Entanglement creation and characterization in a trapped-ion quantum simulator

Transcription

1 Time Entanglement creation and characterization in a trapped-ion quantum simulator Christian Roos Institute for Quantum Optics and Quantum Information Innsbruck, Austria Outline: Highly entangled state or noisy mess? How Trapped-ion to characterize experiments: entangled time states scaleswith and>8 tools ions How Making coherent trapped is the ionsengineered interact with spin-spin each other interaction? Experimental characterization of entangled states Ion number

2 Time Entanglement creation and characterization in a trapped-ion quantum simulator Christian Roos Institute for Quantum Optics and Quantum Information Innsbruck, Austria Outline: Trapped-ion experiments: time scales and tools Making trapped ions interact with each other Experimental characterization of entangled states Ion number

3 Quantum physics with linear ion strings Ion trap Lasers Ion loading ~ minutes Ion creation Ion storage ~ day(s) Individual experiments ~ 20 ms (initialization coherent interaction, + measurement) ~ quantum measurements per day Laser cooling quantized ion motion Coherent excitation spin-spin interactions Ion detection quantum measurements

4 Quantum physics with linear ion strings Trap frequencies: focused laser beam d ~ 5 mm l ~ 700 nm Length scales ion distance d laser wavelength ion localisation > l >> >> 5 mm 700 nm z 0 Bohr radius a 0 10 nm 50 pm Spatially resolved fluorescence Individual addressing No direct state-dependent interactions between ions

5 How to make ions interact State-dependent interactions via Coulomb interaction (collective motional modes) + lasers / m-waves Rydberg interactions coupling to other quantum systems: photons (cavity-qed experiments) atomic quantum gases transmission lines Entangling quantum gates Many-body Hamiltonian

6 Time (ms) Ion number Realization of a transverse-field Ising model

7 Encoding a (pseudo-)spin in a trapped ion 40 Ca + P 1/2 D 5/2 Quantum state detection Qubit/ pseudo-spin 729 nm Quantum state manipulation S 1/2

8 Experimental setup Linear trap with anisotropic harmonic potential: linear strings of up to 20 ions Spatially resolved fluorescence: detection of individual spin states focused steerable laser beam: coherent single-spin manipulation beam switching time ~ 10 ms

9 Measuring spins and spin correlations any correlation can be measured. Measurement of p/2 -pulse (single)-spin rotation + fluorescence measurement

10 Quantum tomography: density matrix reconstruction any correlation can be measured. Quantum tomography: Bloch vector components 1 qubit: N qubits:

11 How to make spins interact with each other Coloumb interaction collective modes of ion motion Coloumb interaction + laser light variable-range effective spin-spin interaction

12 Coupling to transverse vibrational modes 18-ion spectra all modes laser-cooled to ground state Laser Spin-spin interaction by off-resonant laser coupling to vibrational modes Laser

13 ion number Variable-range interactions by coupling to transverse modes Example: 11 ions 'COM' 'Tilt' vibrational mode Spin-spin coupling J ij (Hz) strength decreases with distance antiferromagn. ferromagn. ion number

14 Excitation Protocol: Measurement of the coupling matrix i j 1. Initialize ions in state 2. Switch on Ising Hamiltonian 3. Measure coherent hopping rate ions 6+7 Coupling matrix Time (ms)

15 Spread of correlations after local quenches Ground state: all spins aligned with transverse field 1. Local quench: flip one spin 2. Spread of entanglement 3. Measure magnetization or spin-spin correlations

16 Time Spread of correlations after a local quench 15 ions P. Jurcevic et al., Nature 511, 202 (2014) P. Richerme et al., Nature 511, 198 (2014)

17 Spread of entanglement after a local quench z-magnetization single-site entropy 7 ions a 1.75 density matrix reconstruction of spins ms after the quench P. Jurcevic et al., Nature 511, 202 (2014)

18 Energy Creation of complex N-particle quantum states subspace dimension exponentially with N complex quantum states N/2 excitations 2 excitations quasi-particles: spin waves (delocalized excitation) ground state: all spins aligned with external field

19 Time Characterization of large complex entangled states Quantum state tomography of subsystems 20 ions 1 2 Matrix-product state tomography N. Friis et al., PRX 8, (2018) B. Lanyon et al., Nat. Phys. 13, 1158 (2017) Entropy measurements by random unitaries ms unpublished data 10 ions Complex quantum states as a resource for variational eigensolvers unpublished data 2 ms 1 ms 0 ms T. Brydges et al., manuscript in preparation Ion number

20 1 Spread of entanglement in the system 8 ions Neighbouring spins get entangled and disentangled with correlations spreading further out B. Lanyon, C. Maier et al., Nat. Phys. 13, 1158 (2017) with 20 ions: N. Friis et al., PRX 8, (2018)

21 Time (ms) Entanglement creation across the 8-spin chain z-magnetization dynamics Propagation of spin correlation unpublished data Ion number Ion number Entanglement between the ends of the chain!

22 2 N N-1 Quantum tomography of large multiparticle states with little entanglement N/2 1 0 At early times: n-qubit entangled state, with finite correlation length Strategy: find compact matrix product state representation of the global state by measuring local spin correlations T. Baumgratz et al, PRL 111, (2013) M. Cramer et al, Nat. Comm. 1, 149 (2010)

23 MPS tomography procedure Step 1: Search for MPS state that optimally reproduces the experimentally observed local spin correlations But what does the MPS state tell us about the state in the lab? Step 2: Find a certificate: determine minimum fidelity of the lab state with the MPS state reconstruction

24 B. Lanyon, C. Maier et al., Nat. Phys. 13, 1158 (2017) MPS tomography results for 8-spin quench Dashed lines: MPS reconstruction for model state Data points: Certified minimum fidelity of lab state with MPS state MPS tomography: Resource-efficient in the number of particles but: restricted to states with little entanglement

25 B. Lanyon, C. Maier et al., Nat. Phys. 13, 1158 (2017) State reconstruction: Direct fidelity estimation vs. lower fidelity bounds 14 ions Direct fidelity estimation

26 3 Characterization of complex entangled states How mixed is the quantum state? measure the purity (How close to unitary is the quantum dynamics generating the state?) How much entanglement is generated by non-equilibrium quantum dynamics? measure entanglement entropy von-neumann entropy Renyi entropy measurement of nonlinear functionals of the density matrix

27 Measuring the purity / second Renyi entropy S (2) Measurement options: quantum state tomography resources scale exponentially with system size joint measurement on two copies of a system difficult with ions A. K. Ekert et al., PRL 88, (2002) Islam et al., Nature 528, 77 (2015) random measurement on two virtual copies of a system average over: van Enk, Beenacker, PRL 108, (2012) A. Elben et al., PRL 120, (2018) random gate circuits global or local quenches

28 Random unitaries: single qubit Single qubit length of Bloch vector uniformly distributed Pure state: Fully mixed state:

29 Local random unitaries on multiple qubits Average over random measurements analytically known

30 z-measurement Local random unitaries: experimental realization Single-qubit random unitaries are realized by (a,b,c drawn from suitable distributions) : Concatenation of two such random unitaries to make the experiment robust against calibration errors. Resulting pulse sequence: qubit : global p/2 pulses : single-qubit z-rotations

31 Measurement scheme

32 Purity and Renyi entropy measurement unpublished data 5ms unpublished data 2ms 1ms 0ms Entanglement growth Total purity nearly invariant Number of projective measurements

33 Entropy measurements unpublished data Subsystems acquire high entropies over time (hard to measure) T. Brydges et al, manuscript in preparation

34 Interplay of interactions and disorder long range interaction local disorder potentials Clean system unpublished data unpublished data Disordered system T. Brydges et al, manuscript in preparation Mutual information (correlations) decaying with distance

35 4 Complex quantum states as a resource for variational quantum eigensolvers (VQE) Goal: find the ground state energy of the Hamiltonian Quantum-classical hybrid approach : use quantum co-processor for calculating <H> for a variational state classical computer for updating parameters of the variational state

36 VQEs for spin lattice Hamiltonians Mapping a 1d lattice Schwinger model to a spin lattice Hamiltonian: complicated spin-spin interaction

37 Time Preparing the Schwinger ground state in a trapped-ion experiment and measuring its energy State preparation: Energy measurement: Measurement of all 1- and 2-body correlations for determining : Schwinger target Hamiltonian : trapped-ion spin-spin Hamiltonian

38 Experimental results unpublished data work in progress... Variational optimization

39 Acknowledgments IQOQI Innsbruck (Experiment) P. Jurcevic C. Maier T. Brydges B. Lanyon R. Blatt IQOQI Innsbruck (Theory) University of Ulm University of Strathclyde P. Hauke A. Elben B. Vermersch P. Zoller C. Kokail R. van Bijnen Quasiparticle propagation Random measurements Variational eigensolvers T. Baumgratz M. Holzapfel I. Dhand M. Cramer M. Plenio A. Buyskikh A. Daley University of Vienna N. Friis M. Huber MPS tomography + entanglement witnesses:

40 Trapped-ion quantum simulations Summary and outlook Realization of long range Ising models in trapped ions Fully addressable for up to 20 ions Single-shot measurements of arbitrary spin correlations Entanglement characterization in small subsystems by tomography and random measurements P. Jurcevic et al., Nature 511, 202 (2014) B. Lanyon, C. Maier et al., Nat. Phys. 13, 1158 (2017) N. Friis et al., PRX 8, (2018) T. Brydges et al., in preparation Outlook: Exploration of non-equilibrium quantum dynamics in larger systems Scaling the system up: longer 1d strings, experiments with planar ion crystals