1
Progress in Photonic Quantum Technologies 2026 · IIT Bombay
Making Quantum Light
How do you make a crystal emit exactly one photon — where you want it, in the colour you want?
Anshuman Kumar Laboratory of Optics of Quantum Materials (loqm.tech)
Department of Physics & QuICST, Indian Institute of Technology Bombay
2
WHAT ARE SOLID STATE SINGLE PHOTON SOURCES?
Solid-state single-photon sources are the currency of quantum photonics
Zalogina et al., Laser Photon. Rev. Volume 20, Issue 5 (2026)
Esman et al., ADFM, Volume 34, Issue 30 (2024)
3
WHY SINGLE PHOTONS? Quantum Computing
Solid-state single-photon sources are the currency of quantum photonics
4
WHY SINGLE PHOTONS? Quantum Communication
Solid-state single-photon sources are the currency of quantum photonics
Zahidy et al, npj Quantum Information volume 10, 2 (2024)
Yang et al, Light: Science & Applications volume 13, 150 (2024)
5
WHY HEXAGONAL BORON NITRIDE
Suited for photonic integration
Emitter embedded in bulk crystal
Etch photonic structure around emitter
Risk of damage, low yield
Fabricate photonic structure
Transfer hBN layer
Deterministic hybrid quantum photonics
Conventional paradigm
hBN paradigm
hBN advantage: decoupled emitter creation and nanophotonic fabrication
6
One equation to summarize Quantum Emitter Photonics
Γ = (2π/ℏ) |⟨f|Ĥ |i⟩|² × ρ(ω)
Fermi's golden rule
THE MATRIX ELEMENT
|⟨f|Ĥ|i⟩|²
Everything about the emitter — which defect, where it sits, its colour, its spin.
THE DENSITY OF STATES
ρ(ω)
The photonic modes available to radiate into — reshape it with a nanostructure.
7
THE CHALLENGE
Γ ∝ |M|² ρ(ω)
“Deterministic” means controlling two things at once
A single-photon emitter is only device-ready if you can decide both where it sits and what it is. These are independent problems — and existing maskless routes solve only one at a time.
WHERE
POSITIONAL DETERMINISM
The emitter must land at the optical field maximum — a cavity antinode, a waveguide, a metasurface hot spot. A randomly placed emitter cannot be coupled.
WHAT
SPECTRAL & CHEMICAL DETERMINISM
It must be the same defect every time, at a known, reproducible wavelength. A scatter of uncontrolled species cannot be addressed as a device.
The gap: maskless methods give position or identity — never both at once. That simultaneity is the real bottleneck for scalable emitter arrays.
8
THE LANDSCAPE
Γ = (2π/ℏ) |⟨f|Ĥ |i⟩|² × ρ(ω)
The gap we target: positional accuracy AND chemical selectivity, with device scale yield
Electron irradiation
NEW KNOB: DOSE, THERMAL QUENCHING: Deterministic, high temperature stability
Ion implantation
NEW ION, SPIN RESONANCES, Deterministic spin defect, spectral control
Ga-FIB + C + anneal
NEW APPROACH: Maskless, direct-write, chemically selective — 89% yield.
arXiv:2606.23334
9
Making quantum light part 1
Γ ∝ |M|² ρ(ω)
10
PART 1 · Our CGIB approach
Γ = (2π/ℏ) |⟨f|Ĥ |i⟩|² × ρ(ω)
Carbon-assisted focused-ion-beam engineering: a three-step, maskless flow
The core idea: gallium gives where (position); carbon gives what (the defect family).
AI generated figure
Carbon: the established origin of visible hBN emitters
- no other implanted species produces them.
Mendelson et al., Nat. Mater. 19, 540 (2020)
11
PART 1 · Our CGIB approach
Γ = (2π/ℏ) |⟨f|Ĥ |i⟩|² × ρ(ω)
Carbon-assisted focused-ion-beam engineering: a three-step, maskless flow
Ga FIB milling followed by
selective carbon deposition
Resulted SPE array after 3 step fabrication process
12
PART 1 · Our CGIB approach
Γ = (2π/ℏ) |⟨f|Ĥ |i⟩|² × ρ(ω)
Carbon-assisted focused-ion-beam engineering: a three-step, maskless flow
Ga FIB milling followed by
selective carbon deposition
Statistical Analysis of all the emitters
Second order correlation measurements of representative emitters
13
Making quantum light part 2
Γ ∝ |M|² ρ(ω)
14
PART 2 · Our EBID approach
Γ = (2π/ℏ) |⟨f|Ĥ |i⟩|² × ρ(ω)
Electron beam dose engineering approach: a two-step, maskless flow
The core idea: quantitative single photon purity control -- balance defect creation and carbonaceous contamination
AI generated figure
15
PART 2 · Our EBID approach
Γ = (2π/ℏ) |⟨f|Ĥ |i⟩|² × ρ(ω)
Electron beam dose engineering approach: a two-step, maskless flow
Experiments confirm dose dependent purity
16
PART 2 · Our EBID approach
Γ = (2π/ℏ) |⟨f|Ĥ |i⟩|² × ρ(ω)
Electron beam dose engineering approach: a two-step, maskless flow
Can e-beam-engineered emitters survive thermal processing? Data says ‘yes’
17
Making quantum light part 3
Γ ∝ |M|² ρ(ω)
18
PART 3 · Our KII approach
Γ = (2π/ℏ) |⟨f|Ĥ |i⟩|² × ρ(ω)
Kr+ implantation approach: a route towards spin defects?
From photons to spin: complete quantum functionality in hBN
19
PART 3 · Our KII approach
Γ = (2π/ℏ) |⟨f|Ĥ |i⟩|² × ρ(ω)
Kr+ implantation approach: a route towards spin defects?
Spin Physics: one S=1 or two coupled S=½?
Open question: Spin pair models predict an effective S = ½ with no-resolvable-ZFS acceptor (hyperfine-dominated)
we resolve a finite, small triplet ZFS
AI generated figure
Prof. Dahiwale, SPPU
20
PART 3 · Our KII approach
Γ = (2π/ℏ) |⟨f|Ĥ |i⟩|² × ρ(ω)
Kr+ implantation approach: a route towards spin defects?
The spin signal appears only after krypton implantation
21
PART 3 · Our KII approach
Γ = (2π/ℏ) |⟨f|Ĥ |i⟩|² × ρ(ω)
Kr+ implantation approach: a route towards spin defects?
The spin signal appears only after krypton implantation
Rajaraman, IITB
22
THE SECOND KNOB · NQM TECHNICAL GROUP 3
Γ ∝ |M|² ρ(ω)
A good emitter still needs somewhere to emit into
Everything so far was the emitter — the |M|² half. The other half, ρ(ω), is the photonic structure around it. That is my second NQM track: silicon nitride integrated photonics.
BUILD
PASSIVE Si₃N₄ PLATFORM
Waveguides, microrings, and the ring–bus coupling gaps that set how light enters a resonator. Propagation loss is the figure of merit.
COUPLE
PURCELL ENHANCEMENT
A resonator changes which photon modes exist, so the emitter radiates faster and into the channel you chose. The loop closes.
Status: one patent filed on a monolithic Si₃N₄ microresonator platform with intrinsic emitters, two manuscripts in preparation.
23
FROM LAB TO TECHNOLOGY · NQM + ANRF
What it takes to turn this into a product
The single-photon supply chain is already commercial — Quandela, Aegiq and Sparrow Quantum sell sources and photonic processors. Materials work like ours feeds the front end of that chain.
TRL 4
WHERE THIS WORK SITS TODAY
Three preprints and a filed patent: emitters placed on demand, purity made a dial, a spin signature measured. Validated in the lab.
TRL 5
WHAT IS STILL MISSING
An emitter inside a working circuit, with end-to-end device numbers. Until that exists, this is good physics — not yet a component.
The honest gap: no amount of better material substitutes for emitter-in-circuit integration. That is the next programme, not the next paper.
24
IF YOU WANT TO WORK ON THIS
What this field actually asks of you
Almost nothing in this talk needed exotic mathematics. It needed patience at a microscope, and knowing which single number settles an argument.
LEARN
THE BACKGROUND THAT PAYS OFF
Quantum mechanics and solid state at your UG level. Optics you have touched with your hands. Python for fitting. One simulation tool, properly.
DO
HOW PEOPLE ACTUALLY GET IN
Summer projects, visiting students, project assistantships. Write to groups whose papers you have read, naming the figure you had a question about.
Come with a specific question: loqm.tech · anshuman.kumar@iitb.ac.in
Three knobs on |M|² — and now the ρ(ω) knob too
WHERE + WHAT (Carbon-assisted Ga-FIB)
Maskless, site-correlated SPE arrays, ~89% yield, room-temperature, photostable.
Quality (Electron-dose engineering)
Quantitative single-photon-purity control via dose; emitters survive thermal processing (reversible quenching) → device-integration ready.
SPIN (Kr⁺ implantation)
Chemistry-decoupled native-defect generation; NIR (~830 nm) spin-active V_N–C_B pairs; a direct EPR handle (g ≈ 2.003), consistent with a spatially separated spin pair.
Outlook
Coupling these deterministic emitters to engineered photonic environments
Emission indistinguishability
Engineered spin interaction strength
Thank you — questions welcome
Laboratory of Optics of Quantum Materials (loqm.tech), IIT Bombay · anshuman.kumar@iitb.ac.in