Quantum Optics Seminar by David Gershoni – University of Copenhagen

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Quantum Optics Seminar by David Gershoni

We demonstrated that in quantum dots (QDs), the polarization of a resonantly tuned, single picosecond optical pulse can be used for deterministic writing the bright exciton (electron-hole pair with anti-parallel spin directions) qubit in any desired coherent state [1]. Moreover, we showed that this fundamental optical excitation of matter, with unity integer spin, can be fully controlled coherently (i.e its two level state vector can be "rotated" on its Bloch sphere about any desired axis by any desired angle") using one single short optical pulse [2]. With these advantages the bright exciton forms an unprecedented coherently controlled matter qubit, unavailable in any other studied system [3]. Unfortunately, however, bright excitons have limited applications since their lifetimes are rather short (sub ns) limited by their fast radiative rate

Since light barely interacts with the electronic spin, an electron-hole pair with parallel spin directions is almost optically inactive. Such a pair is called a dark-exciton. The dark exciton is also a spin integer (total spin 2) qubit [4] but since it is optically inactive, its lifetime can be orders of magnitude longer than that of the bright exciton. Recently we have demonstrated that in self assembled quantum dots the DE lifetime (~1 µs) is radiative and that it maintains coherence over at least ~100 ns [5-6]. As such the DE has clear advantages over the bright exciton, provided that it can be externally accessed and controlled, despite its optical inactivity.

We will show that just like the case of the bright exciton, the polarization of a resonantly tuned, single picosecond optical pulse can be used for high fidelity deterministic writing of the dark exciton qubit in any desired coherent state. Thus, the time needed to write the DE qubit is 6 (5) orders of magnitude shorter than its lifetime (coherence time). Similarly, we also show that once "written" the DE spin can be coherently controlled using picosecond long optical pulses.

The advantages of the dark exciton as a matter spin qubit will be discussed.

 

[1 ]  Y. Benny et al. "Coherent Optical Writing and Reading of the Exciton Spin State in Single Quantum Dots" Phys. Rev. Lett. 106, 040504, (2011).

[2 ]  Y. Kodriano et al. " Complete Control of a Matter Qubit using a Single Picosecond Laser Pulse " Phys. Rev. B. 85, 241304(R), (2012).

[3 ]  D. Press et al. " Complete Quantum Control of a single Quantum Dot Spin using Ultrafast Optical Pulses" Nature, 456, 218-221, (2008).

[4 ]  E. Poem et al. "Accessing the Dark Exciton with Light" Nature Physics 6, 993 (2010).

[5 ]  . Schwartz et al. "Deterministic Writing and Control of the Dark Exciton-Spin using Short Single Optical Pulses" Phys. Rev. X. 5, 011009, (2015).

[6 ]   M. Zielinski et al. Atomistic Theory of Dark Excitons in Self-Assembled Quantum Dots of Reduced Symmetry" Phys. Rev. B. 91, 085403, (2015).