QAP - SP1

1. QAP - SP1 Quantum Memories and Interfaces

2. 2hn 2hn QM QM QM QM QM BSM z QND x SP1: Quantum Memories & Interfaces Photons are ideal carrier to transmit quantum information However, some applications rely on storage of quantum information Measurement sensitivity in metrology can be increased using squeezed atomic states created via QND light-atom interaction squeezing of atomic pseudo-spin to improve sensitivity of atomic clocks buffer for quantum information processing quantum repeater - extend transmission span of quantum communication - single photons on demand - buffer (linear optic quantum computation, eavesdropping)

3. SP1: Quantum Memories & Interfaces • WP 1.0 Subproject management, UNIGE, Gisin / Thew • WP 1.1 Rare-Earth-Ion Doped Solids, ULUND, Stefan Kröll • WP 1.2 NV Centres, USTUTT, Jörg Wrachtrup • WP 1.3 Semiconductor Nanotechnology, TREL, Andrew Shields • WP 1.4 Single Trapped Atoms, LMU, Harald Weinfurter • WP 1.5 Room-Temperature Atomic Vapour, NBI, Eugene Polzik • WP 1.6 Cold Atoms, NBI, Eugene Polzik • WP 1.7 Comparison, UNIGE, Nicolas Gisin, Christoph Simon

4. Expected Project Outcomes • Compare, evaluate and analyze different proposed quantum memory techniques and define main criteria. • Demonstrate high-fidelity single-photon wave-packet quantum state storage and recall in a material sample. • Map non-classical photon states onto electronic excitations in atoms and solids, and to store and recall them with high fidelity. • Generation of entangled states, including Schrödinger cat states. • Generate (non-classical), squeezed states via light-atom interactions for metrology, e.g. increased sensitivity of atomic clocks.

5. Objective The goal of this workpackage is to demonstrate efficient storage and reconstruction of single-photon non-stationary light field, e.g. time-bin qubits, in optically dense, rare-earth ion doped solids Where we are going? M1:Optical pumping. (M18) Multipass crystal Pr (Lund) Er doped samples (Unige) M2: Test of multipass crystal. Target an optical density >1 after broadening an isolated spectral structure with an inhomogeneous field. Lund (M21) D1: Demonstration of wave-packet storage and recall in the multipass crystal. Lund (M24) D2: Q. coherence between 2 independent memories. Unige (M24) WP1.1: Rare-earth-ion doped solids • Where are we? • M1.1.1: Evaluate coherence times of doped crystals and fibres. (M6) • Done. • M1.1.2: Demonstrate the possibility to shift a single spectral absorption line in a controlled and reversible way by applying an electric field. (M9) • Done • D1.1.1 Evaluate the suitability of different rare-earth-ion-doped solids for single-photon time-bin qubit storage. (M12) • On target • Pr & Er in crystal and in fibre studied by Lund & Unige. Collaborations : U. Paderborn (D) Workshop : France, Germany, Lund & Unige

6. WP1.1: Rare-earth-ion doped solids Coherence time in Er3+:silica fibre up to 4s measured. Controlled frequency shifts in Er3+:LiNbO3 waveguide using linear Stark effect M.U. Staudt et al. Opt. Comm. 266 720 ((2006) S.R. Hastings-Simon et al., Opt. Comm., 266 716 (2006) Interference measurements of photon echo signals show that storage of time-bin encoding is robust against decoherence in RE-ion doped solids. Visibility 100% M.U. Staudt et al. Submitted PRL

7. Objective The general objective of the WP is to store light states into spin states based on electromagnetically induced transparency (EIT) using Nitrogen-vacancy (NV) defects in diamond as a storage medium . Where we are going? M 1 Evaluate strategy to maximize  transition amplitude on NV centre (M18) M 2 Evaluate optimal optical pulse sequence for electron spin Raman transitions. (M21) D 1 Generation and retrieval of electron spin coherence by optical pulses. (M24) WP1.2: NV Centres • Where are we? • M1.2.1: Evaluate the strategy to achieve spin phase memory times beyond 0.1 ms. (M9). • Done • M1.2.2: Evaluate the achievement of large amplitude EIT (M12). • Done • D1.1.1: Measure phase memory times for ultrapure high NV density diamond (M12). • Done

8. fine structure 3 637 nm signal 2 control 1 ms=0 Quantum memory with paramagnetic defects Implantation of defect in low strain regions yields high optical density and narrow inhomogenous lines: ms=±1 S=1 Storage: electron; nuclear spin states Santori et al. OPTICS EXPRESS 14 (17): 7986 (2006).

9. nP nC mS = 1 mS = 0 Photon strorage by generation of spin coherence with light beams. Step 1: EIT Linewidth of the dip: few MHz << Ghom In collaboration with Ch. Santori (HP, California) and P.R. Hemmer (Texas A&M) Submitted (quant-ph/0607147)

10. 1,06 1,04 Fluorescence, a. u. 1,02 /2  /2 1,00 1 t2 0,98 0,96 0,94 0,92 0 20 40 60 80 100 120 140 160 180 t 1= , µs 2 Spin Coherence Ground state electron spin coherence @T=300K Mechanism: Dipolar coupling of NV spin to other „impurity“ spins in the lattice. Mostly: N, 13C T2=0,35ms T. Gaebel et al. Quant/ph0605038 In collaboration: Prawer et al & Twamley(Aus) and Hemmer (Texas A&M)

11. Objective The goal of this WP is to investigate the possibility of using semiconductor quantum dots as quantum memories as well as different semiconductor systems including single quantum dots, impurity atoms in optical cavities and inhomogeneously broadened ensembles of quantum dots or impurities. Where we are going? M1: Demonstrate charge tuning of quantum dot emission in a structure containing an integrated cavity and a tunnel barrier. (M18) D1: Demonstrate photonic memory in a device containing quantum dots. (M24) WP1.3: Semiconductor Nanotechnology • Where are we? • M 1.3.1: Develop tools for calculating the Q-value and coupling efficiencies of semiconductor cavities. (M9) • Done • M1.3.2: Evalutate techniques for fabrication of semiconductor cavities containing low number of Q dots. (M9) • Done • D 1.3.1: Measure and calculate the input/output coupling efficiencies for a single photon and a quantum dot. (M12) • On target

12. Quantum dots have atomic like properties, they can; confine single electrons for long periods of time, are easy to integrate into existing semiconductor structures which promises good scalability of quantum dot based devices. We are investigating the possibility of using single quantum dots to store qubits. P N N P Quantum Memory with Quantum dots Memory operation The device is biased to return the hole to the quantum dot, radiative recombination restores the photon A single photon is absorbed by the quantum dot and excites an exciton, a bias is applied to remove the hole Writing mode Reading mode

13. Design of microcavities •  cavity between two GaAs/Al0.9Ga0.1As DBRs, bottom 27 pairs, top 20 or 14 pairs • One layer self-assembled InGaAs QDs at cavity center FDTD modelling circular pillar elliptical pillar FIB etching ICP etching

14. FDTD simulations 0.50μm radius micropillar microcavityPlane wave resonance=1001 nm 15 mirror pairs on top and 30 bottom

15. Photon-exciton coupling Micro-cavities can be used to improve the coupling strength between a single photon and a quantum dot • Distributed Bragg Reflectors are grown above and beneath the quantum dots, micro pillars can then be etched. These provide excellent optical confinement and increase collection/absorption efficiency [A.J. Bennett et al. Optics Express 13 50 (2005)] • An exciton-photon coupling efficiency of >97% has been demonstrated • 2D photonic crystals can also provide good optical confinement [D. G. Gevaux et al. APL 88 131101 (2006)]

16. Objective The goal of this WP is focused on a quantum memory consisting of a single long-lived qubit, represented by the quantum state of a single Rubidium atom, trapped in a dipole-trap. State transfer from photonic to atomic qubits and vice versa can thus be achieved via quantum teleportation protocols. Where we are going? M1: Remote state preparation of the atomic quantum memory (M15). M2: Improve coherence time of entangled atom-photon state and verify atom-photon entanglement over 200 m of optical fiber. (M 21) M3: New and optimized single-atom dipole trap which can easly be moved to a distant second lab (M24). D1: Remote preparation of the atomic quantum memory using a teleportation protocol (M15). D2: Observation of atom-photon entanglement over a distance of several hundred metres. (M42). WP1.4: Single trapped Atoms • Where are we? • M 1.4.1: Efficient detection of atomic states (eta>95%) • Done • M 1.4.2: State tomography of an entangled atom-photon state. • Done • D 1.4.1: Observation of atom-Photon entanglement. • Done

17. APD1 STIRAP H-polarised analysed atomic state: APD2 Atom-Photon Correlations (atomic sx basis) • setup • atomic sx basis: probability F=1 photon polarisation angle f (°) J. Volz et al. PRL 96 030404 (2006)]

18. Atom-Photon State Tomography photon 1 7 photon 2 • measured atom-photon density matrix: estimated atom-atom entanglement fidelity: Fat-at = 0.81 > 0.78 J. Volz et al. PRL 96 030404 (2006)]

19. Objective A Memory for a quantum state of light in an atomic ensemble of Cs atoms with a fidelity of up to 70%, had previously been demonstrated. The goal in this WP is to investigate further approaches to the memory, all based on gas in glass cells. Where we are going? D1: Investigation of the ways to improve the fidelity of recording into atomic memory implementing squeezing operations on atoms and light NBI WP1.5: Room Temperature Atomic Vapour • Where are we? • M 1.5.1 Optimization of the experimental light-atoms interaction parameters for the teleportation protocol. (M6) • Done • M 1.5.2 Conclusion on the feasibility of the light-to-atoms quantum teleportation protocol based on a single atomic ensemble in magnetic field. (M9) • Done • D 1.5.1 Measurements of the fidelity of quantum storage of light via light-to-matter teleportation. (12) • Done

20. Light-Matter Q. Teleportation • First quantum teleportation between light and matter - a macroscopic atomic sample. • Distance of 0.5m. • Fidelity of teleportation up to 64% has been achieved.

21. Objective The general goal of this WP is to develop interfaces between light and cold atoms. In addition to a memory of single photon qubits, we want to demonstrate spin squeezing at Cs clock transition based on atom light interaction with the ultimate goal to improve the sensitivity of atom clocks. Where we are going? Investigation of polarization coupling between light and cold dipole trapped Cs atoms for memory applications. NBI WP1.6: Cold Atoms • Where are we? • M 1.6.1 Preparation of the coherent spin state on Cs clock transition. • Done • M 1.6.2 Measurements of the population difference of Cs clock transition levels with the light probe. • Done • D 1.6.1 Quantum non-demolition measurements of the quantum state of cold Cs atoms at the clock transition. • Delayed - Optical density of the sample and stability of the system needs to be improved in order to reach the projection noise level

22. Small kitten Bigger kitten Theory NBI Schroedinger cat states for Quantum Information Networks • Frequency bandwidth ≈ 10 MHz • Perfect Gaussian spatial mode • Tunable to Cs resonance J.S. Neergaard-Nielsen et al. PRL 96 030404 (2006)]

23. Objective The objective of this WP is to compare, evaluate and analyse the different approaches to quantum memory for applications in quantum communication and computation. Where we are going? M1Review of criteria that has to be met by efficient quantum memory and storage devices. D 1 Updated results on the comparison, evaluation and analysis of the different approaches to quantum memory for applications in quantum communication and computation. WP1.7: Comparison • Where are we? • M 1.7.1 Define main criteria that has to be met by efficient quantum memory and storage devices. • Done • M 1.7.2 Quantify performance of quantum storage devices. • 1 month delay • D 1.7.1 Interim results of the comparison, evaluation and analysis of the different approaches to quantum memory for applications in quantum communication and computation. • On target

24. Quantum Memories: Comparison • Key numbers1. Wavelength 2. Numerical aperture3. Bandwidth 4. Storage time5. Reset time 6. Read-out delay7. Read-out jitter 8. Temperature • Figures of Merit1. Bandwidth • storage time2. Prob. of retrieval3. Fidelity of retrieved data4. Storage / Heralding • Intermediate Milestones1. Storage of multiple pulses2. Storage of phase3. Calibration of efficiency, linearity Approaches to Quantum Memories 06 (29 - 30 June) • Goals: • Develop a common means of characterising and comparing the different quantum systems. • Developing collaboration • Within the SP: WP1.1 - WP1.2 • Outside: • REID Workshop (French & German groups) • Paderborn - Supply Doped W/Gs • Structuring the Community Independent input from U. Gdansk and NBI

25. Objective WP “Management” will arrange regular sub-project meetings or workshops. To enable co-ordination of the research effort and to manage the realization of deliverables, as well as raise awareness of the facilities and expertise of the different partners and identify opportunities for collaboration. Meetings will also be open to partners from SP5 (Theory: Architecture) to discuss issues related to WP1.7:Comparison. QAP SP1 : On Target WP1.0: Management

26. Degenerate bright excitons • Typical quantum dots have non-degenerate excitons, limiting the quantum memory schemes which can be used and the bandwidth of photons which can be stored • We can restore this degeneracy by growth or application of an in-plane B-field… …entangled photon emission demonstrates that the degeneracy has been restored. [R.M. Stevenson et al. Nature 439, 179 (2006) R.J. Young et al. New J Phys. 8, 29 (2006) R.M. Stevenson et al. PRB 73 033306 (2006) R.J. Young et al. PRB 72 113305 (2005)]