“Ion Traps for Tomorrow’s Applications”

COST-IOTAEnrico Fermi Summer School, Varenna 2013

David Lucas

University of Oxford, U.K.

Ion Trap Quantum Computing groupwww.physics.ox.ac.uk/users/iontrap

Microfabricated Ion Traps

Lecture 2 outline

1. Motivations for microfabricated traps

2. 3D and 2D microtraps

3. Anomalous heating in traps

4. Near-field microwave techniques

1. Motivations

022

r

eV

m dω =

Ω

1. Tight traps

• easier laser cooling to ground state

• faster quantum logic gates

• easier separation of ions

• interfacing with solid state qubits?

Radial secular frequency:

d: ion-electrode distance scale

Blain et al. 2004

2. Scaling to large arrays of traps

3. Sensing applications

Quantum computing

quantum register

“accumulator”

segmented electrodes

“quantum CCD” architecture – Wineland et al. (1998)

Some microfabricated trap milestones

1990s First wafer traps at NIST

2001 NIST dual-zone trap

2005 Michigan chip trap (semiconductor process)

2006 Michigan T-junction trap

2006 Sandia chip trap (MEMS process)

2006 NIST surface-electrode trap

2008 MIT cryogenic surface traps

2012 NPL 3D silicon trap

2012 Oxford surface trap with integrated microwave elements

ion-electrode distance = 1.2 mmmotional frequencies ~ 1 MHz

UHV < 1x10-11 mbar

10µm

7 mm

10mm

Linear ion trap (“retro” version)

2D and 3D micro traps

Amini et al. 2008 (in “Atom Chips” ed. Vuletic & Reichel)

Univ.Ulm (Schmidt-Kaler group)

Material: evaporated gold on laser-machined alumina waferion-electrode distance 250µmRF drive 25MHz, 140Vtrap depth 76meVradial frequency 1.3MHzheating rate 2.1(3) quanta/ms

Example 3D microfab trap: Ulm

2D (planar) traps

Taken from J Britton’s thesis

Taken from J Britton’s thesis

2D (planar) traps

Material: electroplated gold on quartz (Ti and Ag seed layers)ion-surface distance 150µmRF drive 35MHz, 200Vtrap depth 82meVradial,axial frequencies 3.5MHz, 1.0MHz

filter capacitors

Example 2D microfab trap: Oxford

Trap fabrication process

SEM image of electrode layout

Vacuum System

Calcium ovenmounted atabove chip. Thermal beamis parallel totrap surface.

Imaging throughlarge viewport(conductively coated) Lasers enter and

exitthrough side ports

25-way D-subfor dc electrodes

Trap inUHV-compatibleplastic socket. Vacuum

pumps

RF feedthroughVacuum system

389nm

423nm

continuum

Ca4s2

4s4p

Photo-ionization trap loading

• high absolute efficiency • negligible charging effects

Operating Parameters

RF Amplitude =225 VRF Frequency =25.4 MHZRF Stability Parameter, q = 0.45

Trap Depth = 0.2 eVRadial Secular Frequencies = 4 MHz

-0.24 -3.15 -3.15 -3.15 -0.24

-0.24 -3.15 -3.15 -3.15 -0.24

-1.04

-1.04

rf

rf

0.95 0.95 0.95 0.95 0.95

0

0

rf

rf

1.12 1.12 1.12 1.12 1.12

-0.90

-0.95

rf

rf

5.0 1.9 1.9 1.9 5.0

0.9

rf

rf

-1.03 -1.03 -1.03 -1.03 -1.03

-0.95 -0.95 -0.95 -0.95 -0.95 5.0 1.9 1.9 1.9 5.0

‘Endcap’ Voltagesto produce a500kHz axialsecular frequency

‘Tilt’ voltages to rotate radial normal modes for optimal cooling

x-axis (up-down) micromotioncompensationvoltages (mV per V/m)

y-axis (out of plane) micromotioncompensationvoltages (mV per V/m)

0.9

DC control voltage sets

Micromotion compensation

866nm laser detuning

Trap charging by laser light

- this data for Ca+ at 397nm

- no charging for IR beams (866nm)

- could be worse for UV ions? (e.g. Be+, Mg+)

Junctions

NIST 2008

Michigan 2006

3. “Anomalous” heating

Anomalous heating

elec

tric

fiel

d no

ise

WARNING: do not attempt to reproduce these results at home !!!

Anomalous heating

In situ cleaning 1: pulsed laser cleaning

Allcock et al. NJP 2011

355nm Nd:YAG5ns pulsed100-200 mJ/cm2

In situ cleaning 1: pulsed laser cleaning

Allcock et al. NJP 2011

cleaned zone

control zone before and after cleaning

In situ cleaning 2: Ar+ ion bombardment

Hite et al. PRL 2012

In situ cleaning 2: Ar+ ion bombardment

Hite et al. PRL 2012

elec

tric

fiel

d no

ise

Microwave near-field techniques

Quantum logic with near-field microwaves

C. Ospelkaus et al. Theory: PRL (2008), Experiment: Nature (2011)

static B0

Microwave trap design

50

xy

z

HFSS Simulation

Microwave Testing

Microwave trap design

500um Sapphire substrate for heat dissipation

HFSS simulation of currents

and B-field in trap region

Ion is 75um

off surface

at B-field null

43Ca+ Intermediate Field Hyperfine Qubit

43Ca+ S1/2 Ground State at 146 Gauss

3.2GHz

43Ca+ Intermediate Field Hyperfine Qubit

Use stretch transition

to servo B-field

static B-field (gauss offset from 146G)

c.f. B. Keitch et al. (2007) T2 = 1.2 sec (single 43Ca+ ion, low-field clock state)

C. Langer et al. (2005) T2 = 15 sec (single Be+ ion, intermediate-field clock state)

J. Bollinger et al. (1992) T2 ~ 600sec (~1000 ions, high-field clock state)

43Ca+ qubit: coherence time measurements

with CPMG

sequence

0.93 at 16sec

T2 = 48(10)sec

Randomized benchmarking of single-qubit gates

Recipe (Knill et al. 2007):

Apply random Clifford gates (π/2 pulses) from set x=σX,x=σ-X,y=σY,y=σ-Y

x x y y x x y x y x x y x y y x …

Then randomize again by inserting Pauli gates (pi pulses) randomly chosen

from the set +I,-I,+X,-X,+Y,-Y,+Z,-Z:

x Z x I y X y Z x Y x I y Z x Z y Y x Z x X y X x X y Z y I x …

Finish by rotating the qubit into the measurement (Z) basis:

x Z x I y X y Z x Y x I y Z x Z y Y x Z x X y X x X y Z y I x … y [measure]

Similar implementation to K.Brown et al. (2011)

Randomized benchmarking of single-qubit gates

Prep./readout

error 7x10-4

T 2=50se

c

~160ms

Mean error per gate

= 0.9(3) parts-per-million

gate time (pi/2) = 12µs

Paul trap evolution

~