Session U09: Entangling/2-Qubit Gates

Parallel entangling gates in trapped-ion chains using orthogonal motional modes

Parallel operations are important for both near-term quantum computers and larger-scale fault-tolerant machines because they reduce execution time and qubit idling. We present a pairwise-parallel gate scheme on a trapped-ion quantum computer where entangling gates are driven simultaneously on different sets of orthogonal motional modes in a trapped-ion chain. We demonstrate the utility of this scheme by creating a high-fidelity 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 transverse-field 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 ground-level qubits in ¹⁷¹ Yb+ driven by Raman lasers in this demonstration, but this scheme can be easily applied to other types of trapped-ion qubits and addressed gate schemes, broadly enhancing the capabilities of trapped-ion quantum computers.   

Digital predistortion of optical field of a fast and high-fidelity entangling gate for trapped ions qubits

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 spin-dependent force that mediates qubit-qubit interactions. 

In recent years, there have been many theoretical proposals and experimental demonstrations which have generalized this approach in order to increase the fidelity, robustness, and programmability of the entangling operation. These are all performed by carefully designing the electromagnetic fields which drive the ion chain. However, various components such as amplifiers and modulators, which are used to generate the required field modulations have inherent non-linear responses, resulting in an inaccurate and low-fidelity implementation of the entangling operations.

We propose a method to mitigate this degradation by using digital pre-distortion of the modulating waveform. Specifically, we measure the temporal and amplitude non-linear response of an acusto-optic modulator used to modulate the optical field driving the ion chain, and use it in a feed-forward correction of the desired waveform. 

We measure that the resulting optical field more closely resembles the desired spectrum. Moreover, we use the pre-distorted signal to generate a multi-tone two-qubit entangling gate described in Ref. [1]. We show that our method allows us to utilize all the available optical power in order to drive fast entanglement gates, without incurring fidelity loss due to unwanted non-linear effects. Our method is straightforward to implement, even in complicated waveform modulation, such as Refs. [2,3], which require many driving tones in order to generate multi-qubit robust entanglement gates.

[1] Phys. Rev. Lett. 121, 180502 (2018)

[2] Phys. Rev. A 101, 032330 (2020)

[3] Phys. Rev. Lett. 130, 030602 (2023)   

Realization of a universal two-qubit register for a QCCD-based quantum processor

Single-qubit rotations and two-qubit entangling gates form a universal set of quantum operations. In this work, we realize such a two-qubit register compatible with the quantum CCD architecture. Quantum logic operations are implemented using embedded microwave conductors. Single-qubit gates in a two-ion crystal are performed by addressing each ion individually via a micromotion sideband [1]. The entanglement operation is implemented using an MS-type interaction, where we measure an infidelity approaching 10^-3 using partial state tomography. In addition, we characterize the single-qubit 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)%.

    [1] U. Warring et al.,  Phys. Rev. Lett. 17, 173002 (2013)

    [2] M. Duwe et al., Quantum Sci. Technol. 7, 045005 (2022)

    [3] A. Erhard et al.,  Nat. Commun. 10, 5347 (2019)   

Polarized Particles in a Spin-Transparent Storage Ring as a Quantum Computer

Electrons in spin-transparent storage rings can exhibit a spin-coherencetime of several hours, presenting a compelling platform for quantumcomputing. Spin-polarized electrons are generated by shining circularly-polarized light onto a photocathode, and then injected into the storagering. Then, single-qubit 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 two-qubit 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 spin-entangled electrons couldthen be used as a resource in a measurement-based scheme to performmulti-qubit gates in the storage ring.   

A two-qubit gate in trapped ions using the optical Magnus effect

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 qubit-state 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.

The trapped ions quantum computer has much to gain from the combination with optical tweezers. They can be used to create a programmable quantum simulator [2] as well as a second type of two-qubit logic gate [3] using state dependent optical tweezers in combination with weak oscillating electric fields. By coupling the electric field to the center of mass motion of the whole crystal the proposed gate fidelity is independent on the separation between the targeted ions. Both proposed architectures can compete with state-of-the-art techniques opening the way to new ways of engineering multi-qubit gates in trapped ions using optical tweezers.

[1] M. Mazzanti, R. Gerritsma, R. J. C. Spreeuw and A. Safavi-Naini. arXiv:2301.04668 (2023)

[2] J.D. Espinoza, M. Mazzanti, K. Fouka, R.X. Schüssler, Z. Wu, P.Corboz Phys. Rev. A 104, 013302 (2021).

[3] M. Mazzanti, R.X. Schüssler, J.D. Espinoza,  Z. Wu, R. Gerritsma, A. Safavi-Naini  Phys. Rev. Lett. 127, 260502 (2021).

    

Demonstration of Mølmer–Sørensen Gates Robust to +/-10 kHz Motional Frequency Error

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 Mølmer-Sørensen (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 spin-spin 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.   

Non-stop quantum entangling gate between a stationary ion qubit and a mobile one

Towards the arbitrary scalability of a quantum computer, a quantum charge-coupled 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 non-uniform motion during transportation introduce severe heating issues that cost major overhead in time and laser power for re-cooling and stabilization. To address this issue, we propose a novel entangling scheme between a stationary ion qubit and a non-stopping 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 long-distance 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.   

Quantum Computation and Simulation using Fermion-Pair Registers

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 many-body 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 controlled-phase 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.

    

Real optical π pulses for fast gates in Yb+

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).