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
Introduction
Quantum fluctuations in nature At T=0, thermal excitations vanish The Heisenberg uncertainty principle (1927) rules the physics.
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
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
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 (?)
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?
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?
Our experiment
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
Our set-up : Atom chip experiment Miniaturized magnetic device Transverse motion hω Small structures strong field gradients high ω (~150nK) Effective 1D system
Chip assembly Atom source Electrical connections Chip
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
Density fluctuations measurements Local and direct information pixel : Δ=4.5 µm Armijo, Jacqmin, Kheruntsyan, Bouchoule, PRL (2010)
Density fluctuations measurements Local and direct information pixel : Δ=4.5 µm In a pixel : Armijo, Jacqmin, Kheruntsyan, Bouchoule, PRL (2010)
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)
Thermodynamics of the repulsive 1D Bose gas Lieb-Liniger phase diagram (temperature t ; interactions γ) Ideal Bose gas thermal quantum Quasi-condensate Strong interactions
Thermodynamics of the repulsive 1D Bose gas Lieb-Liniger phase diagram (temperature t ; interactions γ) Ideal Bose gas thermal quantum 2010-2011 Quasi-condensate Strong interactions Armijo, Jacqmin, Kheruntsyan, Bouchoule, PRL (2010) Armijo, Jacqmin, Kheruntsyan, Bouchoule, PRA (2011) Jacqmin, Armijo, Berrada, Kheruntsyan, Bouchoule, PRL (2011)
Thermodynamics of the repulsive 1D Bose gas Lieb-Liniger phase diagram (temperature t ; interactions γ) Ideal Bose gas thermal quantum 2010-2011 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)
Direct detection of quantum fluctuations
Bogolyubov excitations in quasicondensates Bogoliubov spectrum in a BEC or quasi-bec : healing length particles phonons
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
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
Quantum vs thermal fluctuations in a quantum quasicondensate Quantum quasicondensate ξ Microscopic Thermodynamic Thermal phonon wavelength Armijo, PRL (2012)
Direct detection of quantum fluctuations T=18nK T=4.7nK Thermal Quantum 20% Poissonian shot noise Armijo, PRL (2012)
Direct detection of quantum fluctuations T=18nK T=4.7nK Thermal Quantum 20% Armijo, PRL (2012)
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)
Smoking gun : scaling with system size L Thermal + quantum fluctuations Thermal (classical) fluctuations only Die off at small L bad description! Armijo, PRL (2012)
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)
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
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