gradient will vanish exponentially with the system size. As such, any QML algorithm to learn a scrambler will be untrainable. Crucially, in contrast to previously established barren plateau phenomena, which are a consequence of the ansatz structure and parameter initialization strategy, our barren plateaus holds for any choice of ansatz and initialization. Thus, previously proposed strategies for avoiding barren plateaus do not work here. More generally, given the close connection between scrambling and randomness, our no-go theorem also applies to learning random and pseudo-random unitaries. Consequently, our result implies that QML cannot be used to efficiently learn an unknown unitary process, placing a fundamental limit on QML.
2:50pm-3:10pmJeremy Metzner, University of Oregon
Using 'protected' modes in trapped ions to enable mid-algorithm measurements for CVQC*
Abstract Measurements of the motional states of trapped ions require coupling the motion to the ions’ internal spin states. These measurements, however, require detection of spin-dependent fluorescence. Photon scattering, giving rise to fluorescence, causes the ion to recoil, which generally decoheres the ions’ motional modes. This decoherence prevents mid-algorithm measurements, which are necessary for processes that require classical feedback. Overcoming this challenge is likely necessary for the viability of practical continuous variable quantum computing (CVQC) in trapped ions. To address this issue, we are investigating the use of ‘protected’ modes within chains consisting of an odd number of ions, where the center ion has zero displacement (3(N-1)/2 protected modes with N ions). As a demonstration we use a dual-species three-ion chain linear (88Sr+ -40Ca+ - 88Sr+), which enables us to simply address the center ion with global laser fields. We perform measurements of the heating rate and coherence time, via Ramsey interferometry, of these protected modes, to determine how much the decohering effects of photon scattering are suppressed. We are also developing models to minimize the effects of symmetry breaking of the chain due to radiation pressure, and non-linear coupling between modes, on the coherence time of the protected modes. *This research was supported by the U.S. Army Research Office through grant W911NF-19-1-0481.
3:10pm-3:30pmKarthik Chinni, University of New Mexico CQuIC
Reliability of simulation on NISQ-era devices
Abstract The goal of NISQ-era quantum processors is to outperform classical computers on targeted tasks, such as quantum simulation, with moderately sized devices that are well controlled, but lack full fault-tolerant error correction. Here, we consider two different types of errors that could affect the reliability of NISQ devices: a background perturbation that leads to quantum chaos and Trotterization of the unitary dynamical map. In the first case, we consider simulation of the LMG model, which is integrable in the mean-field limit, but becomes chaotic in the presence of a background time-dependent perturbation. Here, we show that the quantities that depend on the global structure of the phase space, such as critical point estimates of the quantum phase transition, are robust to the presence of this perturbation while other aspects of the system such as the mean magnetization that depend on the local trajectories are fragile and cannot be reliably extracted from the simulator. Next, we analyze the effects of Trotterization on the simulation of p-spin models and identify the existence of “dynamical instability regions” in the Trotterized unitary map that are absent in the time evolution operator of the ideal p-spin model. We show that, even in the absence of chaos, Trotter errors proliferate in these dynamical instability regions, as the effective Hamiltonian associated with the Trotterized unitary becomes very different from the target p-spin Hamiltonian.
3:30pm-3:50pmBibek Pokharel, University of Southern California
Dynamically Generated Decoherence-Free Subspaces on Superconducting Qubits
Abstract Decoherence-free subspaces/noiseless subsystems (DFS/NS) preserve quantum information by identifying subspaces/subsystems of the Hilbert space that remain unaffected by decoherence. Identifying DFS/NS codes under collective decoherence is well-understood, and the resultant codes support scalable and universal quantum computation. While most experimental systems, including superconducting qubit-based devices, do not decohere collectively, it is possible to engineer the conditions for collective decoherence using dynamical decoupling (DD) sequences. We report on the creation and verification of DD-assisted DFS/NS codes on quantum processors provided by the IBM Quantum Experience. We compare the performance of a DFS/NS encoded qubit with its unprotected counterpart. We show that qubit lifetime can be improved substantially using DD-assisted DFS/NS codes. Furthermore, we exploit gate set tomography to characterize logical error channels and estimate logical gate error rates for the DFS/NS encoding. When combined with an analysis of qubit lifetimes for multiple simultaneously encoded qubits, we obtain a comprehensive picture of DFS/NS feasibility and scalability on near-term quantum processors.

#### SESSION 4: Talks at Zoom

10:00am-10:30amVladan Vuletić , Massachusetts Institute of Technology
Entangled optical atomic clock
Abstract Entangled states of many particles can be used to overcome limits on measurements performed with ensembles of independent atoms (standard quantum limit). A particularly simple form of entanglement is spin squeezing, where the quantum noise for the variable of interest, e.g., the phase of an atomic clock, is redistributed into another variable. Spin squeezing can be generated by coupling the atomic ensemble to an optical cavity. We report the first sizeable spin squeezing in an optical-clock atom, ytterbium. The squeezing is generated in the electronic ground state, and then transferred onto the clock transition via the clock laser. I will also discuss new results where we create more complex entangled states via atom-cavity interaction, including an effective time-reversal protocol for a many-body Hamiltonian.
10:30am-10:50amJacob Bringewatt, University of Maryland Joint Quantum Institute
Measuring Functions with Quantum Sensor Networks
Abstract We study the problem of optimally measuring analytic functions of field amplitudes with quantum sensor networks with a focus on the effects of interdependence between the various quantities involved in the measurement scheme. We consider such interdependence both at the level of correlations between field amplitudes [Qian et. al., Phys. Rev. A. 103, L030601 (2021)] and at the level of correlations between the functions of these field amplitudes we seek to measure [Bringewatt et. al., Phys. Rev. Res. 3, 033011 (2021)]. In either case, correlations enable more freedom in choosing measurement protocols relative to simpler formulations of the problem. Taking advantage of this interdependence involves common mathematical themes related to the optimal choice of a basis for the problem at hand and reveals connections between ultimate information theoretic bounds and their saturating protocols via linear programming. In addition, our work greatly expands the scope of such protocols to practically relevant settings.
10:50am-11:10amDiego Barberena, University of Colorado JILA
Quantum-enhanced sensing of displacements and electric fields with large trapped-ion crystals
Abstract In this talk we will describe a protocol that performs quantum enhanced sensing of displacements and electric fields in a large crystal of trapped ions (N=150). The protocol uses the center of mass vibrational mode (COM) of the crystal as a high-Q mechanical oscillator and the ions' collective electronic spin, as the effective measurement device. It is implemented by the use of properly detuned laser beams that entangle the oscillator and the collective spin before applying a weak displacement to the oscillator. After a many-body echo that maps the displacement into a spin rotation while canceling both quantum back-action and thermal noise the collective spin is measured. We derive an effective model of coupled oscillators to describe both the spins and the COM of mode and show that the spin-boson entanglement at the core of the protocol can be understood as the squeezing dynamics of this pair of oscillators. We achieve a sensitivity to displacements of 8.8 ± 0.4 dB below the standard quantum limit and about 19 dB below relevant thermal bounds, and a sensitivity for measuring electric fields of 240 ± 40 nV/m in 1 second (240 nVm-1Hz-1/2). With future improvements, electric field sensitivities below 1 nV/m may be possible, which could enable searches for dark matter.
11:10am-11:30amChenxu Liu, Virginia Tech
Protocols for robust generation of microwave photonic graph states from superconducting qubits
Abstract We present protocols that use superconducting qubits for the robust generation of microwave photon cluster and graph states. We consider both fixed-frequency and tunable-frequency transmon qubits as microwave photon emitters and provide the photonic graph state generation circuits. We compare four microwave photonic encoding methods and estimate the photonic graph state fidelity. The generated highly entangled states can be tailored to various quantum information processing tasks, such as robust quantum communication and entanglement generation between different modules of a distributed superconducting qubit processor.
11:30am-11:50amXiaoyue Jin, National Institute of Standards and Technology, Boulder
A versatile parametric coupler between two transmon qubits
Abstract Parametric coupling is a powerful tool that can be used to generate tunable coupling between superconducting qubits via a microwave pump. In this talk, we demonstrate a versatile parametric coupler between two transmon qubits, which can be used to eliminate the residual ZZ coupling between the qubits, to realize a cZ gate by on-resonant parametric coupling, as well as a new kind of cZ gate by off-resonant parametric ZZ manipulation. In the residual ZZ coupling elimination experiment, we show that the upper limit of the effective ZZ coupling after elimination is nominally zero, with an experimental upper limit of 1-10 kHz. Randomized benchmarking experiments show that the on-resonant cZ gate has a fidelity of 99.4% with a gate time of 60 ns, whereas the off-resonant cZ gate has a fidelity of 99.5% with a gate time of 30 ns. We show the gate time dependence of the fidelities of both types of cZ gates and discuss the source of error for those gates.

#### SESSION 6: Talks at Zoom

2:00pm-2:30pmHeather Lewandowski, University of Colorado Boulder
Preparing to enter the quantum workforce
Abstract Quantum sensing, quantum networking and communication, and quantum computing have attracted significant attention recently, as these quantum technologies could offer significant advantages over existing technologies. In order to accelerate the commercialization of these quantum technologies, the workforce must be equipped with the necessary knowledge and skills. Through a study of the quantum industry, in a series of interviews with 21 U.S. companies carried out in Fall 2019 and from a survey administered to 57 companies through the Quantum Economic Development Consortium (QED-C) in Fall 2020, we describe the types of activities being carried out in the quantum industry, profile the types of jobs that exist, and describe the skills valued across the quantum industry, as well as in each type of job. The current routes into the quantum industry are detailed, providing a picture of the current role of higher education in training the quantum workforce.
2:30pm-2:50pmSepehr Nezami, California Institute of Technology
Permanent of random matrices from representation theory: moments, numerics, concentration, and comments on hardness of boson-sampling
Abstract Computing the distribution of permanents of random matrices has been an outstanding open problem for several decades. In quantum computing, "anti-concentration" of this distribution is an unproven input for the proof of hardness of the task of boson-sampling. We study the permanents of random i.i.d. complex Gaussian matrices, and more broadly, submatrices of random unitary matrices. Using a hybrid representation-theoretic and combinatorial approach, we prove strong lower bounds for all moments of the permanent distribution. We provide substantial evidence that our bounds are close to being tight and constitute accurate estimates for the moments.
1. Using the Schur-Weyl duality (or the Howe duality), we prove an expansion formula for the 2t-th moment of |Perm M| when M is a random Gaussian matrix, or a minor of a random unitary matrix
2. We prove a surprising size-moment duality: the 2t-th moment of the permanent of random k by k matrices is equal to the 2k-th moment of the permanent of t by t matrices
3. We design an algorithm to exactly compute high moments of the permanent of small matrices
4. We prove lower bounds for arbitrary moments of permanents of random matrices, and conjecture that our lower bounds are close to saturation up to a small multiplicative error.
5. Assuming our conjectures, we use the large deviation theory to compute the tail of the distribution of log-permanent of Gaussian matrices for the first time.
6. We argue that it is unlikely that the
2:50pm-3:10pmHolly Tinkey, Georgia Institute of Technology
Experimental demonstration of a transport-enabled entangling gate on trapped ions
Abstract We perform a two-qubit entangling Molmer-Sorensen gate by transporting two co-trapped 40Ca+ ions in a linear surface Paul trap through a stationary, bichromatic laser beam. We measure the Doppler shift of the ions during different segments of transport and observe variations in the ion velocity. We correct for these variations using modifications to the temporal interpolation of the moving trap potential (waveform). We compensate for time-dependent ac Stark shifts during transport with two approaches: the first is a static frequency offset applied to both beam tones, ad the second involves dynamic adjustments of the transport waveform to counter the Stark shift with a variable Doppler shift. We compare the performance of these gates to those realized in stationary potentials. This experiment demonstrates the potential of actively integrating transport into quantum information operations.
3:10pm-3:30pmArshag Danageozian, Louisiana State University
Efficiency-Fidelity Trade-off in a Quantum Error Correcting Engine
Abstract Quantum error correction (QEC) is a procedure by which the quantum state of a system is protected against a known type of noise by preemptively adding redundancy to it using an ancillary system. A major type of noise that regularly appears in almost every implementation of quantum computing and QEC is thermal noise, which is also known to play a central role in quantum thermodynamics (QTD). This fact hints at the applicability of certain QTD statements in the QEC of thermal noise. Such statements have been discussed previously in the context of Maxwell's demon. In this article, we view QEC as a quantum heat engine with a feedback controller (demon). The main task of this engine is to correct the effects of the hot bath (thermal noise) by attempting to close its own cycle with respect to the system state, corresponding to a perfect QEC. We derive Clausius' formulation of the second law in the context of this QEC engine operating with general quantum measurements. For efficient measurements and sufficiently low temperatures of the cold bath, we show that this leads to a fundamental trade-off between the fidelity of the error-corrected system state and the super-Carnot efficiencies that heat engines with feedback controllers have been known to possess.
3:30pm-3:50pmQian Yu, University of California Berkeley
Towards a trapped electron quantum computer
Abstract One of the most established physical implementations of quantum computing is trapped ions in Paul traps. Here we study electrons trapped in Paul traps as an attractive alternative to trapped ions. Their extremely light mass leads to faster operations, their simple two-level spin structure avoids leakage into other energy levels, and they can be manipulated with well-established microwave technology, removing some of the optical engineering challenges required to build a large-scale quantum computer with trapped ions. The first step towards this goal is to trap and detect electrons in a Paul trap. This talk will present our recent results on trapping electrons in a room-temperature quadrupole Paul trap. We loaded cold electrons into the trap by photoionization of atomic calcium and successfully confined them with microwave and static electric fields for several tens of milliseconds. A fraction of these electrons remains trapped longer and show no measurable loss for measurement times up to a second. Electronic excitation of the motion reveals secular frequencies which can be tuned over a range of several tens to hundreds of MHz. Our recent work shows that operating a similar electron Paul trap in a cryogenic environment may provide a platform for quantum computing with trapped electrons.

#### SESSION 7: Happy hour at Gather.town

SQuInT Chief Organizer
Akimasa Miyake, Associate Professor
amiyake@unm.edu

SQuInT Co-Organizer
Brian Smith, Associate Professor
bjsmith@uoregon.edu

SQuInT Local Organizers
Philip Blocher, Postdoc
Pablo Poggi, Research Assistant Professor
Tzula Propp, Postdoc
Jun Takahashi, Postdoc
Cunlu Zhou, Postdoc

SQuInT Founder
Ivan Deutsch, Regents' Professor, CQuIC Director
ideutsch@unm.edu