Single Mn atom in CdTe/ZnTe quantum dot as an optical quantum memory

T. Smoleński1, M. Goryca1,2, T. Kazimierczuk1 , J. A. Gaj1,

P. Płochocka2, M. Potemski2,P. Wojnar3, P. Kossacki1,2

1. Institute of Experimental Physics, University of Warsaw2. Grenoble High Magnetic Field Laboratory3. Institue of Physics, Polish Academy of Sciences

OutlineIntroduction – what a quantum dot (QD) is?

Samples – how to observe a single magnetic atom

Experiment 1. Optical reading of information on the Mn

spin state 2. Optical writing and storage of information

Conclusion

Quantum dotsNanostructures, made of two semiconductors

with different energy gaps

10 nm

CdTeZnTe

Inner material, with small energy gap

Outer material, with wide energy gap

ECdTe EZnTe

Exciton in QDTwo types of carriers:

electrons and holes

Jz = ±1/2 Jz = ±3/2+e

-e

Exciton X4 possible spin states of X

Jz = -1

Jz = +1 Jz = -2

Jz = +2

Exciton in QDTwo types of carriers:

electrons and holes4 possible spin states of X

Jz = -1

Jz = +1 Jz = -2

Jz = +2

The way to observe single QDs: photoluminescence measurements !

1.922 1.923

energy (eV)

phot

olum

ines

cenc

e (a

rb.

u.)

QD with a single Mn ionSingle Mn ion: S = 5/2, L = 0Mn has 6 possible spin states

with 6 different Sz values

Mn

En

erg

y

exchange interaction between Mn and X

1.906 1.907 1.908 1.909

energy (eV)

ph

oto

lum

ine

sce

nce

(a

rb.

u.)

SamplesHow to make QDs? We can

do it using MBE method.

GaAs (substrate)

CdTe(buffer)

ZnTeIn an appropriate moment

we open the effusive cell with Mn atoms for a short time.

Number of Mn ions in a single QD ~ 1

ZnTe

CdTe(formation layer)

Experimental setupHow to observe a single QD?

T=1.5K

Tunable dye laser

Monochromator with CCD camera and avalanche photodiode

Criostat with microscope

Pol λ/4

Modulators

Optical QD excitationHow to introduce spin polarized excitons to QD

with single Mn ion?

Light should be circularly polarized (i.e. -s )

We should use … another QD without Mn ion!

Mn

Photoluminescence spectrum

Excitation with unpolarized light = unoriented Mn spin

1.906 1.907 1.908 1.909

energy (eV)

phot

olum

ines

cenc

e (a

rb. u

.)

Mn

Photoluminescence spectrum (2)+

s

ph

oto

lum

inesc

en

ce (

arb

. u

.)

energy (eV)

Excitation with circularly polarized light = we observe Mn spin orientation!

energy (eV)

What is the storage time?

t

Iσ+Excitation

Steady state

What is the storage time?

t

Iσ+Excitation

Steady state

σ-

What is the storage time?

t

Iσ+Excitation

Steady state

I

t

Signal on APD

σ-

What is the storage time?

t

Iσ+Excitation

Steady state

I

t

Signal on APD

σ-

What is the storage time?

t

Iσ+Excitation

Steady state

I

t

Signal on APD

σ-

Storage time determinationExcitation scheme

0

0.5

1.0

0 500 1000 1500 2000

exc 2.5W

time (ns)

PL

inte

nsi

ty (

arb

.u.)

T=1.5K, B=1T

0.8

0.9

1.0

0 100 200

= 0.4 ms

Dark Period (s)

Nor

mal

ized

Pea

k A

mpl

iude

t

Iσ+ σ-

Storage time~ 0.4 ms

Conclusion

We demonstrated optical writing and reading of information on the spin state of a single Mn ion embedded in a CdTe/ZnTe QD

• We shown, that single magnetic atom in a QD is a spin memory with optical writing and readout of information, storage time ~1ms