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- 1. The Future of Quantum Computing Dr. Matthew Broome University of New South Wales Sydney, Australia Art vs Physics, Sydney, 2016 1. What’s weird about quantum? 2. The First Quantum Revolution – wave-particle duality – entanglement 3. The Second Quantum Revolution 4. Are Quantum Computers Useful? 5. Challenges For Optical Quantum Computers QSceptics2013.pptx
- 2. Quantum Weirdness: The First Quantum Revolution
- 3. quantum.info The First Quantum Revolution Two weird things Wave-particle duality Entanglement “…a phenomena which is impossible, absolutely impossible, to explain in any classical way, and which has in it the heart of quantum mechanics. In reality, it contains the only mystery.” Richard Feynman “I would not call that one, but rather the characteristic trait of quantum mechanics, the one that enforces its entire departure from classical lines of thought.” Erwin Schrödinger Nobel Prize 1965 Nobel Prize 1933
- 4. quantum.infoquantum.info Two weird things Wave-particle duality Entanglement “…a phenomena which is impossible, absolutely impossible, to explain in any classical way, and which has in it the heart of quantum mechanics. In reality, it contains the only mystery.” Richard Feynman “I would not call that one, but rather the characteristic trait of quantum mechanics, the one that enforces its entire departure from classical lines of thought.” Erwin Schrödinger Nobel Prize 1965 Nobel Prize 1933 The First Quantum Revolution
- 5. quantum.infoquantum.info Things, stuff … atoms, planets, frogs… know where things are Waves Particles know where things go calm choppy Discrete – can count them waves interfere
- 6. quantum.infoquantum.info 1670 Light travels in straight lines What is light? Newton 1800 Light has interference fringes. Particle Wave Young
- 7. quantum.infoquantum.info 1670 Light travels in straight lines What is light? Newton 1800 Light has interference fringes. Particle Wave Young 1880 Light is alternating electric and magnetic fields. Maxwell
- 8. quantum.infoquantum.info Who is correct?
- 9. quantum.infoquantum.info Who is correct? “a candle burning at a distance slightly exceeding a mile” 1909 Even feeble light sources cause interference fringes G I Taylor
- 10. quantum.infoquantum.info A real single photon experiment Look at screen...
- 11. quantum.infoquantum.info 10 Seconds
- 12. quantum.infoquantum.info 100 Seconds
- 13. quantum.infoquantum.info 1,000 Seconds
- 14. quantum.infoquantum.info 10,000 Seconds
- 15. quantum.infoquantum.info 100,000 Seconds
- 16. quantum.infoquantum.info A real single photon experiment Look at screen... A. Both
- 17. quantum.infoquantum.info Look at screen... What is light? Particle Wave
- 18. quantum.infoquantum.info Look at screen... What is light? Particle Wave Newton Young
- 19. quantum.infoquantum.info Look at screen... What is light? Particle Wave Newton Young
- 20. quantum.infoquantum.info Look at screen... What is light? Particle Wave Newton Young ?
- 21. quantum.infoquantum.info What is light?
- 22. quantum.infoquantum.info What is light? Particle Waveand
- 23. quantum.infoquantum.info What is light? Particle Waveand Depending on what is being measured
- 24. quantum.infoquantum.info “click” “click” What is light? 50% 50% light acts like it is a particle half-silvered mirror
- 25. quantum.infoquantum.info What is light? half-silvered mirror 25% 25% full-silvered mirror half-silvered mirror 50% block light acts like it is a particle
- 26. quantum.infoquantum.info What is light? half-silvered mirror 100% 0% full-silvered mirror full-silvered mirror half-silvered mirror How can a particle do this? Particles can’t … waves can
- 27. quantum.infoquantum.info What is light? 100% 0% + half-silvered mirror full-silvered mirror full-silvered mirror
- 28. quantum.infoquantum.info 100% What is light? 0% Maybe we can trick the photon? light acts like it is a wave half-silvered mirror full-silvered mirror full-silvered mirror
- 29. quantum.infoquantum.info What is light? half-silvered mirror full-silvered mirror full-silvered mirror half-silvered mirror
- 30. quantum.infoquantum.info What is light? 25% 25% block half-silvered mirror full-silvered mirror full-silvered mirror half-silvered mirror
- 31. quantum.infoquantum.info Wave/particle duality half-silvered mirror 100% 0% full-silvered mirror full-silvered mirror half-silvered mirror light can act as a particle or a wave …it depends on the experiment everything neutrons electrons atoms molecules, Na2 bucky ball molecules, C60, C70 viruses… block
- 32. quantum.infoquantum.info I lied.... Not photons....
- 33. quantum.infoquantum.info I lied.... Not photons.... actually electrons.... particles with mass!
- 34. quantum.infoquantum.info The Quantum Revolution Two weird things Wave-particle duality Entanglement “…a phenomena which is impossible, absolutely impossible, to explain in any classical way, and which has in it the heart of quantum mechanics. In reality, it contains the only mystery.” Richard Feynman “I would not call that one, but rather the characteristic trait of quantum mechanics, the one that enforces its entire departure from classical lines of thought.” Erwin Schrödinger Nobel Prize 1965 Nobel Prize 1933
- 35. quantum.infoquantum.info Entanglement: a culinary analogy • Cakes prepared in pairs Q Cakes Inc. • Baked on trips down two conveyor belts • Lucy measures Left conveyor • Ricky measures Right conveyor Kwiat and Hardy, American Journal of Physics 68, 33, (2000)
- 36. quantum.infoquantum.info Entanglement: a culinary analogy • Cakes prepared in pairs Q Cakes Inc. • Baked on trips down two conveyor belts • Lucy measures Left conveyor • Ricky measures Right conveyor • For each cake-pair they each randomly choose to measure EITHER at half-way, the state of the cake Risen / Flat Q Cakes Inc.
- 37. quantum.infoquantum.info Entanglement: a culinary analogy • Cakes prepared in pairs Q Cakes Inc. • Baked on trips down two conveyor belts • Lucy measures Left conveyor • Ricky measures Right conveyor • For each cake-pair they each randomly choose to measure EITHER at half-way, the state of the cake Risen / Flat at end, the taste of the cake Good / Bad • So they measure: state / state (25%) state / taste (25%) taste / state (25%) taste / taste (25%) Q Cakes Inc.
- 38. quantum.infoquantum.info Entanglement: a culinary analogy • Cakes prepared in pairs Q Cakes Inc. • Baked on trips down two conveyor belts • Lucy and Ricky find that 10% of the time both cakes are Risen (RR) Q Cakes Inc.
- 39. quantum.infoquantum.info Entanglement: a culinary analogy • Cakes prepared in pairs Q Cakes Inc. • Baked on trips down two conveyor belts • Lucy and Ricky find that 10% of the time both cakes are Risen (RR) • Whenever Lucy’s cake is Risen, Ricky’s cake tastes Good (RG) Q Cakes Inc.
- 40. quantum.infoquantum.info Entanglement: a culinary analogy • Cakes prepared in pairs Q Cakes Inc. • Baked on trips down two conveyor belts • Lucy and Ricky find that 10% of the time both cakes are Risen (RR) • Whenever Lucy’s cake is Risen, Ricky’s cake tastes Good (RG) • Whenever Ricky’s cake is Risen, Lucy’s cake tastes Good (GR) Q Cakes Inc.
- 41. quantum.infoquantum.info Entanglement: a culinary analogy • Cakes prepared in pairs Q Cakes Inc. • Baked on trips down two conveyor belts • Lucy and Ricky find that 10% of the time both cakes are Risen (RR) • Whenever Lucy’s cake is Risen, Ricky’s cake tastes Good (RG) • Whenever Ricky’s cake is Risen, Lucy’s cake tastes Good (GR) • How often do both Lucy and Ricky find that both cakes taste Good? (GG) Measure taste / taste 25% of time Therefore expect both Good = 25% x 10% = 2.5% (at least) If the cakes are entangled, then quantum mechanics predicts both cakes never taste Good! = 0% Einstein had major problems with this! Q Cakes Inc.
- 42. quantum.infoquantum.info Entanglement: the cake experiment US Patent 6424665 R = -50.9˚ G = 0˚ (horizontal) F = 39.2˚ B = 90˚ (vertical) • Photons entangled in polarisation We found that both cakes never tasted good If there is a hidden variable theory that describes the world, then it requires some faster-than-light aspects… Pretty sure… with 2 reasonable physical assumptions, probability of error is 0.0000…00006% 3,233 zeroes
- 43. quantum.infoquantum.info weird? useful… Quantum technologies use the wave/particle duality
- 44. quantum.infoquantum.info Great... but what is all this good for?
- 45. quantum.infoquantum.info Great... but what is all this good for? The First Quantum Revolution: Using quantum mechanics to predict interesting physical behaviour - Wave/particle duality - Entanglement
- 46. quantum.infoquantum.info Great... but what is all this good for? “...there is a Second Quantum Revolution coming — which will be responsible for most of the key physical technological advances for the 21st Century” Dowling & Milburn, Philosophical Transactions of the Royal Society of London A 361, 1655 (2003) The First Quantum Revolution: Using quantum mechanics to predict interesting physical behaviour - Wave/particle duality - Entanglement
- 47. quantum.infoquantum.info Great... but what is all this good for? Dowling & Milburn, Philosophical Transactions of the Royal Society of London A 361, 1655 (2003) The First Quantum Revolution: Using quantum mechanics to predict interesting physical behaviour - Wave/particle duality - Entanglement The Second Quantum Revolution: Engineered quantum systems - Quantum technologies - Quantum computers - Quantum communications “...there is a Second Quantum Revolution coming — which will be responsible for most of the key physical technological advances for the 21st Century”
- 48. quantum.infoquantum.info Great... but what is all this good for? Dowling & Milburn, Philosophical Transactions of the Royal Society of London A 361, 1655 (2003) The First Quantum Revolution: Using quantum mechanics to predict interesting physical behaviour - Wave/particle duality - Entanglement The Second Quantum Revolution: Engineered quantum systems - Quantum technologies - Quantum computers - Quantum communication Wave-particle duality Entanglement In the last 10 years, >10,000 papers on Quantum Information Science “...there is a Second Quantum Revolution coming — which will be responsible for most of the key physical technological advances for the 21st Century”
- 49. Quantum Computing: The Second Quantum Revolution
- 50. quantum.infoquantum.info Classical computing insoluble soluble insoluble easy Nomenclature easy = “tractable” = “efficiently computable” hard = “intractable” = “not efficiently computable” factoring travelling salesman hard
- 51. quantum.infoquantum.info Quantum computing insoluble soluble insoluble hard Nomenclature easy = “tractable” = “efficiently computable” hard = “intractable” = “not efficiently computable” factoring travelling salesman Quantum computers are interesting physical systems in their own right easy
- 52. quantum.infoquantum.info Quantum computing everywhere? • To build a quantum computer, need 2-level quantum systems — qubits • Over 20 physical systems currently being experimentally investigated
- 53. quantum.info Quantum Qubits quantum.info Computing Classical Bits, binary digits 0 1 0 1 0+1 Gates can be irreversible INPUT OUTPUT A B A AND B 0 0 1 0 1 1 1 0 1 1 1 0 NAND can be reversible Toffoli INPUT OUTPUT A B C A B C 0 0 0 0 0 0 0 0 1 0 0 1 0 1 0 0 1 0 0 1 1 0 1 1 1 0 0 1 0 0 1 0 1 1 0 1 1 1 0 1 1 1 1 1 1 1 1 0 0+i1 Gates must be reversible CNOT INPUT OUTPUT A B A B 0 0 0 0 0 1 0 1 1 0 1 1 1 1 1 0 0+1 0 00+11
- 54. quantum.infoquantum.info Where are we with quantum computers?
- 55. quantum.infoquantum.info - Single photons Where are we with quantum computers now?
- 56. quantum.infoquantum.info Where are we with quantum computers now? - Single photons - Trapped ions
- 57. quantum.infoquantum.info - Single photons - Trapped ions - Superconducting circuits Where are we with quantum computers now?
- 58. quantum.infoquantum.info Where are we with quantum computers now? - Solving small scale problems
- 59. quantum.infoquantum.info Where are we with quantum computers now? - Solving small scale problems Factoring numbers into primes - Shor’s Algorithm insoluble hard factoring travelling salesman easy
- 60. quantum.infoquantum.info Where are we with quantum computers now? - Solving small scale problems Factoring numbers into primes - Shor’s Algorithm insoluble hard factoring travelling salesman easy3x5 = 15
- 61. quantum.infoquantum.info Where are we with quantum computers now? - Solving small scale problems Factoring numbers into primes - Shor’s Algorithm insoluble hard factoring travelling salesman easy3x5 = 15 3x7 = 21
- 62. quantum.infoquantum.info Where are we with quantum computers now? - Solving small scale problems Factoring numbers into primes - Shor’s Algorithm Simulating quantum systems insoluble hard factoring travelling salesman easy3x5 = 15 3x7 = 21
- 63. quantum.infoquantum.info Where are we with quantum computers now? - Solving small scale problems Factoring numbers into primes - Shor’s Algorithm Simulating quantum systems insoluble hard factoring travelling salesman easy3x5 = 15 Lanyon, et al., Nature Chemistry 2, 106 (2010) 3x7 = 21
- 64. quantum.infoquantum.info Where are we with quantum computers now? - Solving small scale problems Factoring numbers into primes - Shor’s Algorithm Simulating quantum systems insoluble hard factoring travelling salesman easy3x5 = 15 Shors algorithm Quantum chemistry Simulating quantum systems... Are quantum computers actually useful? 3x7 = 21
- 65. quantum.infoquantum.info Where are we with quantum computers now? - Solving small scale problems Factoring numbers into primes - Shor’s Algorithm Simulating quantum systems insoluble hard factoring travelling salesman easy3x5 = 15 Shors algorithm Quantum chemistry Simulating quantum systems... Are quantum computers actually useful? YES! 3x7 = 21
- 66. quantum.infoquantum.info Where are we with quantum computers now? - Solving small scale problems Factoring numbers into primes - Shor’s Algorithm Simulating quantum systems insoluble hard factoring travelling salesman easy3x5 = 15 PROVE IT! Shors algorithm Quantum chemistry Simulating quantum systems... Are quantum computers actually useful? YES! 3x7 = 21
- 67. quantum.infoquantum.info Where are we with quantum computers now? - Solving small scale problems Factoring numbers into primes - Shor’s Algorithm Simulating quantum systems insoluble hard factoring travelling salesman easy3x5 = 15 PROVE IT! OK. Shors algorithm Quantum chemistry Simulating quantum systems... Are quantum computers actually useful? YES! 3x7 = 21
- 68. Experimental BosonSampling qt quantum.info lab : AUSTRALIAN RESEARCH COUNCIL CENTRE OF EXCELLENCE FOR ENGINEERED QUANTUM SYSTEMS Small scale ^
- 69. Unsolved Problem in Computer Science
- 70. qt lab The Extended Church-Turing Thesis (ECT) Including quantum computers Why bother building a quantum computer? “All computational problems that are efficiently solvable by realistic physical devices, are efficiently solvable by a probabilistic Turing machine” Statement:
- 71. qt lab The Extended Church-Turing Thesis (ECT) Lots of head scratching.... & No widely accepted proof FOR or AGAINST the ECT - Dershowitz and Falkovich (2012). “A formalisation and proof of the Extended Church-Turing thesis”, arXiv:1207.7148v1 - Feynman, (1982). “Simulating Physics with Computers”, International Journal of Theoretical Physics 21 (6–7): 467–488 - Lloyd, (1996). “Universal Quantum Simulators”, Science 273 (5278) Experience tells us that simulating quantum systems is HARD & - Shor (1994), Proc. 35th Ann. Symp. Found. Comp. Sci., IEEE Comp. Soc. Press, Los Alamitos, California p. 124 Shor’s algorithm is faster than any known classical counterpart
- 72. qt lab Problem? 2. Quantum mechanics is wrong - # qubits >> quantum mechanics breaks -> can’t factor large numbers 1. The Extended Church-Turing thesis is incorrect - cannot efficiently simulate quantum systems using classical computers 3. There is a fast classical factoring algorithm - RSA key encryption breaks down Fast quantum algorithm for factoring numbers into primes - Aaronson, (2004), Limits on Efficient Computation in the Physical World arXiv:quant-ph/0412143 Existence of Shors algorithm poses a Trilemma
- 73. qt lab 1. The Extended Church-Turing thesis is incorrect - cannot efficiently simulate quantum systems using classical computers How do we show this? Build a physical device that computes something KNOWN or STRONGLY BELIEVED to be classically intractable... Fast quantum algorithm for factoring numbers Building scalable quantum computers is HARD! Shor’s Algorithm? Factoring NOT KNOWN to be classically intractable One way to do this:
- 74. Computing With Optical Networks
- 75. 1. The Extended Church-Turing thesis is incorrect - cannot efficiently simulate quantum systems using classical computers Large Scale Demonstration would be Strongest empirical evidence for Complexity of linear optical networks Aaronson, S., and Arkhipov, A., (2011), The Computational Complexity of Linear Optical Networks, Proc. of the 43rd annual ACM symp. on theory of comp., p. 333-342 However, optical networks output distributions that are HARD to calculate on classical computers → BosonSampling If a classical computer can do the same in efficiently the polynomial hierarchy collapses! Main Results qt lab ⋮ Network ⋮ 1 2 3 n !! ! !!! m bosons ⎨ ⎩ ⎧ ⎪ ⎪ ? ? ? ? We measure YES! BosonSampling Can we use linear optical networks to calculate HARD problems? This would make a lot of us really unhappy!
- 76. Remarks on BosonSampling qt lab Classical Computer Universal Quantum Computer BosonSampling is a model of intermediate quantum computation1. Unlike factoring large primes, BosonSampling is KNOWN to be classically HARD 2. Ultimate goal:3. m = 20-30 photons n = m ~ 400-900 modes Large scale BosonSampling --> Quantum Wins 2 Can verify fast quantum experiment using a slow classical simulation Demonstrates that at least ECT is incorrect up to this limit Large scale demonstration would be Strongest empirical evidence against ECT
- 77. Experimental BosonSampling
- 78. 6x6 mode linear optics network Quantum Computer Scientist1 2 U6 ! ! ! !
- 79. Photons 6x6 mode linear optics network Quantum Computer Scientist1 2 U6 ! ! ! !
- 80. Beam-Splitter Network 6x6 mode linear optics network Quantum Computer Scientist1 2 U6 ! ! ! ! Photons
- 81. Beam-Splitter Network 6x6 mode linear optics network Quantum Computer Scientist1 2 U6 ! ! ! ! Photons Photon Detectors
- 82. 1 2 2 Photons in 6 Modes U6 ! ! Text ! ! Quantum Computer Scientist
- 83. 1 2 2 Photons in 6 Modes U6 ! ! Text ! ! U 6 Classical Computer Scientist
- 84. 1 2 2 Photons in 6 Modes U6 ! ! Text ! ! Classical Computer Scientist U 6 Quantum Computer Scientist
- 85. 2 Photons in 6 Modes 1 2 U6 ! ! 1 2 U6 ! ! 1 2 U6! ! ! ! ! ! ! !
- 86. 2 Photons in 6 Modes 1 2 U6 ! ! 1 2 U6 ! ! 1 2 U6 ! ! ! ! ! ! ! !
- 87. 3 Photons in 6 Modes 1 2 3 U6 ! ! ! 1 2 3 U6 ! ! ! 1 2 3 U6 ! ! ! ! ! ! ! ! ! ! ! !
- 88. How do we know we’re sampling the quantum distribution?
- 89. 3 classical states in 6 Modes 1 2 3 U6 ! ! ! 1 2 3 U6 ! ! ! 1 2 3 U6 ! ! ! ! ! ! ! ! ! ! ! ! Similar interference experiment using classical states (lasers) don’t produce the same outputs as single photons Strong evidence to suggest using classical states is EASY to predict Rohde, Ralph, PRA, (2012)
- 90. Experimental imperfections 1 2 3 U6 ! ! ! ! ! ! 1 2 U6 ! ! ! !
- 91. Experimental imperfections Not a perfect single photon source... ... higher order photon emission reduces number state purity
- 92. Challenges for Photonic Quantum Computing
- 93. 45 The Challenges Large Scale Photonic QC Photons Detectors Linear optics Low loss Large networks True single photons fast, efficient, quiet, number-resolving... m = 20-30 photons n = m ~ 400-900 modes Large scale BosonSampling 2
- 94. The Challenges Detectors ≪1 photon dark noise @ 800 nm ⩽ 99±1% Photon-number resolving Efficient Fast Quiet 150-250 ps jitter, 1 ns rise ... 0.4-5.0 µmBroadband ✓ Smith, Devin H., et al. "Conclusive quantum steering with superconducting transition-edge sensors." Nature Large Scale Photonic QC
- 95. Linear optics The Challenges Robust Low loss Reconfigurable Compact Maskless fabrication ✓ Shadbolt, P. J., et al., (2011), Generating, manipulating and measuring entanglement and mixture with a Peruzzo, A., et al., (2010), Quantum walks of correlated photons, Science 389, 5998 Ensure U is HARD To sample MONDAY Sciarrino Large Scale Photonic QC
- 96. Photons The Challenges Current best photon source: spontaneous downconversion BUT Low event probability <10-4 % ... eight-fold events 10/day Higher-order terms: 2-4% 4-photon events lead to 20% gate errors Dousse et al., Nature 466, 217–220 (2010) Excellent spatio-temporal modes, entanglement T<99.9% High event probability 40% BUT Higher-order spatial modes... ~ MUCH better sources are required M. Barbieri, et. al., JMO, (2011) Large Scale Photonic QC
- 97. 45 The Challenges Photons Detectors Linear optics Large Low Loss True single photons fast, efficient, quiet, number-resolving... ✓ ✓ ~ 45 Theory Fault tolerance Imperfect Experiments ~ Large Scale Photonic QC
- 98. Outlook for Quantum Computing
- 99. QIT industry strategy? QC Simulation: semiconductors, … QC Factoring, searching QKD Banks, government 2005 2010 2015 2020 2025 2030 2035 38 Courtesy of R. Beausoleil, HP Labs, Palo Alto
- 100. QIT industry strategy! QC Simulation: semiconductors, … QC Factoring, searching QKD Banks, government 2005 2010 2015 2020 2025 2030 2035 Few-qubit QIP Distributed algorithms & entanglement, economics, … 39 Courtesy of R. Beausoleil, HP Labs, Palo Alto
- 101. quantum.infoquantum.info Where to next? Today The Future

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