“ Anyone who can contemplate quantum mechanics without getting dizzy hasn ’ t understood it. ”

1. “Anyone who can contemplate quantum mechanics without getting dizzy hasn’t understood it.” --Niels Bohr

2. The Quantum Information RevolutionPaul Kwiat DARPA

3. Kwiat’s Quantum Clan (2012)Graduate Students: Rebecca Holmes Aditya Sharma Trent Graham Brad Christensen Kevin Zielnicki Mike Wayne Courtney Byard Undergraduates: Daniel Kumor David Schmid Jia Jun (“JJ”) Wong Cory Alford Joseph Nash David Rhodes Visit Prof: Hee Su Park Post-Doc:Jian Wang

4. A New Science! Quantum Mechanics Information Science 20th Century Quantum Information Science 21st Century

5. Quantum computation Factoring Simulating other quantum systems (>30bits) Ultimate control over Error correction “large” systems Quantum classical Quantum Quantum communciation Information Teleportation Linking separated quantum systems (“q. network”) Quantum cryptography Secure key distribution (even between non-speaking parties) Fundamental physics Entanglement Decoherence Quantum metrology Measurements beyond the classical limit Non-invasive measurements Measurements on quantum systems

6. Quantum Metrology Quantum Cryptography Quantum Computing

7. “Things should be made as simple as possible, but not any simpler.”

8. What I do… Unravel the mysteries of the universe… Quantum Optics Light ???

9. Quantum… very small very big (e.g., “quantum leap”) an unsplittable parcel/bundle of energy a cool buzzword to get more funding, more papers, more people at your Sat. am physics lecture, etc. all of the above

10. 1905: Einstein made a ‘quantum leap’ and proposed that light was really made of particles with tiny energy given by E = h f = h c/l wavelength 6.6 x 10-34J-s frequency

11. Physics 214: Lect. 7Example: “Counting photons” Visible light Power output: P = (# photons/sec) x Ephoton How do we reconcile this notion that light comes in ‘packets’ with our view of an electromagnetic wave, e.g., from a laser?? Partially transmitting mirror Power input How many photons per second are emitted from a 1-mW laser (l=635nm)?

12. 1 mW red laser 3 x 1015 photons/sec = 3,000,000,000,000,000/sec This is an incredibly huge number – your eye basically cannot resolve this many individual photons (though the rods can detect single photons!). And you MAY be able to see just one photon!!

13. Formation of Optical Images • However, for very low light intensities, one can see the statistical and random nature of image formation. • Use an extremely sensitive CCD camera that can detect single photons. A. Rose, J. Opt. Sci. Am. 43, 715 (1953) Exposure time • For large light intensities, image formation by an optical system can be described by classical optics.

14. But how do we *know* there’s only ONE photon… A beamsplitter… • “1” Photon only detected in one output. Equally likely to be transmitted or reflected –cannot tell which. • “0” "God does not play dice with the universe." • Quantum random-number generator! • completely unpredictable • patented • commercially available “It seems to me that the idea of a personal God is an anthropological concept which I cannot take seriously.”

15. “The important thing is not to stop questioning.” -A. Einstein

16. “Yes, yes, already, Warren! … There is film in the camera!” Quantum Interrogation The problem… Measure -film -absorber -atom without -exposing -heating -exciting it

17. WHY was Einstein’s 1905 proposal that light was made of particles such a profound leapthat almost no one believed him? • Because everyone KNEW that light was really waves. • One of the strangest features of QM: all particles can behave like waves…

18. Interference of waves (e.g., water, sound, …)

19. Superposition (adding together) of waves Waves add up: “Constructive interference” Waves cancel: “Destructive interference”

20. Light: Particle or Wave? 1675: Newton “proved” the light was made of “corpuscles” 1818: French Academy science contest • Fresnel proposed interference of light. • Judge Poisson knew light was made of particles: “Fresnel’s ideas ridiculous” If Fresnel ideas were correct, one would see a bright spot in the middle of the shadow of a disk.

21. Judge Arago decided to actually do the experiment… Conclusion (at the time): Light must be a wave, since particles don’t interfere! • Only, now we know that they must!

22. Single-Photon Interference: Exposure time Photons • Question: what if we reduce the source intensity so that at most one particle (photon) is in the apparatus at a time? ? • Answer: Just like in the “optical image formation”, given enough time, the classical interference pattern will gradually build up from a huge # of seemingly random “events”!

23. mirror beam- splitter Optical Interferometers • Interference arises when there are two (or more) ways for something to happen, e.g., sound from two speakers reaching your ear. • An interferometer is a device using mirrors and “beam splitters” (half light is transmitted, half is reflected) to give two separate paths for light to get from the source to the detector. • Two common types: Mach-Zehnder: Michelson : beam- splitter mirror

24. “Yes, yes, already, Warren! … There is film in the camera!” Quantum Interrogation The problem… Measure -film -absorber -atom without -exposing -heating -exciting it

25. The solution…(Elitzur & Vaidman, 1993)Use dual wave-particle nature of quantum objects (“wavicles”) Single photon always shows up at D1(complete destructive interference to D2)

26. Now place an absorbing object in one arm… 50% chance that photon is absorbed by object50% chance it isn’t  25% chance D2 fires “interaction-free measurement” of object

27. Quantum Interrogation • Optimizing reflectivities 50% efficiency • By combining these techniques with the “quantum Zeno effect” (making repeated very weak interactions), the efficiency can in principle be pushed to 100%: no photons absorbed by the absorbing object! • [85% demonstrated to date] • Imaging semi-transparent objects does not readily yield a gray-scale

28. Quantum Metrology WpdrvalL&wz;xcuymnzx Quantum Cryptography Quantum Computing

29. Cryptography: Make messages so that only the intended recipient can understand them… • public key encryption: Standard, but not provably secure; relies on difficulty of factoring (e.g., 15 = 3x5) • secret key encryption: PROVABLY secure as long as • no one else has the key • the key is never reused Quantum Cryptography = Quantum Key Distribution

30. Quantum Cryptography Cryptography KEY: …010001010011101001… XOR(Cipher,Key) Message XOR(Message,Key) Cipher EVE ALICE BOB Cipher: …0110010110100010…

31. Quantum Cryptography:Use a different property of light– polarization! Polarization: --the oscillation direction of the light --property of each photon --can measure with polarizers, calcite, etc. Prob(horizontally polarized photon pass horizontal polarizer): 1 Prob(horizontally polarized photon pass vertical polarizer): 0 Prob(diagonally pol. photon pass horizontal polarizer): 1/2 Prob(diagonally pol. photon pass vertical polarizer): 1/2

32. How to Make “Entangled” Coins “Spontaneous DownConversion”: high energy parent photon can split into two daughter photons (with same polarization) We don’t know WHICH crystal created the pair of photons, but we know they both came from the same crystal  they MUST have the same polarization.

33. What about Eavesdropping? • Eve cannot “tap” the line  photons that don’t make it to Bob are not part of the key • Eve cannot “clone” the photon  forbidden by basic quantum mechanics • Measurements by Eve necessarily have a chance (25%) to disturb the quantum state •  Alice and Bob can detect errors in the key! • If the bit error rate is too high, they simply discard the key. No message is ever compromised.

34. Current Free-Space QKD Distance Record: 144 km between LaPalma and Tenerife Last week news item: they used the entangled photons to teleport the state of a photon between the islands – world distance record for quantum teleportation!

35. QKD Goes Commercial…

36. Quantum Interrogation vs. Quantum Cryptography Since we can seemingly “see” without “looking” using quantum interrogation, does this mean an eavesdropper could use it to defeat quantum cryptography? No! It turns out that even making the gentlest measurement possible, if the eavesdropper gains any information, she disturbs the state. Or if she is so gentle so as not to disturb the state, then she gets no information. Quantum key distribution is secure against any attack allowed by the laws of physics!

37. Imagination is more important than knowledge

39. Moore’s Law Source: Intel

40. The first solid-state transistor (Bardeen, Brattain & Shockley, 1947)

41. The Ant and the Pentium ~100 million transistors INTEL Pentium 4 transistor Size of an atom ~ 0.1nm

42. Binary digit Quantum bit “bit”“qubit” 0, 10101 Physical realization of qubits  any 2 level system 2-level atom: ge spin-1/2:  polarization: HV All 2-level systems are created equal, but some are more equal than others! Quantum communication  photons Quantum storage  atoms, spins Scaleable circuits superconducting systems “Quantum” phenomena Superposition Interference Wave-particle duality Intrinsic randomness in measurement Entanglement

43. “Entanglement”, and the scaling that results, is the key to the power of quantum computing. 1 0 1 • Classically , information is stored in a bit register: a 3-bit register can store one number, from 0 – 7. Quantum Mechanically, a register of 3 entangled qubits can store all of these numbers in superposition: • a | 000 + b | 001 + c | 010 + d | 011 + e | 100 + f | 101 + g | 110 + h | 111 ñ ñ ñ ñ ñ ñ ñ ñ Result: • Classical: one N-bit number -- -- Quantum: 2 (all possible) N-bit numbers N register can simultaneously store • N.B. : A 300- qubit 2300 ~ 1090 numbers

44. That’s a BIG number 1090 = 1,000,000,000,000,000,000, 000,000,000,000,000,000, 000,000,000,000,000,000, 000,000,000,000,000,000,000,000,000,000,000,000 This is more than the total number of particles in the Universe! Some important problems benefit from this exponential scaling, enabling solutions of otherwise insoluble problems.

45. A hard problem: factoring large integers: For example, it is hard to factor 167,659 But an elementary school student can easily multiply389 x 431 = 167,659 This asymmetry in the difficulty of factoring vs. multiplying is the basis of public key encryption, on which everything from on-line transaction security to ensuring diplomatic secrecy depends.

46. Quantum Computing’s “killer app.” Quantum algorithms enable one to factor numbers into their prime constituents MUCH faster: The difficulty (impossibility) of factoring large numbers (and the ease of creating a large number from its factors) is the basis of public key encryption (which nearly everyone uses for secure transmission today).

47. Atom in different energy states: state labels “”, “” energy states: atom in state atom in state atom atom in superposition state shorthand “wave function” representation  =   +  Probability of measuring , P = ||2 Probability of measuring , P= ||2

48. laser |rest + |moving |0 |0 |0 |0 |1 Collective motion: the “quantum data bus” state of motion |rest |0 |0 + |1 |0 |0 |1

49. Science News Quantum Computing Explored Sep 12 2001 @ 08:10 American computer scientists are studying the possibility to build a super fast computer based on quantum physics.

50. Why it might not work… Technology requirements • Set of qubits isolated from environment. • “Quantum information bus” to connect qubits. • Reliable read-out method. Essential Dichotomy Need WEAK coupling to environment to avoid decoherence, but you also need STRONG coupling to at least some external modes in order to ensure high speed and reliability.