Nitrogen-vacancy singlet manifold ionization energy S. A. Wolf1 2I. Meirzada1G. Haim3and N. Bar-Gill1 2 3 1The Racah Institute of Physics The Hebrew University of Jerusalem Jerusalem 91904 Israel

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Nitrogen-vacancy singlet manifold ionization energy

S. A. Wolf,1, 2 I. Meirzada,1G. Haim,3and N. Bar-Gill1, 2, 3

1The Racah Institute of Physics, The Hebrew University of Jerusalem, Jerusalem 91904, Israel

2The Center for Nanoscience and Nanotechnology,

The Hebrew University of Jerusalem, Jerusalem 91904, Israel

3Dept. of Applied Physics, Rachel and Selim School of Engineering, Hebrew University, Jerusalem 91904, Israel

The singlet states of the negatively-charged nitrogen-vacancy centers in diamond play a key role

in its optical spin control and readout. In this work, the hitherto unknown ionization energy of

the singlet is measured experimentally and found to be between 1.91-2.25 eV. This is obtained by

analyzing photoluminescence measurements incorporating spin control and NV charge state dif-

ferentiation, along with simulations based on the nitrogen-vacancy’s master equation. This work

establishes a protocol for a more accurate estimate of this ionization energy, which can possibly lead

to improved read-out methods.

INTRODUCTION

Negatively charged Nitrogen vacancy (NV−) centers

are point defects in diamond which show promising quan-

tum properties essential for various applications ranging

from magnetic sensing to quantum computing. These

applications rely on the NV center’s exceptionally long

coherence times and optical spin readout, even at room

temperature. The properties of the NV center are con-

stantly being studied to improve its coherence time and

readout signal-to-noise ratio (SNR). While most of the

energy levels and transition rates have been measured,

some are still unknown and may have a signiﬁcant im-

pact on our understanding of the system’s dynamics.

The NV center is a defect in diamond where two adja-

cent carbons are replaced by one nitrogen and a vacancy.

The negatively charged NV has an additional electron

which leads to two holes in the valence shell and an eﬀec-

tive two-electron system. The ground state of the NV−

is a spin one triplet with a 2.87 GHz zero-ﬁeld-splitting

between spin projections ms= 0 and ms=±1, and can

be controlled using a resonant microwave (MW) drive.

These spin states are located 2.6 eV below the conduction

band (Fig. 1). The electronically excited triplet state is

located 1.945 eV above the ground state with a lifetime

of ∼13 ns [1, 2]. The radiative transition between the

two triplet states is dominated by the phonon side band

(PSB), with ∼650-800 nm emission. The NV−has an

additional singlet (spin zero) manifold consisting of two

energy levels that are separated by 1.19 eV with an ex-

cited state lifetime of ∼0.1 ns [3–6]. Spin selective inter-

system crossing (ISC) allows the excited triplet to decay

non radiatively to the singlet excited state predominantly

from the ms=±1 spin projection. The ground singlet

state, commonly referred to as a metastable state due

to its long lifetime, ∼300 ns, decays back to the ground

triplet state. Theoretical studies have predicted diﬀerent

values for the energy of the singlet state [7–16]. How-

ever, the energy gap between the singlet manifold and the

conduction band has yet to be measured experimentally,

and is the focus of this work. The NV−can also be ion-

ized optically, and convert to a neutral NV (NV0) center

which has an energy gap of 2.16 eV between it’s ground

and excited states.

FIG. 1. Known energy levels of NV centers.

The most commonly used readout method at room

temperature is based on collecting photoluminescence

(PL) from the PSB while exciting the triplet transition

using the PSB (typically at 532 nm) [17]. Due to the

spin selective ISC, the number of photons emitted de-

pends on the initial spin projection of the ground triplet

state. However, this readout method yields very poor

SNR and requires thousands of repetitions [17]. Diﬀerent

readout methods have been proposed in order to increase

the readout SNR [17, 18], but those readout methods all

rely on the spin-dependent transition to the metastable

singlet state. Better knowledge of the energy levels and

transition rates, and speciﬁcally the unknown singlet en-

ergy, will allow more accurate modeling of the NV dy-

namics and may lead to the development of improved

readout methods. Theoretical studies have predicted var-

arXiv:2210.04171v1 [quant-ph] 9 Oct 2022

FIG. 2. The NV center’s dynamics during the pulse sequence. Blue (orange) dots represent the population with (without) a

πpulse in the initialization step. The faded dots are a reminder of the population before the current pulse. For simplicity,

the ionization pulse shows only the potential ionization from the singlet. Depending on the wavelength, additional transitions

may occur during this pulse such as NV−/NV0excitation and ionization/recombination from the excited states (which are not

shown in this ﬁgure).

ious values for the energy of the singlet state, and experi-

mental data is required in order to constrain this param-

eter [7–14, 16].

In a recent paper [19] an experimental protocol for

ﬁnding the ionization energy of the singlet level was pro-

posed by the authors. The main idea behind this protocol

is to maximally populate the singlet level before apply-

ing an ionization laser pulse at varying wavelengths to

check for ionization (from NV−to NV0). An analysis

of the NV’s master equation predicts signiﬁcantly diﬀer-

ent results between ionization pulse wavelengths below

and above the ionization energy. In this work we report

ﬁrst experimental results towards a direct measurement

of the singlet ionization energy and provide experimen-

tal bounds to the singlet energy. Our measurement and

analysis included important modiﬁcations on the original

experiment proposal.

The manuscript is organized as follows: we ﬁrst detail

the experimental protocol and pulse sequence used. We

then describe the measured results, mostly of the ion-

ization ratio normalized to diﬀerent spin initializations,

as a function of ionization laser power. We present the

detailed measurement results, along with comparisons to

simulations, in separate sections based on the ionization

laser wavelength. Finally, we summarize and conclude.

PULSE SEQUENCE

The pulse sequence in the following measurements be-

gins with an initialization to the ground state’s ms=±1

spin projection, i.e. initialization to spin projection

ms= 0 with a long 532 nm pulse followed by a πpulse

using resonant MW. Next, a short 532 nm population

pulse optimized to maximally populate the ground sin-

glet state is applied (200 µW for 400 ns, see Appendix

B for details), followed by a ∼30 ns delay to allow the

triplet excited states to fully decay to either the singlet

or ground triplet states. At this point the singlet is max-

imally populated and an ionization pulse is applied while

the singlet is still highly populated (∼100 ns). Finally, a

532 nm readout pulse is applied, during which the emit-

ted NV−PL is collected using a 650 nm long-pass ﬁlter

(see ﬁgure 2 in Ref. [19]).

Assuming the singlet manifold lies between the ground

and excited triplet states, the ionization energy of the

ground singlet state is bound between 1.84 eV, corre-

sponding to 674 nm, and 2.6 eV, corresponding to 477

nm [19]. In the following experiments we use a pulsed

supercontinuum laser (NKT Photonics, WL-SC-400-15-

PP) followed by diﬀerent bandpass ﬁlters (BPF) (Blue

- 500±20 nm, Green - 550±20 nm, Red - 650±20 nm,

Long Red - 676±4 nm) and a near-infrared (NIR) 976

nm continuous-wave (CW) laser (BL976-PAG900) for the

ionization pulse. The Long Red ﬁlter and NIR laser are

not expected to ionize from the singlet ground state and

were added as control measurements. The experiments

were conducted on a diamond sample with a high den-

sity of NV centers. During the experiment the PL was

measured as a function of the ionization pulse power. A

reference measurement without a πpulse in the ﬁrst ini-

tialization step (i.e. initialized to ms= 0 in the ﬁrst step

of the experiment) was conducted for every ionization

laser power in order to help isolate the singlet contri-

bution from the rest of the NV dynamics. The results

both with and without the πpulse are normalized by the

PL measurement without an ionization pulse (ionization

pulse power = 0 mW), and with the same initial state.

In the following section we present the PL as a function

of the ionization pulse power with diﬀerent ﬁlters, as well

as the ratio between the experimental results with and

without the πpulse in the initialization step (which will

be referred to as PNP ratio).

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Nitrogen-vacancysingletmanifoldionizationenergyS.A.Wolf,1,2I.Meirzada,1G.Haim,3andN.Bar-Gill1,2,31TheRacahInstituteofPhysics,TheHebrewUniversityofJerusalem,Jerusalem91904,Israel2TheCenterforNanoscienceandNanotechnology,TheHebrewUniversityofJerusalem,Jerusalem91904,Israel3Dept.ofAppliedPhysics,Rachel...

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Nitrogen-vacancy singlet manifold ionization energy S. A. Wolf1 2I. Meirzada1G. Haim3and N. Bar-Gill1 2 3 1The Racah Institute of Physics The Hebrew University of Jerusalem Jerusalem 91904 Israel

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