Controllable Photonic Time-Bin Qubits from a Quantum Dot

Photonic time-bin qubits are well suited to transmission via optical fibers and waveguide circuits. The states take the form $\frac{1}{\sqrt{(2)}}(\alpha |0⟩+e^{i\varphi }\beta |1⟩)$, with $|0⟩$ and $|1⟩$ referring to the early and late time bin, respectively. By controlling the phase of a laser driving a spin-flip Raman transition in a single-hole-charged InAs quantum dot, we demonstrate complete control over the phase, $\varphi$. We show that this photon generation process can be performed deterministically, with only a moderate loss in coherence. Finally, we encode different qubits in different energies of the Raman scattered light, paving the way for wavelength-division multiplexing at the single-photon level.

Popular Summary

Single photons are useful for storing and transmitting information in secure quantum communication and in some implementations of quantum computing. One promising source of single photons is a nanoscale semiconductor particle known as a quantum dot. These dots can produce photons at a very high rate and do not require complicated setups, unlike trapped atoms and ions. Here, we demonstrate an efficient, powerful method for encoding information in a photon generated by a quantum dot.

Time-bin encoding inscribes information in the arrival time of a photon. This encoding scheme is a convenient way to send information over an optical fiber because it does not suffer from decoherence, which degrades the information as it travels.

We borrowed techniques from atomic physics to encode information in the time-bin basis using single photons. Normally, this type of encoding requires the generated photons to be sent through an interferometer and a phase modulator, which results in many of the photons being lost. By moving the location of these components, the photons do not have to be sent through either of them, making it possible to achieve much higher efficiencies. We also take a technique used to send multiple signals along a single fiber and apply it to single photons. This allows us to encode more information per photon.

Our technique allows us to, in principle, deterministically produce arbitrary single-photon time-bin-encoded qubits. These results demonstrate the ability to encode large amounts of information in a single photon, which will be critical for practical quantum communication.

Figure 1

(a) An illustration of our experimental setup. (b) An illustration of a micropillar cavity in a Voigt geometry magnetic field. (c) The energy-level diagram. When the resonant lasers are red or blue detuned from the diagonal transition, the resulting Raman scattered light is similarly detuned. (d) The spectrum of the QD under nonresonant excitation. Polarization filtering ensures that only the vertical transitions are visible, and the long-wavelength transition is clearly enhanced by the cavity mode.

Figure 2

(a) An illustration of the pulse sequence used to demonstrate phase control over a time-bin qubit. (b) The extracted interference fringes for reference (black) and phase modulated (orange) photons. We show the time-resolved plots of the interference between neighboring time bins for the two sequences that allow us to extract the interference fringes and the probabilities of measuring the photon in each time bin. (c) The recorded time-bin qubits mapped onto the Bloch sphere (blue points). The black and orange vectors represent the states generated by reference and example modulated pulse sequences, respectively. (d) The measured interference visibility as a function of photon generation probability (blue data points) and the calculated expected interference visibility for several different radiative decay times (lines).

Figure 3

(a) An illustration of driving the Raman transition with a red- and a blue-detuned laser. (b) A time-resolved plot and fit of the output light when a red-detuned laser is used to drive the Raman transition in time bin 1 and a blue-detuned laser is used to drive the transition in time bin 2, shown for the following situations: The spectral filter is not detuned (top panel); the spectral filter is blue detuned (middle panel); the spectral filter is red detuned (bottom panel). (c) The result of a second-order correlation function measurement on the Raman scattered light generated using two detuned lasers, both of which are pulsed in time bins 1 and 2.