PHYSICAL REVIEW B
covering condensed matter and materials physics
V. Giesz, S. L. Portalupi, T. Grange, C. Antón, L. De Santis, J. Demory, N. Somaschi, I. Sagnes, A. Lemaître, L. Lanco, A. Auffèves, and P. Senellart
Phys. Rev. B 92, 161302(R)
Quantum dots in cavities have been shown to be very bright sources of indistinguishable single photons. Yet the quantum interference between two such bright quantum dot sources, a critical step for photon-based quantum computation, still needs to be investigated. We have reported on such a measurement, taking advantage of a deterministic fabrication of the QD-cavity devices. We show that cavity quantum electrodynamics can efficiently improve the quantum interference between remote quantum dot sources: Poorly indistinguishable photons can still interfere with good contrast with high quality photons emitted by a source in the strong Purcell regime. Our measurements and calculations show that cavity quantum electrodynamics is a powerful tool for interconnecting several quantum dot devices.
Simone Luca Portalupi, Gaston Hornecker, Valérian Giesz, Thomas Grange, Aristide Lemaître, Justin Demory, Isabelle Sagnes, Norberto D. Lanzillotti-Kimura, Loïc Lanco, Alexia Auffèves, and Pascale Senellart.
Bright single photon sources are obtained by inserting solid-state emitters in microcavities. Accelerating the spontaneous emission via the Purcell effect allows both high brightness and increased operation frequency. However, achieving Purcell enhancement is technologically demanding because the emitter resonance must match the cavity resonance. We have shown that this spectral matching requirement is strongly lifted by the phononic environment of the emitter. We study a single InGaAs quantum dot coupled to a micropillar cavity. The phonon assisted emission, which hardly represents a few percent of the dot emission at a given frequency in the absence of cavity, can become the main emission channel by use of the Purcell effect. A phonon-tuned single photon source with a brightness greater than 50% is demonstrated over a detuning range covering 10 cavity line widths (0.8 nm). The same concepts applied to defects in diamonds pave the way toward ultrabright single photon sources operating at room temperature.
Figure: a: Schematic of the model: The quantum dot is considered as a two-level system and the phonon bath and electromagnetic field are described as continua. The density of states of the electromagnetic field is modified by the cavity and is peaked about its resonance frequency. The coupling to the phonons is treated nonperturbatively using the independent bosons model while the coupling to the electromagnetic field is treated in first order perturbation theory. B : brightness (black squares, left scale) and calculated mode coupling (right scale) as a function of the QD-cavity detuning normalized to the cavity linewidth and calculations for different values of the Purcell factor.
Christophe Arnold, Justin Demory, Vivien Loo, Aristide Lemaître, Isabelle Sagnes, Mikhaïl Glazov, Olivier Krebs, Paul Voisin, Pascale Senellart & Loïc Lanco
Entangling a single spin to the polarization of a single incoming photon, generated by an external source, would open new paradigms in quantum optics such as delayed-photon entanglement, deterministic logic gates or fault-tolerant quantum computing. These perspectives rely on the possibility that a single spin induces a macroscopic rotation of a photon polarization. Such polarization rotations induced by single spins were recently observed, yet limited to a few 10−3 degrees due to poor spin–photon coupling. We have reported on the enhancement by three orders of magnitude of the spin–photon interaction, using a cavity quantum electrodynamics device. A single-hole spin in a semiconductor quantum dot is deterministically coupled to a micropillar cavity.
The cavity-enhanced coupling between the incoming photons and the solid-state spin results in a polarization rotation by ±6° when the spin is optically initialized in the up or down state. These results open the way towards a spin-based quantum network.
The prestigious “Médaille d’argent du CNRS” (i.e. CNRS Silver Medal) in physics this year has been appointent to Prof. Pascale Senellart-Mardon for “her contributions in the development of photonic devices for the processing of quantum information… These devices, consisting in solid-state semiconductors quantom dot deterministically coupled to a micropillar optical cavity, represent up to now the brightest emitters of single and indistinguishable photons or entangled photon pais.”
PHYSICAL REVIEW X
C. Arnold, V. Loo, A. Lemaître, I. Sagnes, O. Krebs, P. Voisin, P. Senellart, and L. Lanco
Phys. Rev. X 4, 021004
We have shown that a highly efficient QD-cavity interface makes it possible to monitor in real time single quantum events at the microsecond time scale. This is illustrated here by monitoring in real time single-charge jumps, evidencing a measurement rate 5 orders of magnitudefaster than for previous optics experiments of directsingle-charge sensing. Our technique relies oncoherent reflection spectroscopy, performed with a detectionsetup approaching the shot-noise limit, on a deterministically coupled QD-pillar cavity device, into whichthe incident photons are injected with a high input-coupling efficiency.
This ensures that almost every incident photon interacts with the QD and provides an opticalresponse highly sensitive to the QD transition energy. Single events, corresponding to the capture and release of a single charge by a material defect, are distinctlyidentified with a few microseconds time resolution and with a less than 0.2% error probability. Our measurementsalso reveal a photoinduced acceleration of the chargedynamics.
Figure: (a),(b) Band structures of an InGaAs QD with a nearby loaded or empty trap (c),(d) Typical reflectivity spectra for a loaded and for an empty trap. (e) Scatter plot of measured reflectivity values versus photon energy. (f) Real-time reflectivity signal. Dashed horizontal lines are guides to the eye indicating the two states with reflectivities RL and RE. (g) Histogram of the reflectivity values
A. K. Nowak , S. L. Portalupi, V. Giesz , O. Gazzano , C. Dal Savio , P.-F. Braun , K. Karrai , C. Arnold , L. Lanco , I. Sagnes, A. Lemaître & P. Senellart.
The scalability of a quantum network based on semiconductor quantum dots lies in the possibility of having an electrical control of the quantum dot state as well as controlling its spontaneous emission. The technological challenge is then to define electrical contacts on photonic microstructures optimally coupled to a single quantum emitter. We have developed a novel photonic structure and a technology allowing the deterministic implementation of electrical control for a quantum dot in a microcavity.
We study a λ/2- AlAs cavity surrounded by GaAs/AlGaAs Bragg mirrors, doped in the p-i-n diode configuration. To apply an electric field to the structure, rather than using simple pillar cavities, we use pillars connected to a larger ohmic-contact surface with four 1D-bridges (1μm width). The fundamental mode of the structure is confined in the center of the pillar with a low penetration into the bridges. Quality factors of the connected pillars are similar to the ones obtained for isolated pillars. Deterministic positioning of a single QD at the centre of the connected pillar structure is performed using an advanced single in-situ optical lithography step. Upon the application of a voltage, the QD line is electrically tuned into resonance with the cavity mode. An experimental extraction efficiency as large as 55% is demonstrated.
PHYSICAL REVIEW LETTERS
O. Gazzano, M. P. Almeida, A. K. Nowak, S. L. Portalupi, A. Lemaître, I. Sagnes, A. G. White, and P. Senellart
We have entangled two independent single photons emitted by a quantum dot using a quantum-logic gate.
A quantum controlled NOT (C-NOT) gate is a crucial element for quantum information processing. It is a two-qubit gate that flips the state of a target qubit depending on the state of a control one. The quantum capability of the gate allows creating an entangled two-qubit output state from independent input qubits. We use an ultrabright solid-state single photon source made by inserting a single quantum dots in a pillar microcavity. The truth table of the gate is measured in rectilinear basis and shows a probability of obtaining the correct output of 68% for a source brightness of 0.55 collected photon per pulse. By setting the control photon in diagonal basis, we show that the two output photons can be entangled. The fidelity to the Bell state is above the 0.5 limit for quantum correlation for a source brightness of 50% and reaches 71% for short time delays.
C. Belacel, B. Habert, F. Bigourdan, F. Marquier, J.-P. Hugonin, S. Michaelis de Vasconcellos, X. Lafosse, L. Coolen, C. Schwob, C. Javaux, B. Dubertret, J.-J. Greffet, P. Senellart, and A. Maitre.
We experimentally demonstrate the control of the spontaneous emission rate and the radiation pattern of colloidal quantum dots deterministically positioned in a plasmonic patch antenna. The antenna consists of a thin gold microdisk separated from a planar gold layer by a few tens of nanometers thick dielectric layer.
In collaboration with the groups of Jean Jacques Greffet (LCFIO Palaiseau), Agnès Maitre (INSP Paris) and Benoit Dubertret (LPEM, Paris), we study the emission of colloidal quantum dots inserted in plasmonic optical patch antennas. The plasmonic patch antenna proposed in Phys. Rev. Lett. 104, 026802 (2010), consists in a thin gold microdisk 30 nm above a thick gold layer (fig. 1a), with the emitter positioned in the dielectric spacer (figure a). The small 30 nm separation between the disk and the gold film provide a large confinement of the electromagnetic field in the vertical direction. The finite size of the disk leads to confinement in the plane. Broadband large Purcell factor is theoretically predicted (figure b).
In the present work, we have inserted small clusters of around 50 CdSe/CdS colloidal nanocrystals in the antenna. The cluster presents a cylinder shape with typical lateral radius of 5 nm and height of 13 nm. A deterministic positioning of clusters inside each antenna with a precision of 25nm is obtained using the in-situ lithography technique. As shown in figures c-d, the emitters below the antenna radiate through the entire patch antenna.
The average cluster show an acceleration of spontaneous emission of 80 for vertical dipoles (figure e). The radiation pattern of the antenna is highly directionnal, as measured in figure 1f.
O. Gazzano, S. Michaelis de Vasconcellos, C. Arnold, A. Nowak, E. Galopin, I. Sagnes, L. Lanco, A. Lemaître & P. Senellart
For many applications like long distance quantum communications or for linear quantum computing, the emitted photons need to be indistinguishable. Although quantum dot have been shown to emit indistinguishable photons, combining high brightness and high indistinguishability is not straightforward. Indeed, high brightness requires strong excitation of the system. Thus, many carriers are created in the quantum dot environment leading to pure dephasing.
We have studied the indistinguishability of the single photon source as a function of the source brightness and excitation conditions (figure a). When creating the carriers in the surrounding barriers (green symbols), a high photon indistinguishability (characterized y a mean wavepacket overlap M=0.82) is observed at a source brightness of 30%. When increasing the source brightness, M continuously decreases: additional carriers optically created in the QD surrounding create a fluctuating electrostatic environment.
To circumvent this effect, carriers are directly created in the excited state of the QD (red symbols). Surprisingly, the source indistinguishability is even lower, independently of the source brightness. To combine high brightness with high indistinguishability, we have used a two color excitation scheme (blue symbols): strong pumping directly into an excited QD state together with a weak non-resonant pumping. Doing so, we demonstrate a mean wavepacket overlap as high as 82% for a source brightness of 65%.
PHYSICAL REVIEW LETTERS
V. Loo, C. Arnold, O. Gazzano, A. Lemaître, I. Sagnes, O. Krebs, P. Voisin, P. Senellart, and L. Lanco
Phys. Rev. Lett. 109, 166806
The quantum optical transition of a single quantum dot can be saturated with just one photon. This open the possibility to implement quantum logic gates sending two photons to interact on the quantum dot. However, in the absence of good coupling between the quantum dot and the incident laser beam, such gate would be very inefficient.
Here we insert the quantum dot in a pillar cavity and we take advantage of the good coupling of the pillar cavity mode to the external optical field to demonstrate optical non-linearities for few photons incident on the device.
Measurements performed under continuous wave excitation demonstrate a near-unity input-coupling efficiency is obtained: 95% of the photons sent on the device interact with the QD.
Under pulsed excitation, we demonstratea record nonlinearity threshold at 8 incident photons per pulse.