1 Quantum control of individual electron and nuclear spins in diamond lattice Mikhail Lukin Physics Department, Harvard University Collaborators: L.Childress, M.Gurudev Dutt, J.Taylor, D.Chang, L.Jiang,A.Zibrov (Harvard) A.Sorensen (Niels Bohr Institute) P.Hemmer (Texas A&M) F.Jelezko, J.Wrachtrup (Stuttgart) $ ARO-MURI, NSF, Sloan and Packard Foundations QuickTime and a TIFF (Uncompressed) decompressor are needed to see this picture.
2 Today s talk Introduction: quantum control from AMO to solid-state Quantum control of single electron spins System: electron spin in Nitrogen Vacancy (NV) center in diamond Understanding mesoscopic environment of single electron spin: Quantum control of single nuclear spins Scaling up using quantum optics New ideas and new tools 13 C nuclei as a leading decoherence source Single nuclear spin addressing via quantum back-action from electron Coherent coupling of electron and nuclear spin qubits NMR on a single spin
3 Quantum control: from AMO to solid state Solid-state quantum systems New opportunities: tight confinement, fast operations strong interactions easy entanglement New challenges: understand complex solid-state environment develop quantum control techniques Electron spin as quantum bit Promising, stable qubit candidate We show that interaction with lattice nuclei determine coherence properties and that (nuclear spin) environment can be understood, controlled and utilized as a resource
4 System: NV color center in high-purity diamond Nature s own trapped molecular ion Optical properties Sharp zero-phonon emission 637 nm + phonon sidebands nm Can be excited either by 637 nm or 532 nm Ground state properties non-zero electronic spin (S=1) microwave transition: zero-field splitting Δ=2.88 GHz detectable via state-selective fluorescence Phonon Relaxation S z =1 S z =0 Early work: S.Rand, N.Manson
5 Experimental probing of single centers Permanent magnet 3-axis Hall sensor 20 μm copper wire MW N.A. 1.3 oil immersion objective 532 nm Dichroic fluorescence APD APD Scanning confocal microscope image Single photon source: ~100,000 cts/sec (Paris, Munich,..)
6 Preparation and probing of single spins e 1 Phonon emission e 0 e 1 S Optical excitation: State selective fluorescence Electron spins polarization (>90%) 1 1 enables Manipulation spin states using microwave ESR Detection using spin-dependent fluorescence at room temperature GHz
7 Rabi oscillations of single electron spin First experiments F.Jelezko, J.Wrachtrup (Stuttgart) Contrast limited by competition of optical pumping and shelving effects (improves with efficient collection) Calibrates measurement efficiency for other data
8 Single spin coherence via Ramsey spectroscopy Classic probe of coherence properties 8 MHz MW detuning Overall decay on a 1-2 μs scale Beating of modulation components Each center is different! Signatures of complex, non-markovian environment: can we understand, control it?
9 Mesoscopic environment of NV center Stiff, mostly spinless 12 C lattice Possible contributors to dephasing: Contact hyperfine interaction: single N nuclear spin (I=1) Coupling to remote spins, e.g.: N C 13 nuclear spins (few percent) N (impurity) electron spins: need pure samples Theory: mesoscopic spin bath local (N) non-local nuc effective nuclear field
10 Dephasing of electron spin by mesoscopic environment B nuc S High nuclear temperature => random nuclear field nuc Effect of surrounding spins: Few spins => few modulation frequencies Many spins => dephasing Dephasing: a large number (1000s) of nearby spins contribute for 1.1% abundance 13 C Nuclear spins evolve slowly: long correlation time of the environment
11 Probing single electron coherence via Typical echo decay spin echo B nuc S x E.Hahn 1950s The spin echo signal is only sensitive to changes in the environment which happen faster than τ results in much longer coherence a tool to understand the dynamics of the local environment
12 Single spin echo in magnetic field Echo envelope on longer time scale Big effect: T 2 >200 μs ~ 200 T 2*! periodically collapses and revives Suggests periodic dynamics of electron environment
13 Dynamics of spin environment in B field Main reason for echo evolution: changes in B nuc Nuclear Larmor evolution leads to de-correlation of B nuc re-phasing after one Larmor period Echo revives when waiting interval matches Larmor period! B eff S x L. Childress, M.G.Dutt, J.Taylor et al, Science, in press
14 A picture of single electron environment: 13 C nuclei in diamond lattice 13 C Environment as a resource: can we address and use individual isolated 13 C nuclei in diamond lattice?
15 Coupled electron-nuclear system: more detailed view Hyperfine interaction of electron with single nuclear spin I (j) S I (j) In weak B field electron dynamics is conditional upon nuclear state and vice versa => entanglement Echo signal resulting from single nucleus S j 1 Larmor collapse and revival hyperfine Larmor r j m s 0 Many spins in bath τ Fast modulation (hyperfine) displays coherent electron-nuclear spin dynamics Decoherence!
16 Proximal nuclei are special: back-action from electron Atomic physics of spin bath Two effects of electron on proximal nuclei due to hyperfine field: larger hyperfine splitting enhanced Larmor frequency Hyperfine coupling enhances nuclear magnetic moment Hyperfine interaction mixes electronic and nuclear states: response to B-field given by g-tensor Substantial enhancement even for weak mixing since Proximal nuclei are very special: they precess faster electron moment hyperfine level mixing
17 Coherent dynamics involving individual nuclear spins Larmor envelope L. Childress, M.G.Dutt, J.Taylor et al Fast hyperfine modulation Each NV center has its own, specific dynamics Most display periodic entanglement and disentanglement with (some) isolated nuclear spin(s)
18 Allows to determine coupling constants for individual nuclear spins, information on positions Coupling to proximal 13 C: nuclei-resolving microscope Echo modulation frequencies for different centers Fast component 13 C level mixing and μ enhancement depend on relative distance and B-field orientation relative to S-I axis! Confirms the model
19 Coherent, deterministic coupling to isolated single nuclear spins in diamond lattice S I (j) Theory Addressing of well-defined, isolated nuclear spins possible (up to 3) Coherent, controllable two-qubit (or few qubit!) system L. Childress, M.G.Dutt, J.Taylor, A.Zibrov, F.Jelezko, J.Wrachtrup, P.Hemmer and M.D.L, Science, in press (2006)
20 Watching single nuclear spin precession Pulse sequence to map qubits between electron to nuclear spin MW NV E 20 Gauss π/ω j,0 π/2 π/ω j,1 π/2 Example: single nuclear spin dynamics M.Gurudev Dutt. L.Childress et al polarize Larmor delay (μs) polarize measure nuclear polarization Larmor precession mapping to electron NMR on single, isolated nuclear spin!
21 Nuclear spin as robust quantum memory Second long coherence times are expected for isolated nuclear spins! How well is 13 C really isolated (e.g. from nearby electron)? Example: cycling electron with green light during nuclear precession precession amplitude Preliminary data B = 20 Gauss >200 photons scattered green pulse duration, μs Isolated nuclear qubit: potential for excellent quantum memory
22 Outlook: useful quantum processors from two qubits Quantum emitter Electron spin Nuclear spin Electron entanglement via photon scattering & single photon interference Deterministic quantum gates between remote nuclei via electron-nuclear coupling: high fidelity and rates for modest collection efficiency Operations between any pairs can be performed simultaneously: purely optical scaling for quantum communication & computation Theory: L.Duan, M.Lukin, I. Cirac, P.Zoller Nature 2001, atomic ensembles L.Duan, C.Monroe (ions) L.Childress, J.Taylor, A.Sorensen. MDL PRL 06, quantum repeaters L.Jiang et al, (06) deterministic computation
23 Summary: single spin environment can be understood, controlled and utilized Coherent manipulation of single electron spin using AMO techniques Understanding mesoscopic environment of single electron via spin echo Coherent manipulation of individual proximal nuclei: single spin NMR Quantum optical techniques for wiring up multi-qubit systems - A new frontier on the interface of AMO and cond-mat physics
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