Swing-Up of Quantum Emitter Population Using Detuned Pulses

The controlled preparation of the excited state in a quantum emitter is a prerequisite for its usage as a single-photon source—a key building block for quantum technologies. In this paper, we propose a coherent excitation scheme using off-resonant pulses. In the usual Rabi scheme, these pulses would not lead to a significant occupation. This is overcome by using a frequency-modulated pulse to swing up the excited-state population. The same effect can be obtained using two pulses with different strong detunings of the same sign. We theoretically analyze the applicability of the scheme to a semiconductor quantum dot. In this case, the excitation is several millielectronvolts below the band gap, i.e., far away from the detection frequency, allowing for easy spectral filtering, and does not rely on any auxiliary particles such as phonons. Our scheme has the potential to lead to the generation of close-to-ideal photons.

Popular Summary

Reliable quantum emitters are a prerequisite when building next-generation quantum information technology devices. To emit a single photon at a defined time calls for a deterministic preparation of the excited quantum emitter state. This can be achieved by stimulating the quantum emitter at the transition energy, resulting in the well-established Rabi rotations. However, Rabi cycles only yield a complete inversion at resonance, while detuned pulses do not lead to a significant occupation of the excited state. Up to now, it has been widely believed that a full inversion of a quantum emitter using detuned laser pulses can only be achieved when exploiting auxiliary quasiparticles, as is done, e.g., in phonon-mediated processes.

In our work, we now take a different route and use highly detuned pulses to populate the excited state of the quantum emitter, relying only on the carrier-light interaction. By kicking the system at just the right time, a gradual swing-up of the emitter population is achieved. While a theoretical implementation of our scheme comprises a single frequency-modulated pulse, the swing-up can also be achieved in a two-color scheme by a combination of two state-of-the-art laser pulses. By exciting below the band gap of the materials, absorption-induced heating does not occur. The separation between the energies of excitation and emission of the quantum emitter makes filtering easy.

Our proposal opens up a unique class of preparation schemes for quantum emitters, helping to achieve the perfect protocol to control a single-photon source for quantum technologies.

Figure 1

The Bloch-vector dynamics for (a) off-resonant Rabi oscillations and (b) the SUPER scheme. (c) The dynamics of the excited-state occupation for the SUPER scheme under (d) a time-dependent detuning switching between ${\mathrm{\Delta }}_{\mathrm{high}}=-5.47{\mathrm{\Omega }}_0$ and ${\mathrm{\Delta }}_{\mathrm{low}}=-2.74{\mathrm{\Omega }}_0$. The pulse has a duration of $4.95\pi /{\mathrm{\Omega }}_0$.

Figure 2

The time evolution of the excited-state occupation for (a) $\hslash {\mathrm{\Delta }}_M=2\mathrm{meV},\alpha =6.2\pi$ and (b) $\hslash {\mathrm{\Delta }}_M=1\mathrm{meV},\alpha =30.3\pi$ [red dot in (d); blue dot in (e)]. (c) Spectra of the pulses used in (a) and (b): the dashed line marks the excited-state energy. (d),(e) The final excited-state occupation for different modulation parameters and pulse areas with $\hslash {\mathrm{\Delta }}_C=-6\mathrm{meV}$ and $\sigma =4\mathrm{ps}$. The red lines mark the Rabi frequency at the pulse maximum.

Figure 3

(a) The time evolution of the excited-state occupation for the 2C-SUPER scheme. The inset shows the combined spectrum of the pulses. (b) The normalized laser envelope for the pulse sequence used. The parameters are given in Table 1 for $\hslash {\mathrm{\Delta }}_1=-8\mathrm{meV}$.

Figure 4

The final excited-state occupation for different detunings ${\mathrm{\Delta }}_2$ and time delays $\tau$ of the second pulse. For a negative time difference, the second pulse arrives earlier than the first. (a) Parameters as in Fig. 3. (b) The same as (a) but the second pulse has the same shape as the first one.

Figure 5

The final excited-state occupation for different pulse areas and pulse durations. Here, both pulses always have the same pulse area, $\tau =0$ and ${\sigma }_2=1.5{\sigma }_1$. (a) $\hslash {\mathrm{\Delta }}_1=-5\mathrm{meV}$. (b) $\hslash {\mathrm{\Delta }}_1=-11\mathrm{meV}$.

Figure 6

A comparison of the calculations for $\phi =0$ in Fig. 3 with the result for $\phi =\pi /2$. The inset shows that the final occupation is constant for phases $\phi =0,..,2\pi$.

Figure 7

The final occupation of the excited state for the two-color scheme at lower detunings, with $\tau =0$ and ${\sigma }_2=1.5{\sigma }_1$. (a) $\hslash {\mathrm{\Delta }}_1=-1\mathrm{meV}$. (b) $\hslash {\mathrm{\Delta }}_1=-0.5\mathrm{meV}$.