Integrated Quantum Photonics

Integrated quantum photonics

Besim Mazhiqi

Benni Brecht Christine Silberhorn ICTP Winter College on Optics: Integrated Quantum Optics Quantum Photonics and Information Paderborn University 13/02/2020

Christof Harald Benjamin Eigner Herrmann Brecht

Underlying concepts

Device toolbox

Fabrication technology

HOM-on-chip circuit Why integrated (quantum) photonics?

University of Tokyo, Furusawa group University of Science and Technology of China, Hefei, Pan group History ▶ „Invention“ of Integrated Optics by S.E. Miller [1] ▶ Roadmap from conventional integrated optics to „Integrated Quantum Optics“ ? Quantum

quantum

photon source

quantum

Keep ideas, but adapt them to quantum optics!

[1] S.E. Milller, Bell System Technical Journal, 48, 2059 (1969) Quantum optics “on chip”

▶ Common structure to all quantum experiments ▶ One objective of the Integrated Quantum Optics group:

Integrate as many components as possible (onto a single platform?)

Create Detect Photons Photons

Manipulate Photons Integrated quantum photonics

Photonic quantum simulator

Alán Aspuru-Guzik and Philip Walther, Nature Physics 8, 285–291 (2012) Integrated quantum photonics

Photonic quantum simulator

Advantages:

• Many spatial modes available / controllable • Interferometric stability of large optical system • Compact devices; miniaturaization • „Simple“ operation • High efficiency (?)

Alán Aspuru-Guzik and Philip Walther, Nature Physics 8, 285–291 (2012) Integrated quantum photonics Photonic quantum simulator

Alán Aspuru-Guzik and Philip Walther, Nature Physics 8, 285–291 (2012) Integrated quantum photonics

Photonic quantum simulator

Alán Aspuru-Guzik and Philip Walther, Nature Physics 8, 285–291 (2012) We chose: lithium niobate

Phase- • minimized decoherence shifter ✓ low-loss waveguides Single-photon • stability and scalability conversion ✓ integrated devices Superconducting detectors • photon-pair generation Photon-pair (2) ✓ χ nonlinearity generation

• fast photon routing ✓ fast electro-optic switches Polarization control

Implementation of complex quantum circuits in LN Outline

Underlying concepts

Device toolbox

Fabrication technology

HOM-on-chip circuit Components for integrated quantum circuits

Every single of these components has already been developed and optimized for classical optical applications (telecomms, sensing), but

Re-design / optimization for quantum applications is required. Passive routing – directional couplers

1 3

2 4

Example applications: Polarization Interference Power splitter Wavelength (de-)multiplexer splitter coupler λ , λ P xP 1 2 λ 1 TE,TM TE E1 interference λ (1-x)P 2 TM E2

Example 1 – wavelength demultiplexer

central λ1, λ 2 bendings bendings λ section 1 Lc

▶ Wavelength division demultiplexer to separate S 890 nm from 1320 nm (TM-polarization): radius R λ 2

▶ Weak coupling structure with large separation provides cross-coupling at the longer wavelength and (almost) no coupling at short wavelength

▶ Splitting ratios > 15 dB can be obtained for

couplers with Lc  9000 … 11000 µm Example 2 – polarization splitter

▶ Zero-gap directional coupler acting as polarization splitter at 1550 nm

0 ▶ Low coupling order provides robust and wavelength independent operation Pb 773z -10

▶ Experimental results: -20 TE (meas.) splitting ratios  20 dB achieved TM (meas.) TE (calc.)

splitting ration [dB] ration splitting TM (calc.) for central section length Lc=480 µm -30 excess loss typically below 0.5 dB 350 400 450 500 550 600 650 L [µm] c Active manipulation – electro-optic modulators Nonlinear optical polarization

▶ Nonlinearity in the dielectric polarization

        P(r,t) =  (r,t)E(r,t)    0 P(r,t) =  0 (r,t, E)E(r,t) ▶ Nonlinear polarization is the driving source for the generation of waves at new frequencies

non centrosymmetric crystal required

1 (2) (3) ▶ Taylor expansion of nonlinear polarization 푃 = 휀표(휒 ⋅ 퐸 + 휒 : 퐸퐸 + 휒 ⋮ 퐸퐸퐸+. . . ) electro-optic manipulation: optical & electric fields Electro-optic modulation

▶ Phase modulation via an externally applied voltage

▶ local change of the refractive index field component 1 parallel to c - axis n(x, y) = n3r E (x, y) 2 e 33 y

▶ “Averaging“ via integration over cross section:

2 3 E (x, y) E (x, y) dx dy ne r33 U G  DC, y opt, y neff = −  with  = 2 2 G U E (x, y) dx dy  opt, y

overlap integral ▶ Low driving voltages and high bandwidth Electro-optic modulation

▶ Mach-Zehnder modulator for high- data rate transmission systems [1]

▶ Sophicated electrode design optimized for impedance and velocity matching

▶ RF-operation up to 40 GHz

▶ 5 V drive voltage (@ 40 GHz)

[1] M.M. Howertone et al., IEEE Photon. Technol. Lett. 12 , 792 (2000) Commercial products Polarization converter

Uc 1.2 =22 µm 1.0

0.8 TE

0.6 2 nm TM Theory

0.4 Conversion ▶ Electro-optic wavelength selective polarisation 0.2 conversion 0.0 1582 1584 1586 1588 1590 1592 Wavelength [nm]

▶ „Integrated optical Solc-Filter“ 1.2 TE TM sin2 ▶ Electro-optic coupling via non-diagonal 1.0 0.8 coefficient r51 in a periodically poled waveguide

0.6



▶ Poling period:  = 0.4 Conversion nTM − nTE 0.2

0.0 -10 -5 0 5 10 U [V] c Generation and storage Nonlinear optical polarization

▶ Nonlinearity in the dielectric polarization

non centrosymmetric crystal required

1 (2) (3) ▶ Taylor expansion of nonlinear polarization 푃 = 휀표(휒 ⋅ 퐸 + 휒 : 퐸퐸 + 휒 ⋮ 퐸퐸퐸+. . . ) nonlinear optics: optical & optical fields Selected second-order processes

Second harmonic generation Parametric down-conversion (SHG) (PDC)

s   f SHG = 2 f p i

Sum frequency generation Difference frequency generation (SFG) (DFG)

1 1 DFG

SFG = 1 +2 = 1 −2 2 2 A few words on parametric down-conversion

waveguide Λ 31 µm signal pump

idler

▶ energy and momentum conservation (quasi-phase-matching): 2  p = s +i  =  +  + p s i  ▶ strong confinement of optical waves to small cross sections over long interaction length  high efficiency

2 2   deff PpL

▶ interaction length up to about 90 mm can be possible First demonstrations of guided-wave PDC

[1] B. Hampel, W. Sohler, SPIE Proc. 651 „Integrated Optical Circuit Engineering III“, pp. 229-234 (1986) [2] P. Baldi et al., Electron. Lett. 29, 1539 (1993) Frequency converters

Stationary qubit-systems Photonic state transfer e.g. cold atoms/ions, Bridging the gap by requires solid state systems frequency conversion telecommunication UV-/Visible/NIR wavelengths (IR) Recent examples from the literature

Ionic transitions [1]

Phys. Rev. Lett. 109, 147404 (2012)

369.5 nm 369.5

Energy

422 422 nm

557 557 nm 606 606 nm

[2] 637 nm 710 710 nm 710 710 nm [4] Nature Photon 4, 786 (2010) New J. Phys. 16, 113021 (2014) Telecombands 1310nm 1550nm [3]

[1] [2] [3] [4] [5] UPB Recent quantum frequency converters [5]

Phys. Rev. Applied 10, 044012 (2018)

Opt. Exp. 22, 11205 (2014) Interfacing Dielectric endfacet coatings

Ti:PPLN Coatings waveguide Pigtailing (waveguide-fibre coupling)

▶ Coupling between waveguide modes and single- mode fibers ▶ Good mode overlap of waveguide modes with standard single mode fibers:

▶ Coupling loss < 1 dB possible without any specific tailoring of waveguide modes © Paderborn University: Besim Mazhiqi ▶ Optimization of waveguide fabrication parameter can further minimize coupling losses Outline

Underlying concepts

Device toolbox

Fabrication technology

HOM-on-chip circuit Waveguide fabrication Waveguide fabrication

Titanium-indiffused waveguide fabrication

Titanium deposition and photoresist spin coating

Photolithographic patterning

Ti-etching and photoresist removal

Ti-indiffusion Waveguide indiffusion

▶ Diffusion furnance: Centrotherm

▶ Waveguide NIR

1060ºC for 8h in O2

▶ Waveguide MIR

1060ºC for 31h in O2 and Ar

Diffusion in Pt-Box avoid of out-diffusion protection against particles Mode size measurements

He-Ne laser

M1 Pinhole

FWHM: wg = 6.8µm, hg = 4.8µm EDFA BPF

M3 M2 Polariser L1 L2

CCD camera FWHM: wg,hg = 6.1µm Fabry-Perot loss measurement

He-Ne laser ECDL

M1 M2 Polariser L1 L2

Pinhole )

InGaAs arb.units

Detector Intensity ( Intensity

Time(s) R. Regener, W. Sohler, Appl. Phys. B 36, 143-147 (1985) Periodic poling Periodic poling of lithium niobate

Ferroelectric Phase:

➢ stacking of Li+ and Nb5+ relative to O2- leads Ps to spontaneous polarization (PS)

external electric PS field

➢ E =21 kV/mm O C Li

Nb ➢ periodic PS-inversion domain grating

(2) ➢ domain grating  -grating

(compare V. Gopalan et al.) Periodic poling of lithium niobate

Periodic domain inversion

Photoresist coating

Photolithographic patterning

Domain growth under pulsed HV application

Periodically inverted domains Example waveguide

Lithium niobate waveguide with 4.5µm poling Mode profiles @ 1550nm

(z-cut Ti:LiNbO3) 20 µm TE-mode

2 ▶ Both polarizations guided 6.0 x 4.2 µm TM-mode ▶ Low propagation losses: ~ 0.02 dB/cm ▶ Coupling to single-mode fibre: efficiency >85%

▶ Linear and nonlinear characterization 4.3 x 2.9 µm2 (losses, modes, SHG/SFG/PDC) Dielectric coatings Dielectric coating technique

Ion Assisted Deposition (IAD) Mirrors and partial reflectors

Quarter wave stack

A quarter wave stack consisting of Reflectivity spectra of quarter wave layers with equal optical thickness stacks consisting of 15 pairs of

Ta 2O5/SiO2 and TiO2/SiO2

[P.W. Baumeister “Optical coating technology”, SPIE press monograph, PM 137, Washington 2004] AR/HR coatings Our coating machine View inside the coating machine Sample holder

calotte

quartz oscillators

neutralizer

TiO2 SiO2

e-gun I ion source e-gun II Outline

Underlying concepts

Device toolbox

Fabrication technology

HOM-on-chip circuit Some quantum devices

Integrated N00N source using a non-linear coupler Photon triplet generation Plug & Play single photon source

R. Kruse et al, Phys. Rev. A 92, 053841 (2015)

N. Montaut et al, Phys. Rev. Applied 8, S. Krapick et al, Opt. Exp. 24, 2836 (2016) 024021 (2017)

HOM on a chip Polarization entanglement source

L. Sansoni et al, Quant. Inf. 3, 5 (2017)

K.-H. Luo et al, Sci. Adv. 5, eaat1451 (2019) HOM interference = two-photon interference at balanced (50/50) beamsplitters

destructive interference

t

time delay Dt C. K. Hong, Z. Y. Ou, L. Mandel, Phys. Rev. Lett. 59, 2044 (1987) Typical bulk optics setup Our HOM chip

How do we implement a time-delay on chip? Birefringence is a friend, not a foe…

0 휏 ∆휏

K.-H. Luo et al, Sci. Adv. 5, eaat1451 (2019) Birefringence is a friend, not a foe…

0 ∆휏 • Additional PC for swapping polarizations to synchronize both photons

K.-H. Luo et al, Sci. Adv. 5, eaat1451 (2019) Tuneable on-chip delay

• Additional PC for swapping polarizations to synchronize both photons

• Segmented polarization converters

K.-H. Luo et al, Sci. Adv. 5, eaat1451 (2019) HOM on a chip

➢ Tuneable time delay ➢ Dip visibility −ퟏ ~ ퟏퟐ 퐩퐬 ퟗퟑ ± ퟐ %

K.-H. Luo et al, Sci. Adv. 5, eaat1451 (2019) Where to in the future?

+ = LNOI

Ti:PPLN circuits have a limited Silicon photonics has a huge number of components due to the integration density but no second- low integration density. order nonlinearity.

N. C. Harris et al., Optica 5, 1623-1631 (2018) Where to in the future?

+ = LNOI

Ti:PPLN circuits have a limited Silicon photonics has a huge number of components due to the integration density but no second- low integration density. order nonlinearity.

N. C. Harris et al., Optica 5, 1623-1631 (2018) Summary

Underlying concepts

Device toolbox

Fabrication technology

HOM-on-chip 20 µm circuit Thank you for your attention

Quantum Networks Technology Benjamin Brecht Christof Eigner Sonja Barkhofen Laura Padberg Jan Sperling Felix vom Bruch Michael Stefszky Viktor Quiring Vahid Ansari Raimund Ricken Syamsundar De Thomas Nitsche Melanie Engelkemeier Devices Jano Gil Lopez Harald Herrmann Johannes Tiedau Kai-Hong Luo Nidhin Prasannan Matteo Santandrea Rene Pollmann Marcello Massaro Sebastian Brauner Christian Kießler

Silberhorn group