Direct observation of quantum phonon fluctuations in ultracold 1D Bose gases

Transcription

1 Laboratoire Charles Fabry, Palaiseau, France Atom Optics Group (Prof. A. Aspect) Direct observation of quantum phonon fluctuations in ultracold 1D Bose gases Julien Armijo* * Now at Facultad de ciencias, Universidad de Chile 22/11/2012, Congreso SOCHIFI, La Serena, Chile

2 Introduction

3 Quantum fluctuations in nature At T=0, thermal excitations vanish The Heisenberg uncertainty principle (1927) rules the physics.

4 Quantum fluctuations in nature At T=0, thermal excitations vanish The Heisenberg uncertainty principle (1927) rules the physics. Spontaneous emission of excited atoms Lifetime of excited state Δt Linewidth of the transition Δν = h/δt

5 Quantum fluctuations in nature At T=0, thermal excitations vanish The Heisenberg uncertainty principle (1927) rules the physics. Spontaneous emission of excited atoms Casimir force (1948) Lifetime of excited state Δt Linewidth of the transition Δν = h/δt Attraction between 2 metallic plates

6 Quantum fluctuations in nature At T=0, thermal excitations vanish The Heisenberg uncertainty principle (1927) rules the physics. Spontaneous emission of excited atoms Casimir force (1948) Lifetime of excited state Δt Linewidth of the transition Δν = h/δt Hawking radiation Black hole evaporation due to virtual particles Attraction between 2 metallic plates Also : Quantum phase transitions Dark energy (?)

7 Quantum fluctuations in nature At T=0, thermal excitations vanish The Heisenberg uncertainty principle (1927) rules the physics. Spontaneous emission of excited atoms Casimir force (1948) Quantum fluctuations present in any system But : never observed directly (=microscopically) in a continuous field. measurements so far concerned integral quantities LifetimeAll of excited state Δt Linewidth of the Δν = h/δt attransition the thermodynamic (=macroscopic) scale Hawking radiation Attraction between 2 metallic plates Black hole evaporation Also : Quantum phase transitions Dark energy?

8 Quantum fluctuations in nature At T=0, thermal excitations vanish The Heisenberg uncertainty principle (1927) rules the physics. Spontaneous emission of excited atoms Casimir force (1948) Quantum fluctuations present in any system But : never observed directly (=microscopically) in a continuous field. measurements so far concerned integral quantities LifetimeAll of excited state Δt Linewidth of the Δν = h/δt attransition the thermodynamic (=macroscopic) scale Attraction between 2 metallic plates In ultracold clouds we could detect them directly Hawking radiation Black hole evaporation Also : Quantum phase transitions Dark energy?

9 Our experiment

10 Quantum gases : ultracold atoms High T billiard balls Low T wave packets 1995 : Bose-Einstein Condensation (BEC) T=T c BEC formation λ db ~ d : quantum degeneracy T=0 giant matter wave JILA Very clean systems, highly controllable (density, temperature, ) Ideal to study condensed matter physics! Collective wavelike behaviour, superfluidity (1911), fluctuations

11 Our set-up : Atom chip experiment Miniaturized magnetic device Transverse motion hω Small structures strong field gradients high ω (~150nK) Effective 1D system

12 Chip assembly Atom source Electrical connections Chip

13 Experimental routine (15 s) 300 K 1. Magneto-optical trap Atom chip 2. magnetic microtrap 3. Evaporation (2-3s) 87Rb atoms 4. Absorption picture 10 µk 10 nk

14 Density fluctuations measurements Local and direct information pixel : Δ=4.5 µm Armijo, Jacqmin, Kheruntsyan, Bouchoule, PRL (2010)

15 Density fluctuations measurements Local and direct information pixel : Δ=4.5 µm In a pixel : Armijo, Jacqmin, Kheruntsyan, Bouchoule, PRL (2010)

16 Density fluctuations measurements Local and direct information pixel : Δ=4.5 µm In a pixel : Local Density Approximation (LDA) : Each pixel is in equilibrium with rest of the gas (= reservoir) Fluctuation-dissipation theorem: Compressibility Equation of State n(µ,t) Armijo, Jacqmin, Kheruntsyan, Bouchoule, PRL (2010)

17 Thermodynamics of the repulsive 1D Bose gas Lieb-Liniger phase diagram (temperature t ; interactions γ) Ideal Bose gas thermal quantum Quasi-condensate Strong interactions

18 Thermodynamics of the repulsive 1D Bose gas Lieb-Liniger phase diagram (temperature t ; interactions γ) Ideal Bose gas thermal quantum Quasi-condensate Strong interactions Armijo, Jacqmin, Kheruntsyan, Bouchoule, PRL (2010) Armijo, Jacqmin, Kheruntsyan, Bouchoule, PRA (2011) Jacqmin, Armijo, Berrada, Kheruntsyan, Bouchoule, PRL (2011)

19 Thermodynamics of the repulsive 1D Bose gas Lieb-Liniger phase diagram (temperature t ; interactions γ) Ideal Bose gas thermal quantum Quasi-condensate Strong interactions Quantum fluctuations and anticorrelations dominate ( antibunching ) Armijo, Jacqmin, Kheruntsyan, Bouchoule, PRL (2010) Armijo, Jacqmin, Kheruntsyan, Bouchoule, PRA (2011) Jacqmin, Armijo, Berrada, Kheruntsyan, Bouchoule, PRL (2011)

20 Direct detection of quantum fluctuations

21 Bogolyubov excitations in quasicondensates Bogoliubov spectrum in a BEC or quasi-bec : healing length particles phonons

22 Bogolyubov excitations in quasicondensates Bogoliubov spectrum in a BEC or quasi-bec : healing length particles phonons T Q Thermal occupation number If ε k >> k B T, n k <<1 quantum fluctuations dominate

23 Bogolyubov excitations in quasicondensates Bogoliubov spectrum in a BEC or quasi-bec : healing length particles phonons cf. harmonic oscillator : T Q Thermal occupation number Q If ε k >> k B T, n k <<1 quantum fluctuations dominate hω

24 Quantum vs thermal fluctuations in a quantum quasicondensate Quantum quasicondensate ξ Microscopic Thermodynamic Thermal phonon wavelength Armijo, PRL (2012)

25 Direct detection of quantum fluctuations T=18nK T=4.7nK Thermal Quantum 20% Poissonian shot noise Armijo, PRL (2012)

26 Direct detection of quantum fluctuations T=18nK T=4.7nK Thermal Quantum 20% Armijo, PRL (2012)

27 Direct detection of quantum fluctuations T=18nK T=4.7nK Thermal Quantum 20% Up to 20% of fluctuations observed are quantum phonons Armijo, PRL (2012)

28 Smoking gun : scaling with system size L Thermal + quantum fluctuations Thermal (classical) fluctuations only Die off at small L bad description! Armijo, PRL (2012)

29 Recent related works Observation of quantum fluctuations of ultracold bosonic atoms in optical lattice Endres et al., Science (2011) Studies of density fluctuations / correlations in situ : 2D bosons (Hung et al., Nature (2011)), Fermions (Sanner et al., Mueller et al., PRL (2010)), Optical lattices : Gemelke et al. Nature (2009), Sherson et al. Nature (2010)

30 Conclusion Record low T and high sensitivity measurements First microscopic detection of collective quantum fluctuations in any continuous field More spectacular effects are still to see : e.g., excess of fluctuations at short distance compared to thermodynamic value Thermal + quantum Thermodynamic

31 Thanks Atom chip team I. Bouchoule T. Jacqmin T. Berrada Collaborators/discussions A. Sinatra (LKB, ENS Paris) K. Kheruntsyan (U. Queensland, Australia) Electronics Mechanics Micro-fabrication Optics F. Moron A. Guilbaud B. Ea-Kim G. Colas A. Villing P. Roth F. Delmotte M. Lamarre L. Jakuboviez