Public trial lecture: A review of the state of the art for optical computer Presented on the 6 th of May 2011 Andrea

1. Public trial lecture: A review of the state of the art for optical computer Presented on the 6th of May 2011 Andreas Kimsås Department of Telematics

2. Agenda • Introduction • Definition, Motivation, History & Classification • Special purpose optical computers • Linear optics, miniaturisation, application • General purpose optical computer • Logic classes, optical ‘’transistors’’ • Quantum computer • Principles, Method & Properties • Summary • References

3. Motivation, Evolution & Classification Introduction

4. Definition «An optical computer is a physical information processing device that uses photons to transport data from one memory location to another, and processes the data while it is in this form» – Naughton & Woods, 2009 • It is not necessarily programmable • It may be part of a larger (electronic) system • Logic gates may be actuated using a different technology • Optical transport is necessary, but not sufficient

5. Comparison optics/electronics

6. Optical computer research • Greater progress in other fields • Weak effects • Integration • Speed • Moore’s law lives on • Integration • A story of advances and setbacks

7. Classification

8. Classic Linear Optics & Refinements Special purpose computers

9. Using classic linear optics Perfect shuffle –permutes an analogue/digital address into any other adress in (logN)2 steps. Requires additional capability of exchanging nearest neigbours High spatial 3D paralellism Applications All-optical label swapping Image analysis (rotation) Ref. & figure from Lohmann, 1985

10. Micro-lenses (lenslets) • Classic Lens (a) - Fresnel Zone Plate (b) – SWG(c) • Subwavelength grating technical data: • Thickness 150 nm, λ=1550nm • Focal length 17.3 mm, diameter 0.3mm • Applications: • Micro-scale classical optics, • Replace Bragg reflector in Vertical Cavity Surface Emitting Laser (VCSEL) Figure from Chrostowsky (2010)

11. Micro & Nano-scale devices Ultralow threshold VCSEL laser. Threshold of 180nA @ 50K, 287nA @ 150K. GaAs active region is confined by quantum dots. All-optical recirculation buffer Active MZI element is a GaAs modulator Silicon waveguides on Silicon substrate MZI switches. Loop delay = 2.6m or 13 ns Ref: Burmeister (2008), Paniccia (2010), Ellis (2011)

12. 3R optical regeneration • Lithographically etched, modelocked ring laser • Retiming and reshaping • Amplification performed by SOA • Used for clock recovery in all-optical networks Ref. and figure: Koch et al. (2008)

13. Optical matrix multiplier 16 Teraflops computation capability Low power consumption (<1W) Spatial light modulator (SLM) modulated @ 125 MHz (5ns) SLM matrix is multiplied with VCSEL input vector (1x256x8bits).(256x256x8bits) Multiplication is the interaction of a vector element and a pixel Addition is the superposition of intensities Applications: Co-processor Image processing Element for: correlaton, convolution, Fourier transform, Hamiltonian path problem ++ Figure from Tamir et al. - 2009 Ref: Caulfield (2010), Tamir (2009).

14. Versatility of matrix multiplier General form Notes: Binary OR requires boolean interpretation of addition output. Binary AND uses the SLM matrix and input vector as inputs

15. Logic schemes & devices General purposeOptical computer

16. Conventional and Directed logic circuits Conventional Directed Refs.: Dadamundi (2005), Hardy et al. (2007) Very common in ICs of today CMOS transistor is the building block Energy is dissipated at (almost) every step Sequential stabilizaton of gates Min Clock period = longest path delay + SUM of ALL rise/fall times Inspired by Fredkin gates Cross-Bar switch is main element The same light pulse along the path, main energy cost is to set the switch All gates can be set at the same time ! Min. Clock period = longest path delay + switch reconfiguration time

17. Directed logic circuit examples Refs.: Hardy et al. (2009) A XOR B Initialised with True=(1,0) F(A) is cross if A=True F(A) is bar if A=False A OR B Feedback is required At most 3 levels is required to perform any logical operation Algebraic functions Not yet defined for directed logic Matrix operations ??

18. Mechanical switch example 2x2 MEMS add-drop switch (cross-bar) 20μs switching time Collimating lenses are required for coupling Extendable to several WDM channels Micro electromechanical mirror (MEMS) High spatial paralellism: 100 x100 (Fijutsu) Footprint about 8 x 6 x 3 cm3 Ref. and figures Wu & Solgaard (2006).

19. Mechanical switch example 2x2 MEMS add-drop switch (cross-bar) 20μs switching time Collimating lenses are required for coupling Extendable to several WDM channels Micro electromechanical mirror (MEMS) High spatial paralellism: 100 x100 (Fijutsu) Footprint about 8 x 6 x 3 cm3 Ref. Wu & Solgaard (2006).

20. Guided wave switches Directed Coupler type Phased Array Switch Ref. : Caulfield (2010) & Tanamura et al. 2011 Essex M.Sc course notes Phase-shift e.g. via electro-optic effect. Top, three couplers, bottom Mach-Zender Interferometer Based on coupled mode theory Electronically induced phase shift Footprint 4x3 mm2 , 1x16 switch with 24 array waveguides 11 & 5 ns rise/fall time.

21. Other basic switching elements Refs.: Borella (1997), Hardy (2007), Yang (2010), & Essex M.Sc. Course notes

22. All-optical SOA logic • Pump, SOA and BPF is used for logic via non-linear effects (XGM,XPM & FWM) • 40 GHz speed demonstrated! • Cascadability demands VOA (stable output power), fast wavelength tuning, polarization control and is limited by amplifer noise. Ref. and figure: Zhang (2009)

23. Principles & Advantages Quantum computing

24. Basic elements Any distinguishable quantity can be used to encode the qubit value, e.g. polarisation, time bin or space State of a qubit is fully described as a sum of vector elements: a|H> + b|V>, with a2 + b2 = 1 State changed though phase shifts or by switching in space. Linear optical elements can be used without major problems (determinsitic), but loss and noise will destroy the system Non-linear effects are far too weak to be used; was the phase shift in the MZI applied or not? Refs. O’Brien (2007) & Thompson (2011). Figures from O’Brien

25. Entangled states The system should work as a CNOT gate, without using non-linear effects. Beam splitters act on qubits inside the probabilistic gate and creates a total of 16 output combinations. The CNOT is sucessful for one combination, measured by single-photon detection at each detector In a cascade the probability of success at all CNOTs decreases exponentially! Partily soved via quantum teleportation. If successful, the target qubit is output to the next stage. Ref and figures from O’Brien (2007)

26. Summary • Paralellism is a key property for high performance optical computing • NP to P computational complexity • Reduces footprint • Optical signal processing is useful for special purpose applications; e.g. for optical networking • The general purpose computer should not be an optical blue-print of electronic systems • Poor cascadability is currently the main impediment to general purpose optical computer • Quantum effects enables small, power-efficient and computationally efficient algorithms, but a realization is far from immediate

27. Properties of a quantum computer • Power efficient • Very compact (but not much smaller than other) • Spatial paralellism represents a speedup • Probabilistic overhead is compensated for by «quantum speed-up». Ignoring overhead gives exponential speedup for some specific problems. • Identify states that correspond to a certain problem is difficult • Known applications: • Factorisation problem • Fourier transform • Probabilistic database search

28. References • Naughton, T. J. and D. Woods (2009). “Optical Computing”. Encyclopedia of Complexity and Systems Science. R. A. Meyers, Springer New York • Lohmann, A. W. (1986). "What classical optics can do for the digital optical computer." Appl. Opt. 25(10): 1543-1549 • Abdeldayem, H., D. Frazier, et al. (2008). «Recent Advances in Photonic Devices for Optical Super Computing». Optical SuperComputing. S. Dolev, T. Haist and M. Oltean, Springer Berlin / Heidelberg. • Chrostowski, L. (2010). "Optical gratings: Nano-engineered lenses." Nat Photon 4(7): 413-415. • Dandamudi, S. (2005). Digital Logic Circuits. Guide to Assembly Language Programming in Linux, Springer US: 11-44 • Ellis, B., M. A. Mayer, et al. (2011). "Ultralow-threshold electrically pumped quantum-dot photonic-crystal nanocavity laser." Nat Photon 5(5): 297-300 • Tamir, D. E., N. T. Shaked, et al. (2008). Electro-Optical DSP of Tera Operations per Second and Beyond (Extended Abstract). Proceedings of the 1st international workshop on Optical SuperComputing. Vienna, Austria, Springer-Verlag: 56-69. • Hardy, J. and J. Shamir (2007). "Optics inspired logic architecture." Opt. Express 15(1): 150-165 • Wu, M. C., O. Solgaard, et al. (2006). "Optical MEMS for Lightwave Communication." J. Lightwave Technol. 24(12): 4433-4454. • Caulfield, H. J. and S. Dolev (2010). "Why future supercomputing requires optics." Nat Photon 4(5): 261-263. • Borella, M. S., J. P. Jue, et al. (1997). "Optical components for WDM lightwave networks." Proceedings of the IEEE 85(8): 1274-1307. • Yang, W., Y. Liu, et al. (2010). "Wavelength-Tunable Erbium-Doped Fiber Ring Laser Employing an Acousto-Optic Filter." J. Lightwave Technol. 28(1): 118-122.

29. References II • Koch, B. R., A. W. Fang, et al. (2008). All-Optical Clock Recovery with Retiming and Reshaping Using a Silicon Evanescent Mode Locked Ring Laser. Optical Fiber communication/National Fiber Optic Engineers Conference • Paniccia, M., (2010), Integrating silicon photonics, Nature Photonics, Interview | Focus, Vol. 4. • Burmeister, E. F., J. Mack, et al. (2008). SOA Gate Array Recirculating Buffer for Optical Packet Switching, Optical Society of America • Zhang, X., J. Xu, et al. (2009). All-Optical Logic Gates Based on Semiconductor Optical Amplifiers and Tunable Filters. Optical SuperComputing. S. Dolev and M. Oltean, Springer Berlin / Heidelberg. 5882: 19-29. • Tucker, R. S. (2006). "The Role of Optics and Electronics in High-Capacity Routers." Lightwave Technology, Journal of 24(12): 4655-4673 • Thompson, M. G., A. Politi, et al. (2011). "Integrated waveguide circuits for optical quantum computing." Circuits, Devices & Systems, IET 5(2): 94-102 • O'Brien, J. L. (2007). "Optical Quantum Computing." Science 318(5856): 1567-1570

30. Optical label processing Klonidis et al. Muligens 1 slide av Leuthold