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ISING MACHINESAnil PrabhakarDept. of Electrical EngineeringIIT Madraswww.ee.iitm.ac.in/~anilpr
47-779: Quantum Integer Programming
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Outline
• Some properties of light• Superposition, Entanglement• Wave – Particle Duality• Classical versus Quantum Experiments
• Photonic Ising Machines• Multiplexed in Time – Poor Man’s Ising Machine• Multiplexed in Space – Spatial Light Modulation
• Chip-based Ising Computing Machines• Electronic oscillators• Magnetic oscillators
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Properties of Light
• Polarization• Color
• Wavelength, Frequency• Phase • Spatial Modes
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How does Quantum Computing work?• Classical bits versus quantum qubits
47-779: Quantum Integer Programming
A B0 00 11 01 1
2 bits of informationState of A and of B
A B probability0 0 α0 1 β1 0 γ1 1 δ
4 bits of informationSuperposition state
• Extend this to 3 qubits – 8 bits of information• n-qubits will have 2^n bits of information
Waves interfere, we use them for computation
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How does Photonic Computing work?
• Double slit experiment – wave particle duality• Analogy – massively parallel computation
47-779: Quantum Integer Programming
(a) Particles (b) Waves (c) Particles as Waves
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Testing a Single Photon Source
• Hong-Ou-Mandel Interferometer
HOM dip as a function of T
47-779: Quantum Integer Programming
Coincidence of counts shows a non-classical dip
ωp
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How does a laser work?
• Optical wave (photons) oscillating inside a resonant cavity
47-779: Quantum Integer Programming
Stimulated Emission is in phase with the incoming photon
A laser is Coherent.All the photons are locked in phase.
Can measure a coherence time.
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How do we resolve photon numbers?
• Redistribution into different spatial or temporal bins
Pulses with 5 photons
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Temporal redistribution47-779: Quantum Integer Programming
• Average power reduced by half for each circulation
• Synchronized detection using gating at GAPD
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Detection probabilities
• Decreasing detection probabilities with subsequent redistribution
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Predicting detections
• If it is classical, we can predict patterns• 10…, 11• 100…, 101, 110, 111…• 1000, 1001, 1010, 1011 … etc
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The Ising Model
• Popular model in Statistical Physics• Phase Transition• Percolation Theory• Ferromagnetic vs Antiferromagnetic
47-779: Quantum Integer Programming
coupling J e -J e J
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Bloch Wall
• Spins will orient themselves based on the minimum energy configuration
• Costs less energy to create a Wall than to flip one spin
47-779: Quantum Integer Programming
E = 0
E = e-J
E = e-2J
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The Poor Man’s Ising Machine
• Mach Zender Modulator with optoelectronic feedback• Bohm et al, Nature Communications, 10:3538,2019.
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The Optical-Electronic-Optical Model
• Self bias term α• Coupling coefficient β• Weights between spins J• What does the optics do?
• Nonlinear function – cos2
47-779: Quantum Integer Programming
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MZM Transfer Function
• Find the correct bias point, so we have two solutions - bifurcation
47-779: Quantum Integer Programming
xn
xn+1
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Pitchfork Bifurcation
• Nonlinear function has two solutions• Uncoupled spins
47-779: Quantum Integer Programming
xn
xn+1
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Pitchfork Bifurcation
• With 100 spins• Tune α to observe the bifurcation
47-779: Quantum Integer Programming
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Square Lattice
• 10 x 10 lattice, α = 0.25, β = 0.29• Can observe domain walls where spins are aligned up• Lowest energy for the checkerboard pattern
47-779: Quantum Integer Programming
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The Poor Man’s Ising Machine
• Not quite Poor….about $10,000
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The Poor Man’s Ising Machine
• It is actually a little more complicated• Needed an optical isolator, a polarization controller, and an optical amplifier
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16 x 16 spin lattice
• Nearest Neighbours on a square lattice• Gautham and Parth, IIT Madras
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Iteration
Spi
n Va
lue
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Spin lattice (16x16, 24x24)47-779: Quantum Integer Programming
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24 x 24 spin lattice
• Discontinuities are under investigation• Gautham and Parth, IIT Madras
47-779: Quantum Integer Programming
Iteration
Spi
n Va
lue
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How well does it scale?
• Photonic Chips, NUS,Singapore
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Spatial Light Ising Machine (SLIM)
• Optical beams have spatial divergence• Interactions are between different rays - phase
47-779: Quantum Integer Programming
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Understanding Phase
• Different rays of light accumulate a different phase based on their propagation distance from the source
• Think of diffraction in 2-D, you will see rings
47-779: Quantum Integer Programming
Diffraction from a single slit
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Fourier Optics
• The 4-f system• Every optical ray in 2-D interacts with every other ray• A lens in 2-D acts as a Fourier Transformer, gives spatial frequency content
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Spatial Filters
• Easy to design high pass, low pass and vertical pass filters
47-779: Quantum Integer Programming
Blurred
Edge detection
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• Amplitude mask• Phase plane (SLM)• Camera• Feedback
47-779: Quantum Integer Programming
Far field intensity
Fourier transform of a rectangle
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SLIM Algorithm
• Computer-generated holographic optical tweezer arrays• Dufresne et al, Rev. Sci. Instr., 72, 2001
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What are its limitations?
• We need Φ(x,y)• Write an image to the SLM• Read 1000 x 1000 pixels off a camera
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Convergence Issues• We lose phase information with a camera
• SLM is slow and resets every so often
• 500 iterations takes 1 hour
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Chip based Ising Computing Machine• CICM oscillators can be optical, magnetic or electronic
47-779: Quantum Integer Programming
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Magnetic Memory, and Oscillators
The “free” magnet aligns itself towards a preferred direction determined by the injected current
47-779: Quantum Integer Programming
The “free” magnet oscillates about an effective field (analogous to a top precessing in a gravitational field)
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Coupled Nano Oscillators
• Arrays of oscillators can lock together
• How do we change the coupling?
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State of the Art
• Can do weighted oscillator networks – vowel recognition• Romera et al, Nature 563, 2018
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Acknowledgements
• Gautham, Parth, Vikram and Vignesh, • Dr. Nitin Chandrachoodan, IITM
• Dr. Aaron Danner, Prof. Hyunsoo Yang, NUS
• Prof. Sridhar Tayur, CMU
• Centre for Quantum Information, Communication and Computing (quantum.iitm.ac.in)
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