High-efficiency Cooper pair splitting demonstrated by two-particle conductance resonance and positive noise cross-correlation

Abstract

Entanglement is at the heart of the Einstein-Podolsky-Rosen paradox, where the non-locality is a necessary ingredient. Cooper pairs in superconductors can be split adiabatically, thus forming entangled electrons. Here, we fabricate such an electron splitter by contacting an aluminium superconductor strip at the centre of a suspended InAs nanowire. The nanowire is terminated at both ends with two normal metallic drains. Dividing each half of the nanowire by a gate-induced Coulomb blockaded quantum dot strongly impeds the flow of Cooper pairs due to the large charging energy, while still permitting passage of single electrons. We provide conclusive evidence of extremely high efficiency Cooper pair splitting via observing positive two-particle correlations of the conductance and the shot noise of the split electrons in the two opposite drains of the nanowire. Moreover, the actual charge of the injected quasiparticles is verified by shot noise measurements.

ARTICLE

Received 8 May 2012 | Accepted 25 Sep 2012 | Published 6 Nov 2012

Entanglement is at the heart of the Einstein-Podolsky-Rosen paradox, where the non-locality is a necessary ingredient. Cooper pairs in superconductors can be split adiabatically, thus forming entangled electrons. Here, we fabricate such an electron splitter by contacting an aluminium superconductor strip at the centre of a suspended InAs nanowire. The nanowire is terminated at both ends with two normal metallic drains. Dividing each half of the nanowire by a gate-induced Coulomb blockaded quantum dot strongly impeds the ow of Cooper pairs due to the large charging energy, while still permitting passage of single electrons. We provide conclusive evidence of extremely high efciency Cooper pair splitting via observing positive two-particle correlations of the conductance and the shot noise of the split electrons in the two opposite drains of the nanowire. Moreover, the actual charge of the injected quasiparticles is veried by shot noise measurements.

DOI: 10.1038/ncomms2169

High-efciency Cooper pair splitting demonstrated by two-particle conductance resonance and positive noise cross-correlation

Anindya Das, Yuval Ronen, Moty Heiblum, Diana Mahalu, Andrey V. Kretinin & Hadas Shtrikman

Braun Center for Submicron Research, Department of Condensed Matter Physics, Weizmann Institute of Science, Rehovot 76100, Israel. Correspondence and requests for materials should be addressed to M.H. (email: mailto:[email protected]

Web End [email protected] ).

NATURE COMMUNICATIONS | 3:1165 | DOI: 10.1038/ncomms2169 | http://www.nature.com/naturecommunications

Web End =www.nature.com/naturecommunications 1

& 2012 Macmillan Publishers Limited. All rights reserved.

ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms2169

Two particles are entangled if a detection or a manipulation of the quantum state instantaneously affects the other quantum state. Hence, being non-local, the entanglement

of two separated particles must involve simultaneous, non-local, measurements. Such two-particle state can be achieved, in principle, via particles interaction or by breaking apart a composite quantum object. For example, fully entangled photons are readily provided by low-efciency parametric down-conversion of higher energy photons13. Such a feat is not readily

available for electrons. However, the closest electrical analogue to the high energy photons are Cooper pairs in a superconductor, being a natural source of entangled electron pairs. Splitting them adiabatically may give birth to entangled electron pairs. Indeed, it had been predicted and measured that Cooper pairs, emanating from a superconductor, can split into two normal metallic leads in the so-called cross Andreev reection process411. Such process

can be conclusively veried by observing positive coincident arrival events, namely, positive cross-correlation of current uctuations in two separated normal metallic leads that collect the split pairs1218. The main difculty in identifying such process

is the overwhelming ux of Cooper pairs that enters the normal leads via direct Andreev reection (the proximity effect). Such an experiment was attempted by Wei et al.10, where cross-correlation measurements were performed in an all metallic system (Al superconductor and Cu normal metal) without quantum dots (QDs) at very low frequencies (26 Hz) at a temperature of0.30.4 K. The large 1/f noise, the relatively high temperature and a dominant Cooper pair transport compromised the obtained data. Replacing each of the normal metallic leads with a QD in the Coulomb blockade regime (see ref. 12), performed by Hofstetter et al.8, indeed suppressed Cooper pairs transport, but lacked to prove coincidence splitting.

Here, we provide results of coincidence measurements by observation of positive cross-correlation of current uctuations. These are reinforced by simultaneous non-local conductance measurements on both sides of the nanowire. Quenching superconductivity with a weak magnetic eld suppressed the positive correlations. We obtained a splitting efciency, dened as the ratio between single-electron to two-electron transport, as high as B100%.

ResultsExperimental setup. Figure 1 shows a SEM image of our device, as well as a schematic illustration of the measurement setup. A 50-nm diameter InAs nanowire, grown by a high purity Au-assisted MBE process19, was suspended on Au pillars above a conducting Si substrate coated with 150 nm SiO2. A

superconducting aluminium strip (S), B100 nm wide, was intimately contacted at the centre of the nanowire, separating it into two equal sections, each B200 nm long, with two terminating gold ohmic contacts serving as drains (D). Aside from the conducting Si substrate, which served as a global gate (GG), two narrow metallic gates, some 50 nm wide, were used to tune the local chemical potential on each side of the nanowire. Although the local gates (positively biased) accumulated electron puddles, the GG (negatively biased) induced barriers on the sides of each puddle, thus forming two QDs on both sides of the superconducting contact. Although currents were amplied with a room temperature current amplier at B575 Hz, current uctuations (broad band auto-correlation or shot noise) and their cross-correlation, were rst ltered by an inductor-capacitor (LC) resonant circuit tuned to 725 kHz (bandwidth B100 kHz);

amplied by a home-made cold (1 K) preamplier cascaded by a room temperature amplier, and nally measured by a spectrum analyser or an analogue cross-correlation setup. More

details of the fabrication process and the measurement techniques are given in the Methods section.

One side transport and charge measurement. With all three gates unbiased, the InAs nanowire conductance is n-type with an approximate electron density of 5 106 cm 1. The differential

conductance of one side of the wire, say the left side, when the right side is pinched-off by its local gate (RG), is measured at 10 mK as a function of its gate voltage (VLG), whereas the GG is grounded. The conductance varies around 2 e2/h (Fig. 2a) characteristic of Fabry-Perot type oscillations19. Note that

conductance exceeding 2e2/h for the rst subband indicates the presence of Andreev reections with a barrier near the SInAs interface (with maximum of 4 e2/h). Under similar conditions, the non-linear differential conductance as a function of bias VSD is

shown in Fig. 2b,c for two values of VLG (C and A in Fig. 2a). The gate voltage VLG mainly controls the barrier near the SInAs interface, shifting the linear conductance from high at point C to low at point A, with a strikingly different non-linear conductance in the two points. At point C, with a linear transmission probability t* 2.6/4 0.65, the conductance drops with biasas

expected for a diminishing tunnelling probability of Cooper pairs as the bias approaches half the superconducting gap (D).

Alternatively, at point A, with t* 0.4, the conductance

increases with bias and peaks at VSD D. Here single electron

tunnelling dominates. The superconducting gap 2D B220 meV is noted by the dotted line. A perpendicular magnetic eld quenches the non-linear differential conductance with a critical eld B B0.12 T.

For the above conditions, with the barrier at the SInAs interface, the injected current is carrying shot noise, which depends linearly on the injected current (I) and the tunnelling

RT CA

VLG VGG

VRG

SiO2

Si GG

RT CA

1 K VA

1 K VA

GG

LG RG

NW

N N S

N N

S

VDC+VAC LG RG

Au Au Au

Figure 1 | Device and measurement setup. An SEM image, with false colour enhancement, of the working suspended InAs-based splitting device. Scale bar, 200 nm. The nanowire is connected in its centre with a superconducting Al contact (S) and two normal Au contacts (N) each on either sides of the nanowire. Inset shows a cross-sectional schematic view. The superconducting contact is biased by a voltage source and the currents at the two normal drains are measured by room temperature current ampliers (RT CA). Current uctuations are measured by cold voltage ampliers (1 K VA) with an inductance-capacitance (LC) resonant circuits at their input. The switching between RT CA and 1 K VA is done by a low-temperature relay.

2 NATURE COMMUNICATIONS | 3:1165 | DOI: 10.1038/ncomms2169 | http://www.nature.com/naturecommunications

Web End =www.nature.com/naturecommunications

& 2012 Macmillan Publishers Limited. All rights reserved.

NATURE COMMUNICATIONS | DOI: 10.1038/ncomms2169 ARTICLE

a d

3

2

1

6

5

4

3

2

1

0

3

C

A

B

G(e2/h)

2

1

0

G(e2/h)

2 kFdr

kFdr2 e 2dr

px ; with 1E

1 pD

1U, where x is the coherence

length of a Cooper pair, kF, the Fermi wave vector and dr r1 r2, the distance between the emerging split electrons;

with all values related to the proximity region in the InAs12.

As dr is likely to be smaller than the superconductor width, we believe that its suppression factor is not important, leading to ZD(pD/G)2 B1.

Non-local conductance measurements. We begin with non-local conductance measurements by forming two QDs on both sides of the nanowire. Applying a large negative voltage to the GG (VGG 15 V), while keeping the local gates (VLG and VRG) at

small negative voltage, induces two barriers surrounding each of the two electron puddles. Starting, with the left QDL, although the right side of the nanowire is blocked, the conductance peaks as a function of VLG are solely due to Cooper pairs transport. Such transport takes place likely due to sequential tunnelling of single particles by higher order process within a time scale smaller than the coherence time of a Cooper pair; leading to a nite conductance proportional to (G/D)2 (ref. 12).

Now when the right side of the nanowire is also allowed to conduct, Cooper pairs splitting can take place, thus also enhancing current on the left side. The largest one-electron transport on both sides is expected when the two QDs are at resonance (Fig. 3a), which is actually a two-particle conductance resonance12. Such non-local conductance measurement is shown in Fig. 3b. We simultaneously measure the conductance of both sides of the nanowire by two individual current ampliers. In Fig. 3b, a colour plot of GL is plotted by scanning the VRG for different xed values of VLG. The solid red line towards the left side of the colour plot as a function of VLG is the measured GL of one Coulomb blockade peak due to Cooper pairs transport through QDL when the right side is blocked, ICP. The blue line on the top panel is the local conductance of QDR as a function of VRG when the left side is blocked. Tunning to VLG 0.558 V

(the dashed white line in Fig. 3b) and scanning VRG leads to the non-local GL (projected red line). Conductance is enhanced (DG, due to single electron transport, Ie) by as much as 0.18 e2/h (marked by a asterik in Fig. 3b), corresponding to the conductance peaks of QDR with VRG. The efciency of splitting, dened as Z Ie/ICP DG/G, is proportional to tR/tL for the left

side. For tuning VLG 0.558 V, the efciency is B70% (DG/

G 0.18/0.26) and is more than 100% when VLG is set to off

resonance (see Supplementary Fig. S4a).

Similarly, non-local enhancement of GR induced by VLG takes place (see Supplementary Fig. S4b). A full representation of the non-local conductance, Cooper pair splitting currents, between the left QD (Coulomb blockade peak at VLG 0.557 V) and the

right QD (Coulomb blockade peak at VRG 0.21 V) is shown

in the two colour plots in Fig. 3c; with DGL (VLG, VRG) (top

representation) and DGR (VLG, VRG) (bottom representation). Note that the apparent DGL is bigger than DGR. The two-particle conductance resonance can be seen in Fig. 3c, where each single particle transport is non-local, as it depends on the transmission of both QDs (tLtR). Here, we have shown the resonance of split electrons with an energy close to the Fermi energy. In the presence of magnetic eld (B0.2 T), the superconductivity quenches and the non-local conductance diminishes (Fig. 3c). The residual non-local conductance, in the form of a weak sawtooth-like dependence, is the familiar detection behaviour of electron occupation in QDs25. Owing to the proximity between the two dots (B300 nm), the left dot senses the potential swing in

0.4

0.3

0.2

0.1 0 276 135

VLG (V)

VSD (V)

0V SD (V)

128 254

b

c

300 200 100 00 100 200 300

300 200 100 0 100 200 300

e

2.62.42.2 2

1.8

2.2 2

1.81.6

G(e2/h)

G(e2/h)

C

27 A2 Hz1 )

S I(0) (10

A

20 10 0

ISD (nA)

10 20

Figure 2 | Andreev reection and charge measurement. (a) Differential conductance (G) of the left side of the nanowire as a function of left local gate voltage (VLG) when the right side is pinched off. (b,c) Bias (VSD)-dependent differential conductance at points C and A, respectively. In C, the conductance is characteristic of Andreev reection in a superconductor normal (SN) junction. In A, it is characteristic of a tunnelling in a superconductorinsulatornormal (SIN) junction. Dashed lines border the superconducting gap (2D). (d) Non-linear conductance and (e) auto correlation signal (shot noise) as a function of current (ISD) for magnetic

elds, B 0 and B 0.2 T. Solid lines are theoretical predictions at

temperature, T 10 mK. Charge is 2e for VSDoD (blue line) and e for

VSD4D (red line).

charge (e*)2023. The low frequency spectral density of the excess noise (shot noise above the Johnson-Nyquist and environment noise) in the single InAs channel takes the form: Si(0) 2e*I(1 t*)F(T), with t* t for electrons and t* t2 for

Cooper pairs, and F(T) cothz 1/z, with z e*VSD/kBT.

Determination of the non-linear conductance (Fig. 2b,c) is crucial for accurate excess noise value as it affects the background noise (composed of thermal and current noise of the preamplier24); as explained further in the Supplementary Fig. S2. In Fig. 2e, we plot Si(0) as a function of I for zero magnetic eld B and for B B0.2 T. The blue and red solid lines are the 10-mK predictions for t* 0.4, e* 2e and t 0.63, e* e, respectively;

demonstrating an excellent quantitative agreement with the data (black circles). The distinct change of slope (from e* 2e to

e* e) nicely corresponds to D that was deduced from the con

ductance (Fig. 2d). A perpendicular small magnetic eld (B0.2 T) quenched the superconductivity with the excess noise nicely agreeing with e* e across the full biasing range.

QD and Cooper pair splitting. We now turn to study the efciency of Cooper pair splitting. Introducing a Coulomb blockaded QD on each side of the nanowire is expected, under suitable conditions, to suppress Cooper pair transport due to the dots relatively large charging energy U. Preventing single electron injection from the superconductor necessitates, eVSD, kBToD,

while quenching of Cooper pairs transport through the QD requires eVSDoU. The characteristic energies of each QD is determined by measuring the non-linear differential conductance as a function of the DC bias (VSD) and the local gate voltage (via the so-called diamond structure; see Supplementary Fig. S3). We estimated the average charging energy at U 810 meV and the

single particle level broadening at G B200300 meV. Under these conditions, with U4D but GED4kBT, two-sequential-electron

transport, proportional to (G/D)2, is barely suppressed12. The efciency, dened as the ratio of splitted current/ Cooper pair current, Z Ie/ICP, can be expressed as

Z 2E2G2 sin

NATURE COMMUNICATIONS | 3:1165 | DOI: 10.1038/ncomms2169 | http://www.nature.com/naturecommunications

Web End =www.nature.com/naturecommunications 3

& 2012 Macmillan Publishers Limited. All rights reserved.

ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms2169

a

c

0.4

0.2

G

R

GL

G

(e2/h)

GL

GR

N N

S

0.562

0.560.5580.5560.554

0.205

0.21

0.215

V RG (V)

VLG (V)

b

0.9

0.6

0.3

0

GR ( e

2/h)

d

0.5

0.4

0.3

0.2

*

GL (e2/h)

G (e2/h)

2 / h )

0.3

0.6

G L (e

2 / h )

0.4

G L (e

GR ( e

2/h)

0.2

0.2

0.2

0

0.55

VLG VRG

0.555

0.3

0.25

VRG (V)

0.2

VLG (V)

0.56

0.25

0.2

0.3

VRG (V)

Figure 3 | Non-local conductance measurement. (a) Band diagram of the system aligned for maximum Cooper pair splitting. In the non-local measurement, we look at the conductance of one side of the nanowire (with an embedded QD) as a function of local gate voltage applied to the QD on the other side. (b) Colour plot of the left side conductance GL by scanning the right local gate voltages, VRG for different xed values of left local gate voltages,

VRG. The solid red line towards the left side of the colour plot is a Coulomb blockade peak of QDL due to Cooper pairs transport when the right side is blocked. The projected red line on the top panel is the non-local conductance GL measured as a function of VRG (for the white dashed line at

VLG 0.558 V), with peaks enhanced in a corresponding manner to the conductance peaks of QDR (blue line on the top panel). (c) 2D colour plot of

currents due to Cooper pair splitting through the left QD (DGL) and the right QD (DGR scaled up by 2). The red and blue lines at top left and top right

panel are the Coulomb blockade peaks of QDL and QDR near VLG 0.557 V and VRG 0.21 V, respectively. (d) Non-local signal of GL (red line) as a

function of VRG for VLG 0.558 V at B 0.2 T. The blue line is the local GR as a function of VLG.

the right dot when an electron is added to it, thus affecting its conductance.

Cross-correlation measurements. Measuring positive cross-correlation of current uctuations in the two drains, assuring coincident clicks, provides direct test for the existence of split Cooper pairs (as the Andreev reections on each side are uncorrelated). To measure the cross-correlation, the current uctuations were rst amplied by a home-made cooled preamp, with the amplied signals fed to an analogue signal multiplier at 725 kHz. Starting with an unbiased device, the uncorrelated background noise in both drains was nulled (being only some 23% of the actual auto-correlated back ground noise due to cross talk, see Supplementary Fig. S5). In Fig. 4a the cross-correlation, measured with the two dots around their respective resonances (VLG 0.557 V and VRG 0.21 V), is displayed for

VSD 20, 10 and 5 mV DC. The cross-correlation is positive and

is highest when the two dots are at resonance; in full agreement with the nonlocal conductance measurement (Fig. 3c). The dependence of the cross-correlation signal on VSD, for the two

QDs at resonance, is shown in Fig. 4c. Applying a perpendicular magnetic eld, B 0.2 T, quenches the superconductivity and

thus eliminates the (positive) correlation between the drains current uctuations (Fig. 4b).

DiscussionAccording to our model, the non-local conductance is expected to be proportional to tLtR, and DGR DGL. This was not observed.

Although we do not understand the reason for this discrepancy, it might be related to a reduction in the two-electron transport, be it Cooper pairs or sequential two-electron transport, accompanying the single electron transport. Near the resonance of the QD, charge uctuations and thus wide frequency range potential uctuations are dominant and can partly dephase the neighbouring dot (see such saw-tooth behaviour in Fig. 3d), possibly affecting the higher order two-electron transport.

The spectral density of the cross-correlation signal at zero temperature is given by SCC oDILDIR4 D 2eICAR(1 t), with

ICAR the single electron current (due to cross-Andreev reections) on one side17. As ICAR/IAR B0.14 (0.04/0.3 seen in Fig. 3c for the

4 NATURE COMMUNICATIONS | 3:1165 | DOI: 10.1038/ncomms2169 | http://www.nature.com/naturecommunications

Web End =www.nature.com/naturecommunications

& 2012 Macmillan Publishers Limited. All rights reserved.

NATURE COMMUNICATIONS | DOI: 10.1038/ncomms2169 ARTICLE

b c

a G (e2/h) G (e2/h)A2Hz1

A2Hz1

0.40.2

1029

0.40.2

1029

12

G

R

10

GR

10

GL

G

10

Spectral density ( 1029 A 2 Hz 1 )

2

0

L

8

6

4

2

0

0T

0.085T

0.2T

8

6

4

2

0

8

6

4

20 V

10 V

5 V

0.56

0.205

0.559

0.557

0.555 0.215

0.205 0.21

2 20 0 20

0.21

VSD (V)

VLG (V)

0.555 0.215

V RG (V)

VLG (V)

V RG (V)

Figure 4 | Noise cross correlation. Cross-correlated noise measured by a cross-correlation setup. The current uctuations from the normal contacts are ltered by two individual inductor-capacitor (LC) circuits with a matched resonant frequency of 725 kHz with the amplied signals (by cold ampliers) fed into an analogue signal multiplier (at 725 kHz with a band width of 100 kHz). (a) Positive cross-correlation signals for bias voltages, VSD 20, 10 and 5 mV

between left side (Coulomb blockade peak of the QDL at left local gate voltage, VLG 0.557 V) and right side (Coulomb blockade peak of the QDR at

right local gate voltage, VRG 0.21 V). (b) Cross-correlation signal for magnetic elds, B 0, 0.085 and 0.2 T (VSD is kept at 20 mV). (c) Power spectral

density of cross-correlation, SCC as a function of VSD, measured when both dots are at resonance. Error bars are calculated from the standard deviations.

resonance peaks near VLG 0.557 V and VRG 0.21 V) and

IAR 500 pA at VSD 30 mV, ICAR 70 pA. Hence, we expect SCC

B1.7 10 29 A2 Hz 1. However, the experimentally obtained

SCC B710 10 29 A2 Hz 1 is more than four times higher

than the estimated value. This discrepancy may be attributed to an under-estimated value of Cooper pair splitting efciency that is deduced from the non-local conductance measurement.

In conclusion, we have shown Cooper pair splitting efciency with ratio approaching unity of split pairs vs unsplit pairs by measuring average and time-dependent cross-correlations of two electron transport. To prove the entanglement, one still needs to show Bells inequality by measuring the coherence and spin correlation using ferromagentic contacts.

Methods

Device fabrication. The sample was fabricated on a thermally oxidized Si. The nanowire was suspended on three gold pillars, B50 nm high. Two lower gold pillars, B25 nm high, provided local gating on both sides of the wire. After wires spreading from the ethanol solution, the source and drain regions were etched by ammonium polysulphide ((NH4)2Sx1.5M) to remove the native oxide, and were

immediately transferred into the evaporation chamber. For normal-metallic contacts 5/100 nm Ti/Au were evaporated, whereas for the superconducting contact 5/ 100 nm Ti/Al was used.

Measurement technique. The superconducting contact was biased by a voltage source, DC or AC (the voltage divider was placed on the cold nger, see Supplementary Fig. S1), with an AC excitation voltage of B2 mV. Conductance measurements were conducted at a rather low frequency (room temperature current preamplier at 575 Hz; gain 107, input impedance B200 ohm and current noise B50 fA/

Hz

p ), as well as at a higher frequency (cooled to 1 K voltage preamplier at 725 kHz, gain 2.5, voltage noise B500 pV/

Hz

p and current noise

p ). At the input of the cold preamplier, a LCR circuit determined the frequency 725 kHz and the bandwidth (100 kHz). The amplied signal is fed to a room temperature NF amplier, followed by a spectrum analyser with a bandwidth of 30 kHz. To switch between the two measurement systems a Relay, placed at the base temperature, was operated by a 100-ms, 1 V pulse. For cross-correlation measurements, we employed an analogue cross-correlation setup, where the signals from the two NF ampliers were multiplied and measured by a digital multimeter (at a bandwidth of 100 kHz).

References

1. Einstein, A., Podolsky, B. & Rosen, N. Can quantum-mechanical description of physical reality be considered complete? Phys. Rev. 47, 777780 (1935).

2. Aspect, A., Grangier, P. & Roger, G. Experimental realization of Einstein-Podolsky-Rosen-Bohm Gedanken experiment: a new violation of Bells inequalities. Phys. Rev. Lett. 49, 9194 (1982).

3. Ou, Z. Y. & Mandel, L. Violation of Bells inequality and classical probability in a two-photon correlation experiment. Phys. Rev. Lett. 61, 5053 (1988).

4. Beckmann, D., Weber, H. B. & Lohneysen, H. V. Evidence for crossed Andreev reection in superconductor-ferromagnet hybrid structures. Phys. Rev. Lett. 93, 197003 (2004).

5. Russo, S., Kroug, M., Klapwijk, T. M. & Morpurgo, A. F. Experimental observation of bias-dependent nonlocal Andreev reection. Phys. Rev. Lett. 95, 027002 (2005).

6. Zimansky, P. C. & Chandrasekhar, V. Nonlocal correlations in normal-metal superconducting systems. Phys. Rev. Lett. 97, 237003 (2006).

7. Kleine, A., Baumgartner, A., Trbovic, J. & Schonenberger, C. Contact resistance dependence of crossed Andreev reection. Europhys. Lett. 87, 27011 (2009).

8. Hofstetter, L., Csonka, S., Nygard, J. & Schonenberger, C. Cooper pair splitter realized in a two-quantum-dot Y-junction. Nature 461, 960963(2009).

9. Herrmann, L. G., Portier, F., Roche, P., Yeyati, A. L., Kontos, T. & Strunk, C. Carbon nanotubes as cooper-pair beam splitters. Phys. Rev. Lett. 104, 026801 (2010).

10. Wei, J. & Chandrasekhar, V. Positive noise cross-correlation in hybrid superconducting and normal-metal three-terminal devices. Nat. Phys 6, 494498 (2010).

11. Hofstetter, L. et al. Finite-bias cooper pair splitting. Phys. Rev. Lett. 107, 136801 (2011).

12. Recher, P., Sukhorukov, E. V. & Loss, D. Andreev tunneling, Coulomb blockade, and resonant transport of nonlocal spin-entangled electrons. Phys. Rev. B 63, 165314 (2001).

13. Torrs, J. & Martin, T. Positive and negative Hanbury-Brown and Twiss correlations in normal metal-superconducting devices. Eur. Phys. J. B 12, 319322 (1999).

14. Choi, M. S., Bruder, C. & Loss, D. Spin-dependent Josephson current through double quantum dots and measurement of entangled electron states. Phys. Rev. B 62, 13569 (2000).

15. Lesovik, G. B., Martin, T. & Blatter, G. Electronic Entanglement in the Vicinity of a Superconductor. Eur. Phys. J. B 24, 287290 (2001).

16. Chevallier, D., Rech, J., Jonckheere, T. & Martin, T. Current and noise correlations in a double-dot Cooper-pair beam splitter. Phys. Rev. B 83, 125421 (2011).

17. Bignon, G., Houzet, M., Pistolesi, F. & Hekking, F. W. J. Current-current correlations in hybrid superconducting and normal-metal multiterminal structures. Europhys. Lett. 67, 110116 (2004).

18. Melin, R., Benjamin, C. & Martin, T. Positive cross correlations of noise in superconducting hybrid structures: roles of interfaces and interactions. Phys. Rev. B 77, 94512 (2008).

B20 fA/

Hz

NATURE COMMUNICATIONS | 3:1165 | DOI: 10.1038/ncomms2169 | http://www.nature.com/naturecommunications

Web End =www.nature.com/naturecommunications 5

& 2012 Macmillan Publishers Limited. All rights reserved.

ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms2169

19. Kretinin, A., Popovitz-Biro, R., Mahalu, D. & Shtrikman, H. Multiple Fabry-Perot conductance oscillations in suspended stacking-Faults-free InAs nanowires. Nano Lett. 10, 34393445 (2010).

20. de-Picciotto, R. et al. Direct observation of a fractional charge. Nature 389, 162164 (1997).

21. Jehl, X., Sanquer, M., Calemczuk, R. & Mailly, D. Detection of doubled shot noise in short normal-metal superconductor junctions. Nature 405, 5053 (2000).

22. Leoch, F., Hoffmann, C., Sanquer, M. & Quirion, D. Doubled full shot noise in quantum coherent superconductor-semiconductor junctions. Phys. Rev. Lett. 90, 067002 (2003).

23. Choi, B.-R. et al. Shot-noise and conductance measurements of transparent superconductor two-dimensional electron gas junctions. Phys. Rev. B 72, 024501 (2005).

24. Bid, A. et al. Observation of neutral modes in the fractional quantum Hall regime. Nature 466, 585590 (2010).

25. Field, M. et al. Measurements of Coulomb blockade with a noninvasive voltage probe. Phys. Rev. Lett. 70, 13111314 (1993).

Acknowledgements

We thank Yunchul Chung for contributing to the cross-correlation measurement. We also thank Hyungkook Choi, Nissim Ofek, Ron Sabo, Itamar Gurman and Hiroyuki Inoue for their technical help. We are grateful to Christian Schnenberger and Yuval Oreg for their helpful discussions. M.H. acknowledges partial support from the European Research Council under the European Communitys Seventh Framework Programme (FP7/2007-2013)/ERC Grant agreement no. 227716, the Israeli Science Foundation (ISF),

the Minerva foundation, the German Israeli Foundation (GIF), the German Israeli Project Cooperation (DIP) and the US-Israel Bi-National Science Foundation (BSF). H.S. acknowledges partial support from the Israeli Science Foundation Grant 530-08 and Israeli Ministry of Science Grant 3-66799.

Author contributions

A.D. and Y.R. contributed to the sample design, device fabrication, setup, data acquisition, analysis and paper writing. M.H. contributed to the design, data interpretation and paper writing. D.M. contributed to electron beam lithography, lithography technique. A.V.K. contributed to fabrication techniques, initial measurements and comments on the manuscript. H.S. contributed to MBE growth and structural study of InAs nanowires by Au-assisted VLS, discussions and manuscript editing.

Additional information

Supplementary Information accompanies this paper at http://www.nature.com/naturecommunications

Web End =http://www.nature.com/ http://www.nature.com/naturecommunications

Web End =naturecommunications

Competing nancial interests: The authors declare no competing nancial interests.

Reprints and permission information is available online at http://npg.nature.com/reprintsandpermissions/

Web End =http://npg.nature.com/ http://npg.nature.com/reprintsandpermissions/

Web End =reprintsandpermissions/

How to cite this article: Das, A. et al. High-efciency Cooper pair splitting demonstrated by two-particle conductance resonance and positive noise cross-correlation. Nat. Commun. 3:1165 doi: 10.1038/ncomms2169 (2012).

6 NATURE COMMUNICATIONS | 3:1165 | DOI: 10.1038/ncomms2169 | http://www.nature.com/naturecommunications

Web End =www.nature.com/naturecommunications

& 2012 Macmillan Publishers Limited. All rights reserved.