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Quantum Photonics & Nanophotonics

Quantum Dot Single-Photon Source Lab

Model self-assembled InAs/GaAs quantum dots, DBR micropillar Purcell cavities ($F_P > 10$), Hanbury Brown-Twiss sub-poissonian antibunching ($g^{(2)}(0) < 0.01$), Hong-Ou-Mandel indistinguishability ($V_{\text{HOM}} > 95\%$), and 20-year quantum photonics foundry economics.

Jump to Visualizer ↓ View 20-Yr DCF ↓
Purcell Factor (FP)
12.4×
Spontaneous Acceleration
Emission Lifetime (τsp)
82 ps
Bulk GaAs: 1,100 ps
Extraction Efficiency (η)
64.8%
First-Lens Collection
Multiphoton Rate (g(2)(0))
0.008
HBT Antibunching Dip
Indistinguishability (VHOM)
96.8%
Hong-Ou-Mandel Dip
Single-Photon Rate
51.4 MHz
80 MHz Pulsed Laser
Optical Linewidth
8.4 μeV
4.2 K Homogeneous
20-Yr Cleanroom NPV
$34.80M
Discount Rate: 9.0%

Quantum Dot & Cavity Parameters

930.0 nm
900 nm (Near-IR) 980 nm (InGaAs)
14,500
4,000 (Broad / Low Loss) 30,000 (Ultra-High Q)
2.2 μm
1.4 μm (Small Vmode) 4.0 μm (Low Scatter)
0.0 μeV
-80 μeV Resonant (0) +80 μeV
4.2 K
1.6 K (Sub-Kelvin base) 25.0 K (Phonon dephasing)
22
18 (High outcoupling) 28 (Higher Q / Lower escape)
1.00 π
0.5 π (Under-driven) 1.0 π (Ideal Rabi) 1.8 π (Re-excitation)
80.0 MHz
20 MHz 150 MHz

Photonics Foundry Commercial Drivers

18
6 Units / yr 48 Units / yr
$480k
$250k $850k
12.8
$7.0M (Pilot Line) $22.0M (High Volume)
16.0%
9.0%

Panel 1: DBR Micropillar Cavity Heterostructure

Cavity QED Purcell Boost
Distributed Bragg reflector (DBR) micropillar with embedded single InAs quantum dot at the central antinode, accelerating emission via Purcell factor $F_P$.

Panel 2: Quantum Optics Coherence & HOM Interference

HBT & Two-Photon Dip
Hanbury Brown-Twiss antibunching dip ($g^{(2)}(0) \approx 0$) proving sub-Poissonian single-photon emission alongside Hong-Ou-Mandel two-photon bunching visibility $V_{\text{HOM}}$.

Panel 3: 20-Year Cleanroom Foundry DCF Curve

Commercial Payback
Cumulative free cash flow trajectory demonstrating capital breakeven, turnkey photonics hardware margins, and service contracts.

20-Year Commercial Quantum Photonics Cash Flow Projection

Hardware Foundry & Support Contracts
Year Units Shipped Gross Revenue Operating Expenses Net Cash Flow Cumulative Cash Flow

Solid-State Single-Photon Sources & Quantum Information

1. Stranski-Krastanov Growth & Discrete Energy States

Semiconductor quantum dots form spontaneously when InAs is deposited on GaAs with a 7.2% lattice mismatch. Above a critical monolayer thickness (~1.7 ML), strain relaxation drives the 2D wetting layer to transition into coherent 3D nanometer-sized pyramids or lens-shaped islands. Carriers are confined in all three spatial dimensions on a length scale smaller than the exciton Bohr radius (~10 nm), collapsing the continuous energy band into discrete atomic-like $s$-shell and $p$-shell energy levels with sub-10 μeV natural linewidths at liquid helium temperatures.

2. Purcell Enhancement in DBR Micropillars

In bulk semiconductor, the high refractive index of GaAs ($n \approx 3.45$) traps over 98% of emitted photons via total internal reflection. Etching circular micropillars through planar GaAs/AlAs distributed Bragg reflector stacks forms a 3D optical resonator. Tuning the micropillar cavity into resonance with the $s$-shell exciton transition boosts the spontaneous emission rate by the Purcell factor $F_P = \frac{3}{4\pi^2} (\frac{\lambda}{n})^3 \frac{Q}{V}$. Accelerated radiative decay shortens the exciton lifetime from 1,100 ps to under 80 ps, heavily outcompeting non-radiative acoustic phonon dephasing and channeling emitted photons into a low-divergence Gaussian spatial mode with extraction efficiency exceeding 60%.

3. Hanbury Brown-Twiss Sub-Poissonian Statistics

The fundamental benchmark of single-photon emission is the second-order intensity correlation function $g^{(2)}(\tau) = \langle I(t) I(t+\tau) \rangle / \langle I(t) \rangle^2$. When driven by resonant optical $\pi$-pulses, the quantum dot undergoes deterministic single-cycle Rabi inversion from ground $|g\rangle$ to single-exciton $|X\rangle$. The state collapses upon emitting a single photon and cannot emit a second photon until the next laser pulse arrives. In a Hanbury Brown-Twiss interferometer, this manifests as a dramatic vanishing coincidence dip at zero delay: $g^{(2)}(0) < 0.01$, confirming deterministic sub-Poissonian photon emission.

4. Hong-Ou-Mandel Indistinguishability & Quantum Computing

Linear optical quantum computing (LOQC) and measurement-based photonic cluster state generation require sequentially emitted photons to be indistinguishable. When two identical single photons interfere at a symmetric 50:50 beamsplitter, the two probability amplitudes where both photons are transmitted or both reflected have opposite signs and destructively cancel ($|1,1\rangle \to \frac{1}{\sqrt{2}}(|2,0\rangle - |0,2\rangle)$). This Hong-Ou-Mandel (HOM) bunching effect produces a deep zero-delay coincidence minimum with visibility $V_{\text{HOM}} > 95\%$, governed by the ratio of radiative decay rate to pure dephasing rate ($\gamma_{\text{sp}} / (\gamma_{\text{sp}} + 2\gamma_{\text{deph}})$).

Concept Check: Quantum Dot Nanophotonics Quiz

1. How does cavity Purcell enhancement protect photon indistinguishability against pure dephasing?



2. What physical outcome occurs during a Hong-Ou-Mandel experiment when two indistinguishable single photons arrive simultaneously at a 50:50 beamsplitter?



3. Why is liquid helium cryogenic cooling (T ≈ 4 K) mandatory for high-visibility quantum dot single-photon emission?



Frequently Asked Questions

An epitaxial quantum dot is a nanometer-scale semiconductor island (such as InAs inside a GaAs matrix) grown via Stranski-Krastanov self-assembly. Due to three-dimensional carrier confinement, its electronic density of states becomes atomic-like with discrete delta-function energy levels. When resonantly driven by a short laser pi-pulse, the quantum dot creates a single electron-hole pair (exciton) that radiatively recombines, deterministically emitting exactly one photon per pulse.

Embedding the quantum dot at the antinode of a distributed Bragg reflector (DBR) micropillar optical microcavity with quality factor Q and small mode volume V enhances the local vacuum field density of states. This yields a Purcell factor FP = (3 / (4 * pi^2)) * (lambda / n)^3 * (Q / V). Purcell enhancement accelerates spontaneous emission by over 10 times (shortening lifetime from 1.1 ns to under 100 ps) and funnels over 60% of emitted photons into a collimated single-mode spatial beam.

In a Hanbury Brown-Twiss (HBT) interferometer, the emitted light is split by a 50:50 beamsplitter onto two single-photon detectors. The normalized coincidence rate g(2)(tau) measures the photon statistics. For a true single-photon source, an emitted photon cannot split, leading to zero simultaneous coincidences at zero delay (g(2)(0) approaching 0, strictly under 0.05). In contrast, Poissonian laser light gives g(2)(0) = 1 and thermal chaotic light gives g(2)(0) = 2.

Hong-Ou-Mandel (HOM) two-photon interference tests whether successively emitted photons are quantum-mechanically identical across polarization, spatial mode, and spectral wavepacket envelope. When two indistinguishable photons simultaneously arrive at a symmetric beamsplitter, destructive quantum interference causes both photons to bunch and exit the same output port together. Achieving HOM visibility above 95% is the foundational benchmark required for linear optical quantum computing, photonic cluster state generation, and quantum repeaters.

A commercial quantum dot single-photon foundry requires approximately $12.8 million in initial capital expenditure. Major investments include solid-source molecular beam epitaxy (MBE) with in-situ wafer monitoring ($4.2M), electron-beam lithography and deep cryogenic ICP-RIE etching for smooth micropillar sidewalls ($3.4M), low-vibration closed-cycle 4K confocal cryostats with superconducting nanowire detectors ($3.0M), and automated fiber-pigtailed optical packaging stations ($2.2M).

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