Study on Bose-Einstein Condensation of Positronium

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1 Study on Bose-Einstein Condensation of Positronium K. Shu 1, T. Murayoshi 1, X. Fan 1, A. Ishida 1, T. Yamazaki 1,T. Namba 1,S. Asai 1, K. Yoshioka 2, M. Kuwata-Gonokami 1, N. Oshima 3, B. E. O Rourke 3, R. Suzuki 3 1 Dept. of Physics, Graduate School of Science, and International Center for Elementary Particle Physics (ICEPP), The University of Tokyo, 2 Photon Science Center (PSC), Graduate School of Engineering, The University of Tokyo, 3 National Institute of Advanced Industrial Science and Technology (AIST) 14th International Workshop on Slow Positron Beam Techniques & Applications JAPAN 1

2 Ps - BEC Ps: The lightest atom Advantage Very high critical temperature of BEC e.g.) /cm 3 Ø Ps is the best candidate for the first BEC with anti-matter Novel applications of Ps - BEC: p Precise measurements of anti-matter gravity p Realization of 511 kev laser using decaying gamma rays ) 3 Density (/cm Rb 1 H Ps BEC phase over the lines 3 1 Goal Critical Temperature (K) 2 2

3 Challenges: High Density and Cooling u Difficult because of Short lifetime as 142 ns Two Challenges in lifetime p High density 18 /cm 3 p Cooling K ) 3 Density (/cm Rb 1 H Ps Goal Current Currently 15 /cm 3 *1, 150 K *2 8 BEC phase over the lines Both need much improvement! 4 9 *1 : S. Mariazzi et al. Phys. Rev. Lett. 4, (20). *2 : D. B. Cassidy et al. physica status solidi 4, 3419 (2007) Critical Temperature (K) 3 2

4 Our strategy: Two-Stage Cooling Thermalization and Laser Cooling Using two cooling processes: efficient in different Ps temperature region Described in K. Shu et al. J. Phys. B 49, 4001 (2016). 7 spin-polarized positrons/bunch focused into 0 nm Positron beam Laser beams from six directions e + Internal void 0 nm 0 nm 0 nm 4000 spinpolarized Ps created 4 x 18 /cm 3 Magnified view 1 K cavity made by silica (SiO 2 ) which is transparent to the laser 1. Down to 300 K: Energy exchange by collisions with cold silica (thermalization process) 2. Down to K: Laser cooling (Doppler cooling) 4

5 Model of the Simulation Cold silica wall 1. Collisions with the silica walls (Thermalization) 2. Ps-Ps two-body elastic scatterings Ps Ps Variables for each Ps Velocity Internal state 1. 1s 2. 2p 3. decayed 3. Photon absorptions/emissions Variables of every Ps are tracked at the same time 3 interactions are integrated Details are in a poster session by A. Ishida today 5

6 Thermalization Model and Parameters The classical model by Nagashima et al. well-tested for Ps in large pores from Y. Nagashima et al. Phys. Rev. A, 52, 258(1995) ) de av dt = 2 LM 2mPs E av ( E av 3 2 k BT E av : Ps average kinetic energy, m Ps : Ps mass, L : Mean free path of scatterings, M : Effective of mass of scatteing bodies, T : Temperature M depends on kinetic energy of Ps Estimated by various experiments Estimation of the dependence: M(E) = p ( + p * exp. / 0 Legends: DBS(1987): T. Chang et al. Phys. Lett. A 126, 189 ACAR(1995): Y. Nagashima et al. Phys. Rev. A 52, 258 ACAR(1998): Y. Nagashima et al. J. Phys. B 31, 329 2γ/3γ(2013): K. Shibuya et al. Phys. Rev. A 52, 258 Silica Effective Mass M (a.m.u) 3 2 DBS(1987) ACAR(1995) ACAR(1998) 2γ/3γ(2013) Uncertainty of M Energy (ev) Measured values superimposed 6

7 Evaluation of Ps Thermalization Included Classical model with three M estimation The two body elastic scatterings An initial condition Ø Ps initial kinetic energy: 0.8 ev from Y. Nagashima et al. Phys. Rev. A 52, 258(1995) Ø An initial number of Ps: 4000 Ø Silica cavity: 0 nm x 0 nm x 0 nm,1k Temperature (K) 3 2 Cooled to 300 K in 0 ns Cooling to K is too slow Ø Laser cooling is important after 300K 300 K in 0 ns Slow Best fit Uncertainty Fast of M Very slow for full thermalization Time (ns) Time evolution of Ps temperature ü Precise measurement is necessary 7

8 Laser cooling of Ps E Ps internal state 2p (τ=3.2 ns) 5.1eV = 243 nm=1.23 PHz (UV) 1s Ps Resonance between 1s 2p will be used Cooled in every 3.2 ns x 2 = 6.4 ns in average 8

9 Requirements for the Laser: Long pulse Two requirements for efficient cooling especially for Ps: 1 Long pulse Ps cooling takes the order of the lifetime (>0 ns) Laser should be pulsed with 300 ns width (much longer than usual) Pulse energy is 40 µj to saturate cooling cycle Intensity (Arb.) µj pulse 300 ns e + injection at t= Time (ns) Laser intensity vs Time 9

10 Requirements for the Laser: Broad bandwidth & Frequency shift Two requirements for efficient cooling especially for Ps: 2 Broad bandwidth & Frequency shift Doppler broadening for Ps is quite large because of its light mass c.f. 500 GHz at 300 K Broad bandwidth To cool Ps with various velocities Intensity (Arb.) For hot Ps t=0 ns 140 GHz 1s-2p Resonance For cold Ps Frequency shift To follow smaller Doppler shift of cold Ps Laser Frequency Detune (GHz) Frequency Profile of the Cooling Laser

11 Requirements for the Laser: Broad bandwidth & Frequency shift Two special requirements for efficient Ps cooling: 2 Broad bandwidth & Frequency shift Doppler broadening for Ps is quite large because of its light mass c.f. 240 GHz at 300 K Broad bandwidth To cool all of Ps with various velocities Frequency shift To follow smaller Doppler shift of cooled Ps Intensity (Arb.) t=300 ns 60 GHz shift in 300 ns 1s-2p Resonance Laser Frequency Detune (GHz) Frequency Profile of the Cooling Laser 11

12 Laser Specifications Summary of required specifications Pulse energy 40 µj Center frequency Frequency detune (D(t)) Bandwidth(2σ) Time duration(2σ) 1.23 PHz - D(t) D(0 ns)=300 GHz D(300 ns)=240 GHz Very special specs! Especially: fast and well-controlled frequency shifting in pulsed mode New trial for optics!!! 140 GHz 300 ns Beam waist(2σ) 200 µm 12

13 Design of the Laser System Newly designed for Ps cooling mj injection seeded Ti:sapphirelaser Ti:Sapphire Q-switched Pump Laser (c) EOM2 Sideband Generator (b) EOM1 Frequency Shifter (a) mw SHG 4 THz 820 THz THG 40 µj 1.23 PHz(by THG) 13

14 Design of the Laser System Newly designed for Ps cooling mj injection seeded Ti:sapphirelaser Ti:Sapphire Q-switched Pump Laser 1 (c) EOM2 Sideband Generator (b) EOM1 Frequency Shifter (a) mw (c) 20GHz SHG f (b) 20GHz 4 THz 820 THz (b) 20GHz (a) THG CW 40 µj 1.23 PHz(by THG) f Idea of the design: 1. Controlingfrequency in Continuous Wave mode with a third frequency Both of shifting and broadening are possible by EOMs 14 f (a) 4 THz

15 Design of the Laser System Newly designed for Ps cooling mj 2 injection seeded Ti:sapphirelaser Ti:Sapphire Q-switched Pump Laser (c) EOM2 Sideband Generator (b) EOM1 Frequency Shifter (a) 1 mw 4 THz 40 µj SHG THG 820 THz 1.23 PHz(by THG) Idea of the design: 2. Seeding the CW laser into Ti:Sapphireto generate pulsed laser Generate pulsed laser with controlled frequency 15

16 Development of the cooling laser The initial part (CW seed laser) 4 THz (red) light 4 THz Output Laser Diode opnext HL7301MG (InGaAsP) ECDL-Box Gratings 8cm Home-made External Cavity Diode Laser Compact To controller Oscillating at desired ~4 THz Powerful enough (~ mw) 16

The initial part (CW seed laser) Laser Diode opnext HL7301MG (InGaAsP) 0.002 4 THz (red) light 0 0.002 0.004 0.006 Gratings0.

Time (s) 0 0.2 Next: 0.4 Ø Designing Ti:Sapphire To controller 0.6 8cm oscillator for 300 ns pulse 0.

17 The initial part (CW seed laser) Laser Diode opnext HL7301MG (InGaAsP) THz (red) light Gratings0.2 LD frequency drift (GHz) 4 THz Output Temperature drift (K) Development of the cooling laser ECDL-Box Time (s) Next: 0.4 Ø Designing Ti:Sapphire To controller 0.6 8cm oscillator for 300 ns pulse 0.8 Home-made External Cavity Diode Laser 1 Oscillating at desired ~4 THz and injection seeding Powerful enough (~ mw) 1.2 Compact Time (s) 17

18 Evaluation of the Cooling Temperature (K) 3 2 Without the laser Arb. t=380 ns 70 K Time (ns) Time evolution of Ps temperature Velocity (km/s) Ps velocity distribution (Calculated by simulated temperature) Without the laser, cooling is too slow (by Best fit estimation of M) 18

19 Evaluation of the Cooling Temperature (K) 3 2 Without the laser Arb. t=380 ns t=380 ns 11 K With laser With the laser Time (ns) Time evolution of Ps temperature 70 K Velocity (km/s) Ps velocity distribution (Calculated by simulated temperature) Without the laser, cooling is too slow (by Best fit estimation of M) With the laser, cooling from ~300 K is accelerated 19

20 Evaluation of the Cooling Temperature (K) 3 2 Without the laser Arb. t=380 ns t=380 ns t=450 ns 7 K (broad) + 30% condensate (peak) Critical temperature With the laser Time (ns) Time evolution of Ps temperature Velocity (km/s) Without the laser, cooling is too slow (by Best fit estimation of M) With the laser, cooling from ~300 K is accelerated Compared with the critical temperature, BEC transition will happen at 400 ns! 11 K 70 K Ps velocity distribution (Calculated by simulated temperature) 20

21 Roadmap 1. Precise Ps Doppler spectroscopy (in 2 years) l l Establishment of a methodology for cold Ps Solving uncertainty of Ps thermalization process with silica 2. Ps laser cooling (in 4 years) l Development of the laser system with long pulse, wide bandwidth, frequency shift 3. Developing a dense positron system l 7 e + into 0 nm diameter in a nanoseconds bunch 4. Ps BEC l Combing all the technologies! 21

22 Principle of Doppler-sensitive two-photon spectroscopy of Ps Conventional technique is not precise for K Ps or BEC We will use Doppler-sensitive two-photon spectroscopy(new for Ps) 4nm pulsed laser Ps E -0.76eV -1.7eV 3s e + e - Two photons of 4 nm wavelength 1. Excited to 3s by absorbing co-propagating two photons 2. Ionize 3s-Ps to free e + and e - 3. The e + annihilates into γ-rays -6.8eV 1s Doppler effect is doubled because two-photon: sensitive to Ps temperature Resonance can be measured via SSPALS ( D. B. Cassidy et al. App. Phys. Lett. 88, 1945(2006) ) 22

23 Laser Requirements for Doppler Spectroscopy Laser spec. for temperature measurement Wave length Pulse energy Time duration Bandwidth(FWHM) Freq. tunable range 4 nm 1 mj A few ns 25 GHz 150 GHz beam waist (2σ) 850 µm Repetition rate 0 Hz Visible wavelength Easily achievable bandwidth Ø Design of the laser is under study Excited Ps ratio (SHG of Ti:Sapphirelaser is a promising candidate) Plan to n Confirm feasibility of the method n Measure thermalization process of Ps with cold silica 0 χ 2 / ndf 3.5 / 4 Constant ± Mean ± Ps Temperature (K) ± Frequency (GHz) Expected resonance curve for K Ps with 7 Ps in total, at t=300 ns 23

24 Goal for Dense Positron Goal for Ps-BEC Current at AIST microbeam 1 stage brightness-enhancement-system Number of positrons Beam waist Pulse width Positron Energy 7 e + /bunch < 0 nm < 5 ns < 5 kev Number of positrons 4 e + /bunch Beam waist 25 µm Pulse width ~ µs Positron Energy 0.5 ~ 30 kev Principle of positron focusing (brightness enhancement): Focusing lens bunched e + beam Re-moderator Re-moderated positrons are Perpendicular to surface Monochromatic Energy With focused waist N. Oshima et al. J. Appl. Phys. 3, (2008). 24

25 Planned Upgrade of Positron Beam Two necessary improvements: State-of-the-art Buncher for spin-polarized positron 8 e + /bunch 2x 9 e + /bunch Beam compression ratio by Focusing lens 1/ 1/20-1/30 With 20% re-moderation efficiency, 7 e + in 0 nm by 3-stage brightness-enhancement-system Goal Silica target 1 n=2x 9 e + φ=15 mm Ø Buncher and beam optics are now under detailed consideration 25 4x 8 e µm 8x 7 e + 40 µm 2x 7 e + 2 µm 2x 7 e + 0 nm At target

26 Summary We proposed and evaluated a new experimental scheme shown to be possible to realize Ps-BEC by: 1. Thermalizationprocess with cold silica (down to 300 K) 2. Laser cooling (down to K) The Cooling laser is special for: 1. Long pulse 300 ns time duration 2. Broad bandwidth 140 GHz 3. Frequency shift 60 GHz New optics for efficient cooling CW seed ECDL is ready Next: Ti:Sapphireinjection seeding laser including frequency shift Several tasks are under detailed study and preparation: 1. Doppler-enhanced 1s - 3s spectroscopy using two-photon excitation 2. Designing a buncher and beam optics for 7 spin-polarized e + /bunch in 0 nm waist by 20 times more e + & factor 2 strong focusing 26

Study on positronium Bose-Einstein condensation

Study on positronium Bose-Einstein condensation Akira Ishida a,*, K. Shu a, T. Murayoshi a, X. Fan a, T. Namba a,s. Asai a, K. Yoshioka b, M. Kuwata-Gonokami a, N. Oshima c, B. E. O Rourke c, R. Suzuki