Bulletin of the American Physical Society
54th Annual Meeting of the APS Division of Atomic, Molecular and Optical Physics
Volume 68, Number 7
Monday–Friday, June 5–9, 2023; Spokane, Washington
Session U09: Entangling/2Qubit Gates 
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Chair: Spencer Fallek, Raytheon Room: 206 D 
Thursday, June 8, 2023 2:00PM  2:12PM 
U09.00001: Parallel entangling gates in trappedion chains using orthogonal motional modes Yingyue Zhu, Alaina M Green, Nhung H Nguyen, Cinthia Huerta Alderete, Norbert M Linke, Elijah Mossman Parallel operations are important for both nearterm quantum computers and largerscale faulttolerant machines because they reduce execution time and qubit idling. We present a pairwiseparallel gate scheme on a trappedion quantum computer where entangling gates are driven simultaneously on different sets of orthogonal motional modes in a trappedion chain. We demonstrate the utility of this scheme by creating a highfidelity GHZ state in one step using parallel gates with one overlapping qubit and show its advantage for long circuits by running a digital quantum simulation of a transversefield Ising model. With essentially no overhead apart from additional initial cooling, this method effectively extends the available gate depth by up to a factor of two. We use groundlevel qubits in ^{171} Yb+ driven by Raman lasers in this demonstration, but this scheme can be easily applied to other types of trappedion qubits and addressed gate schemes, broadly enhancing the capabilities of trappedion quantum computers. 
Thursday, June 8, 2023 2:12PM  2:24PM 
U09.00002: Digital predistortion of optical field of a fast and highfidelity entangling gate for trapped ions qubits Jovan Markov, Yotam Shapira, Nitzan Akerman, Roee Ozeri Tapped ions qubits are a leading quantum computing platform. In these systems, entangling gates are performed by driving the normal modes of motion of the ion chain, generating a spindependent force that mediates qubitqubit interactions. 
Thursday, June 8, 2023 2:24PM  2:36PM 
U09.00003: Realization of a universal twoqubit register for a QCCDbased quantum processor Hardik Mendpara, Nicolas PulidoMateo, Markus C Duwe, Giorgio Zarantonello, Ludwig Krinner, Christian Ospelkaus Singlequbit rotations and twoqubit entangling gates form a universal set of quantum operations. In this work, we realize such a twoqubit register compatible with the quantum CCD architecture. Quantum logic operations are implemented using embedded microwave conductors. Singlequbit gates in a twoion crystal are performed by addressing each ion individually via a micromotion sideband [1]. The entanglement operation is implemented using an MStype interaction, where we measure an infidelity approaching 10^3 using partial state tomography. In addition, we characterize the singlequbit gates using a randomized benchmarking protocol and obtain an infidelity of 3.8(4)*10^3. Finally, we characterize the quantum processor in a computational context using the cycle benchmarking protocol [3]. As a preliminary result, we obtain a composite process fidelity of 96.6(4)%. 
Thursday, June 8, 2023 2:36PM  2:48PM 
U09.00004: Polarized Particles in a SpinTransparent Storage Ring as a Quantum Computer Riad S Suleiman, Matt Grau, Vasiliy S Morozov Electrons in spintransparent storage rings can exhibit a spincoherencetime of several hours, presenting a compelling platform for quantumcomputing. Spinpolarized electrons are generated by shining circularlypolarized light onto a photocathode, and then injected into the storagering. Then, singlequbit rotations can be implemented by a pulsedsolenoid, and readout of the spin is done using a Mott polarimeter.However, a signifi cant question of the viability of storage rings as aquantum computing platform remains: to date, there is no demonstrationof a twoqubit gate. In this talk, I will explore the possibility of using anentangled train of light pulses impinging on a photocathode to produceelectrons with entangled spins. These spinentangled electrons couldthen be used as a resource in a measurementbased scheme to performmultiqubit gates in the storage ring. 
Thursday, June 8, 2023 2:48PM  3:00PM 
U09.00005: A twoqubit gate in trapped ions using the optical Magnus effect Matteo Mazzanti, Rene Gerritsma, Robert J C Spreeuw, Arghavan SafaviNaini We present a novel implementation of quantum logic gates in trapped ions using tightly focused optical tweezers [1]. Such tightly focused tweezers exhibit strong a polarization gradient at their focus. This can be used to generate qubitstate dependent forces on trapped ions in order to engineer a novel type of quantum logic gate. Interestingly these forces lay on the plane perpendicular to the direction of propagation of the tweezers that generate them. This greatly simplifies the required optical system and allows for new ways of coupling to motional modes of an ion crystal. We show that the proposed gate does not require ground state cooling in order to achieve high fidelities. 
Thursday, June 8, 2023 3:00PM  3:12PM 
U09.00006: Demonstration of a fast π2ππ Rydberg entangling gate for finite blockade Daniel C Cole, Eric Copenhaver, Garrett T Hickman, David Mason, Woo Chang Chung, Martin T Lichtman We describe an entangling gate for two atomic qubits based on the interaction between Rydberg states. This protocol can produce any desired twoqubit controlledphase gate when combined with singlequbit rotations. The gate adapts the approach of the classic π2ππ Rydbergblockade entangling gate to the finiteblockade regime. Rydberg excitation pulses are applied sequentially to the control, target, and control atoms, respectively. The first (third) pulse transfers population in one of the control atom’s qubit states to (back from) a Rydberg level. The second pulse drives a qubittoRydberg Rabi oscillation from one of the target atom’s qubit states. The detuning of this oscillation depends on whether the control atom was transferred to the Rydberg level, which would impose a shift on the target atom’s qubittoRydberg transition frequency. One can engineer the target atom’s qubittoRydberg Rabi oscillations so that the target atom always returns to the qubit state through appropriate choice of the detuning, frequency, and duration of the oscillation.
We demonstrate this gate protocol in a neutral atom quantum processor based on an array of Cesium atom clockstate qubits individually trapped in optical tweezers. For gate characterization, we use a square array of four qubits. We calibrate the gate parameters and additional singlequbit rotations to realize controlledZ gates for each nearestneighbor pair. These gates are implemented with a ratio of blockade strength to target atom qubitRydberg Rabi frequency of about 1.2. We perform quantum circuits to produce entangled Bell states, and we observe a maximum (mean) fidelity over the four qubit pairs of 92.8 (6) % (91.8 (3) %). Accounting for the measured fidelity of initial qubitstate preparation, we infer a maximum (mean) controlledZ gate fidelity across qubitpairs of 96.4 (7) % (95.3 (4) %). This protocol relaxes requirements on both blockade strength and addressing symmetry for highfidelity entangling gates between neutralatom qubits.

Thursday, June 8, 2023 3:12PM  3:24PM 
U09.00007: Demonstration of Mølmer–Sørensen Gates Robust to +/10 kHz Motional Frequency Error Matthew N Chow, Brandon P Ruzic, Ashlyn D Burch, Megan K Ivory, Daniel S Lobser, Melissa C Revelle, Christopher G Yale, Susan M Clark Two of the most significant barriers to trapped ion quantum computing are scalability and susceptibility of the entangling gates to technical noise. In this work, we make substantial advancement towards addressing these issues by designing and testing a MølmerSørensen (MS) entangling gate that is robust against a dominant noise source – variation in the motional mode frequencies. Our gate uses a simple Gaussian pulse shape that exponentially suppresses displacement (loop closure) errors. This gives us the freedom to tackle sensitivity of the gate's spinspin rotation angle to detuning error by operating at a frequency that balances the contribution of multiple motional modes. The resulting 'balanced Gaussian' MS gate is broadly robust to motional frequency offset errors, and we experimentally demonstrate <1% drop in fidelity over a +/10 kHz range in frequency error. Further, we numerically study the scalability of our design and find the gate retains its robustness against frequency error on chains of up to 33 ions. 
Thursday, June 8, 2023 3:24PM  3:36PM 
U09.00008: Nonstop quantum entangling gate between a stationary ion qubit and a mobile one WenHan Png, Ting Hsu, TzeWei Liu, MingShien Chang, GuinDar Lin Towards the arbitrary scalability of a quantum computer, a quantum chargecoupled device (QCCD) based on ion shuttling has currently been regarded as one possible route. However, detaching an ion from and merging it to an ion array and driving nonuniform motion during transportation introduce severe heating issues that cost major overhead in time and laser power for recooling and stabilization. To address this issue, we propose a novel entangling scheme between a stationary ion qubit and a nonstopping mobile one, where the latter can be kept in uniform motion without causing heat, and theoretically demonstrate a gate error of the order of magnitude 10^{4} based on current technologies. This scheme enables efficient and economic quantum operations and facilitates longdistance entanglement distribution, where stationary trapped ion arrays form memory units with mobile ions acting as information carriers flying over them, making possible a revolutionary alternative to the QCCD architecture. 
Thursday, June 8, 2023 3:36PM  3:48PM 
U09.00009: Quantum Computation and Simulation using FermionPair Registers Di Luo, Xiangkai Sun, Soonwon Choi Fermion pairs have recently been shown as a scalable and robust quantum register with long coherent time, providing promising applications for quantum information processing. In this work, we develop schemes for programmable universal quantum computation and quantum manybody simulations based on the quantum register of fermion pairs. We utilize the fermion hopping J to design the SWAP gate, which serves as a prelimary for applying gates between arbitrary two qubits. We engineer an effective Hamiltonian using J and the tunable Feshbach interaction U in a perturbative regime J « U, which can generate the controlledphase gate with infidelity scaling as (J/U)^{4}. By modulating the U term, the platform can be also used for 2D quantum Ising model simulations. To calibrate quantum gates and probe quantum dynamics, we further propose a new shadow process tomography protocol with minimal experimental requirements and high sampling efficiency. Our work opens up a number of new opportunities for quantum computation and simulation with fermion pair registers. 
Thursday, June 8, 2023 3:48PM  4:00PM 
U09.00010: Real optical π pulses for fast gates in Yb^{+} Erik W Streed, Kenji Shimizu, Jordan Scarabel, Mirko Lobino The coherent momentum kick from counterpropagating optical π pulse pairs forms the basis for fast two qubit quantum gate protocols, where the gate speed is not limited by the confinement strength. Towards this aim we demonstrate coherent optical excitation of an 171Yb+ ion using single picosecond pulses at 370 nm with a maximum population transfer of 94(1%), limited by imperfections in the pulse detuning (est. 33 GHz from resonance on a 339 GHz FWHM near transform limited pulse, 3GHz/hr drift) and intensity stability (2.4% pulse to pulse). Expected gate fidelities for two fast gate pulse schemes were simulated using measured laser noise parameters and found to be 32% (GZC n=1) and 77% (Duan n=1). 
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