Optical Pumping of a Single Electron Spin Bound to a Fluorine Donorin a ZnSe NanostructureDarin J. Sleiter,*,† Kaoru Sanaka,†,‡ Y. M. Kim,¶ Klaus Lischka,¶ Alexander Pawlis,*,¶,†

and Yoshihisa Yamamoto†,‡

†E. L. Ginzton Laboratory, Stanford University, Stanford, California 94305, United States‡National Institute of Informatics, Hitotsubashi 2-1-2, Chiyodaku, Tokyo 101-8403, Japan¶Department of Physics, University of Paderborn, Warburger Strasse 100, 33098 Paderborn, Germany

ABSTRACT: Here we demonstrate optical pumping of asingle electron within a semiconductor nanostructurecomprised of a single fluorine donor located within a ZnSe/ZnMgSe quantum well. Experiments were performed to detectoptical pumping behavior by observing single photons emittedfrom the nanostructure when the electron changes spin state.These results demonstrate initialization and read-out of theelectron spin qubit and open the door for coherent opticalmanipulation of a spin by taking advantage of an unconventional nanostructure.

KEYWORDS: Optical pumping, donor, qubit, initialization, spin measurement

Many proposed schemes for quantum informationprocessing require scalable and homogeneous localized

quantum bits (qubits) that can efficiently interface with singlephoton flying qubits.1−5 Trapped ion qubits are extremelyhomogeneous and have a nearly perfect optical quantumefficiency,6−8 but scalability is inherently difficult due to theneed to trap and cool the ions. On the other hand,semiconductor nanostructures provide a much more scalableenvironment for quantum information. Electron spin qubitstrapped in self-assembled InGaAs quantum dots (QDs) have ahigh optical quantum efficiency and fast manipulation times,but suffer from inhomogeneity due to their wide distribution insize when grown naturally.9−12 Charged nitrogen-vacancy (NV)centers in diamond are more homogeneous than quantum dots,but are more difficult to structure than more conventionalsemiconductors and have low optical quantum efficiency.13−16

Electron spins bound to individual donors in direct-bandgapsemiconductors, however, are naturally homogeneous with ahigh-optical quantum efficiency and in addition provide ascalable system.17−21

Spin manipulation in QDs is typically carried out via acoupled optical lambda system provided by the ground-statetransition of the QDs. A single electron trapped by a fluorinedonor in ZnSe forms a similar lambda system as used in QDsand has many attractive characteristics. The optical quantumefficiency of the donor system is very close to unity21 and canbe used to generate triggered single photons.19 Furthermore, ithas recently been shown that photons from two independentdonor nanostructures that each contain an isolated fluorineatom can be indistinguishable and entangled throughpostselection.22 While coupling between the electron spinand the nuclear spin of the host crystal is the leadingdecoherence-causing mechanism in QDs,23 isotopic purification

can be used to deplete the ZnSe host crystal of nuclear spins.This is not possible in III−V semiconductors but has been verysuccessful in extending the decoherence times of electrons indiamond and Si.25,26 Furthermore, F has a natural 100%abundance of spin-1/2 nuclei, which could be used as anadditional qubit naturally coupled to the electron qubit.18,27,28

Finally, it has been recently shown that an ensemble of fluorinedonors in bulk ZnSe features long electron-spin dephasingtimes T2*, greater than 30 ns for temperatures up to 40 K.29

Optically active donors have also been successfully implantedusing ion implantation, which could eventually lead todeterministically placing single donors in specific locations.20

All of these characteristics make the ZnSe/F system anappealing qubit candidate, particularly for quantum repeaterschemes.Here we present the experimental demonstration of optical

pumping of the spin of a single electron trapped by a fluorinedonor placed within a ZnSe/ZnMgSe quantum well nanostruc-ture. This can be used to initialize the spin qubit to a knownstate, which is the first step of most quantum informationprocessing schemes,1,2,5 and to provide a measurement (read-out) of the spin state. Here, we first describe the optical systemof the donor-bound electron along with the nanostructure usedto isolate the electron. Next, we present the evidence indicatingthat we address a single F-bound electron. Finally, we describethe optical pumping experiments we performed and discuss theexperimental results that clearly indicate single electron opticalpumping.

Received: October 2, 2012Revised: November 20, 2012Published: December 7, 2012

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When replacing a Se atom in ZnSe, a F atom will act as adonor (Figure 1a). The donor nucleus provides a shallow

potential well that is able to trap the donated electron with abinding energy of 29.3 meV in bulk,2,5,30 creating the neutraldonor state (D0) shown in Figure 1c. At sufficiently lowtemperatures (all of the experimental data presented here wereobtained at temperatures between 2 and 5 K), a F donor cantrap an exciton in addition to the electron with a binding energyof 5 meV in bulk,24 creating the donor-bound exciton state(D0X). In bulk ZnSe, the D0X to D0 transition has an energy of2.8 eV.24 Note that these emission and binding energies can benoticeably larger when the fluorine donors are located withinthe excitonic confinement of a quantum well (QW).Therefore, we combine the donor with a QW to form a

donor-QW hybrid nanostructure that retains much of thedesirable features of the donor system while allowing us toisolate a single donor-bound electron. The ZnSe/F nanostruc-ture design used here (Figure 1b) is the same described in ref18, where the isolation of a single donor-bound electron wasconfirmed. The sample used in the experimental resultspresented here consisted of a 2-nm ZnSe QW delta-dopedwith fluorine and enclosed by Zn1−xMgxSe (x ≈ 17%) claddinglayers grown by molecular-beam epitaxy (MBE). The fluorineareal δ-doping concentration was approximately 3 × 1010 cm−2,resulting in 2.4 donors on average per 100 nm diameter mesaafter structuring. The mesas were separated by 10 μm, ensuringwe could optically isolate photoluminescence (PL) from theindividual mesa nanoemitters.When placed in a magnetic field, both the D0X and D0 states

split due to Zeeman splitting (Figure 1d). In the D0 state, thespin-1/2 electron splits into two states. In the D0X state, thetwo electrons combine together to form a spin singlet, and sothe spin of the state is determined entirely by the spin of the

hole. In our samples, the light-hole states are split off to higherenergy by confinement and strain in the nanostructure, leavingonly the heavy-hole ±3/2 spin states to be considered. Theoptical system of interest is thus composed of two D0 states andtwo D0X states. The orientation of the magnetic fielddetermines the allowed optical transitions.18 Unless otherwisenoted, all of these results were obtained in Voigt geometry,where the magnetic field is perpendicular to the growthdirection and the optical access direction. Because of the fullyconnected pair of lambda systems in Voigt geometry, the spinof the D0 electron can be manipulated optically by transitionsthrough the D0X state.To confirm the presence of isolated F donors, we observed

the PL spectra of the nanodevices using a spectrometer andabove-band (∼410 nm) pulsed excitation (Figure 2). We first

looked for a mesa with a single peak in the PL spectra with theexpected separation from the free exciton peak (5−15 meV)that is associated with the F-bound exciton transition in a QW.Saturation of this peak with increased pump power wasobserved, indicating that the excited state has a finite lifetimeand number of occupiable states. Next, we used magneto-photoluminescence spectra to confirm the Zeeman splittingand polarization selection rules of the donor-bound excitontransition in both Faraday and Voigt geometry. We determinedthe electron and hole g-factors to be |ge| = 1.3 ± 0.3, |gh

⊥| = 0.0± 0.1 and |3gh

∥ − ge| = 0.9 ± 0.1, which is in good agreementwith previous results.18 Finally, we observed a value of 0.22 ±0.06 in the normalized two photon correlation function g(2)(τ)at zero delay (without background subtraction), well below the0.5 threshold for a single emitter. Together, these pieces ofevidence, all taken from the same mesa, give very highconfidence that this individual peak corresponds to the D0X toD0 transition of a single F-bound electron.Optical pumping was performed in Voigt geometry with a

magnetic field of 7 T by applying a continuous wave (CW)laser resonant with one of the D0X optical transitions, shown inFigure 3a. If the electron begins in state |1⟩, it will be excited to|e1⟩, where it will then decay back into either |1⟩ or |0⟩. If itfalls into |1⟩, it will be re-excited to |e1⟩ and will eventually endin |0⟩. If the electron begins in |0⟩, the laser is far off resonancewith any of the optical transitions connected to that state, andthe electron will remain in state |0⟩. In order to observe theoptical pumping, we collected photons from the |e1⟩ → |0⟩transition. A single photon is emitted on this transition for each

Figure 1. (a) Crystal structure showing F acting as a donor whenreplacing a Se atom. (b) Schematic of the nanostructure, showing theQW with delta-doped F in the center. After etching into 100 nmmesas, there are only 2.4 donors per nanostructure on average. Thepassivation coating is added in order to protect the structure duringthermal cycles. (c) Level diagram of the QW showing the D0 and D0Xstates. The dotted lines indicate the energies of the lowest QWparticle-in-a-box states. (d) Zeeman splitting of the D0 and D0X statesin Voigt geometry with a magnetic field of 7 T. The dotted lines showthe allowed optical transitions and H/V indicates the respectivephoton polarization.

Figure 2. Simplified diagram of the experimental setup. The optical-pumping and spin-randomization lasers are incident on thenanostructure in a 4 K cryostat. Emitted photons are polarizationfiltered with a polarizing beam splitter (PBS) and frequency filteredusing a pair of optical gratings and slits and then detected by either asingle photon-counting module (SPCM) or a spectrometer.

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optical pumping event. By accumulating photons from manyindividual optical pumping experiments, we determined theoptical pumping rate and final population of state |0⟩.The timing of each individual optical pumping experiment

was determined by the repetition rate of our pulsed spin-randomization laser. Every 13 ns, a picosecond above-band(∼410 nm) laser pulse puts the electron into either |e0⟩ or |e1⟩,randomly selected with approximately equal probability. Theelectron then decays into either |0⟩ or |1⟩, thus randomizing thespin state regardless of which state the electron was in prior tothe spin-randomization pulse. The resonant spin-pumping laseris kept on continuously. Photons from the |e1⟩ → |0⟩ transitionare filtered out by polarization using a polarizing beam splitter(PBS) and by frequency using a pair of optical gratings andslits, as shown in Figure 2, and are then sent to either aspectrometer or a single photon counting module (SPCM).While the SPCM has timing resolution and better quantumefficiency, the additional grating and slits of the spectrometersignificantly reduced the background noise due to photonsscattered from the optical-pumping laser, which was separatedby only 150 GHz from the signal photons.For each 13-ns experiment, assuming the electron within the

nanostructure is excited every pulse, there is a 25% chance thata photon will be emitted from the |e1⟩ → |0⟩ transition due tospontaneous emission just after the above-band pulse. In theabsence of optical pumping, this is the only way a photon canbe emitted from that transition. There is also a 50% chance thatthe electron will decay to |1⟩ following the above-band pulse. Ifoptical pumping is strong enough, an electron in state |1⟩ willalways be optically pumped within the 13-ns experimentalwindow, emitting a photon along the |e1⟩ → |0⟩ transition.Therefore, 75% of the experiments will emit a single photonwith strong optical pumping, compared to only 25% inexperiments without optical pumping. The ratio γ between

the count rate of single photons when the optical-pumpinglaser is on versus when it is off is a good measure of opticalpumping, and it can take a value between 1 (no opticalpumping occurring) and 3 (saturated optical pumping).This increase in the average number of photons emitted per

experiment has been observed in our sample. In one set ofexperiments using the spectrometer, we measured 30.2 ± 0.8average counts/sec with only the spin-randomization laser on,13.0 ± 0.6 average counts/sec with only the optical-pumpinglaser on, 95.0 ± 3.0 average counts/sec with both lasers on, anda background of −0.2 ± 0.1 average counts/sec with both lasersoff. Note that these measurements were taken within the linearresponse region of the spectrometer CCD and the count ratewas calibrated against an optical power meter so that thespectrometer counts were proportional to the actual photoncounts. In addition, the average measurement values andstandard deviation were calculated statistically using multipledata sets, resulting in standard deviations smaller than the shot-noise expected for the CCD. Using the data from theseexperiments, we measure the optical pumping ratio γ to be(95.0−13.0)/(30.2 + 0.2) = 2.7 ± 0.1, indicating near-saturation optical pumping. We observed an optical pumpingratio larger than 2.3 when pumping on any of the four opticaltransitions, showing the fully connected nature of the opticalsystem.The reason we observe γ ≈ 2.7 instead of 3 is due to the

relationship between the power of the two laser incident on thenanostructure. At low spin-randomization laser powers, theD0X state is not created after every pulse, and so the electronstate and the experiment are not reset. This serves to increasethe average length of the experiment T and does not change thepumping rate from |1⟩ → |0⟩. However, at low optical-pumpinglaser powers, the rate of optical pumping R is slow, reducing theprobability of an electron in |1⟩ being pumped into |0⟩ withinthe experimental window. Together, R and T determine γ.

γ = + − −e1 2(1 )TR(1)

α=R P1 (2)

β=

−T

13[ns]1 P2 (3)

where P1 is the power of the CW optical-pumping laser and P2is the average power of the pulsed spin-randomization laser. αand β are parameters that determine the relative effect of eachlaser (note that β must be positive and less than 1). Theexperimental power dependence of γ is shown in Figure 3c,d.The theory curves are from eq 1 with α and β as fittingparameters.Optical pumping is also a resonant effect, as seen in Figure

3b. In this set of data, the pulsed laser was held fixed while theoptical-pumping laser was set at different wavelengths withconstant pumping power. The best-fit curve is a Gaussian witha full-width at half-maximum (fwhm) of 58 GHz. This is 25times larger than the lifetime-limited line width of 2.3 GHz (fora 70-ps lifetime19). The Gaussian shape and large line widthsuggest fast spectral diffusion is occurring, resulting in time-ensemble averaged measurements. The cause of the spectraldiffusion has not been systematically studied, but wehypothesize it is caused by Stark shifting from chargefluctuations due to the combination of surface charge trapscreated through mesa structuring and the large number of freecarriers created with the above-band laser pulses. The power

Figure 3. Optical pumping behavior. (a) Level diagram of thenanostructure-bound electron showing which transition is opticallypumped and which transition is monitored for single photon emission.(b) The optical pumping ratio γ versus the wavelength of the optical-pumping laser. γ is the ratio of the single photon emission count ratewith the optical-pumping laser on to the count rate with the optical-pumping laser off. (c) γ versus the power of the optical-pumping laserwith the spin-randomization laser power fixed. (d) γ versus the powerof the spin-randomization laser with the optical-pumping laser powerfixed.

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density of the above-band laser pulses incident on the donorused in the results presented here was 104−105 W/cm2. This is2−3 orders of magnitude larger than the power density used inref 19, where the line width was nearly lifetime-limited sinceindistinguishability was observed. Both experimental results areconsistent with our hypothesis for the cause of the large linewidth seen here.We also observed the time-dependent behavior of optical

pumping, as shown in Figure 4. The data show an initial pulse

due to photons collected from the spontaneous emission fromthe |e1⟩ → |0⟩ transition after the spin-randomization pulse,followed by 13 ns of optical pumping. The signature of opticalpumping is observed in the difference between the dataobtained with both lasers on, Figure 4b, and the data obtainedwith just the spin-randomization laser on, Figure 4a.The data were modeled using a Monte Carlo simulation

which followed the state trajectory of the electron opticalsystem both with and without optical pumping (Figure 4,red curves). The simulation first initialized the electron into |e0⟩or |e1⟩ with equal probability at t = 0. It was then transferredbetween the four states according to the allowed transitionrates, and the time of jumps between |e1⟩ and |0⟩ was recordedas well as the final electron state after 13 ns. The allowedtransitions were (1) Rsp(|ei⟩ → |j⟩) (i,j = 0,1) spontaneousemission from either D0X state |ei⟩ into either D0 state |j⟩ withequal rate determined by the measured excited state lifetime ofτ = 70 ps19 and (2) Rst(|ei⟩ ↔ |j⟩) stimulated emission along allfour transitions due to the optical-pumping laser.

τ| ⟩ → | ⟩ =R e j( )

12isp (4)

| ⟩ ↔ | ⟩ = σ−ΔR e j RXe( )ist/22 2

(5)

where R is the effective optical pumping rate and X accounts forthe relative polarization of the optical-pumping laser and theoptical transition. σ is the standard deviation associated with the58 GHz Gaussian line width determined in Figure 3 and Δ isthe energy detuning between the laser and the opticaltransition. The laser was on resonance with the |e1⟩ → |0⟩transition and had a polarization of |H⟩ with an experimentallymeasured extinction ratio of 2000. The detuning Δ from theother transitions was determined by the measured electron andhole g-factors. A large background count rate due to scatteringof photons from the optical-pumping laser into the SPCM ispresent in the optical-pumping data, which could not becompletely filtered out. Therefore, the mean of the background,

measured with just the resonant laser on, was added to theexperimental data with no optical pumping and to thesimulated data before fitting.This Monte Carlo simulation was used to create a series of

simulated data sets by varying two parameters: the total numberof simulated experiments N, and the optical pumping rate R.The maximum-likelihood method was then used to find thebest fit to the experimental data, and the χ2 test was used todetermine an interval of confidence.31 From this analysis, weobtained a maximum likelihood pumping rate of 0.14 GHz with95% confidence that the pumping rate is between 0.008 and0.75 GHz. The likelihood sharply drops at even lower pumpingrates, giving 99% confidence that the pumping rate is greaterthan 0.006 GHz. We can infer from the best-fit simulation thatthe rate of transition from |1⟩ to |0⟩ was 1/15 ns−1, and thatafter 13 ns of optical pumping, the electron ends in state |0⟩ in79% of the experiments. This rate and resulting population waslimited by the available power of the optical-pumping laserincident on the sample in our setup and can likely be increasedin future experiments. Simulations with higher opticalpumping powers suggest this rate could be increased to greaterthan 1 ns−1, which is eventually limited by the lifetime of theD0X state.In conclusion, we have experimentally demonstrated optical

pumping of the single electron bound to a fluorine donor in aZnSe QW nanostructure through the D0X excited state toprepare one particular spin state of the electron qubit. Theexperiment shown here serves to initialize a spin qubit based ona single donor-bound electron in addition to indicating whichstate a qubit was in prior to optical pumping. Furthermore, thelatter shows that electrons bound to a ZnSe/F QWnanostructure have the same state structure and opticaltransitions as electrons in InGaAs quantum dots, which havebeen successfully used to perform single-qubit operations inthat system. The opportunities for nuclear spin depletion of thehost crystal and donor ion implantation, and the possibility ofusing the donor nucleus as a naturally coupled long-lived spinqubit makes the ZnSe/F nanostructure a particularly appealingqubit alternative to QDs. Future experiments will be focused oncoherent manipulation of the electron spin using fast opticalpulses and measurement of decoherence times in bothepitaxially doped and ion implanted nanostructures.

■ AUTHOR INFORMATION

Corresponding Author*E-mail: (D.J.S.) dsleiter@stanford.edu; (A.P.) apawlis@mail.upb.de.

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTS

This research was supported by the German ResearchFoundation (DFG), the NTT Basic Research Laboratories,the Commissioned Research of the Nation Institute ofInformation and Communications Technology (NICT), bythe Ministry of Education, Culture, Sports, Science andTechnology (MEXT), and through the Funding Program forWorld-leading Innovative Research and Development onScience and Technology (FIRST). We would also like tothank Zhe Wang for assistance in finding an appropriate mesa.

Figure 4. Experimental (blue) and best-fit simulated (red) photonarrival times during a 13 ns window in the case of (a) no opticalpumping and (b) optical pumping behavior. The simulated data wereaveraged over 10× more experiments than the experimental data,resulting in the reduced variance.

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