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Preparation and storage of frequency-uncorrelatedentangled photons from cavity-enhancedspontaneous parametric downconversionHan Zhang1†, Xian-Min Jin1†, Jian Yang1†, Han-Ning Dai1, Sheng-Jun Yang1, Tian-Ming Zhao1, Jun Rui1,

Yu He1, Xiao Jiang1, Fan Yang2, Ge-Sheng Pan1, Zhen-Sheng Yuan1, Youjin Deng1, Zeng-Bing Chen1,

Xiao-Hui Bao1, Shuai Chen1, Bo Zhao3* and Jian-Wei Pan1*

The preparation and storage of photonic entanglement arecentral to the achievement of scalable linear optical quantumcomputation1–3 (LOQC). The most widely used photonicentanglement source (a spontaneous parametric downconver-sion (SPDC) source)4,5 is not directly suitable for storage,because its working frequency bandwidth is significantlylarger than any available quantum memory. To remedy thisproblem, cavity-enhanced narrow-band SPDC sources6–12 havebeen developed. However, the storage of cavity-enhancednarrow-band entangled photons has not yet been achieved.Also, the spectral correlations between the entangled photonscan make them practically useless for scalable LOQC5,13,14.Here, we report the preparation and storage of frequency-uncorrelated narrowband (5 MHz) entangled photons from acavity-enhanced SPDC source. The frequency correlationbetween the entangled photons is eliminated by changing thecontinuous UV pumping beam to short pulses. The storage ofthe polarization state of a single photon, and of a photonentangled with another flying in the fibre, is demonstrated. Ourwork demonstrates a quantum interface between narrow-bandentangled photons from cavity SPDC and atomic quantummemory, and thus provides an important tool towards the achieve-ment of all-optical quantum information processing.

Scalable linear optical quantum computation (LOQC) hasattracted a great deal of interest since the pioneering work byKnill, Laflamme and Milburn (KLM)1. The scheme was laterdramatically improved by Browne and colleagues, who proposed aresource-efficient protocol for the experimental implementation ofLOQC2. Central to such an efficient protocol are the preparationand storage of photonic polarization-entangled pairs.

As the most common photonic entanglement source, the spon-taneous parametric downconversion (SPDC) source has beenexploited to demonstrate the principles of LOQC15,16. Moreover,heralded polarization-entangled pairs, the key element ofBrowne’s protocol, have been generated17,18. However, it remainschallenging to implement LOQC because of the absence of suitablequantum memory for the SPDC photons. The SPDC source has abandwidth on the order of terahertz, significantly beyond theworking bandwidth of available quantum memories such asatomic memory (megahertz to gigahertz19). To store the SPDCphotons directly, one must use an extremely narrow frequencyfilter to eliminate the unwanted frequency components20. Thisleads to very low efficiency.

A more elegant solution is to develop a high-brightness narrow-band SPDC source, which can be achieved by placing a nonlinearcrystal in an optical cavity. In this way, the bandwidth of the outputphotons is limited by the cavity linewidth and thus can match thebandwidth of the quantum memory. Meanwhile, the entanglementgeneration rate is greatly improved by cavity enhancement.Significant progress has been made in this regard6–8,10,12. Post-selected, single-mode, cavity-enhanced, narrow-band polarization-entanglement sources have recently been demonstrated9,11.However, the storage of cavity-enhanced SPDC photons in atomicquantum memory requires a combination of highly complextechniques and therefore remains challenging.

Furthermore, the narrow-band polarization-entangled photonsdemonstrated to date cannot be used to implement scalable LOQC,because the photons are usually frequency-correlated. The photonpairs are generated in many correlated spatiotemporal modes, andthe detection of one photon projects the other photon into a mixedstate. However, when two photons from independent sources aresubject to a Hong-Ou-Mandel (HOM) interferometer, a unityvisibility can be achieved only if the photons are in a pure and indis-tinguishable spatiotemporal mode13,14. A detailed analysis14 furthershows that, when polarization-entangled pairs are used to implementthe fusion gates, the frequency correlation can significantly reducethe fidelity of the generated entangled states. Therefore, preparingpolarization-entangled photons in a pure spatiotemporal modewithout spectral correlation is crucial to scalable LOQC.

Here, we report the preparation and storage of a frequency-uncorrelated cavity-enhanced SPDC photonic entanglement witha linewidth of 5 MHz. The narrowband polarization-entangledphotons are produced by applying pump light to a type-II nonlinearcrystal in a cavity and aligning the generated twin photons at apolarizing beamsplitter (PBS). Eliminating frequency correlationand preparing entangled photons in a pure spatiotemporal modeis achieved by choosing a suitable pump pulse. Storage of cavity-enhanced entangled photons is demonstrated by using electromag-netically induced transparency (EIT). A single photon with arbitrarypolarization is stored, and the average fidelity of keeping thepolarization is about 92% for a storage time of up to 200 ns.Furthermore, polarization-entangled photon pairs are prepared,and one of the photons is stored in atomic ensembles while theother keeps flying in an optical fibre. Violation of the Clauser–Horne–Shimony–Holt (CHSH) Bell inequality is clearly observedfor storage times up to 200 ns.

1Hefei National Laboratory for Physical Sciences at Microscale and Department of Modern Physics, University of Science and Technology of China, Hefei,Anhui, 230026, PR China, 2Physikalisches Institut, Universitat Heidelberg, Philosophenweg 12, D-69120 Heidelberg, Germany, 3Institute for TheoreticalPhysics, University of Innsbruck, A-6020 Innsbruck, Austria; †These authors contributed equally to this work. *e-mail: bo.zhao@uibk.ac.at; pan@ustc.edu.cn

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The experimental set-up is illustrated in Fig. 1, where polariz-ation-entangled photons are prepared and stored in the sourceand memory laboratories, respectively. In the source laboratory,UV pump light was applied to a periodically poled KTiOPO4(PPKTP) crystal inside a cavity with linewidth g≈ 5 MHz. Thisproduced narrow-band photon pairs, for which the centralfrequency v0 was tuned to match the D1 line of 87Rb. A single-mode output was realized by choosing a suitable cavity-lengthdifference for different polarizations and using an actively stabilizedfilter cavity. Entanglement was post-selected by interfering the twinphotons at a PBS (see Supplementary Information).

Preparing entangled photons in a pure spatiotemporal modewithout spectral correlation can in principle be achieved by a spec-tral filter5, but this only keeps a small fraction of the generatedphoton pairs and thus has very limited efficiency. For conventionalbroadband SPDC, an improved method is to control the modalstructure of the photon-pair emission21 or engineer the density ofstates of the optical field using a microcavity22. In our case, thegenerated entangled photons have an extremely narrow linewidthg≈ 5 MHz, and thus we can eliminate the spectral correlation bychoosing a pulsed pump light with linewidth s . g (seeSupplementary Information).

To quantitatively analyse the remaining frequency correlationand the purity14,21 of the spatiotemporal modes of the photons, weperformed a HOM-type experiment to demonstrate non-classicalinterference between a pulsed-pumped SPDC source and an

independent weak coherent light at a single photon level23 (seedetails in Supplementary Information). We chose the duration ofthe UV pump pulse to be Tp¼ 40 ns (s≈ 9.4 MHz). Photon 1 isdetected where the detection window is larger than the duration ofthe wave packet and does not provide temporal filtering. Heraldedphoton 2 is subject to a HOM interferometer together with theweak coherent light, which is prepared to have a bandwidth matchingwith the entangled photon pairs. Note that no additional frequencyfilter is applied to the SPDC photons to ensure indistinguishabilitybetween the SPDC photons and single photons in the weak coherentlight. We obtained a raw three-photon visibility of 0.57+0.02, largerthan the classical bound 0.5, which is clear evidence of non-classicalinterference. After subtracting the background noises induced byhigher-order events, we obtained a net three-photon visibility ofV¼ 0.88+0.05. Such a high net visibility indicates that heraldedphoton 2 and the single photon in the weak coherent light arehighly indistinguishable. We can also obtain a lower bound ofthe purity21 of the spatiotemporal mode of the heralded photon ofP . 2V 2 1 ≈ 0.76.

To demonstrate the storage of photons with atomic memory,photon 1 was guided to a polarization analyser through a 60 msingle-mode fibre, while photon 2 was directed to the memorylaboratory through a 20 m single-mode fibre for further operation.We shall call photon 1 the flying photon and photon 2 the signalphoton. In the memory laboratory, a cigar-shaped laser-cooled87Rb atomic cloud was confined in a rectangular magneto–optical

Memory lab

Source lab

Narrow-band entanglement

20 m fibre

60 m fibre

Atomic filter

Ensemble 2

σ+σ+

σ+σ+

Ensemble 1

FP cavity

PBS2PBS1

Piezo

D4

D3

D1

D2PPKTP

Cavity

V

HH

V

10 m

Control field

1

2

Lock out Lock in

Prism QWPHWP PBS

U mode

D mode

1

Figure 1 | Experimental set-up. In the source laboratory, a pair of photons is generated by applying UV pump light to a periodically poled KTiOPO4 crystal

(PPKTP) inside a cavity. The flying photon is coupled into a 60-m-long single-mode fibre and guided to a polarization state analyser, which consists of a

half wave plate (HWP), a quarter wave plate (QWP), a polarizing beamsplitter (PBS) and single photon detectors. The signal photon is directed through a

20-m-long fibre to the memory laboratory located 10 m away. In the memory laboratory, a cold atomic cloud trapped in an MOT serves as the quantum

memory for the signal photon, with spatial modes U and D selected as two ensembles for storage of the vertical (V) and horizontal (H) polarization

components, respectively. The path-length difference between the U and D modes is set to zero and actively stabilized. The two polarization modes are both

transferred to be sþ for storage. A strong control light is applied at an angle of 28 relative to the two spatial modes. The signal photon is stored and read out

of the atomic ensemble by changing the strength of the control light. The retrieved signal photon is transmitted through a Fabry–Perot cavity and an atomic

filter cell to absorb the leakage from the control light, and then subject to a polarization analyser for state analysis.

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trap (MOT) and served as an EIT-based quantum memory24–30.The dark MOT technique31 was applied to further increase theoptical depth to 55. The control field was applied along the long axisof the atomic cloud. The up (U) and down (D) modes, tilted 28from the direction of the control field, were coupled to single-modefibres and used as two separated atomic ensembles for differentpolarizations (Fig. 2a).

To characterize the storage capability of the atomic memory, astandard EIT experiment was carried out. For a control light witha Rabi frequency of 12.6 MHz and a coherent 50-ms-long probelight, we observed an EIT window of 5.5 MHz and a delay timeof 200 ns (Fig. 2b), corresponding to a delay–bandwidth productof 7. We then cut the UV pump light into pulses with durationTp¼ 100 ns and performed a stop-light experiment on the Umode. Thanks to the high delay–bandwidth product, we couldfully compress the pulse into the atomic ensemble with negligibleleakage. After a controllable delay, the stored pulse was read out,and the leakage efficiency and overall storage efficiency for differentstorage time measured (Fig. 2c).

We were then ready to perform a storage experiment for theSPDC photons, for which Tp¼ 30 ns was chosen. First, a signal

photon triggered by detecting the flying photon was stored. Thepolarization was predetermined to be H by means of a polarizerbefore PBS1. The signal photon was stored in ensemble 2 by adiaba-tically switching off the control light and was read out by turning iton after a controllable delay. We measured the cross-correlationg 13

(2)¼ p13/( p1p3), where p1( p3) is the probability of detecting aphoton on detector D1 (D3) and p13 is the coincidence probabilitybetween D1 and D3. In the experiment, our optical parametricoscillator works at a regime far below threshold, and the stategenerated is a two-mode squeezed state with excitation probabilitymuch smaller than 1. In this case, the non-classical correlationof two photons means g13

(2) . 2 (refs 32 and 33). The single-photon quality of the triggered signal photon can be estimated bya¼ 4/( g13

(2) 2 1), and thus g13(2) . 5 implies a , 1 and the single-

photon quality is well preserved27,32,33 (see SupplementaryInformation). The measured cross-correlation versus storage timeis shown in Fig. 3a, where g13

(2) ≈ 10 for a storage time up to 1 ms.We then demonstrated the capability to coherently store a single

photon with arbitrary polarization. We prepared six polarizationstates: four linear states H| l, V| l, +| l = H| l + V| l

( )/

��2

√, and

two circular states R| l, L| l = H| l + i V| l( )

/��2

√. In the experiment,

−40 −30 −20 −10 0 10 20 30 400

300

600

900

1,200

1,500

1,800

2,100

Tras

mitt

ed c

ount

s

Detuning (MHz)

Control field

U

b

2°

5P1/2 F = 2

5S1/2 F = 1

5S1/2 F = 2

Cont

rol fi

eld

−200 0 200 400 600 800 1,000 1,200

Reference

Store 200 ns

Store 400 ns

Store 600 ns

Store 800 ns

Store 1,000 ns

Time (ns)EfficiencyLeakage

× 0.2

0.45 %

100 %

0.58 %

0.50 %

0.35 %

0.53 %

9.74 %

9.7 %

8.06 %

7.22 %

6.19 %

5.5 MHz

OD = 55

Single photon

D

a

c

Figure 2 | Characterizations of storage medium. a, Illustration of laser configuration. Inset: relevant atomic levels of 87Rb and associated light fields.

b, Transmission spectra of coherent 50-ms-long probe light versus probe detuning from the |5S1/2,F¼ 1l�|5P1/2,F¼ 2l transition in the presence (red

circles) and absence (blue circles) of the control field with a Rabi frequency of 12.6 MHz. The optical depth (OD) derived from the absorption profiles is 55

and the EIT widow is �5.5 MHz. Each data point represents an average of 20 experimental trials. c, Coincidence rates between D1 and D3. The reference

coincidence (without storage) has been multiplied by factor of 0.2 for the purpose of illustration. Leakage efficiency is defined as the ratio of coincidence

during storage and before storage, and overall storage efficiency is defined by the ratio of the coincidence after retrieval and before storage.

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the H and V components of a polarization state were coherentlyseparated by PBS1 and directed to the D and U modes, respectively.After being stored in the atomic ensembles for a controllabledelay, the H and V components were read out and combined atPBS2 to restore the original polarization state. The performance ofsuch a quantum memory can be justified by the storage fidelity ofpolarization F¼ |kcin|coutl|2. The results after a storage timeof 200 ns are shown in Fig. 3b–d, which yields an average fidelityof (92.4+1.2)% over the six initial states.

Finally, we stored a signal photon entangled with the flyingphoton. The polarizer before PBS1 was removed, and the flyingphoton was undetected until the signal photon was fully stored. Inthis case, the photonic entanglement C| l1,2 = (|Hl1|Vl2 + |Vl1|Hl2)/

��2

√was mapped to atom–photon entanglement C| l1,2 =

(|Hl1|Ul2 + |Vl1|Dl2)/��2

√during storage, and was converted

back after retrieval. We emphasize that the polarization of thesignal photon is entangled with the flying photon during thewhole storage process. To justify the performance of the quantummemory, we measured the flying photon in the |Hl(|þ l) basisand the retrieved signal photon in the cos u|Hlþ sin u|Vl basis

with varying polarization angle u. The normalized polarization cor-relation E(u) versus u was calculated and the results for a storagetime of 200 ns are shown in Fig. 3e, giving a visibility about 81%.

We further demonstrated the quality of the entanglementafter storage by means of a clear violation of the CHSH-type Bellinequality34. We measured the correlation function E(u1,u2), with u1(u2) being the polarization angles for the flying (signal) photon, andcalculated quantity S¼ |E(u1,u2) 2 E(u1,u ′

2)þ E(u ′1,u2)þ E(u ′

1,u ′2)|.

The angle arrangement was set as (u1,u′1,u2,u′

2)¼ (08,458,22.58,67.58),and S was measured for (i) both photons in the source laboratoryand (ii) the signal photon stored for 200 ns. The S values areshown in Table 1, which clearly violate the CHSH inequality.

Table 1 | Measured violation of Bell inequality.

t (ms) S (CHSH) s.d.

(i) 0 2.59(5) 11.8(ii) 0.2 2.54(13) 4.2

s.d., standard deviation; t, storage time.

0 1,000 2,000 3,000 4,0000

2

4

6

8

10

12

Storage time (ns)

−45°

+45°+45°

Initial stateRetrie

ved state−45°

L

RRInitial state Retrie

ved stateL

V

HHInitial state

Retrieved state

V

a b

c

d

e

0 20 40 60 80 100 120 140 160 1800.0

0.2

0.4

0.6

0.8

1.0

Pola

rizat

ion

corr

elat

ion

Project angle for retrieved photon (deg)

g 13(2)

Figure 3 | Performance of the quantum memory. a, Cross-correlation g13(2) versus storage time. b–d, Fidelities after a storage time of 200 ns for six initial

polarization states: 0.954+0.026 (|Hl), 0.989+0.010 (|Vl), 0.909+0.027 (|þ l), 0.889+0.037 (| 2 l) 0.920+0.031 (|Rl) and 0.881+0.035 (|Ll).

e, Normalized polarization correlation for the retrieved entanglement. The red (blue) curve represents that the polarization of the flying photon is projected

onto |þ l (|Hl). Error bars represent+1 standard deviation.

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In summary, we have experimentally achieved the preparationand storage of frequency-uncorrelated, cavity-enhanced, narrow-band polarization-entangled photons. Some aspects remain to beimproved. For example, the polarization entanglement is post-selected, the storage time is only of the order of a few hundrednanoseconds and the overall storage efficiency is less than 10%. Anon-post-selected polarization entanglement source can beimplemented by combing two type-II SPDC sources interferometri-cally on a PBS4. The relatively short storage time and the low storageefficiency can be improved by better compensating the magneticfield or by using magnetic field-insensitive states33. Our workopens up new possibilities for the proof-in-principle demonstrationof LOQC with SPDC and atomic quantum memories, such as thepreparation of large-scale graph states35, the demonstration of aquantum repeater node36, the implementation of linear-opticalone-way quantum computing37 and the implementation ofentanglement-enhanced measurement38.

Received 30 September 2010; accepted 25 July 2011;published online 11 September 2011

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AcknowledgementsThis work was supported by the National Natural Science Foundation of China, theNational Fundamental Research Program of China (grant no. 2011CB921300), the ChineseAcademy of Sciences, the Austrian Science Fund, the European Commission through theEuropean Research Council Grant and the Specific Targeted Research Projects of HybridInformation Processing.

Author contributionsX-M.J., J.Y., H.Z., Z.-B.C., Y-J.D., X-H.B, S.C., B.Z. and J-W.P. designed the experiment.H.Z., X-M.J., J.Y., H-N.D., S-J.Y., T-M.Z., J.R., Y.H., X.J., F.Y., G-S.P., Z-S.Y. and S.C.performed the experiment and analysed the data. X-M.J., J.Y., H.Z., Y-J.D., X-H.B, B.Z. andJ-W.P. edited the manuscript.

Additional informationThe authors declare no competing financial interests. Supplementary informationaccompanies this paper at www.nature.com/naturephotonics. Reprints and permissioninformation is available online at http://www.nature.com/reprints. Correspondence andrequests for materials should be addressed to B.Z. and J.W.P.

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