Quantum applications and spin off discoveries in rare earth crystals

Transcription

1 Quantum applications and spin off discoveries in rare earth crystals Stefan Kröll Dept. of Physics, Lund University Knut och Alice Wallenbergs Stiftelse Funded by the European Union

2 Rare earth doped crystals Rare earth crystal quantum properties Quantum cryptography and quantum memories Building materials with new properties Quantum memory spin off 1 Quantum memory spin off 2

3 Outline Rare earth crystals and quantum properties Long distance quantum cryptography and quantum memories Building materials with new properties Quantum memory spin off 1 Quantum memory spin off 2

4 Quantum state superpositions Quantum information tasks generally requires superposition of states Complicated quantum information tasks benefit from systems where superposition remains over long times Atoms/molecules/ions in vacuum

5 The closely spaced atoms in a solid continuously senses each other Rare earth crystals inner shell transitions can have sub khz linewidth Optical superpositions may last in ms They are efficient optical interfaces to much more longlived states

6 Optically addressable nuclear spins in a solid with a six-hour coherence time, Zhong et al., Nature 517, 177 (2015), Sellars Lab, Canberra Nuclear spin state superposition can be >6 hours A crystal travelling at 9 kilometers per hour will have lower decoherence with distance than light in an optical fibre.

7 Outline Rare earth crystals and quantum properties Long distance quantum cryptography and quantum memories Building materials with new properties Quantum memory spin off 1 Quantum memory spin off 2

8 Quantum cryptography Today commercial equipment for quantum cryptography is available for distances up to about km First time used in

9 Long distance quantum cryptography requires Quantum Repeaters (QR) A Length L, transmission probability10-10 B QR QR QR QR Alice Bob Unknown quantum states can neither be copied (cloned) nor read

10 Long distance quantum cryptography requires Quantum Repeaters (QR) Classical communication lines QR Quantum measurement device QM QM Quantum communication lines QM Quantum memory

11 Afzelius Group, Gisin Lab, Geneva Quantum storage of photonic entanglement in a crystal, Nature 469, 508 (2011) Heralded quantum entanglement between two crystals, Nature Photonics, 6, 234 (2012)

12 Outline Rare earth crystals and quantum properties Long distance quantum cryptography Building materials with new properties Quantum memory spin off 1 Quantum memory spin off 2 Zinc blende GaP nanowires, NanoLund, Lund

13 Engineering materials in frequency space Conceptual picture of crystal 3+ Pr : Y2 SiO 5

14 Engineering materials in frequency space Absorption Γ hom 1kHz Γ inhom 5GHz Frequency 3+ Pr : Y 2 SiO 5

15 Engineering materials in frequency space Γ inhom 5GHz Pr 3+ : Y 2 SiO 5 Γ hom 1kHz Absorption ν 0 Excited state Frequency ν 0 Optical pumping Two ground state Hyperfine levels

16 Engineering materials in frequency space Γinhom 5GHz Pr 3+ : Y 2 SiO 5 Γ hom 1kHz Absorption B ν 0 Excited state Frequency ν 0 A C Two ground state Hyperfine levels

17 A narrow spectral transmission window is created Absorption (αl) αl L crystal length α absorption coefficient Γ Frequency (MHz)

18 Index of refraction in the vicinity of an absorption line R W Boyd, JOSA B, 28, A38 (2011)

19 A narrow spectral transmission window is created Speed of light in transmission window is 1-50 km/s Absorption (αl) Γ αl Index of refraction L crystal length α absorption coefficient Frequency (MHz)

20 Outline Rare earth crystals and quantum properties Long distance quantum cryptography Building materials with new properties Quantum memory spin off one Laser frequency stabilization

21 Significance of laser frequency stabilization Frequency is the quantity that we can measure most accurately Our ability to measure frequency (and time) accurately e.g. impacts todays communication and GPS systems 16 digit accuracy enables tests of general relativity, gravity wave detection, measurements of the constants of nature, etc.

22 Laser stabilization 101 Mirrors Vibrations Laser Temperature changes Control system Frequency reference

23 Laser stabilization 101 Brownian movement of the atoms 1 Average movement < 1 fm or < 1 proton radius 1. Numata, K. et. al. Thermal-noise limit in the frequency stabilization of lasers with rigid cavities Phys. Rev. Lett., 2004, 93,

24 How to improve the cavity frequency stability Slow light material dν ν = dd L v g v p ~10 4 ~10 8 L v g = group velocity of light v = light frequency v p = c n = phase velocity of light dν ν = dd L c = speed of light in vacuum n = index of refraction

25 Outline Rare earth crystals and quantum properties Long distance quantum cryptography Building materials with new properties Quantum memory spin off one Laser frequency stabilization Quantum memory spin off two Deep tissue medical imaging

26 A narrow spectral transmission window is created Absorption (αl) αl L crystal length α absorption coefficient Γ Frequency (MHz)

27 Ultrasound optical tomography Ultrasound ν S Tissue Spectral Filter Laser Input ν L + ν S ν L + ν S ν L - ν S ν L ν L + ν S ν L Tumours Ultrasound attenuation coefficient in tissue 0.3 db/(cm MHz) Detector Absorption Frequency

28 Penetration depth & S/N for the two different techniques Measurements of blood oxygenation front of the heart muscle Spatial resolution: 30 elements, ~3 mm each, measurement time, 250 ms

29 Outline Rare earth crystals and quantum properties Long distance quantum cryptography Building materials with new properties Quantum memory spin off one Laser frequency stabilization Quantum memory spin off two Deep tissue medical imaging

30 Lars Rippe Andreas Walther Qian Li Mahmood Sabooni Adam Nilsson Yupan Bao Ivan Sytsevich Theodor Strömberg

31 End