Robust quantum storage with three atoms

The processing of quantum information — be it for quantum communication, forquantum key distribution, or for quantum computation — has experienced tremen-dous progress over the past deca

THREE ATOMS

Han Rui

NATIONAL UNIVERSITY OF SINGAPORE

2012

I hereby declare that the thesis is my original work and it has been written

by me in its entirety I have duly acknowledged all the sources of

information which have been used in the thesis

This thesis has also not been submitted for any degree in any university

previously

Han Rui

10 Jan 2013

and teachers

First of all, I would like to thank my supervisor Prof Berthold-Georg Englert forhis tireless support thoughout my undergraduate study and four years of Ph.D.candidature in Singapore I am deeply grateful for your invaluable guidance, aswell as your passion in Physics that inspires and encourages me always Thank youfor the unconditional support and freedom that is provided through the years.Thank you, Ng Hui Khoon for collaborating with me on much of the work

in the past few years, and also for your patience in teaching me and making ourdiscussions effective and enjoyable Thank you, Jun Suzuki for the support on thework of quantum storage and guiding me at the beginning of my Ph.D candidature.Thank you to Niels L¨orch and Vanessa Paulisch for working with me during yourinternships in Singapore

A special thank to Marta Wolak for making the office cheerful, to Shang Jiangweifor being supportive always and to Lee Kean Loon for many very helpful discussions

I would like to express my gratitude to all my colleagues who gave me the possibility

to complete this thesis, especially to Philippe Raynal, Arun, Christian Miniatura,Benoˆıt Gr´emaud, Paul Constantine Condylis, Mile Gu and our dearest “boss” OlaEnglert

I would like to take this opportunity to convey my sincere thanks to J´anosBergou, Hans Briegel, Kae Nemoto, Maciej Lewenstein and Gerd Leuchs for thewonderful hospitality and enlightening discussions during my visit to their groups.Thanks to Tan Hui Min Evon, Wang Yimin, Li Wenhui and Wu Chunfeng forvarious support, help and the enjoyable female physicists gatherings

My great appreciation also goes to my dearest friends in Singapore: Lan Tian,Wang Huidong, Zhao Xue and Lai Sha Thank you for being such wonderful friendsfor years and making my spare time splendid and colorful

Last but not least, I would like to thank my parents and Zhao Pan, Guo Qiyou,Zhang Yin for their great supports from China

careful reading and useful comments that helped in improving this thesis.

In this thesis, we present a systematic construction of a reference-frame-free (RFF)qubit in the noiseless subspace for a system of three identical spin-j atoms The ex-plicit example of three spin-1/26Li atoms trapped in an optical lattice is studied todemonstrate the robustness of the RFF qubit storage The resulting coherence timecan be many days and the fidelity of 99.99% is maintained for 2 hrs, with conserva-tively estimated parameters, making RFF qubits of this kind promising candidatesfor quantum information storage units A qubit preparation scheme using the Ry-dberg blockade mechanism is presented, and the scheme is numerically proven to

be robust with a modest estimation of about 98% of the preparation fidelity withexisting technologies The excitation of an atom from the ground state to the Ryd-berg state is done with a stimulated Raman transition, which is a powerful tool forthe manipulation of atoms In the last part of this thesis, a new methodology forstudying the three-level Raman transition in a single atom is presented Solutionsmore accurate than those relying on the conventional adiabatic elimination methodare obtained without increasing the computational complexity by much This newmethod can also be applied to the multi-atom Rydberg excitation that is used forthe RFF state preparation

Acknowledgments i

1.1 Quantum computation and quantum memories 1

1.2 Physical implementation of qubits 3

1.3 Quantum information with Rydberg atoms 7

1.4 Stimulated Raman transition 10

2 Construction of the RFF qubit 13 2.1 RFF states 14

2.1.1 RFF qubit from three spin-1/2 particles 14

2.1.2 RFF qubit from four spin-1/2 atoms 18

2.1.3 RFF qubit from three spin-1 atoms 19

2.2 Physical carrier and geometry 22

2.2.1 Neutral atoms in an optical lattice 22

2.2.2 Ions in a linear trap and other systems 30

2.3 Summary 31

3 Robustness of the RFF Qubit 33 3.1 Three spin-1/26Li in equilateral triangle configuration 35

3.1.1 Noise model for the magnetic field 35

3.1.2 Lithium-6 38

3.1.3 Master equation 42

3.1.4 Time dependence of RFF-qubit variables 46

3.1.5 Compare to decoherence of a single-atom qubit 47

3.2 Non-ideal geometry 50

3.2.1 Center-of-mass probability distribution 50

3.2.2 Distortion of equilateral triangle geometry 52

3.2.3 Time dependence of RFF-qubit variables 54

3.2.4 Compensating for triangle distortions 59

3.3 Robustness of the RFF qubit from four spin-1/2 atoms 60

3.3.1 The 2D square geometry 60

3.3.2 The 3D pyramid geometry 63

3.4 Alternatives 65

3.4.1 RFF qubit from three 87Rb atoms 65

3.4.2 Ions in a linear trap 69

3.5 Summary and discussion 71

4 State preparation of the RFF qubit 75 4.1 Rydberg Blockade 75

4.2 General Scheme of RFF State Preparation 79

4.3 Choice of atoms 86

4.3.1 Three6Li atoms 87

4.3.2 Three87Rb atoms 89

4.3.3 Three40Ca+ ions 89

4.4 Robustness of RFF State Preparation 91

4.4.1 Error analysis 91

4.4.2 Numerical simulation 97

4.5 Summary 103

5 Raman-type Transitions 105 5.1 The three-level system 106

5.2 Adiabatic Elimination 109

5.2.1 The methodology 109

5.2.2 Light shift 111

5.2.3 Problems with adiabatic elimination 113

5.3 Without adiabatic elimination 115

5.3.1 General methodology 115

5.3.2 Resonant two-photon transitions (δ = 0) — exact solution 117

5.3.3 Non-resonant two-photon transitions (δ6= 0) 120

5.3.4 Discussion 126

5.4 Multi-atom collective Rydberg transitions 129

5.4.1 Two-atom collective Rydberg excitation 130

5.5 Summary 138

6 Conclusion and outlook 141

A Reduced dipole matrix element 145

B Unitarity of the approximate evolution operators 151

2.1 Six coplanar laser beams consist of two sets of three coherent beams;the angle between beams within each set is 2π/3 The respectivewave vectors have lengths |k1| = |k2| = |k3| = 2π/λ and |k4| =

|k5| = |k6| = 2π/λ0 Different lattice structures can be created byalternating the phases of the laser beams For the lattice of ourdesign, we keep the set of beams with wavelength λ to be in phaseand the phases for the other three beams with wavelength λ0 are2π/3, 0, and−2π/3 23

2.2 The left-hand side is a contour plot of the potential energy V (x, y)

in Eq (2.30) with (E0/E00)2 = 3 and λ/λ0 = 5 One unit is equal

to λ/2π in the plots Red-detuned lasers (δ < 0) are used Thedarker is the color, the higher is the potential at the region Theplot on the right-hand side shows the potential maxima produced bythe two sets of laser beams individually; the big circles indicate thepotential maxima produced by lasers of frequency λ and the smallcircles indicate the potential maxima produced by lasers of frequency

λ0 25

2.3 The left-hand side shows the potential along the vertical cut-off line

in plot (b) of Fig 2.2; and the left-hand side shows the potentialalong the horizontal cut-off line in plot (b) of Fig 2.2 25

2.4 Six coplanar laser beams consist of a set of three coherent beamsindicated by red arrows and another set of three coherent beamsindicated by blue arrows The angle between neighboring beams isπ/3 26

2.5 The left hand side is a contour plot with lasers set in the configurationshown in Fig 2.4 with V1/V2= 3 and λ/λ0 = 4/√

3 26

2.6 Three ions trapped in a linear Paul trap 31

3.1 Three 6Li atoms are trapped at the corners of an equilateral gle The probability clouds indicate the center-of-mass distributionswhose spread w is about one-sixteenth of a, the distance between theatoms 39

trian-3.2 Ground-state hyperfine levels of the neutral6Li atom The f = 3/2quartet is separated from the f = 1/2 doublet by a transition fre-quency of 228.2 MHz Three 6Li atoms confined to their f = 1/2ground states serve as the spin-1/2 particles from which the RFFqubit is constructed 39

3.3 Level scheme for the effective three-atom Hamiltonian of Eq (3.22).The separations are not drawn to scale: ~ω0 is many orders of mag-nitude larger than~Ω The two J = 1/2 levels are degenerate dou-blets; this degeneracy is exploited for the encoding of the robustsignal qubit 41

3.4 Comparison of the data from a numerical simulation with the ical results of Eqs (3.46) Curve “a” displays PJ=1/2

analyt-t, curves “b”showhΣ1itforhΣ1i0 = 1, and curve “c” is forhΣ3it withhΣ3i0 = 1.The crosses are from a simulation of the dynamics, averaged over

1000 runs The solid-line curves represent the analytical results ofEqs (3.46) The dotted “b” curve shows what one would get for

hΣ1it if Ω vanished rather than being large on the scale set by τ ; weobserve that the inter-atomic dipole-dipole interaction acceleratesthe decay of hΣ1it For the parameter values used throughout thepaper, the time range is 3×1010s (roughly 1000 years); see Eq (3.35)for the value of τ 47

3.5 Fidelity of the RFF qubit For Ω2/Ω1 = 10−4, the plots show F (t) of

Eq (3.89) and its lower bound of Eq (3.90) for t < 45× 2π/Ω1 (topplot), for t < 150× 2π/Ω1 (inset in the top plot), and for t < 2π/Ω2(bottom plot) Curve ‘a’ is the lower bound on F (t); the other threecurves are for Σ(ϕ)1

0 = 0.4 and s(0) = 1 (curve ‘b’), s(0) = 0.8(curve ‘c’), and s(0) = 0.6 (curve ‘d’) One can clearly see the small-amplitude short-period oscillations and the large-amplitude long-period oscillation For the parameter values used throughout thepaper, the respective time ranges are 2, 7, and 450 hours We have

Ω1 ≈ Ω, with Ω = 2π × 6mHz given in Eq (3.20) 58

3.6 RFF qubit constructed from four spin-1/2 atoms (i) Two-dimensionalsquare configuration The dipole-dipole interaction is unavoidablyunbalanced here because the distance for the two diagonal pairs islarger than the distance for the four edge pairs (ii) Three-dimensionalpyramidal configuration Here, if the height h is chosen such thatb/a = 0.661, the effective dipole-dipole interaction has equal strengthfor all six pairs of atoms 61

3.7 Lower bounds for the fidelity F (t) (solid curve) and the expectationvalue hPJ=0it (dashed curve) for the RFF qubit constructed fromfour spin-1/2 atoms at the corners of a perfect square The fidelity ofFig 3.5 (three atoms with non-ideal geometry) would be indiscerniblefrom F (t) = 1 here 63

3.8 Energy level structure of87Rb ground state 65

3.9 Apply the bias magnetic field in the ez direction to the line of atoms;when (ez· n)2= 1/3, the effective dipole-dipole interaction vanishes.The atoms do not need to be placed at equal distance 70

4.1 Energy levels of the coherent excitations of two-atom systems Plot

‘a’ shows the energy level for the double excitation to a low-lyingexcited state|ei; plot ‘b’ shows the energy level for double excitation

to a Rydberg state|ri, where the energy of state |rri is shifted awayfrom 2E by ∆E given in Eq (4.5), and the true energy level isindicated by the red curves 77

4.2 Coherently driving the transition between the ground state and aRydberg state in a three-atom system; only single Rydberg excitationcan be obtained owing to the Rydberg-blockade mechanism 79

4.3 Optical pumping of spin-F atoms to the S1/2ground state with mf =

F by applying a σ+ pumping light field that is coupled to the P1/2excited state 81

4.4 Population transfer between the ground state |g+i and the excitedstate |ri with a stimulated Raman transition via the intermediatestate|ei 81

4.5 Population transfer between the ground state |g−i and the excitedstate|ri with a Raman transition via the intermediate state |ei 82

4.6 The geometrical phase of state|+, 0i is imprinted by shining a laserwith wave vector k0 at incident angle θ Angle θ is determined by

Eq (4.13) 83

4.7 Top view of the trio of atoms and two applied coherent laser beamswith wavevectors k0 and k1 and polarization vectors 0 and 1, asgiven in Eqs (4.15) and (4.16) 85

4.8 Energy diagram of the6Li 2s and 2p levels [1] 87

4.9 Optical transition diagram of the RFF state preparation with 6Liatoms 88

4.10 Energy diagram of the 5s and 5p levels of 87Rb 89

4.11 Optical transition diagram of the RFF state preparation with 87Rbatoms 90

4.12 Optical transition diagram of the RFF state preparation with40Ca+ions 90

4.13 The geometry of the three atoms and the incident laser labeled by

k0, 0 with imperfection 96

4.14 Two-atom energy level structure near the |43s, 43si state 99

4.15 The population of the states during a 2π excitation pulse, for ∆ =800MHz, Ω0 = 120MHz and Ω1 = 80MHz The black curve showsthe population of the ground state |gggi; the blue curve shows thepopulation of the collective single-Rydberg-excitation state; the greencurve shows the population of the state with a single collective ex-citation to the intermediate 5p1/2 state; the red curve shows thepopulation of states with one atom excited to 43s1/2 and one atomexcited to 5p1/2 The populations of the other states are very smalland not shown on this plot 100

4.16 Frequency histogram for the fidelities of the state preparation over

200 numerical simulations The errors of the parameters are domly selected from normal distributions with the error ranges stated

ran-in this section 101

4.17 The upper plot shows frequency histogram of the fidelities of ing state |0i + |1i over 100 numerical simulations The lower plotshows population of the states during a 2π excitation pulse, for

prepar-∆ = 800MHz, Ω0 = 80MHz and Ω1 = 40MHz The black curveshows the population of the ground state|gggi; the blue curve showsthe population of the collective single-Rydberg-excitation; the greencurve shows the population of single collective excitation to the in-termediate 5p1/2 state; and the red curve shows the population ofstates with one atom excited to 43s1/2 and one atom excited to 5p1/2.102

5.1 Level scheme of a typical Raman transition (a) shows the level ture of a Λ-type Raman transition and (b) shows the level structure

struc-of a cascade-type Raman transition Ω0 and Ω1 denote the Rabifrequencies of the individual two-level transitions, ∆ denotes the de-tuning of the laser from the transition frequency of the excited stateand δ is the detuning of the two-photon transition The requirement

is that the detuning ∆ is much larger than the Rabi frequencies sothat the excited state|ei is not significantly populated 107

5.2 Fidelity between the two states at later times evolving with the exactHamiltonian but taking different δ values given by Eq (5.17) and

Eq (5.20) ∆ = 400Mhz and |Ω1| = 40Mhz are fixed and theredifferent curves are for different ratio of|Ω0/Ω1| 112

5.3 Population distribution for a single-atom Raman transition in timewhen δ = 0 The solid curves show the exact solution, and the dashedcurves the adiabatic-elimination approximation The initial state is

|0i The red curves are for the ground-state population |c0(t)|2, theblue curves for |c1(t)|2, and the orange curves report |ce(t)|2, thepopulation in the excited state The detuning is ∆ = 400 MHz for allplots; the top left plot is for|Ω0| = |Ω1| = ∆/10 = 40 MHz; the topright plot is for|Ω0| = ∆/10 = 40 MHz and |Ω1| = ∆/16 = 25 MHz;the bottom plot is for|Ω0| = |Ω1| = ∆/4 = 100 MHz 119

5.4 Plots of populations obtained from different zeroth-order solutions

of the Lippmann-Schwinger equations Blue curves give the exactresults from numerical simulation using the Hamiltonian HI; greencurves show solutions from the symmetric approximation eU0(S)(t); redcurves show solutions from eU0(R)(t); and orange curves show solutionsfrom eU0(L)(t) The parameters are ∆ = 400 MHz,|Ω0| = ∆/2, |Ω1| =3∆/10 and δ =−Ω†σ3Ω/(4∆) = |Ω1|2− |Ω0|2
/(4∆) = −16 MHz.The effective Rabi frequency is ΩR = 27.8 MHz, about 7% of ∆.Initially, we have c0(0) = 1 and c1(0) = ce(0) = 0 The curvesstarting at 1 show the approximations for

c0(t) 2

; the curves thatstart at 0 and rise to 1 are for

c1(t) 2

; and the curves that start at

0 and never exceed small values are for ... fast quantum gates and generating entanglement with Rydberg atoms ? ?atoms with one or more electrons in a highly excited Rydberg state with a largeprinciple quantum number n

Neutral atoms. .. criticalchallenge for building quantum computer with trapped atoms will be to preservethe high-fidelity control in a system with a large number of atoms

Quantum dots and N-V centers in... noise The robustnessstudy would apply also to RFF qubits made of other types of atoms, and thedecoherence is expected to be of a similar scale

Quantum computing with neutral atoms has