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Quantum Optics



Photonic quantum information |  Indistinguishable single photons |  Superconducting photo-detectors

The general framework of this research is quantum information, whose goal is to take advantage of the possibilities offered by quantum mechanics to process information in a more efficient way

(for a tutorial see http://www.qubit.org/ ).

top of the page1- Photonic quantum information

A proposal for conditional quantum logic gates using single photons and only linear optics has recently come out [Knill]. The principle is depicted in figure 1. The grey box contains only linear optics, that is eventually polarisation sensitive beam splitters. This box is fed by indistinguishable single photons. Top photons carry information (for example polarisation encoded) and make up the logic gate input, while the bottom ones are ancillary photons. The setting of the linear optics components in the grey box entangle the output channels in such a way that if the ancillary detectors are triggered in a given configuration, the output state of the gate is projected in the state corresponding to the desired logic function applied on the input Qbits.

The operation of these gates is thus conditional, since it relies on an event post-selection. The entanglement created in the gray box arises from multiphoton interference effects [Hong] that require indistinguishable single photons. The photo-detectors have to be very efficient and to offer the ability to resolve the number of photon in a pulse.

Principle of an all-optical quantum logic gate

Fig.1:Principle of an all-optical quantum logic gate

Our project aims at the physical implementation of such a logic gate, and is divided in a part on single photon generation and another part of their detection.

top of the page2- Indistinguishable single photons

2.1 Presentation

Triggered single photon source can be realized by collecting the fluorescence of a single quantum dot. Such sources can be used in quantum cryptography or, when the spectrum is lifetime-limited, to implement quantum logical gates. Additionally, triggered emission of pairs of entangled photons will be experimentally investigated.

2.2 Goals

This project aims at the implementation of single-mode single photons from II-VI semiconductor quantum dots in order to realize quantum logical gates based on multiphoton interferometry [Knill]. Additionally, the production of triggered pairs of entangled photons by a single quantum dot will be investigated. This effect [Benson] has never been experimentally observed.

More generally, our goal is to build photon sources enabling multiphoton interferometry experiments for applications in the field of quantum information (quantum gates, quantum teleportation).

2.3 Single photons from semi-conducting quantum dots

Quantum dots are nano-islands (10 nm) of a semiconductor embedded in another semiconducting material (Fig. 1-a). Electron-hole pairs are trapped in these islands that behave almost as artificial atoms. With an adequate pulsed excitation, the fluorescence light of a single of these nano-objects can be observed. It is then possible to detect, at a given wavelength, a light pulse containing at most one photon [Gérard]. To enhance the collection efficiency, the quantum dot may be inserted in a 3D microcavity (cf Fig. 1-b)

Left, InAs quantum dots visualized with an electron beam microscope. Right, 3D microcavity made of a micropillar containing quantum dots sandwiched inbetween two Bragg mirrors of about 10 layers.

Fig.2:Left, InAs quantum dots visualized with an electron beam microscope. Right, 3D microcavity made of a micropillar containing quantum dots sandwiched inbetween two Bragg mirrors of about 10 layers.
(Photos, Jean-Michel Gérard)

 

2.4 Two-photon interference with two independent photons, application to quantum logical gates.

When the single photon spectrum is lifetime limited, the successive photons can become indistinguishable. It becomes then possible to implement a two-photon experiment in which two photons coming from both side of a 50/50 beamsplitter emerge always together on the same side (cf Fig.2) [Hong]. This indistinguishability condition on independent single photons imposes rather severe constraints on the coherence of the light emission process [Santori1]. This multiphoton interference effect is at the heart of the proposition of quantum logical gates using only linear optics by Knill et al [Knill].

Fig.3:When two identical photons are impinging on both side of a 50/50 beamsplitter, the probability of having them emerging on opposite sides is zero, owing to destructive interference of the probability amplitude. The two photons emerge always together on the same side.

2.5 Triggered pairs of entangled photons

It is possible to inject exactly two electron-hole pairs in a single quantum dot. By resonant excitation of the biexciton. It has already been experimentally shown that the two photons emitted are correlated in time [Moreau] and in polarisation [Santori2]. Their entanglement has been predicted [Benson] but never observed. This experimental demonstration requires the preservation of the spin coherence during the radiative cascade. A good understanding of the spin dephasing mechanisms in quantum dots is therefore necessary.

Fig. 4:When the spin coherence is preserved during the radiative cascade,
the two emitted photons are polarization entangled |σ+σ->+|σ-σ +.

2.6 Support

We acknowledge financial support from CNRS and French Ministry of Research.

top of the page3- Superconducting photo-detectors

3.1 Presentation

The goals of this project are the fabrication and the characterisation of superconducting photons counters. These bolometric photo-detectors are made of an ultrathin (<10 nm) film of superconducting material, whose properties are altered by the absorption of a single photon. The expected behaviour makes them very attractive for application in photonic quantum information, such as quantum cryptography and all-optical quantum logic gates. These detectors should exhibit better quantum efficiencies and dark counts than currently available photons counters at telecom wavelength (1.3 and 1.55 microns). [Verevkin]. Furthermore they should offer the possibilities of non-linear detection and photon number resolution.

Fig.5:

3.2 Operating principle

These detectors are made of a superconducting strip of a few microns long, 250 nm wide and 6 nm thick. At the temperature of liquid helium (4K), the strip (usually in NbN that has a critical temperature Tc=10 K) is biased by a current just below the critical current. The absorption of a photon by the strip rises locally its temperature. This creates a non-superconducting hot spot [Kadin]. The current is then forced to flow around this hot spot and the current density can then reach the critical current. The strip becomes resistive and a short (30 ps) voltage pulse appears across the strip.

3.3 Properties

Depending on the gap between the current bias and the critical current, the superconducting-normal transition can require the absorption of one or several photons. A non-linear photo-detector sensitive to more-than-two photon pulses can then be realized [Gol'tsman]. This property offers interesting possibilities for original quantum optics experiments based upon multiphoton interferences in which the lambda/N wavelength of N photon Fock state can be visualized. These effects may have useful applications in improving the resolution of optical lithography.

Furthermore, it appears that the electrical pulse shape in the linear regime depends on the energy of the light deposited on the film and therefore on the number of photons [Gol'tsman]. This photon number resolution feature has yet never been demonstrated, and is a key feature for the implementation of all-optical quantum logic gates [Knill]

3.4 Technology

* Fabrication

The epitaxial growth of ultrathin NbN film (<10 nm) is carried out at CEA Grenoble by J.C. Villegier et al on sapphire or MgO at 600°C. The 250 nm stripe etching is done by reactive ion etching (RIE) [Villegier]

* Photonic engineering

To improve the quantum efficiency of these detectors, the reflection and transmission coefficients can be tailored by 2D or 3D light confinement techniques to enhance the photon absorption probability. These techniques (cavities, plasmons) [Glass] have been widely used for semi-conducting detectors. These aspects are done in collaboration with CEA-LETI in Grenoble and P. Lalanne (LCFIO) in Orsay.

3.5 Support

We acknowledge support from the French Ministry for research and the Rhône-Alpes Region.

·         References « Photonic quantum information »

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·         References « Single photons »

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·         References « Superconducting detectors »

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