Atom trifft Photon. Rydberg blockade. July 10th 2013 Michael Rips

Transcription

1 Atom trifft Photon Rydberg blockade Michael Rips

2 1. Introduction Atom in Rydberg state Highly excited principal quantum number n up to 500 Diameter of atom can reach ~1μm Long life time (~µs ~ns for low excited atoms) Large dipole moment (scaling with n7!) Dipole interaction between to Rydberg atoms scales with n11 2

3 1. Introduction Blockade Energy shift ΔE caused by dipole interaction 3

4 1. Introduction What's interesting about Rydberg atoms? Ability to entangle two or more neutral atoms Preparation of Qubits Implementation of quantum-gates quantum computing 4

5 Outline 1.Introduction 2.Collective excitation in the Rydberg regime (experiment: Gaëtan et al. 2009) 3.Rydberg blockade between two atoms (experiment: Urban et al. 2009) 4.CNOT-gate between two atoms (experiment: Isenhower et al. 2010) 5.Summary & References 5

6 Outline 1.Introduction 2.Collective excitation in the Rydberg regime (experiment: Gaëtan et al. 2009) 3.Rydberg blockade between two atoms (experiment: Urban et al. 2009) 4.CNOT-gate between two atoms (experiment: Isenhower et al. 2010) 5.Summary & References 6

7 2. Collective excitation in the Rydberg regime Collective excitation in the Rydberg regime (experiment: Gaëtan et al. 2009) Demonstration of a procedure to deterministically entangle two rubidium atoms. Rydberg blockade effect to achieve entangled state. 7

8 2. Collective excitation in the Rydberg regime Experimental setup CCD (charge-cupled device) camera to measure atom position in trap. APDs (Avalanche-Photodiodes) to check if trap is empty or not 8

9 2. Collective excitation in the Rydberg regime Excitation by two (simultaneous!) laserpulses: x-direction 795nm, π z-direction 474nm, σ+ term scheme of rubidium 87 9

10 2. Collective excitation in the Rydberg regime Excitation by two (simultaneous!) laserpulses: x-direction 795nm, π z-direction 474nm, σ+ Detuning δ: First pulse detuned 400Mhz to blue side term scheme of rubidium 87 Necessary to avoid population of intermediate state 10

11 2. Collective excitation in the Rydberg regime Time sequence of the experiment Cooling Ground state preparation Rydberg excitation ~600µs <500ns Dipole traps time Observing trap state with fluorescence induced by cooling laser light. If atom is lost Rydberg atom 11

12 2. Collective excitation in the Rydberg regime Excitation probability outside the blockade regime, R=18µm Red and green:single atom excitation, second atom is absent Blue: Product of red and green Black: Collective excitation of two atoms. 12

13 2. Collective excitation in the Rydberg regime Excitation probability inside the blockade regime, R=3.6µm Red and green:single atom excitation, second atom is absent Blue: Product of red and green Black: Collective excitation of two atoms. 13

14 2. Collective excitation in the Rydberg regime 14

15 2. Collective excitation in the Rydberg regime New 2-atom state in Rydberg regime: entanglement 15

16 2. Collective excitation in the Rydberg regime state coupled to ground state effective Rabi-frequency 16

17 2. Collective excitation in the Rydberg regime state coupled to ground state effective Rabi-frequency 2-level system! 17

18 2. Collective excitation in the Rydberg regime Measurement of entanglement Red line: 1-atom system (2nd trap empty) Blue line: 2-atom system 18

19 2. Collective excitation in the Rydberg regime Measurement of entanglement Red line: 1-atom system (2nd trap empty) Blue line: 2-atom system 19

20 3. Rydberg blockade between two atoms Experimental setup Counter-propagating laser beams 780nm 480nm for excitation. Distance between control and target atom amounts about 10µm. Each atom excited by own laser beam! 20

21 3. Rydberg blockade between two atoms Rydberg state and ground state form 2-level system with Rabifrequency. 21

22 3. Rydberg blockade between two atoms Rydberg state and ground state form 2-level system with Rabifrequency If Control atom is in state level shift B state. Target atom feels is blocked. 22

23 3. Rydberg blockade between two atoms Probability of ground state population on target site depending on duration of target excitation. Control atom in ground state Control atom in Rydberg state 23

24 3. Rydberg blockade between two atoms Phase shift of target atom while 2π-pulse depends on state of control atom. 24

25 3. Rydberg blockade between two atoms Time sequence of the experiment 25

26 3. Rydberg blockade between two atoms Unitary transformation 26

27 3. Rydberg blockade between two atoms Unitary transformation 2-atom states: 27

28 3. Rydberg blockade between two atoms Unitary transformation 2-atom states: controlled-z (CZ) gate 28

29 3. Rydberg blockade between two atoms Unitary transformation 2-atom states: controlled-z (CZ) gate CZ gate can be converted into a controlled-not (CNOT) gate 29

30 4. CNOT-gate between two atoms CZ gate conversion to controlled-not (CNOT) gate: apply π/2 rotations between and on target atom before and after the interaction. π/2 pulse control target 30

31 4. CNOT-gate between two atoms Hadamard controlled-z (H-CZ) CNOT gate 31

32 5. Summary and references Summary: Quantum gates with Rydberg atoms already realized Potential to become basic tool in quantum-information processing Fidelity has to be improved Biggest error source at the moment: atom loss during gate operations 32

33 4. Summary and references References: Gaëtan et al. Observation of collective excitation of two individual atoms in the Rydberg blockade regime, nature physics (2009) Urban et al. Observation of Rydberg blockade between two atoms, nature physics (2009) Isenhower et al. Demonstration of a Neutral Atom Controlled-NOT Quantum Gate, Physical Review Letters (2010) Saffman et al. Quantum information with Rydberg atoms, reviews of modern physics, volume 82 (2010) 33