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Optics Express

  • Editor: J. H. Eberly
  • Vol. 1, Iss. 10 — Nov. 10, 1997
  • pp: 284–292
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Characterizing the coherence of Bose-Einstein condensates and atom lasers

R. J. Dodd, Charles W. Clark, Mark Edwards, and K. Burnett  »View Author Affiliations


Optics Express, Vol. 1, Issue 10, pp. 284-292 (1997)
http://dx.doi.org/10.1364/OE.1.000284


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Abstract

For a dilute, interacting Bose gas of magnetically-trapped atoms at temperatures below the critical temperature T0 for Bose-Einstein condensation, we determine the second-order coherence function g(2)(r1, r2) within the framework of a finite-temperature quantum field theory. We show that, because of the different spatial distributions of condensate and thermal atoms in the trap, g(2)(r1, r2) does not depend on |r1 - r2| alone. This means that the experimental determinations of g(2) reported to date give only its spatial average. Such an average may underestimate the degree of coherence attainable in an atom laser by judicious engineering of the output coupler.

© Optical Society of America

1. Introduction

The experimental demonstration of Bose-Einstein condensation (BEC) in magnetically-trapped atomic gases [1

1. M. H. Anderson, J. R. Ensher, M. R. Matthews, C. E. Wieman, and E. A. Cornell, “Observation of Bose-Einstein condensation in a dilute atomic vapor,” Science 269, 198 (1995). [CrossRef] [PubMed]

, 2

2. K. B. Davis, M.-O. Mewes, M. R. Andrews, N. J. van Druten, D. S. Durfee, D. M. Kurn, and W. Ketterle, “Bose-Einstein condensation in a gas of sodium atoms,” Phys. Rev. Lett. 75, 3969 (1995). [CrossRef] [PubMed]

, 3

3. C. C. Bradley, C. A. Sackett, and R. G. Hulet, “Bose-Einstein condensation in lithium: observation of limited condensate number,” Phys. Rev. Lett. 78, 985 (1997). [CrossRef]

] has focused interest on the prospects for constructing a source of coherent matter waves, or “atom laser.” Indeed, a prototype device of this type has recently been demonstrated, [4

4. M.-O. Mewes, M. R. Andrews, D. M. Kurn, D. S. Durfee, C. G. Townsend, and W. Ketterle, “An output coupler for Bose condensed atoms,” Phys. Rev. Lett. 78, 582 (1997). [CrossRef]

] and the first-order coherence properties of BEC atom-sources have been demonstrated by the observation of interference fringes in collisions of separately prepared BECs. [5

5. M. R. Andrews, C. G. Townsend, H.-J. Miesner, D. S. Durfee, D. M. Kurn, and W. Ketterle, “Observation of interference between two Bose condensates,” Science 275, 637 (1997). [CrossRef] [PubMed]

] There is some controversy over the use of the laser analogy to describe such systems, [6

6. H. M. Wiseman, “Defining the (atom) laser,” Phys. Rev. A (1997 in press). [CrossRef]

] in part because the theory of coherence has not yet been as fully developed for matter-wave as for optical-wave sources. Nevertheless, the rapid progress of this field has motivated practical definitions of matter-wave coherence based on optical analogies, and recent papers have presented determinations of coherence functions of BEC systems based on measurements of three-body decay rates [7

7. E. A. Burt, R. W. Ghrist, C. J. Myatt, M. J. Holland, E. A. Cornell, and C. E. Wieman, “Coherence, correlation and collisions: what one learns from Bose-Einstein condensates from their decay,” Phys. Rev. Lett. 79, 337 (1997). [CrossRef]

] and measurements of the release energies of trapped gases.[8

8. W. Ketterle and H.-J. Miesner, “Coherence properties of Bose condensates and atom lasers,” Phys. Rev. A 57, 3291 (1997). [CrossRef]

] We believe that a first-principles model of matter-wave coherence is needed to provide a framework within which the results of such experiments can be quantified. This paper presents an outline of such a theory, and describes several qualitative implications that the theory presents for future experiments.

We proceed by outlining the quantum field theory of the second-order coherence function, treating the equal-time or stationary case as this is of greatest interest to current experiments. The formalism is applied in explicit calculations of two cases: an ideal Bose gas in a spherical trap; and an interacting 87Rb gas in the JILA TOP trap. [1

1. M. H. Anderson, J. R. Ensher, M. R. Matthews, C. E. Wieman, and E. A. Cornell, “Observation of Bose-Einstein condensation in a dilute atomic vapor,” Science 269, 198 (1995). [CrossRef] [PubMed]

] Similar results obtained for the third-order coherence function will be presented elsewhere.

2. Coherence of matter waves

When a Bose gas undergoes condensation there are changes in its first-order coherence function. In fact, the appearance of the condensate induces off-diagonal, long-range order (ODLRO), as explained by Penrose and Onsager. [10

10. O. Penrose and L. Onsager, “Bose-Einstein condensation and liquid helium,” Phys. Rev. 104, 576 (1956). [CrossRef]

] In company with the optical case we come across the point of view [11

11. D. F. Walls, “Evidence for the quantum nature of light,” Nature 280, 451 (1979). [CrossRef]

] that a coherent laser field cannot be distinguished through its first-order coherence function from a filtered chaotic field of a thermal source: for example, one sees the same Young’s double-slit interference patterns from a laser as from a conventional light source. In the case of the gas we might then be led to observe that a condensate is, as far as its first-order, equal-time, matter-wave coherence function is concerned, equivalent to a filtered (but remarkably intense) chaotic deBroglie wave source.

The second-order coherence function arises in optics as a descriptor of the correlation between two separate photon-detection events, and, as emphasized by Walls [11

11. D. F. Walls, “Evidence for the quantum nature of light,” Nature 280, 451 (1979). [CrossRef]

], it is among the most elementary constructs that provides a clear contrast between quantum-mechanical and classical descriptions of the radiation field. So also in the case of atoms, the second-order coherence function shows unequivocal effects of Bose-Einstein condensation; thus it highlights the distinctions of the quantum-field-theoretical properties of a BEC vs. those of matter waves from a thermal source.

We now derive the zero-separation, equal-time, second-order coherence function for matter fields, presenting it in terms equivalent to those used by Glauber [12

12. R. Glauber, “Optical coherence and photon statistics,” in Quantum Optics and Electronics, C. DeWitt, A. Blandin, and C. Cohen-Tannoudji, eds. (Gordon and Breach, New York, 1965).

] to treat optical fields. We work within the framework of the Popov approximation to Hartree-Fock-Bogoliubov theory, [13

13. A. L. Fetter, “Nonuniform states of an imperfect Bose gas,” Ann. Phys (NY) 70, 67 (1972). [CrossRef]

, 14

14. E. M. Lifshitz and L. P. Pitaevski, Statistical Physics Part 2 (Butterworth Heinemann, Oxford, 1995).

, 15

15. V. N. Popov, Functional Integrals and Collective Modes (Cambridge University Press, New York, 1987), Chapter 6.

, 16

16. A. Griffin, “Conserving and gapless approximations for an inhomogeneous Bose gas at finite temperatures,” Phys. Rev. B 53, 9341 (1996). [CrossRef]

] as defined by Griffin. [16

16. A. Griffin, “Conserving and gapless approximations for an inhomogeneous Bose gas at finite temperatures,” Phys. Rev. B 53, 9341 (1996). [CrossRef]

] As discussed in Sec. 4, this theory has been found to give an excellent description of the thermodynamics of the partially-condensed Bose gas, and, at least for condensate fractions above about 50%, a good account of condensate collective excitation frequencies. However, we note that the theory of the finite-temperature Bose gas still has many open questions.

The second-order coherence function is defined as [12

12. R. Glauber, “Optical coherence and photon statistics,” in Quantum Optics and Electronics, C. DeWitt, A. Blandin, and C. Cohen-Tannoudji, eds. (Gordon and Breach, New York, 1965).

]

g(2)(r1,r2)=ψ̂(r1)ψ̂(r2)ψ̂(r2)ψ̂(r1)ψ̂(r1)ψ̂(r1)ψ̂(r2)ψ̂(r2),
(1)

where ψ̂ (r i) is the Bose field operator which annihilates an atom at position r i. The Bose field operators obey the usual commutation relations

[ψ̂(r1),ψ̂(r2)]=δ(r1r2),[ψ̂(r1),ψ̂(r2)]=[ψ̂(r1),ψ̂(r2)]=0.
(2)

Under the Popov approximation, the confined Bose gas is portrayed as a thermodynamic equilibrium system under a restricted grand canonical ensemble [17

17. M. Houbiers and H. T. C. Stoof, “Stability of Bose condensed atomic 7Li,” Phys. Rev. A 54, 5055 (1996). [CrossRef] [PubMed]

] whose thermodynamic variables are N, the total number of trapped atoms; T, the absolute temperature; and either N 0, the number of condensate atoms, or μ, the chemical potential. The system Hamiltonian has the form

KHμN=drψ̂(r)(H0μ)ψ̂(r)
+U02drψ̂(r)ψ̂(r)ψ̂(r)ψ̂(r),
(3)

where H0=ħ22M2+Vtrap(r) is the bare trap Hamiltonian, with trapping potential given by V trap(r) = M (ωρ2 ρ 2 + ωz2 z 2) /2, with ω ρ and ωz = √8ωρ being the radial and axial trap frequencies; and U 0 = 4πħ 2 a/M is a measure of the interaction strength between atoms, with a being the scattering length for zero-energy binary atomic collisions.

The Bose field operator is written as the sum of a c-number condensate wave function, ψ(r), and a fluctuation term, ψ͂(r) which can be decomposed into a sum of quasi-particle modes

ψ̂(r)=ψ(r)+ψ͂(r)=ψ(r)+j[uj(r)αj+vj*(r)αj].
(4)

where αj and αj are operators that, respectively, create and destroy the jth mode. These operators satisfy the following commutation relations

[αj,αk]=δjk,[αj,αk]=[αj,αk]=0.
(5)

When the quasi-particle amplitudes, uj (r) and vj (r) satisfy Eqs. (13), the Popov Hamil-tonian has the diagonal form

K̂=jEjαjαj.
(6)

The ensemble average of an operator Ô is given by [13

13. A. L. Fetter, “Nonuniform states of an imperfect Bose gas,” Ann. Phys (NY) 70, 67 (1972). [CrossRef]

]

Ô=Tr[ÔeβK̂]Tr[eβK̂].
(7)

From this equation it is clear that 〈ψ͂〉 = 0 and products of three ψ͂ operators will vanish as well as they consist of sums containing products either one or three quasi-particle creation and annihilation operators.

We decompose the field operator and expand the numerator in Eq. (1) evaluated at zero separation (r 1 = r 2r) and take the ensemble average. Under the Popov approximation anomalous averages such as 〈ψ͂ψ͂〉 are assumed negligible, after some algebra we find that

g(2)(r,r)=1+1n(r){2ψ(r)2n͂(r)+n͂2(r)}.
(8)

In the above equation, (r) = 〈ψ͂(r)ψ͂(r)〉 is the thermal-atom density and n(r) = |ψ(r)|2 + (r) is the total density. The coherence function can then be written in the simple form

g(2)(r,r)=2f2(r),
(9)

where f(r) = |ψ(r)|2 /n(r), the spatially resolved condensate fraction, is the ratio of the condensate density to the total density at position r. This equation is relevant to recent experiments such as the determination of decay rates [7

7. E. A. Burt, R. W. Ghrist, C. J. Myatt, M. J. Holland, E. A. Cornell, and C. E. Wieman, “Coherence, correlation and collisions: what one learns from Bose-Einstein condensates from their decay,” Phys. Rev. Lett. 79, 337 (1997). [CrossRef]

, 18

18. Yu. Kagan, B. V. Svistunov, and G. V. Shlyapnikov, “Effect of Bose condensation on inelastic processes in gases,” JETP Lett. 42, 210 (1985).

, 19

19. H. T. C. Stoof, A. M. L. Janssen, J. M. V. A. Koelman, and B. J. Verhaar, “Decay of spin-polarized atomic hydrogen in the presence of a Bose condensate,” Phys. Rev. A 39, 3157 (1989). [CrossRef] [PubMed]

] and expansion energies. [8

8. W. Ketterle and H.-J. Miesner, “Coherence properties of Bose condensates and atom lasers,” Phys. Rev. A 57, 3291 (1997). [CrossRef]

] It also agrees in the appropriate limit with that for a homogeneous gas, [19

19. H. T. C. Stoof, A. M. L. Janssen, J. M. V. A. Koelman, and B. J. Verhaar, “Decay of spin-polarized atomic hydrogen in the presence of a Bose condensate,” Phys. Rev. A 39, 3157 (1989). [CrossRef] [PubMed]

] where f, the condensate fraction, is independent of r; in that case, g (2) is simply a function of temperature T. However, as we have suggested above and shall show below, in current experiments f(r) depends strongly upon r. Note that g (2)(r,r) = 2 for a thermal gas without condensation and g (2)(r, r) = 1 for a pure BEC.

3. Second-order coherence of the ideal trapped Bose gas

f(r)=[1N0k=1eβk(ε0μ)[1e2βkħω]3/2e(rd)2{1tanh(βkhω2)}]1,
(10)

where N 0 is the number of condensate atoms, ε0=32ħω is the ground-state energy of a single atom in the trap, β = 1/kBT with kB being the Boltzmann constant, μ is the chemical potential, and d=ħMω is the characteristic length scale of the single-atom ground state wavefunction, which in this case is the same as the condensate wavefunction (in the discussion of anisotropic traps, our definition of d uses the geometric mean of the trap frequencies). In the low-temperature limit βħω ≫ 1, we find that f(r) reduces to a form in which we can identify a characteristic length, R(T), at which f(R) = 1/2:

R(T)=deβħω2(ln[N0]2)1/2,
(11)

where Figure 1 shows a false-color plot of g (2)(r) = g (2)(r,r) vs. reduced temperature, T/T 0, for N = 40, 000 87Rb atoms in a spherical trap with v = ω/(2π) = 200 Hz. Here T 0 is the critical temperature for an ideal trapped gas in the semiclassical limit;[21

21. S. R. de Groot, G. J. Hooyman, and C. A. ten Seldam, “On the Bose-Einstein condensation,” Proc. R. Soc. London, Ser. A 203, 266 (1950). [CrossRef]

, 22

22. V. Bagnato, D. E. Pritchard, and D. Kleppner, “Bose-Einstein condensation in an external potential,” Phys. Rev. A 35, 4354 (1987). [CrossRef] [PubMed]

] the transition temperatures of interacting, finite-N systems are somewhat lower than T 0. The sharp blue → red transition shows that R(T) defines an appropriate length scale over a large range of temperatures, even though it was derived only in the low temperature limit, Tħω/kB , with N 0 ≫ 1.

Figure 1. A false-color plot of g (2)(r)vs. reduced temperature, T/T 0, and radial trap coordinate r, for N = 40000 87Rb atoms in a spherical trap with v = 200 Hz. The coherence length R(T) is given by Eq. 11. Blue corresponds to g (2) = 1 viz. coherence characteristic of a laser source; red to g (2) = 2 viz. coherence characteristic of a thermal source. It is apparent that R(T) defines the typical length scale over which laser-like coherence is maintained.

4. Interacting trapped gas

We have computed the second-order coherence function for a confined, interacting Bose gas using the Hartree-Fock-Bogoliubov (HFB) theory within an approximation originally introduced by Popov. [15

15. V. N. Popov, Functional Integrals and Collective Modes (Cambridge University Press, New York, 1987), Chapter 6.

, 16

16. A. Griffin, “Conserving and gapless approximations for an inhomogeneous Bose gas at finite temperatures,” Phys. Rev. B 53, 9341 (1996). [CrossRef]

, 23

23. D. A. W. Hutchinson, E. Zaremba, and A. Griffin, “Finite temperature excitations of a trapped Bose gas,” Phys. Rev. Lett. 78, 1842 (1997). [CrossRef]

] The HFB-Popov theory is a finite-temperature extension of Bogoliubov mean-field theory, which provides self-consistent treatment of the condensed and thermal components of the gas. The zero-temperature limit of this theory, which leads to the familiar Gross-Pitaevskii (GP) equation, has been found to describe accurately experimental values of condensate geometries, [24

24. M. J. Holland and J. Cooper, “Expansion of a Bose-Einstein condensate in a harmonic potential,” Phys. Rev. A 53, R1954 (1996) [CrossRef] [PubMed]

, 25

25. M. Edwards, R. J. Dodd, C. W. Clark, P. A. Ruprecht, and K. Burnett, “Properties of a Bose- Einstein condensate in an anisotropic harmonic potential,” Phys. Rev. A 53, R1950 (1996). [CrossRef] [PubMed]

] excitation frequencies, [26

26. M. Edwards, P. A. Ruprecht, K. Burnett, R. J. Dodd, and C. W. Clark, “Collective excitations of Bose-Einstein condensates,” Phys. Rev. Lett. 77, 1671 (1996) [CrossRef] [PubMed]

, 27

27. S. Stringari, “Collective excitations of a trapped Bose-condensed gas” Phys. Rev. Lett. 77, 2360 (1996) [CrossRef] [PubMed]

, 28

28. Y. Castin and R. Dum, “Bose-Einstein condensates in time-dependent traps,” Phys. Rev. Lett. 77, 5315 (1996). [CrossRef] [PubMed]

] and internal energies [29

29. M. J. Holland, D. S. Jin, M. L. Chiofalo, and J. Cooper, “Emergence of interaction effects in Bose-Einstein condensation,” Phys. Rev. Lett. 78, 3801 (1997). [CrossRef]

] for very cold condensates. The finite- temperature HFB-Popov treatment gives excellent agreement with experimental data on condensate fractions and transition temperatures [30

30. S. Giorgini, L. P. Pitaevskii, and S. Stringari, “Condensate fraction and critical temperature of a trapped interacting Bose gas,” Phys. Rev. A 54, R4633 (1996). [CrossRef] [PubMed]

, 31

31. A. Minguzzi, S. Conti, and M. P. Tosi, “The internal energy and condensate fraction of a trapped interacting Bose gas,” J. Phys.: Condens. Matter 9, L33 (1997). [CrossRef]

]; as for collective excitation frequencies, it appears to agree well with experiments for temperatures corresponding to thermal gas fractions of up to about 50%, though its validity at higher temperatures is uncertain. We have reported details of this approach and its comparison with experiment in a separate publication, [32

32. R. J. Dodd, M. Edwards, C. W. Clark, and K. Burnett, “Collective excitations of Bose-Einstein condensed gases at finite temperatures,” Phys. Rev. A (in press).

] and only summarize the underlying ideas here.

We treat systems of several thousand 87Rb atoms in the JILA TOP trap. Following the approach of Sec. 2, we decompose the Bose field operator into a c-number condensate wave function plus an operator describing the non-condensate part: ψ̂(r) = N01/2 ϕ(r) + ψ͂(r) and insert into Eq. (3). Linearization of this equation in the operators ψ͂(r), leads [15

15. V. N. Popov, Functional Integrals and Collective Modes (Cambridge University Press, New York, 1987), Chapter 6.

, 16

16. A. Griffin, “Conserving and gapless approximations for an inhomogeneous Bose gas at finite temperatures,” Phys. Rev. B 53, 9341 (1996). [CrossRef]

, 23

23. D. A. W. Hutchinson, E. Zaremba, and A. Griffin, “Finite temperature excitations of a trapped Bose gas,” Phys. Rev. Lett. 78, 1842 (1997). [CrossRef]

, 32

32. R. J. Dodd, M. Edwards, C. W. Clark, and K. Burnett, “Collective excitations of Bose-Einstein condensed gases at finite temperatures,” Phys. Rev. A (in press).

] to a generalized GP equation for ϕ(r)

{H0+U0[N0ϕ(r)2+2n͂(r)]}ϕ(r)=μϕ(r),
(12)

and to equations for quasi-particle amplitudes uj (r) and vj (r),

Luj(r)+N0U0ϕ(r)2vj(r)=Ejuj(r)
Lvj(r)+N0U0ϕ(r)2uj(r)=Ejvj(r),
(13)

where where LH 0 + 2U 0 n(r) - μ and the Ej , the quasi-particle energies, are determined as eigenvalues. The quasi-particle amplitudes generate ψ͂(r) via ψ(r) = Σj(uj (r)αj + vj*(r)αj), where αj , αj are quasi-particle annihilation and creation operators satisfying the usual Bose commutation relations.

The density of the thermal component of the gas (r) can be written in terms of the quasi-particle amplitudes as

n͂(r)=j{[uj(r)2+vj(r)2]Nj+vj(r)2},
(14)

where Nj = (eβEj - 1)-1 . The total number of trapped atoms, N, is given by

N=drn(r)=N0+drn͂(r).
(15)

Equations (12), (13), (14), and (15) form a closed system of equations that we refer to as the “HFB-Popov” equations (our version of Eq. (13) differs from that of Hutchinson et al. [23

23. D. A. W. Hutchinson, E. Zaremba, and A. Griffin, “Finite temperature excitations of a trapped Bose gas,” Phys. Rev. Lett. 78, 1842 (1997). [CrossRef]

] via a sign change in the definition of vj (r)). Numerical solution of these equations proceeds by choice of state variables {T, μ, N}: for fixed T and μ, N is determined by solving the HFB-Popov equations, iteratively, to self-consistency.

Figure 2 shows the coherence function g (2)(r,r) that emerges from such calculations for a sample of 2000 87Rb atoms in the JILA TOP trap with ωρ /(2π) = 74 Hz at T = 40 nK, a temperature at which the condensate fraction is approximately 50%. The solid line shows the value of g (2) for the interacting sample, calculated by the HFB-Popov theory, and the dashed line shows the same calculation for a noninteracting sample. As can be seen, the repulsive atomic interactions extend the coherence length of the sample. This is due to two mechanisms: repulsive interactions between atoms of the condensate cause it to swell; and condensate atoms repel thermal atoms, leading to a more sharply defined variation of f(r). These mechanisms are clearly manifested in the animated sequence of figures that follows below. Near the trap center, however, the ideal gas has a coherence closer to unity, since the peak density of a ideal gas condensate is substantially higher than that of an interacting one.

Figure 2. A plot of g (2)(r) versus r/d for 2000 87Rb atoms in the JILA TOP trap at 40 nK, with ωρ /(2π) = 74 Hz. The figure displays the variation in the plane z = 0, with r being the cylindrical radius.
Figure 3. Animation of surface plots of the scaled total densities, n(r)d 3 , versus ρ/d and z/d, for 2000 87Rb atoms in the JILA TOP trap with vρ = 74 Hz, at temperatures as labelled in each frame. The critical temperature T 0 for this system is ≈ 59 nK. The height of the surface is proportional to the density, the peak value displayed in these frames being ≈ 5 × 1013cm-3; the (dimensionless) z and ρ coordinates attain maximum values of 6 and 12, respectively; and the color shading represents the zero-separation, second-order coherence function of the system, g (2)(r,r). Blue indicates high coherence (g (2) ~ 1), while red indicates low coherence (g (2) ~ 2). [Media 1]

The above animation shows that the onset of a region containing coherent atoms is quite sudden and tends to occupy a large volume as T decreases. Furthermore, it is clear from this sequence that the condensate density (roughly the blue regions) far exceeds the thermal-atom density. The repulsion mechanisms discussed above are clearly in play.

5. Implications for experiment

We have shown that for a dilute Bose gas of magnetically-trapped atoms at temperatures below the critical temperature T 0, the second-order coherence function g (2)(r 1, r 2) does not depend on |r 1 - r 2| alone, as would be the case for a homogenous system. Instead, it exhibits significant dependence upon the trap spatial coordinates, and it reflects the relative purity of the condensate. For cases of repulsive interaction, regions of high relative purity are more extensive than those encountered in the ideal Bose gas.

Ketterle and Miesner [8

8. W. Ketterle and H.-J. Miesner, “Coherence properties of Bose condensates and atom lasers,” Phys. Rev. A 57, 3291 (1997). [CrossRef]

] have noted that for a zero-range atomic pair interaction, the mean-field energy U of a Bose gas satisfies U ∝ ∫ d r g (2)(r,r) n 2(r). Experimental measurement of U can be made by observing the expansion of the gas upon dropping the trap. [29

29. M. J. Holland, D. S. Jin, M. L. Chiofalo, and J. Cooper, “Emergence of interaction effects in Bose-Einstein condensation,” Phys. Rev. Lett. 78, 3801 (1997). [CrossRef]

] If it is assumed that, as is the case in a homogeneous system, g (2)(r, r) is independent of r, i.e. g (2)(r,r) = g (2)(0), then g (2)(0) can be obtained directly from experimental measurements of U and n(r). A similar idea is employed by Burt et al., [7

7. E. A. Burt, R. W. Ghrist, C. J. Myatt, M. J. Holland, E. A. Cornell, and C. E. Wieman, “Coherence, correlation and collisions: what one learns from Bose-Einstein condensates from their decay,” Phys. Rev. Lett. 79, 337 (1997). [CrossRef]

] who infer a value of g (3)(0) from a three-body decay rate. However, according to the picture we have developed in this paper, the release-energy measurements actually determine a spatially-averaged value of g (2)(r, r). Thus, the analysis of Ref. [8

8. W. Ketterle and H.-J. Miesner, “Coherence properties of Bose condensates and atom lasers,” Phys. Rev. A 57, 3291 (1997). [CrossRef]

] gives g (2)(0) = [∫ d r g (2)(r,r) n 2(r) / [∫ d r n 2(r)], rather than a value of g (2) that characterizes the sample as a whole. For very cold samples that are nearly pure condensates, the density-weighted variance in the distribution of g (2)(r, r) will be small, but quantitative investigations of the coherence properties of partially-condensed gases will have to account for effects of spatial dependence.

The expanded volume of high coherence that is induced by the condensate-cloud repulsion mechanism can have a positive impact on atom-laser design. For example, one scheme for out-coupling atoms from a condensate involves focussing two lasers into the condensate to cause transitions between trapped and untrapped magnetic sublevels via a two-photon Raman transition, and can in principle allow beam extraction from specific regions of the trap. Thus, even if one can only generate large condensates at relatively high temperatures, it may still be possible to extract a relatively pure condensate even in the presence of a substantial thermal component of the gas. In addition, we note that the effective segregation of condensate and non-condensate atoms may have favorable implications for loss rates associated with depolarizing atomic collisions. Two-body collisional rates are proportional to g (2), so there is a higher rate of collisional loss per unit density in the thermal cloud than in the condensate.

Acknowledgements

Footnotes

Also at the Physics Laboratory, National Institute of Standards and Technology, Technology Administration, U. S. Department of Commerce, Gaithersburg, MD 20899.

References and links

1.

M. H. Anderson, J. R. Ensher, M. R. Matthews, C. E. Wieman, and E. A. Cornell, “Observation of Bose-Einstein condensation in a dilute atomic vapor,” Science 269, 198 (1995). [CrossRef] [PubMed]

2.

K. B. Davis, M.-O. Mewes, M. R. Andrews, N. J. van Druten, D. S. Durfee, D. M. Kurn, and W. Ketterle, “Bose-Einstein condensation in a gas of sodium atoms,” Phys. Rev. Lett. 75, 3969 (1995). [CrossRef] [PubMed]

3.

C. C. Bradley, C. A. Sackett, and R. G. Hulet, “Bose-Einstein condensation in lithium: observation of limited condensate number,” Phys. Rev. Lett. 78, 985 (1997). [CrossRef]

4.

M.-O. Mewes, M. R. Andrews, D. M. Kurn, D. S. Durfee, C. G. Townsend, and W. Ketterle, “An output coupler for Bose condensed atoms,” Phys. Rev. Lett. 78, 582 (1997). [CrossRef]

5.

M. R. Andrews, C. G. Townsend, H.-J. Miesner, D. S. Durfee, D. M. Kurn, and W. Ketterle, “Observation of interference between two Bose condensates,” Science 275, 637 (1997). [CrossRef] [PubMed]

6.

H. M. Wiseman, “Defining the (atom) laser,” Phys. Rev. A (1997 in press). [CrossRef]

7.

E. A. Burt, R. W. Ghrist, C. J. Myatt, M. J. Holland, E. A. Cornell, and C. E. Wieman, “Coherence, correlation and collisions: what one learns from Bose-Einstein condensates from their decay,” Phys. Rev. Lett. 79, 337 (1997). [CrossRef]

8.

W. Ketterle and H.-J. Miesner, “Coherence properties of Bose condensates and atom lasers,” Phys. Rev. A 57, 3291 (1997). [CrossRef]

9.

R. J. Dodd, K. Burnett, M. Edwards, and C. W. Clark, “Two-gas description of dilute Bose- Einstein condensates at finite temperature,” Phys. Rev. A (submitted).

10.

O. Penrose and L. Onsager, “Bose-Einstein condensation and liquid helium,” Phys. Rev. 104, 576 (1956). [CrossRef]

11.

D. F. Walls, “Evidence for the quantum nature of light,” Nature 280, 451 (1979). [CrossRef]

12.

R. Glauber, “Optical coherence and photon statistics,” in Quantum Optics and Electronics, C. DeWitt, A. Blandin, and C. Cohen-Tannoudji, eds. (Gordon and Breach, New York, 1965).

13.

A. L. Fetter, “Nonuniform states of an imperfect Bose gas,” Ann. Phys (NY) 70, 67 (1972). [CrossRef]

14.

E. M. Lifshitz and L. P. Pitaevski, Statistical Physics Part 2 (Butterworth Heinemann, Oxford, 1995).

15.

V. N. Popov, Functional Integrals and Collective Modes (Cambridge University Press, New York, 1987), Chapter 6.

16.

A. Griffin, “Conserving and gapless approximations for an inhomogeneous Bose gas at finite temperatures,” Phys. Rev. B 53, 9341 (1996). [CrossRef]

17.

M. Houbiers and H. T. C. Stoof, “Stability of Bose condensed atomic 7Li,” Phys. Rev. A 54, 5055 (1996). [CrossRef] [PubMed]

18.

Yu. Kagan, B. V. Svistunov, and G. V. Shlyapnikov, “Effect of Bose condensation on inelastic processes in gases,” JETP Lett. 42, 210 (1985).

19.

H. T. C. Stoof, A. M. L. Janssen, J. M. V. A. Koelman, and B. J. Verhaar, “Decay of spin-polarized atomic hydrogen in the presence of a Bose condensate,” Phys. Rev. A 39, 3157 (1989). [CrossRef] [PubMed]

20.

R. Feynman, Statistical Mechanics (W. A. Benjamin, Reading, MA, 1972).

21.

S. R. de Groot, G. J. Hooyman, and C. A. ten Seldam, “On the Bose-Einstein condensation,” Proc. R. Soc. London, Ser. A 203, 266 (1950). [CrossRef]

22.

V. Bagnato, D. E. Pritchard, and D. Kleppner, “Bose-Einstein condensation in an external potential,” Phys. Rev. A 35, 4354 (1987). [CrossRef] [PubMed]

23.

D. A. W. Hutchinson, E. Zaremba, and A. Griffin, “Finite temperature excitations of a trapped Bose gas,” Phys. Rev. Lett. 78, 1842 (1997). [CrossRef]

24.

M. J. Holland and J. Cooper, “Expansion of a Bose-Einstein condensate in a harmonic potential,” Phys. Rev. A 53, R1954 (1996) [CrossRef] [PubMed]

25.

M. Edwards, R. J. Dodd, C. W. Clark, P. A. Ruprecht, and K. Burnett, “Properties of a Bose- Einstein condensate in an anisotropic harmonic potential,” Phys. Rev. A 53, R1950 (1996). [CrossRef] [PubMed]

26.

M. Edwards, P. A. Ruprecht, K. Burnett, R. J. Dodd, and C. W. Clark, “Collective excitations of Bose-Einstein condensates,” Phys. Rev. Lett. 77, 1671 (1996) [CrossRef] [PubMed]

27.

S. Stringari, “Collective excitations of a trapped Bose-condensed gas” Phys. Rev. Lett. 77, 2360 (1996) [CrossRef] [PubMed]

28.

Y. Castin and R. Dum, “Bose-Einstein condensates in time-dependent traps,” Phys. Rev. Lett. 77, 5315 (1996). [CrossRef] [PubMed]

29.

M. J. Holland, D. S. Jin, M. L. Chiofalo, and J. Cooper, “Emergence of interaction effects in Bose-Einstein condensation,” Phys. Rev. Lett. 78, 3801 (1997). [CrossRef]

30.

S. Giorgini, L. P. Pitaevskii, and S. Stringari, “Condensate fraction and critical temperature of a trapped interacting Bose gas,” Phys. Rev. A 54, R4633 (1996). [CrossRef] [PubMed]

31.

A. Minguzzi, S. Conti, and M. P. Tosi, “The internal energy and condensate fraction of a trapped interacting Bose gas,” J. Phys.: Condens. Matter 9, L33 (1997). [CrossRef]

32.

R. J. Dodd, M. Edwards, C. W. Clark, and K. Burnett, “Collective excitations of Bose-Einstein condensed gases at finite temperatures,” Phys. Rev. A (in press).

OCIS Codes
(020.7010) Atomic and molecular physics : Laser trapping
(030.1640) Coherence and statistical optics : Coherence
(140.3320) Lasers and laser optics : Laser cooling

ToC Category:
Focus Issue: Fluctuations and oscillations of Bose-Einstein

History
Original Manuscript: September 10, 1997
Published: November 10, 1997

Citation
Robert Dodd, Charles Clark, Mark Edwards, and Keith Burnett, "Characterizing the coherence of Bose-Einstein condensates and atom lasers," Opt. Express 1, 284-292 (1997)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-1-10-284


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References

  1. M. H. Anderson, J. R. Ensher, M. R. Matthews, C. E. Wieman, and E. A. Cornell, "Observation of Bose-Einstein condensation in a dilute atomic vapor," Science 269, 198 (1995). [CrossRef] [PubMed]
  2. K. B. Davis, M.-O. Mewes, M. R. Andrews, N. J. van Druten, D. S. Durfee, D. M. Kurn, and W. Ketterle, "Bose-Einstein condensation in a gas of sodium atoms," Phys. Rev. Lett. 75, 3969 (1995). [CrossRef] [PubMed]
  3. C. C. Bradley, C. A. Sackett, and R. G. Hulet, "Bose-Einstein condensation in lithium: observation of limited condensate number," Phys. Rev. Lett. 78, 985 (1997). [CrossRef]
  4. M.-O. Mewes, M. R. Andrews, D. M. Kurn, D. S. Durfee, C. G. Townsend, and W. Ketterle, "An output coupler for Bose condensed atoms," Phys. Rev. Lett. 78, 582 (1997). [CrossRef]
  5. M. R. Andrews, C. G. Townsend, H.-J. Miesner, D. S. Durfee, D. M. Kurn, and W. Ketterle, "Observation of interference between two Bose condensates," Science 275, 637 (1997). [CrossRef] [PubMed]
  6. H. M. Wiseman, "Defining the (atom) laser," Phys. Rev. A (1997 in press). [CrossRef]
  7. E. A. Burt, R. W. Ghrist, C. J. Myatt, M. J. Holland, E. A. Cornell, and C. E. Wieman, "Coherence, correlation and collisions: what one learns from Bose-Einstein condensates from their decay," Phys. Rev. Lett. 79, 337 (1997). [CrossRef]
  8. W. Ketterle and H.-J. Miesner, "Coherence properties of Bose condensates and atom lasers," Phys. Rev. A 57, 3291 (1997). [CrossRef]
  9. R. J. Dodd, K. Burnett, M. Edwards, and C. W. Clark, "Two-gas description of dilute Bose-Einstein condensates at finite temperature," Phys. Rev. A (submitted).
  10. O. Penrose and L. Onsager, "Bose-Einstein condensation and liquid helium," Phys. Rev. 104, 576 (1956). [CrossRef]
  11. D. F. Walls, "Evidence for the quantum nature of light," Nature 280, 451 (1979). [CrossRef]
  12. R. Glauber, "Optical coherence and photon statistics," in Quantum Optics and Electronics, C. DeWitt, A. Blandin, and C. Cohen-Tannoudji, eds. (Gordon and Breach, New York, 1965).
  13. A. L. Fetter, "Nonuniform states of an imperfect Bose gas," Ann. Phys. (NY) 70, 67 (1972). [CrossRef]
  14. E. M. Lifshitz and L. P. Pitaevski, Statistical Physics Part 2 (Butterworth Heinemann, Oxford, 1995).
  15. V. N. Popov, Functional Integrals and Collective Modes (Cambridge University Press, New York, 1987), Chapter 6.
  16. A. Griffin, "Conserving and gapless approximations for an inhomogeneous Bose gas at finite temperatures," Phys. Rev. B 53, 9341 (1996). [CrossRef]
  17. M. Houbiers and H. T. C. Stoof, "Stability of Bose condensed atomic 7 Li," Phys. Rev. A 54, 5055 (1996). [CrossRef] [PubMed]
  18. Yu. Kagan, B. V. Svistunov, and G. V. Shlyapnikov, "Effect of Bose condensation on inelastic processes in gases," JETP Lett. 42, 210 (1985).
  19. H. T. C. Stoof, A. M. L. Janssen, J. M. V. A. Koelman, and B. J. Verhaar, "Decay of spin-polarized atomic hydrogen in the presence of a Bose condensate," Phys. Rev. A 39, 3157 (1989). [CrossRef] [PubMed]
  20. R. Feynman, Statistical Mechanics (W. A. Benjamin, Reading, MA, 1972).
  21. S. R. de Groot, G. J. Hooyman, and C. A. ten Seldam, "On the Bose-Einstein condensation," Proc.R. Soc. London,Ser. A203, 266 (1950). [CrossRef]
  22. V. Bagnato, D. E. Pritchard, and D. Kleppner, "Bose-Einstein condensation in an external potential," Phys. Rev. A 35, 4354 (1987). [CrossRef] [PubMed]
  23. D. A. W. Hutchinson, E. Zaremba, and A. Griffin, "Finite temperature excitations of a trapped Bose gas," Phys. Rev. Lett. 78, 1842 (1997). [CrossRef]
  24. M. J. Holland and J. Cooper, "Expansion of a Bose-Einstein condensate in a harmonic potential," Phys. Rev. A 53, R1954 (1996) [CrossRef] [PubMed]
  25. M. Edwards, R. J. Dodd, C. W. Clark, P. A. Ruprecht, and K. Burnett, "Properties of a Bose-Einstein condensate in an anisotropic harmonic potential," Phys. Rev. A 53, R1950 (1996). [CrossRef] [PubMed]
  26. M. Edwards P. A. Ruprecht, K. Burnett, R. J. Dodd, and C. W. Clark, "Collective excitations of Bose-Einstein condensates," Phys. Rev. Lett. 77, 1671 (1996) [CrossRef] [PubMed]
  27. S. Stringari, "Collective excitations of a trapped Bose-condensed gas" Phys. Rev. Lett. 77, 2360 (1996) [CrossRef] [PubMed]
  28. Y. Castin and R. Dum, "Bose-Einstein condensates in time-dependent traps," Phys. Rev. Lett. 77, 5315 (1996). [CrossRef] [PubMed]
  29. M. J. Holland, D. S. Jin, M. L. Chiofalo, and J. Cooper, "Emergence of interaction effects in Bose-Einstein condensation," Phys. Rev. Lett. 78, 3801 (1997). [CrossRef]
  30. S. Giorgini, L. P. Pitaevskii, and S. Stringari, "Condensate fraction and critical temperature of a trapped interacting Bose gas," Phys. Rev. A 54, R4633 (1996). [CrossRef] [PubMed]
  31. A. Minguzzi, S. Conti, and M. P. Tosi, "The internal energy and condensate fraction of a trapped interacting Bose gas," J. Phys.: Condens. Matter 9, L33 (1997). [CrossRef]
  32. R. J. Dodd, M. Edwards, C. W. Clark, and K. Burnett, "Collective excitations of Bose-Einstein condensed gases at finite temperatures," Phys. Rev. A (in press).

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