TLS Dynamics in a Superconducting Qubit Due to Background Ionizing Radiation Ted Thorbeck1Andrew Eddins2Isaac Lauer1Douglas T. McClure1and Malcolm Carroll1 1IBM Quantum IBM T.J. Watson Research Center Yorktown Heights NY 10598 USA

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TLS Dynamics in a Superconducting Qubit Due to Background Ionizing Radiation

Ted Thorbeck,1, ∗Andrew Eddins,2Isaac Lauer,1Douglas T. McClure,1and Malcolm Carroll1

1IBM Quantum, IBM T.J. Watson Research Center, Yorktown Heights, NY 10598, USA

2IBM Quantum, MIT-IBM Watson AI lab, Cambridge MA, 02142, USA

(Dated: October 11, 2022)

Superconducting qubit lifetimes must be both long and stable to provide an adequate founda-

tion for quantum computing. This stability is imperiled by two-level systems (TLSs), currently a

dominant loss mechanism, which exhibit slow spectral dynamics that destabilize qubit lifetimes on

hour timescales. Stability is also threatened at millisecond timescales, where ionizing radiation has

recently been found to cause bursts of correlated multi-qubit decays, complicating quantum error

correction. Here we study both ionizing radiation and TLS dynamics on a 27-qubit processor, repur-

posing the standard transmon qubits as sensors of both radiation impacts and TLS dynamics. Unlike

prior literature, we observe resilience of the qubit lifetimes to the transient quasiparticles generated

by the impact of radiation. However, we also observe a new interaction between these two processes,

“TLS scrambling,” in which a radiation impact causes multiple TLSs to jump in frequency, which

we suggest is due to the same charge rearrangement sensed by qubits near a radiation impact. As

TLS scrambling brings TLSs out of or in to resonance with the qubit, the lifetime of the qubit

increases or decreases. Our ﬁndings thus identify radiation as a new contribution to ﬂuctuations in

qubit lifetimes, with implications for eﬀorts to characterize and improve device stability.

I. INTRODUCTION

The drive to build a superconducting quantum com-

puter has led to rapid increases in both the number of

qubits in a device and the lifetimes, T1, of those qubits.

The increase in lifetimes has been remarkable given the

sensitivity of the qubits to environmental noise [1]. This

sensitivity can be harnessed by using the qubits as sensors

to better understand the noise. For example, two-level

systems (TLSs) are currently a dominant loss mechanism

in superconducting qubits, but superconducting qubits

are also useful to study TLSs [2]. A key diagnostic tool

has been TLS spectroscopy, in which T1is measured as

the frequency of the qubit is swept [3–7]. An individual

TLS can be resolved as a dip in T1as the qubit is tuned to

the TLS frequency. Spectroscopy has revealed that TLSs

can drift, appear, and disappear over time [6–8]. When

a TLS moves in to resonance with a qubit, T1can be

suppressed by an order of magnitude in a modern device

[7, 8]. This instability in T1is a threat to quantum com-

puters: when lifetimes decrease, either the computation

suﬀers or the device must be taken oﬄine and retuned,

potentially by changing the qubit frequency or by re-

learning a noise model for error mitigation [9]. Therefore

understanding TLS dynamics is important to improving

superconducting quantum processors. Prior work on TLS

dynamics has focused on the interactions between TLSs

[10, 11]. In this model each high-frequency TLS (i.e. TLS

that are near resonant with the qubit frequency) is cou-

pled to many low-frequency TLSs that occasionally ﬂip

states due to the ambient thermal energy, perturbing the

high-frequency TLS. Evidence for this model has been

observed in both superconducting qubits [6, 7, 12–14] and

∗ted.thorbeck@ibm.com

resonators [11, 15–17]. However, TLS-TLS interactions,

via either electric or elastic dipole interactions, are very

short range [12, 18] compared to the large size of the

transmon capacitor paddles (∼100 µm) or the Joseph-

son junction (∼100 nm), so it is unlikely that multiple

high-frequency TLSs would interact with the same set

of low-frequency TLSs. Therefore the interacting defect

model would struggle to explain simultaneous dynamics

in multiple high-frequency TLSs.

Improvements in T1have helped reveal previously un-

observable decoherence mechanisms, like the impact of

ionizing radiation [19–21]. Ionizing radiation generates

quasiparticles (QPs) that poison the device, creating a

transient dip in T1, but the magnitude and duration of

the dip vary widely in the literature [19–23]. On multi-

qubit devices the QPs can poison many qubits at the

same time, potentially the entire chip, generating cor-

related errors that quantum error correction algorithms

struggle to correct [22–25]. Unfortunately background

radiation is ubiquitous. γ-rays and cosmic ray muons

cannot be easily shielded, and small amounts of αand β

radiation sources could even lurk inside the device pack-

aging. In one recent experiment, background radiation,

which hit the chip on average every 10 s, suppressed T1

to less than 1 µs, and rendered the entire chip unus-

able for about 100 ms [23, 26]. Therefore drastic steps

have been proposed to mitigate these potentially catas-

trophic events such as moving the experiments deep un-

derground [21], adding potentially lossy QP traps [24, 27–

32], and distributing the quantum information across

multiple chips [33, 34].

In this paper, we repurpose the standard qubits of a 27-

qubit IBM Quantum Falcon R6 processor as multiphysics

sensors – electrometers to detect the impact of radiation

and spectrometers to monitor TLS dynamics – to study

the eﬀect of radiation not only on the qubits but also on

the TLS. In section II, we report localized, multi-qubit

arXiv:2210.04780v1 [quant-ph] 10 Oct 2022

Interposer

a) b)

Qubit Chip

qubit

Nb film

e/h electron/hole

QP phonons bump bond

Legend:

Impact Transient

defect

Quasistatic

γ-ray

TLS

FIG. 1. Cartoon of a typical γ-ray impact. The device consists of a qubit chip bump bonded onto an interposer. Each chip

includes a thin, patterned niobium ﬁlm (gray) on a silicon substrate (pink). a) A γ-ray Compton scatters oﬀ of a silicon atom,

ejecting an electron. b) Transient response of the device after the impact. (Defects and TLSs are not shown in b) to avoid

clutter.) The electron scatters oﬀ of other atoms in the substrate generating electron-hole pairs. Relaxation and recombination

of the electrons and holes creates phonons that propagate outward, potentially for several millimeters, creating QPs when they

hit either the ground plane or a qubit. The bump bonds provide both a lower gap material, which may trap QPs, and a

thermalization path for the phonons. c) After the transient response most of the charges recombine, but some of the electrons

and holes get trapped at defects, resulting in a long-lasting charge rearrangement, which is sensed by both the local TLS and

by the oﬀset-charge on the qubits.

oﬀset-charge jumps consistent with the impact of radia-

tion. In contrast to previous reports [22–24], in section

III we observed minimal reduction in T1during impact,

showing the potential for superconducting qubits to be

robust against ionizing radiation. No special measures

were taken to shield the device or to mitigate QPs, sug-

gesting that a combination of materials, packaging, and

architecture determines the qubit’s susceptibility to ra-

diation. However, we sometimes observed an unexpected

long-lasting change in T1after the radiation impact. In

section IV, we show that an impact can scramble the TLS

spectrum, by which we mean that several TLSs that are

near resonant with the qubit either appear, disappear, or

change frequency at the same time. Because scrambling

involves multiple high-frequency TLSs, the interacting

defect model is insuﬃcient to explain these dynamics. In

section V we suggest that the oﬀset-charge jumps and

TLS scrambling can both be explained by charge rear-

rangement after the radiation impact event. We thus

identify a new mechanism contributing to ﬂuctuations in

T1over time, deﬁning a new focus for research to improve

stability of superconducting quantum processors.

II. CHARGE DETECTION OF RADIATION

IMPACT EVENTS

First we brieﬂy review the dynamics of radiation im-

pacting a superconducting qubit chip as described in the

literature [19, 22–24, 35]. We did not deliberately in-

troduce any radioactive sources [19, 21], so we consider

only background radiation. Multiple layers of shielding

protect our device from low-energy photons such as ther-

mal radiation from the higher temperature stages [36–38],

and also attenuate any αor βradiation coming from out-

side the qubit packaging. However, γ-rays and cosmic ray

muons or neutrons cannot be easily shielded, and small

amounts of αand βsources can exist in the qubit pack-

aging. Here we will discuss the impact of a γ-ray, but the

impact of other forms of radiation would be similar. The

likely radioisotopes in the vicinity, such as 40K, 232Th,

and 238U in building materials like concrete, emit γ-rays

with an energy of order 1 MeV [19, 21, 22]. At these en-

ergies, Compton scattering is the most likely interaction

of the γ-ray with the substrate, as shown in Fig. 1(a).

During Compton scattering, the γ-ray ionizes a silicon

atom, ejecting an electron with of order 100 keV of en-

ergy [22]. This high-energy electron initiates a cascade

process, scattering oﬀ of atomic electrons and generating

a large number of electron-hole pairs (Fig. 1(b)). As the

electrons and holes relax and recombine they emit pho-

tons and phonons. The phonons downconvert in energy

until they travel ballistically through the silicon, poten-

tially for several millimeters [29]. When a phonon reaches

the superconductor at the surface of the silicon, it can

break Cooper pairs, generating QPs, which are a source

of loss for the qubits. Because the phonons can travel

for millimeters, many qubits may be simultaneously poi-

soned by the QPs, resulting in correlated energy relax-

ation events [22–24]. As shown in Fig. 1(c) the electrons

and holes that do not quickly recombine will diﬀuse with

a characteristic trapping length of a few hundred microns

until becoming trapped in the substrate or at the surface,

resulting in a long-lasting charge rearrangement that can

be sensed by nearby qubits.

Radiation impact events can be detected using dif-

ferent techniques, so we choose based on the capa-

bilities of our quantum processor. The low-Q(Q ∼

1200) resonators used for fast readout ruled out im-

pact detection methods based on changes in kinetic in-

ductance [20, 21, 39]. Another method, monitoring for

bursts of correlated qubit decays indicative of QP poison-

ing, has been demonstrated in a similar scale quantum

processor[23]; however, we attempted this method and

were not able to resolve any events. Instead, following

[22], we used the qubits to sense the abrupt changes in

the local charge environment caused by an impact. The

energy levels of the transmon, which retains the charge

qubit Hamiltonian, weakly depend on the local charge

environment, quantiﬁed by the oﬀset-charge, ng0. There-

fore a change in oﬀset-charge causes a small change in the

qubit frequency, which can be detected by a Ramsey mea-

surement. We did not make oﬀset-charge sensitive trans-

mons, so our qubits do not have enough charge dispersion

to easily detect an oﬀset-charge jump [22]. However, the

higher energy levels have larger charge dispersion (Fig.

2b), so we used the ef qubit subspace instead [40].

ng0

00.5

-0.5

ħωge

ħωef

|e⟩

|f⟩

M0 Xef/2 -X

Xef/2 Xef

TRamsey= 2 μs

if M0=0

if M1=0

Energy

P(M1=1)

Delay:

40 μs

FIG. 2. Ramsey-based oﬀset-charge jump measurement.

a) Measurement conditional Xprepares the qubit in |ei.

Xef /2−Idle −Xef /2 performs a ﬁxed delay Ramsey se-

quence on the ef transition, with an idle time of 2 µs. The

sequence -X, Xef maps the ef subspace to the ge subspace for

Ramsey measurement M1. A conditional Xthen returns the

qubit to |ei, and a 40 µs delay between repetitions provides

a ﬁxed-delay T1measurement using the outcome of M0. b)

Exaggerated transmon energy level diagram showing the de-

pendence on the unitless and periodic oﬀset-charge, ng0. For

any ng0there are two ef-transition frequencies (purple), one

for each QP parity, symmetrically detuned about the mean

transition frequency ¯ωef . c) Bloch sphere illustration of the

qubit state evolution during the early part of the Ramsey

sequence, showing rotation along the equator at equal rates

but in opposite directions depending on the QP parity (d)

A single qubit trace of P(M1 = 1) as a function of experi-

ment time, showing a jump 23 s into the experiment. Here

the 1 million repetitions were grouped into 200 time bins to

compute probabilities.

The experiment to detect an oﬀset-charge jump is

shown in Fig. 2(a). First, measurement M0 followed

by a conditional Xpulse if the qubit is in |giprepares

the qubit in |ei. An Xef /2 pulse rotates the qubit from

|eito the equator of the ef Bloch sphere, after which

a ﬁxed-delay Ramsey experiment is performed. Driv-

ing the Xef /2 pulses at the mean transition frequency

¯ωef causes the transmon to evolve around the equa-

tor of the Bloch sphere during TRamsey at the detun-

ing ωef (p, ng0)−¯ωef , where pis the QP parity. Be-

cause we only want to detect oﬀset-charge jumps, and

not QP parity ﬂips, here the two QP parities are sym-

metrically detuned above and below ¯ωef , so they yield

the same measurement result (Fig. 2(b,c)). Similar se-

quences, replacing the second Xef /2 with Yef /2, have

been used to measure the QP parity [40, 41]. At the

end of the experiment we map the state back to the ge

subspace and measure the qubit (M1), where P(M1 =

1) = (1 + cos(ef cos(2πng0)TRamsey/2))/2, in the ab-

sence of decoherence. Our transmons had ef charge dis-

persions ef /2π∼800 kHz. For ﬁxed TRamsey, oﬀset-

charge jumps appear as abrupt jumps in P(M1 = 1), as

shown in Fig. 2(d). Because the relationship between

P(M1=1) and ng0is nonlinear and periodic, we cannot

determine the magnitude of ∆ng0, meaning we cannot

use the size of the jumps on diﬀerent qubits to predict

the location of the impact.

We ran the jump detector simultaneously on qubits

across the chip to identify radiation impact events. Fig.

3(a) shows the connectivity of half of the qubit chip; the

other half is similar. Black lines between qubits indi-

cate couplings via bus resonators that mediate two-qubit

gates. These couplings induce a small shift in the qubit

frequency that depends on the state of its neighbors. To

avoid contamination of the Ramsey phase by this inter-

action, we restricted the simultaneous jump detection to

a set of 17 non-neighboring qubits (0, 2, 4, 5, 6, 9, 10, 11,

13, 15, 16, 17, 20, 21, 22, 24, 26) while the remaining 10

qubits were idled. The results from one run of the jump

detector are shown in Fig. 3(b). About 11 s into the run,

simultaneous jumps can be seen on qubits 0, 2, 4, 6, 9, 10

while no jumps were observed on qubits 5, 11, 13 or any

of the qubits on the left half of the chip (not shown). The

qubits that jumped were localized to a small area of the

chip, with a radius of a few millimeters, as shown by the

colored background in Fig. 3(a). Random coincidence

cannot explain simultaneous jumps on so many qubits in

a small portion of the device (App. A 1), so these qubits

must be sensing a common change in the local environ-

ment. While models of oﬀset-charge drift have tradition-

ally focused on local charge rearrangement very close to

an island such as a TLS dipole ﬂip, metallic grain charg-

ing or a ﬂuctuating patch potential [42–46], these models

fail to explain simultaneous discrete jumps on qubits that

are millimeters apart and well isolated by ground planes

(Fig. 1). We thus attribute these multi-qubit jumps to

radiation generating large charge rearrangements in the

substrate, which are not as eﬀectively screened by the

ground plane. As further evidence that the jumps we ob-

serve are due to radiation, in App. A 2 we show that the

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TLSDynamicsinaSuperconductingQubitDuetoBackgroundIonizingRadiationTedThorbeck,1,AndrewEddins,2IsaacLauer,1DouglasT.McClure,1andMalcolmCarroll11IBMQuantum,IBMT.J.WatsonResearchCenter,YorktownHeights,NY10598,USA2IBMQuantum,MIT-IBMWatsonAIlab,CambridgeMA,02142,USA(Dated:October11,2022)Superconductingq...

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TLS Dynamics in a Superconducting Qubit Due to Background Ionizing Radiation Ted Thorbeck1Andrew Eddins2Isaac Lauer1Douglas T. McClure1and Malcolm Carroll1 1IBM Quantum IBM T.J. Watson Research Center Yorktown Heights NY 10598 USA.pdf

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TLS Dynamics in a Superconducting Qubit Due to Background Ionizing Radiation Ted Thorbeck1Andrew Eddins2Isaac Lauer1Douglas T. McClure1and Malcolm Carroll1 1IBM Quantum IBM T.J. Watson Research Center Yorktown Heights NY 10598 USA

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