Two-photon interference from a quantum emitter in hexagonal boron nitride Clarisse Fournier1 S ebastien Roux12 Kenji Watanabe3 Takashi Taniguchi4 St ephanie Buil1 Julien Barjon1 Jean-Pierre Hermier1 Aymeric Delteil1

2025-04-24 0 0 945.14KB 7 页 10玖币

Two-photon interference from a quantum emitter in hexagonal boron nitride

Clarisse Fournier1, S´ebastien Roux1,2, Kenji Watanabe3, Takashi Taniguchi4,

St´ephanie Buil1, Julien Barjon1, Jean-Pierre Hermier1, Aymeric Delteil1

1Universit´e Paris-Saclay, UVSQ, CNRS, GEMaC, 78000, Versailles, France.

2Universit´e Paris-Saclay, ONERA, CNRS, Laboratoire d’´etude des microstructures, 92322, Chˆatillon, France.

3Research Center for Functional Materials, National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan

4International Center for Materials Nanoarchitectonics,

National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan

aymeric.delteil@usvq.fr

Recently discovered quantum emitters in two-dimensional (2D) materials have opened new per-

spectives of integrated photonic devices for quantum information. Most of these applications require

the emitted photons to be indistinguishable, which has remained elusive in 2D materials. Here, we

investigate two-photon interference of a quantum emitter generated in hexagonal boron nitride

(hBN) using an electron beam. We measure the correlations of zero-phonon-line photons in a Hong-

Ou-Mandel (HOM) interferometer under non-resonant excitation. We ﬁnd that the emitted photons

exhibit a partial indistinguishability of 0.44 ±0.11 in a 3 ns time window, which corresponds to a

corrected value of 0.56 ±0.11 after accounting for imperfect emitter purity. The dependence of the

HOM visibility on the width of the post-selection time window allows us to estimate the dephasing

time of the emitter to be ∼1.5 ns, about half the limit set by spontaneous emission. A visibility

above 90 % is under reach using Purcell eﬀect with up-to-date 2D material photonics.

PACS numbers:

Two-photon interference is essential for many photonic

implementations of quantum information protocols, from

linear optical quantum computing [1] to distant entangle-

ment generation [2–4] and quantum communication [5].

The indistinguishability of two single-photon pulses –

which quantiﬁes their ability to interfere – results in the

so-called Hong-Ou-Mandel (HOM) eﬀect [6]. The latter

refers to the fact that perfectly indistinguishable pho-

tons simultaneously reaching the two input ports of a

beamsplitter always exit the beamsplitter from the same

output port [7]. Experimental observation of the HOM

eﬀect between consecutive photons from a quantum emit-

ter constitutes an important milestone in the use of

a physical system for the generation of scalable pho-

tonic qubits. Among the physical systems able to gener-

ate indistinguishable photons, solid-state single-photon

emitters (SPEs) have been widely investigated due to

their potential for integration in photonic devices [8].

Thus, photon indistinguishability has been experimen-

tally demonstrated with III-V semiconductor quantum

dots [9–12] and color centers in three-dimensional wide

bandgap crystals [13–15].

In turn, recently discovered quantum emitters in

2D materials, comprising trapped excitons in transi-

tion metal dichalcogenides [16–20] and color centers in

hBN [21–23], have raised a growing interest owing to the

perspectives of extreme miniaturization and integration

into complex heterostructures [24] – yet without demon-

stration of two-photon interference to date. Among these

systems, a recently discovered family of hBN SPEs stands

out – a class of blue emitting color centers (abbrevi-

ated B-centers in the following) that can be generated

at controlled locations using an electron beam. Their

zero-phonon-line (ZPL) center wavelength is consistently

found within 3 meV around 436 nm [25–27]. Several

studies have already demonstrated their spectral stabil-

ity, narrow linewidth, brightness and single-photon emis-

sion up to room temperature [25, 26, 28].

In this letter, we characterize two-photon interference

of light emitted by an individual B-center. Our sam-

ple consists of a single hBN crystal grown using high

pressure, high temperature conditions [29], that we ex-

foliated on a SiO2(285 nm)/Si substrate. We generate

a SPE ensemble in a commercial scanning electron mi-

croscope (SEM) using a slightly defocused electron beam

(diameter ∼300 nm) under 15 kV acceleration voltage

and 10 nA current, following [25] (ﬁgure 1a). We subse-

quently characterize the sample in a confocal microscope

operating in a helium closed-cycle cryostat, keeping the

sample at 4 K. The sample is optically excited by a pulsed

diode laser of 405 nm wavelength, 850 µW power and

80 MHz repetition rate (ﬁgure 1b) that is focused on the

sample using a microscope objective of numerical aper-

ture 0.8. The SPE luminescence is collected through the

same objective, and coupled into a single-mode ﬁber. In

the following, we focus on an individual SPE with a ZPL

centered at 436.24 nm. Figure 1c shows a low resolution

spectrum of the SPE, exhibiting the usual spectral shape

of the B-centers, which comprises a narrow ZPL (40 % of

the emission) and an acoustic phonon sideband (60 %).

We ensure that spectral diﬀusion of the ZPL is limited,

as shown ﬁgure 1d, where the wavelength ﬂuctuations are

contained below 20 pm.

Figure 2a depicts the experimental setup used for two-

photon interference characterization. The photolumi-

nescence is collected in a single-mode ﬁber that chan-

arXiv:2210.05590v2 [quant-ph] 28 Apr 2023

440 460 480 500

Wavelength (nm)

0.0

0.2

0.4

0.6

0.8

1.0

Counts (arb. u.)

436.0 436.5

Wavelength (nm)

100

150

200

250

Time (s)

436.0 436.5

0.0

0.5

Counts (arb. u.)

FIG. 1: (a) Irradiation by a 15 kV electron beam generates

B-centers in a multilayer hBN crystal. (b) Energy levels of

the SPE: A 405 nm laser excites the emitter. Non-radiative

relaxation occurs, followed by emission of a photon at 436 nm.

the acoustic phonon sideband can be observed. Inset: High

resolution spectrum, limited by the spectrometer resolution of

about 100 µeV. The phonon pedestal of the main panel is no

more visible due to the higher resolution. (d) High resolution

spectra as a function of time measured with 2 s integration

time during 5 min, showing the stability of the SPE.

nels the photons to a delayed Mach-Zehnder interferom-

eter. The ZPL is ﬁltered using a transmission grating

of 1379 grooves per millimeter. Together with subse-

quent coupling to single-mode ﬁbers, it implements a

narrow bandpass ﬁlter of 100 GHz bandwidth. This spec-

tral width is larger than the time-averaged linewidth but

much narrower than the width of the acoustic phonon

sideband of about 7.8 meV (1.9 THz). A polarizer en-

sures that the input photons have a well-deﬁned lin-

ear polarization at the input port of the delayed Mach-

Zehnder interferometer. One of the arms is delayed by

the same amount as our repetition period (12.5 ns) using

a 2.6 m ﬁber, such that two consecutively emitted pho-

tons can simultaneously impinge on the beamsplitter. A

liquid crystal retarder is inserted in the other arm to ro-

tate the photon polarization by 90 degrees when suited,

allowing photon polarization at the two input ports of

the second beamsplitter to be either identical (parallel)

or orthogonal. The total count rate of the ZPL photons

at the output ports is about 1200 counts per second. Fig-

ure 2b shows a histogram of the photon detection times

at the output ports. We ﬁrst consider photon detections

occurring during the ∆t= 3 ns time window highlighted

by the orange shadowing on ﬁgure 2b, that is located

after the laser pulse of width 550 ps (gray shadowing

on ﬁgure 2b). Figure 2c shows the second-order photon

correlations measured in a Hanbury Brown and Twiss

conﬁguration of the interferometer (i.e. in a single arm).

The relative height of the center period allows to infer

the photon purity of g(2)

HBT(0) = 0.14 ±0.03, limited by

reminiscent background signal and dark counts.

We then measure the photon coincidences in the HOM

conﬁguration of the interferometer. Figure 2d gives

the normalized coincidences measured during 36 hours

while alternating between parallel and orthogonal po-

larizations using the liquid crystal retarder, consider-

ing photons detected during the same time window of

width ∆t. The signiﬁcant reduction of the center period

value g(2)

HOM,k(0) = 0.32±0.05 in the parallel polarization

case as compared with the orthogonal case g(2)

HOM,⊥(0) =

0.58 ±0.07 is a signature of photon coalescence. The raw

(uncorrected) degree of indistinguishability of the emit-

ted photons is then given by the interference visibility

deﬁned as VHOM = 1 −g(2)

HOM,k(0)/g(2)

HOM,⊥(0). In our

case, we ﬁnd VHOM = 0.44 ±0.11. In the case of single

photons with ideal purity (i.e. g(2)

HBT(0) = 0), this quan-

tity ranges between VHOM = 0 (perfectly distinguishable

photons) and VHOM = 1 (perfectly indistinguishable pho-

tons). When multiple detection events are not negligible,

the theoretical bounds of g(2)

HOM,k(0) and g(2)

HOM,⊥(0) are

oﬀset upwards [9], such that the corrected visibility reads

Vcorr

HOM =1+2g(2)

HBT(0)VHOM. In the case of the stud-

ied SPE, the theoretical upper (resp. lower) bounds ac-

counting for our ﬁnite value of g(2)

HBT(0) are indicated by

light (resp. dark) gray bars on the center period of the

histogram shown ﬁgure 2d. Accordingly, we ﬁnd a cor-

rected HOM visibility of Vcorr

HOM = 0.56 ±0.11. This num-

ber is comparable with the indistinguishability of non-

resonantly excited quantum dots [9, 11] or color centers

in wide gap 3D semiconductors [15]. This observation of

HOM interference from single photons emitted by a 2D

material quantum emitter constitutes the main result of

our study, and suggests possible practical applications

of hBN for optical quantum information, upon further

improvement of the visibility.

The limited value of the corrected HOM visibility can

be possibly attributed to fast dephasing of the optical

dipole. The total dephasing rate of an optical transition

can be written γ= Γsp/2 + γ∗, where Γsp = 1/T1is

the spontaneous emission rate, and γ∗= 1/T ∗

2denotes

the rate of dephasing caused by reservoirs other than

the vacuum electromagnetic ﬁeld. In the pulsed regime,

only γ∗causes a reduction of the photon indistinguisha-

bility [30, 31]. An expected consequence of dephasing

is that extending the integration window ∆twould de-

grade the HOM visibility by allowing a larger delay time

between detected pairs [7, 32–35]. It is then in princi-

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Two-photoninterferencefromaquantumemitterinhexagonalboronnitrideClarisseFournier1,SebastienRoux1;2,KenjiWatanabe3,TakashiTaniguchi4,StephanieBuil1,JulienBarjon1,Jean-PierreHermier1,AymericDelteil11UniversiteParis-Saclay,UVSQ,CNRS,GEMaC,78000,Versailles,France.2UniversiteParis-Saclay,ONERA,CNRS,L...

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Two-photon interference from a quantum emitter in hexagonal boron nitride Clarisse Fournier1 S ebastien Roux12 Kenji Watanabe3 Takashi Taniguchi4 St ephanie Buil1 Julien Barjon1 Jean-Pierre Hermier1 Aymeric Delteil1

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