High-fidelity trapped-ion qubit operations with scalable photonic modulators C. W. Hogle1D. Dominguez1M. Dong2 3A. Leenheer1H. J. McGuinness1B. P. Ruzic1M. Eichenfield1 4and D. Stick1

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High-ﬁdelity trapped-ion qubit operations with scalable photonic modulators

C. W. Hogle,1, ∗D. Dominguez,1M. Dong,2, 3 A. Leenheer,1H. J.

McGuinness,1B. P. Ruzic,1M. Eichenﬁeld,1, 4, †and D. Stick1

1Sandia National Laboratories, Albuquerque, New Mexico 87185, USA

2The MITRE Corporation, 202 Burlington Road, Bedford, Massachusetts 01730, USA

3Research Laboratory of Electronics, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA

4Wyant College of Optical Sciences, University of Arizona, Tucson, Arizona 85721, USA

(Dated: August 28, 2023)

Experiments with trapped ions and neutral atoms typically employ optical modulators in order

to control the phase, frequency, and amplitude of light directed to individual atoms. These elements

are expensive, bulky, consume substantial power, and often rely on free-space I/O channels, all of

which pose scaling challenges. To support many-ion systems like trapped-ion quantum computers

or miniaturized deployable devices like clocks and sensors, these elements must ultimately be mi-

crofabricated, ideally monolithically with the trap to avoid losses associated with optical coupling

between physically separate components. In this work we design, fabricate, and test an optical

modulator capable of monolithic integration with a surface-electrode ion trap. These devices consist

of piezo-optomechanical photonic integrated circuits conﬁgured as multi-stage Mach-Zehnder mod-

ulators that are used to control the intensity of light delivered to a single trapped ion on a separate

chip. We use quantum tomography employing hundreds of multi-gate sequences to enhance the

sensitivity of the ﬁdelity to the types and magnitudes of gate errors relevant to quantum computing

and better characterize the performance of the modulators, ultimately measuring single qubit gate

ﬁdelities that exceed 99.7%.

INTRODUCTION

Since their inception, microfabricated ion traps [1, 2]

have grown more advanced in their fabrication and ge-

ometry [3], demonstrated transport capabilities necessary

for quantum computing [4–6], and been used for high ﬁ-

delity multi-ion experiments [7, 8]. In 2016, researchers

demonstrated integrated waveguides to deliver light to

individual ions within a trap [9], culminating in the de-

livery of all wavelengths needed for probing a trapped

ion [10], including those used for high-ﬁdelity entangling

gates [11]. More recently researchers have integrated sin-

gle photon detectors into traps operating at both cryo-

genic [12] and room temperatures [13, 14].

One of the remaining challenges to making larger

trapped-ion quantum computers is addressing the optical

input/output (I/O) scaling constraints [15]. While mod-

ern processors can support billions of transistors with

only thousands of electrical I/O, quantum computers re-

quire a controllable signal for every qubit. In trapped-

ion experiments with integrated waveguides these optical

signals are often delivered to the chip from edge-coupled

ﬁber arrays. This poses a scaling mismatch; while the

number of qubits scales with the chip area, the avail-

able optical I/O only scales with its perimeter. Elec-

trical I/O faces a similar challenge, but demonstrations

with through-substrate vias [16] and fanout of the input

signals [3] to co-wired electrodes have both achieved good

performance and constitute practical solutions. Optical

∗cwhogle@sandia.gov

†eichenﬁeld@arizona.edu

I/O cannot directly employ the same fanout strategy be-

cause of the sensitivity to site-to-site deviations. Manip-

ulating the quantum state of an ion with a laser requires

precise individual signal control to adjust for expected

natural variations (e.g. diﬀerent attenuations in separate

waveguides and outcouplers) as well as the need to turn

optical signals on and oﬀ at a single ion level within an

algorithm.

A way to maintain signal control while preserving the

beneﬁts of fanout involves fabricating optical modulators

with amplitude and phase control on the same chip as the

trap, conceptually shown in Fig. 1a. Then a single optical

launch onto the chip can fanout using waveguide beam

splitters, with each individual line controlled separately

by an optical modulator. Note that this does not entirely

solve the scaling problem, but pushes it into the electrical

domain where a unique electrical signal is required per

qubit. This challenge is more manageable as wirebond or

through-substrate via density can be much higher than

the density of edge-coupled optical ﬁbers, and in addition

on-chip digital and analog circuitry can be used [17] to

further reduce the electrical I/O. Another technique for

shifting the modulation burden onto electronics uses ion

shuttling to control the laser amplitude [18] and phase

[19].

The performance requirements for these optical mod-

ulators include the ability to control the amplitude and

phase of light with minimal optical loss, achieve switching

speeds faster than the fastest gate times (∼1µs for single

qubit gates), support optical powers required for single

and two qubit gates (1 to 10 mW, and potentially higher

depending on the ion species and gate time), achieve high

extinction ratios, and perform consistently with low er-

rors. They must be able to be co-fabricated with an ion

arXiv:2210.14368v2 [quant-ph] 24 Aug 2023

INPUT OUTPUT

ϕ1ϕ2

v1 v2

-v1 -v2

FIG. 1: a. Conceptual rendering showing how on-chip modulators can be used to reduce the number of optical I/O

needed to cool, manipulate, and detect trapped ions. In this ﬁgure blue light for cooling and detecting ions is split

four ways from a single input and controlled by independent modulators associated with a single site, and similarly

for other wavelengths. In the experiments described here, the modulators are fabricated on a separate chip from the

trap for testing their performance. b. The topology of the serial MZIs that comprise the full MZM switch, along

with the couplers and push-pull mechanism that deforms the arms of each MZI in opposite directions by switching

the ground and applied voltage. c. An optical micrograph of the MZM. Each MZI (just the meander section) is

340 µm wide by 440 µm tall. The four squares on the left are the diﬀractive incouplers, with the inner two used as

input and the outer two as output for a ﬁber V-groove array. Deformations that exist in the zero applied voltage

state are visible at the corners of the meander waveguide sections.

trap and directly interface with on-chip waveguides. The

modulators must also be CMOS (complementary metal-

oxide-semiconductor) compatible so as not to preclude

co-fabrication with other integrated technologies (e.g.

on-chip electronics and detectors) in a volume CMOS

foundry. Finally, it is desirable if the same technology

can support multiple wavelengths (UV to IR), operate

with modest voltages (10’s of volts), and operate at both

room and cryogenic temperatures. While frequency mod-

ulation may be useful, in principle the laser light that

is launched on the chip does not need to be frequency

tuned on an ion-by-ion basis, provided the relevant en-

vironmental parameters are constant across all ions and

the same types of operations are applied at the same

time. In some cases, like optical qubits with a magneti-

cally sensitive transition, this imposes limits on magnetic

ﬁeld variation between locations that use the same source

laser.

Piezoelectrically actuated modulators are a promising

candidate based on these criteria, and also because they

employ a modulation mechanism that is eﬀectively ag-

nostic to the waveguide material and can therefore sup-

port the wide range of wavelengths needed for ion trap-

ping. Here we design and fabricate an optical modu-

lator that uses co-integrated piezoelectric actuators and

waveguides conﬁgured in a Mach-Zehnder Interferometer

(MZI) to shift the optical phases between the interfer-

ometer arms. The materials and fabrication process are

compatible with CMOS in general and surface ion traps

in particular [3], though in this work the modulators were

fabricated as separate devices from the trap and used to

control the amplitude of light that was then directed to

a single trapped ion via ﬁbers and free space optics.

Other materials, notably lithium niobate (LN) [20–22],

have been used to make modulators that meet most of

these requirements and could be used in similar visible-

light and atomic applications. LN has a high electro-optic

coeﬃcient that supports small footprint and low voltage

devices (VπL= 1.6V·cm [23]). While the AlN modu-

lators have a higher voltage length product (VπL= 12

V·cm at the beginning of the experiments), the ability

to meander the waveguides on top of the AlN structure

allows for comparable overall sizes (see Fig. 1), albeit

higher operating voltages. The primary advantage of

AlN modulators over LN modulators is their direct in-

tegrability with a microfabricated ion trap. Since LN

is not a CMOS compatible material, thin ﬁlm LN mod-

ulators must be heterogeneously integrated using wafer

bonding after CMOS processing is complete. There have

been many successful demonstrations of this technique

and it could be used to hybrid integrate LN modulators

with an ion trap to achieve the same I/O beneﬁts as de-

scribed. However it would pose other constraints related

to processing and the vertical position of waveguides in

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High-fidelitytrapped-ionqubitoperationswithscalablephotonicmodulatorsC.W.Hogle,1,∗D.Dominguez,1M.Dong,2,3A.Leenheer,1H.J.McGuinness,1B.P.Ruzic,1M.Eichenfield,1,4,†andD.Stick11SandiaNationalLaboratories,Albuquerque,NewMexico87185,USA2TheMITRECorporation,202BurlingtonRoad,Bedford,Massachusetts01730,US...

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High-fidelity trapped-ion qubit operations with scalable photonic modulators C. W. Hogle1D. Dominguez1M. Dong2 3A. Leenheer1H. J. McGuinness1B. P. Ruzic1M. Eichenfield1 4and D. Stick1.pdf

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High-fidelity trapped-ion qubit operations with scalable photonic modulators C. W. Hogle1D. Dominguez1M. Dong2 3A. Leenheer1H. J. McGuinness1B. P. Ruzic1M. Eichenfield1 4and D. Stick1

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