1. Experimental quantum information processing - the of the art Nadav Katz A biased progress report Contact: katzn@phys.huji.ac.il 02-6584133 Quantum computation workshop, Jan. 2015 What is a quantum information and why do we want to process it? Different models – gates, cluster, adiabatic, topological. Different realizations – photons, atoms, ions, semi- conductors and superconductors. Outlook and future directions

2. Motivation Single atom decays – cat dies! Wait for one half-life We NEVER see such macroscopic superpositions – why? Dual/related problem (Feynman): exponential computational overhead for simulating many-body quantum systems Quantum Information Processing – Practical advantages over classical info. ($$) Relates to quantum phase transitions and computation complexity How big a Schrödinger kitten can we build? Aristotle: Nature abhors a vacuum coherence Schrödinger:

3. Some comments about QC It is mainstream physics to assume it is possible with established error-correction codes (is nature malicious/ingenious?) Failure actually implies fundamentally new physics regarding decoherence (no evidence for this in known physics) QC is not magic: General/Generic unitary evolution of a many-body is still exponentially hard to simulate In the presence of symmetry and structure, sometimes dramatic speedup is predicted

4. Classical bit: definite 0 or 1 + 5V V out Transistor Logic: 0 = 0 volts 1 = 5 volts Storage of Information: Bits Quantum bits: superpose 0 or 1 H atom wavefunctions : 01 Example:

5. Bloch representation Control: resonant (or close to resonant) pulses can be visualized as a rotation! Geometrical picture: Useful for any two-level system

6. Qubit Characterization T 2 ~350ns Meas. time T 1 ~450ns 0100200300400500600 time [ns] T  ~100ns Rabi time x  /2 time x  /2 yy Ramsey Echo time xx lifetime P1P1 0 1 1 0 1 1 0 1 0 Data from 2007…

7. Entanglement Require 2 N complex numbers to specify a general N-qubit state! Many (most actually) such states are not separable = entangled Qubit 1Qubit 2Qubit 3Qubit 4 Classic 2-qubit example: Bell state Such (anti-)correlations are normally generated by interactions (gates). Resource for secure communication (not discussed)

8. Gate model (DiVincenzo criteria) Classical Computation: not and                               Quantum Computation: Initialize state Y i = |000..0> Logic via series of operations: State Manipulation (1 qubit) Controlled not (2 qubit) Final state measurement Measure qubits of state Y f Coherence:  t coherence / t logic ~ number logic operations  > 10 2 for error correction                                     } + linear superposition controlbit Initialize state Logic Output result Logic errors: Error correction possible

9. Need to be clever When we measure – we want to see something interesting (and not some random, useless state out of 2 N )… Deutsch-Josza’s algorithm – find the parity of a function (exponentially fast!) Shore’s algorithm – find the prime factors of a number (exponentially fast!) Grover’s algorithm – check if a database contains an element (poly-faster) These are famous, but there are some more (Eigenvalue estimation, random walk, Boson sampling hidden subgroup)… Well-known quantum computing algorithms: IMPORTANT: Error correction can make it work even if gates are not perfect! (Shor+many others…)

10. Qubit progress Remarkable progress of the past 15 years Already passed the fault tolerant threshold From Devoret and Schoelkopf, Science (2013)

11. Alternative computational models Cluster states Single photons Adiabatic Topological All are theoretically equivalent – but experimentally VERY different… Briegel & Raussendorf (2001) Farhi (2001) Kitaev (1997) Exponentially degenerate ground state (phases) with large gap. Braiding particles evolves the state. Generate a massively entangled initial state (c-not gates between nodes in graph, compute by measuring in a specific order) Knill, Leflamme, Milburn (2001) Using single photons (if you have them!) and linear optics – Scalable QIP is possible! D-wave (??) Slowly evolve the Hamiltonian to remain in the ground state

12. Experimental Quantum Information Processing (QIP) a perplexing flora and fauna of different systems NMR Quantum optics Trapped ions Neutral atoms Josephson superconducting qubits Quantum dots ??

13. Experimental QIP – a guide for the perplexed Smaller Ions Neutral Atoms NMR Semiconductor Spins Quantum Dots and defects Superconducting Circuits Easier to isolate Harder to couple Easier to couple & construct Harder to isolate Bigger NMR: 2 to 7 qubits; scalability? Ions: up to 14 qubits + scalable Many technical issues still unsolved Dots: LONG T 1 and T 2 Coherent Oscillations Coupling? Little dissipation Reasonable coherence Coupling 9 qubits demonstrated Goal - reach the fault tolerant threshold – F  99 % photons Excellent single qubit coupling hard… Sinlge photon/graph states

14. Recent results Photons – 8 photon cluster states (2012) (Jian-Wei Pan group) Ions –99.93% fidelity of 1-qubit and 2-qubit gate demonstrated (Lucas group 2014): Coherent 14 and 6 ion states demonstated (Blatt/Wineland)

15. Recent results – cont. Atoms – Mott insulator + controlled collisons + site addressing (Bloch group) Semiconductors – even denominator fractional Hall states demonstrated Heiblum group (2010) A possible model system for topological QIP.

16. Recent results – cont (2). Superconductors – (1) surface code fault tolerance demonstrated (Martinis, 2013) (2) errors suppressed by logical qubits – for the first time! (Martinis 2014)

17. Recent results – cont (2b). Superconductors – (2) errors suppressed by logical qubits – for the first time! (Martinis 2014)

18. Recent results – cont (3). Superconductors – Circuit cavity electrodynamics Schoelkopf (2010) Martinis (2010-2013), Katz (2013-2014) Simmonds (2007) Generation of Fock states up to N=16, with full state tomography

19. Outlook - hybrids Cavity – qubit interfaces will improve Mechanical – qubit interfaces Kimbel (2008), Dayan (2014) Yamamoto (2006-2008) Lehnert (2008-2013) Can we make a mechanical S-cat? Yes we can!

20. Summary Exciting new computational models – better suited for implementation Experimental control/coherence of quantum systems is steadily growing Expect very exciting advances in the next decade…