PHYSICAL REVIEW LETTERS

Entangling Quantum-Logic Gate Operated with an Ultrabright Semiconductor Single-Photon Source

O. Gazzano, M. P. Almeida, A. K. Nowak, S. L. Portalupi, A. Lemaître, I. Sagnes, A. G. White, and P. Senellart

http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.110.250501

DOI:http://dx.doi.org/10.1103/PhysRevLett.110.250501

 

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.

Controlling Spontaneous Emission with Plasmonic Optical Patch Antennas

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.

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.

Bright solid-state sources of indistinguishable single photons

O. Gazzano, S. Michaelis de Vasconcellos,  C. Arnold,  A. Nowak,  E. Galopin,  I. Sagnes,  L. Lanco,   A. Lemaître  & P. Senellart

http://www.nature.com/ncomms/journal/v4/n2/full/ncomms2434.html

doi:10.1038/ncomms2434

 

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

Optical Nonlinearity for Few-Photon Pulses on a Quantum Dot-Pillar Cavity Device

V. Loo, C. Arnold, O. Gazzano, A. Lemaître, I. Sagnes, O. Krebs, P. Voisin, P. Senellart, and L. Lanco

Phys. Rev. Lett. 109, 166806

http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.109.166806

DOI:http://dx.doi.org/10.1103/PhysRevLett.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.

 

PHYSICAL REVIEW LETTERS

Evidence for Confined Tamm Plasmon Modes under Metallic Microdisks and Application to the Control of Spontaneous Optical Emission

O. Gazzano, S. Michaelis de Vasconcellos, K. Gauthron, C. Symonds, J. Bloch, P. Voisin, J. Bellessa, A. Lemaître, and P. Senellart

http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.107.247402

DOI:http://dx.doi.org/10.1103/PhysRevLett.107.247402

 

 

Light can be confined at the interface between a distributed Bragg reflector and 2D metallic layer: the confinement arises, on one side, from the metal’s negative dielectric constant, and on the other side, from the DBR’s stop band.

Here, we show strong three-dimensional confinement of a Tamm plasmon with a very simple microstructure consisting of a thin gold microdisk on top of a planar GaAs/AlGaAs Bragg mirror. The Tamm plasmon (TP), formed at the interface between DBR and metal, is laterally confined to the dimensions of the gold disk, resulting in a discrete mode spectrum with quality factors of up to 1200 .

The modes exhibit a zero in-plane wave vector, allowing for an excellent coupling to quantum dot excitons and the vertical emission of photons. With the in-situ lithography technique, we couple single quantum dots to the confined Tamm plasmon modes. We observe an acceleration of the spontaneous emission if the quantum dot transition is at resonance with the mode, while a remarkably strong inhibition of the spontaneous emission (by a record factor of up to 40) is measured if the quantum dot transition is off-resonance.

 

 

 

Ultrabright source of entangled photon pairs

Adrien Dousse, Jan Suffczyński, Alexios Beveratos, Olivier Krebs, Aristide Lemaître, Isabelle Sagnes, Jacqueline Bloch, Paul Voisin & Pascale Senellart

Nature 466, 217–220 (08 July 2010) doi:10.1038/nature09148

 

At the very heart of applications such as quantum cryptography, computation and teleportation lies a fascinating phenomenon known as “entanglement”. Two photons are entangled if the properties of one depend on those of the other, whatever the distance separating them. A new source of entangled photons twenty times brighter than all existing systems has been developed by a team from the Laboratoire de Photonique et de Nano-structures (LPN) of CNRS. This novel device is capable of considerably boosting the rate of quantum communications and constitutes a key component in future quantum logic processes. These results are published in the journal Nature on the 8 July 2010.

Take a photon in Paris and another in Tokyo: if they are “entangled”, they are interdependent and measuring the properties of one makes it possible to know the properties of the other instantly, whatever the distance separating them. This mysterious property, known as “entanglement”, has far reaching application potential in information fields such as quantum cryptography, quantum computation and quantum teleportation. Normally, researchers use sources of entangled photon pairs that are easy to put in place (a laser transforming a photon into two photons of different color) but with very low brightness: less than one pulse out of 100 actually contains a pair of entangled photons, which considerably restricts the rate of any quantum communication. In addition, the size of such sources means they cannot be easily integrated into microsystems.

 

A team from the Laboratoire de Photonique et de Nano-structures (LPN) of CNRS has developed a light source of entangled photons twenty times brighter than all existing systems. The researchers have invented a novel “photonic molecule” system in which a semiconductor quantum dot(1) emits a pair of entangled photons per excitation pulse. This photonic molecule constitutes a trap for each of the photons of the pair and allows them to be collected efficiently. This new source operates at a rate of one pair of photons collected every 8 pulses (compared to less than one pair every 100 pulses so far). In the long run, the researchers should be able to reach a rate close to one pair of photons per pulse. This device could make it possible to manufacture electroluminescent diodes of entangled photon pairs, with rates close to one gigahertz (in other words around on billion hertz). Moreover, the LPN scientists have also shown that the use of this “photonic molecule” concept allows the quality of the entanglement of the emitted photon pairs to be improved.

© Jean-Louis Le Hir

This image represents, in the top right hand corner, the new component produced in this experiment: two pillars of micrometric size are coupled to form the “photonic molecule”.
The semiconductor quantum dot (of nanometric size) is inserted into one of the pillars (visible as the bright spot in the right hand pillar). The lower part of the image shows the radiation pattern of the entangled photons emitted by the component.

 

To fabricate efficient devices, we need to make sure that a quantum dot is coupled to a single mode of the electromagnetic field.
We can approach such ideal situation by controlling the quantum dot spontaneous emission, through cavity quantum electrodynamics.
To do so, the quantum dot must be in both spectral and spatial resonance with the cavity mode. Yet, quantum dots usually grow with random spatial positions and a wide distribution of their spectral properties. As a result, usual nanofabrication techniques present fabrication yields in the low 10-4.

We have invented a technique which enables to fabricate as many as desired ideally coupled quantum dot-cavity devices, with a fabrication yield close to unity.
Our technique relies on a low temperature photolithography, with an in situ monitoring of the quantum dot emission.
The emission monitoring makes it possible to select the quantum dot with desired optical properties, to measure the quantum dot spatial location with 50 nm accuracy and to expose the photoresist to define a disk centered on the QD. The hole in the resist will later be used as a mask for the micropillar etching, ensuring that the quantum dot will be located at the maximum of the fundamental optical mode.

Dozens of micropillar microcavities with diameters ranging from 1 to 2.3 µm are fabricated. Each of them embeds a spectrally resonant quantum dot in its center. Using this technique, we have demonstrated on demand control the spontaneous emission for one or two QDs coupled to a cavity mode, with an acceleration of spontaneous emission close to 10. Increasing the quality factor of the cavity mode, we have recently demonstrated scalable implementation of strongly coupled QD cavity devices.

 

Further reading

 

“Controlled Light-Matter Coupling for a Single Quantum Dot Embedded in a Pillar Microcavity Using Far-Field Optical Lithography”, A. Dousse, L. Lanco, J. Suffczynski, E. Semenova, A. Miard, A. Lemaître, I. Sagnes, C. Roblin, J. Bloch, P. Senellart, Phys. Rev. Lett. 101, 267404 (2008)

 

“Scalable implementation of strongly coupled cavity-quantum dot devices” , A. Dousse, J. Suffczynski, R. Braive, A. Miard, A. Lemaître, I. Sagnes, L. Lanco, J. Bloch, P. Voisin, P. Senellart, Appl. Phys. Lett. 94, 121102 (2009)