Quantum communications

Transcription

1 Quantum communications Quantum teleportation Trapping of single atoms Atom-photon entanglement Entanglement of remote single atoms Elementary quantum network

2 Telecommunication today Secure communication today does not concern government or military applications

3 Towards quantum internet How to interconnect quantum systems (qubits, atoms, spins, ) over long distances?

4 Communicating with quantum state Send a message encoded in a quantum state Reading by destroying an information: intrinsically secure way of transmitting Ψ = a 0 b Qubit A: Communicate with single photons? Problem: decoherence Qubit B:. Photons are lost. States are destroyed o eavesdropping? o absorption? o eavesdropping? o decoherence?

5 Communicating with entangled states: Quantum teleportation Teleportation is often associated with instantaneous transport over long distances No matter is teleported between the input and output, only quantum information As we know from relativity: information can not be transmitted faster then the speed of light Teleportation can not be used for superluminal information exchange gewöhnliche Wunder (usual miracle) Project QUIMP Erbium and SC devices C. Bennett et al., Phys. Rev. Lett. 70, (993)

6 Quantum teleportation Bell state measurement Ψ Ψ Φ Φ classical information ψ = a b Ψ ± = ±, BSM Φ = ( ± ) ± W, ( ) U D 3 ψ = a b Initial photon W Project QUIMP Erbium and SC devices Source of entangled pairs Qubit is encoded into polarization Total wave function of 3 photons is projected onto Bell state basis The result is transmitted onto station D N. Gizin and R. Thew, Nature Photon., 65 (007)

7 Quantum Teleportation: experiment Simultaneous observation of correlations and anti-correlations Transfer of the quantum state of one photon into another one

8 Teleportation of state over 40 km X.S. Ma et al., Nature 489, 69 (0)

9 Teleportation of state over 40 km

10 Teleportation of state over 40 km X.S. Ma et al., Nature 489, 69 (0)

11 Teleportation of state over 40 km

12 Year 84. Congress of Vienna.

13 Preußischer optischer Telegraph Berlin-Koblenz optical telegraph. Length 588 km. Delivery time of a letter: 7 min. Post: 3-4 days. Transmission rate: 5 bit/min

14 Concept of Quantum repeaters H. Briegel et al. Phy.Rev.Lett. 8, 593 (998) N. Sanguard et al., Rev. Mod. Phys. 83, 33 (0)

15 Quantum entanglement between two or more atoms.

16 Principle of atom-photon entanglement Ψ = ( ), σ, σ Ψ = ( ) σ σ z z

17 Atom-photon entanglement: the atomic qubit. Presence of stable ground state and suitable spin states. The qubit have to form Λ-system with an excited state to form entanglement 3. Presence of optical transitions for cooling and preparation 4. Optical transitions must have wavelength suitable for fiber optics The choice of the experiment: Single 87 Rb atom V. Rosenfeld, PhD-Thesis, München 008

18 How to trap a single neutral atom: Optical dipole trap 0.65 mk V. Rosenfeld, PhD-Thesis, München 008

19 Trapping of single atom: apparatus Dipole trap: P = 30 mw at 854 nm, NA=0.38 Simultaneous MOT and DT operation Fluorescence detection: MOT is off Atom is confined within 3-4 sec

20 Trying to entangle remote single atoms

21 The entanglement sequence

22 STimulated Raman Adiabatic Passage How to detect atomic state? Select the proper measurement basis STIRAP pulses pumps the population into F= state ψ i D = sinθ z e ϕ cosθ z Resonant pulse F= F = to throw atom from the trap Projection onto dark state,>

23 Entanglement verification Ψ = (, σ, σ ) By rotating λ/(β) and λ/4(γ) we select projection basis of the photon. ± - APD or APD Atomic state after the detection of a photon look like replica of the photon state X V X H Atom-photon state correlation depends on the rotation angles β or γ J. Volz et al., Phys.Rev.Lett. 96, (006)

24 Create atom-photon entanglement Choose the proper basis Measure coincidence HV or HV Measure atom-atom correlation Entanglement of remote single atoms ( ) ( ) ( ) ( ) x x x x AA Ph x x z z H V V H V H = Ψ = Ψ = Ψ = Ψ σ σ J. Hofmann et al., Science 337, 7 (0)

25 Atom-atom correlations obtained after Bell state projection of the photons onto state Ψ (A and B) and Ψ (C and D) Ψ ± = ( ) ± x x x x

26 Wiring up remote atoms: Quantum network To send a quantum state between remote quantum bits (atoms in cavity) J. Kimble, Nature 453, 03 (008) S. Ritter et al, Nature 484, 95 (0)

27 Wiring up remote atoms: protocol Atom is coupled to optical cavity D = cosθ a 0 sinθ b Stimulated Raman Adiabatic Transfer cosθ = Ω ( t) g Ω = 0, t = 0 D = a 0 Ω >> g, t, D = b J. Kimble, Nature 453, 03 (008)

28 Universal quantum network node Single photons with temporal envelope Level scheme of Rb atom and storage and read-out processes Second-order correlation function proves that single photons are produced. Quantum tomography of process

29 Atom-photon elementary quantum network