L. de Santis, C Anton, B. Reznychenko, N. Somaschi, G. Coppola, J. Senellart, C. Gomez, A. Lemaitre, I. Sagnes, A. G. White, L. Lanco, A. Auffeves, P. Senellart.
Nature Nanotechnology 12, 663–667 (2017)
A strong limitation of linear optical quantum computing is the probabilistic operation of two-quantum bit gates  based on the coalescence of indistinguishable photons. A route to deterministic operation is to exploit the single-photon nonlinearity of an atomic transition. Through engineering of the atom-photon interaction, phase shifters, photon filters and photon-photon gates have been demonstrated with natural atoms. Proofs of concept have been reportedwith semiconductor quantum dots, yet limited by ine_cient atom-photon interfaces and dephasing. We have fabricated a highly efficient single-photon filter based on a large optical non-linearity at the single photon level, in a near-optimal quantum-dot cavity interface. When probed with coherent light wavepackets, the device shows a record nonlinearity threshold around 0.3 incident photons. We demonstrate that directly reected pulses consist of 80% single-photon Fock state and that the two- and three-photon components are strongly suppressed compared to the single-photon one.
Figure : (a) Measured and (c) calculated reflectivity of a 125 ps coherent pulse (resonant at the QD transition) as function of the incident average photon-number. The straight lines in panel (a) are guide to the eyes showingthe nonlinear threshold. (b) Measured and (d) calculated time-integrated second-order correlation function, g(2)(0), asa function of average photon number. (e) Fraction of single photon (black symbols) and of coherent light (open symbols) in the reflected beam.
V. Giesz, N. Somaschi, G. Hornecker, T. Grange, B. Reznychenko, L. De Santis, J. Demory, C. Gomez, I. Sagnes, A. Lemaître, O. Krebs, N. D. Lanzillotti-Kimura, L. Lanco, A. Auffeves & P. Senellart
In a quantum network based on atoms and photons, a single atom should control the photon state and, reciprocally, a single photon should allow the coherent manipulation of the atom. Both operations require controlling the atom environment and developing efficient atom–photon interfaces, for instance by coupling the natural or artificial atom to cavities. Before our work, much attention had been drown on manipulating the light field with atomic transitions, recently at the few-photon limit. We have studied the reciprocal operation and demonstrated the coherent manipulation of an artificial atom by few photons. We study a quantum dot-cavity system with a record cooperativity of 13. Incident photons interact with the atom with probability 0.95, which radiates back in the cavity mode with probability 0.96. Inversion of the atomic transition is achieved for 3.8 photons on average, showing that our artificial atom performs as if fully isolated from the solid-state environment.
Figure: (a) Emission intensity as a function of the excitation power (top axis) and the average photon number n (bottom axis) sent on the device for two pulse durations (12 and 56 ps). (b) Calculated emission as a function of n for two pulse durations. (c) Calculated probability to find the QD in its excited state after the pulse as a function of n for 12 and 56 ps pulses.
Juan C. Loredo, Nor A. Zakaria, Niccolo Somaschi, Carlos Anton, Lorenzo de Santis, Valerian Giesz, Thomas Grange, Matthew A. Broome, Olivier Gazzano, Guillaume Coppola, Isabelle Sagnes, Aristide Lemaitre, Alexia Auffeves, Pascale Senellart, Marcelo P. Almeida, and Andrew G. White
OPTICA, Vol 3, Issue 4, pp. 433 – 440 (2016)
The desiderata for an ideal photon source are high brightness, high single-photon purity, and high indistinguishability. Defining brightness at the first collection lens, these properties have been simultaneously demonstrated with solid-state sources; however, absolute source efficiencies remained close to the 1% level and indistinguishability had only been demonstrated for photons emitted consecutively on the few-nanoseconds scale. We demonstrate solid-state single-photon sources with scalable performances. In one device, an absolute brightness at the output of a single-mode fiber of 14% and purities of 97.1%–99.0% are demonstrated. When nonresontantly excited, it emits a long stream of photons that exhibit indistinguishability up to 70%—above the classical limit of 50%—even after 33 consecutively emitted photons with a 400 ns separation between them. Resonant excitation in other devices results in near-optimal indistinguishability values: 96% at short timescales, remaining at 88% in timescales as large as 463 ns after 39 emitted photons. The performance attained by our devices brings solid-state sources into a regime suitable for scalable implementations.
Figure: Temporal-dependent indistinguishability under strictly resonant excitation. Two-photon interference histograms with Device 2 of parallelly polarized photons at (a) Δt=12.2 ns and (b) Δt=158.5 ns, under a pi-pulse preparation. (c) Second-order autocorrelation measurement at pi-pulse. (d) Indistinguishability between a first and n-th consecutive emitted photon for two. Indistinguishability remains robust in the temporal domain, decreasing only by 4.4% in 159 ns or by 8.3% in 463 ns.
The scaling of optical quantum technologies requires efficient, on-demand sources of highly indistinguishable single photons. We had demonstrated in 2013 that semiconductor quantum dots inserted into photonic structures were ultrabright single-photon sources, yet the indistinguishability wass limited by charge noise. Parametric downconversion sources provide highly indistinguishable photons but are operated at very low brightness to maintain high single-photon purity.
Until this work, no technology had provided a bright source generating near-unity indistinguishability and pure single photons. We have fabricated such devices made of quantum dots in electrically controlled cavities. Application of an electrical bias on the deterministically fabricated structures is shown to strongly reduce charge noise. Under resonant excitation, an indistinguishability of 0.9956 ± 0.0045 is demonstrated with g(2)(0) = 0.0028 ± 0.0012. The photon extraction of 65% and measured brightness of 0.154 ± 0.015 make this source 20 times brighter than any source of equal quality. This new generation of sources opens the way to new levels of complexity and scalability in optical quantum technologies.
Figure: a: Schematic of the devices under study: a micropillar coupled to a QD is connected to a surrounding circular frame by four one-dimensional wires. The top p-contact is defined on a large mesa adjacent to the frame. The n-contact is deposited on the back of the sample. b: Photoluminescence map of a connected device: the bright emission at the centre of the device arises from the deterministically coupled QD. c,d: Correlation histograms measuring the indistinguishability of photons successively emitted by the QD3. The photons are sent to the HOM beamsplitter with the same polarization (c) or orthogonal polarization (d). e : Summary of the source properties as a function of excitation power: from top to bottom: purity (g(2)(0)); indistinguishability (M); and brightness (collected photons per pulse).
Figure: Comparison of state-of-the-art QD-based single-photon sources prior to our work (blue symbols, the corresponding reference is indicated in the label), high-quality SPDC heralded single-photon sources (grey symbols) and the devices reported in our Nature Photonics 2016 (red symbols).
L. C. Loredo, M. A. Broome, P. Hilaire, O. Gazzano, I. Sagnes, A. Lemaitre, M. P. Almeida, P. Senellart, A. G. White.
A BosonSampling device is a quantum machine expected to perform tasks intractable for a classical computer, yet requiring minimal non-classical resources as compared to full-scale quantum computers. Photonic implementations to date employed sources based on inefficient processes that only simulate heralded single-photon statistics when strongly reducing emission probabilities. BosonSampling with only single-photon input has thus never been realised. Here, we report on a BosonSampling device operated with a bright solid-state source of highly-pure single-photon Fock states: the emission from an efficient and deterministic quantum dot-micropillar system is demultiplexed into three partially-indistinguishable single-photons, with purity 1−g (2)(0) of 0.990 ± 0.001, interfering in a 6×6 linear optics network. Our demultiplexed source is orders-of-magnitude brighter than current heralded multi-photon sources based on spontaneous parametric down-conversion, allowing us to complete the BosonSampling experiment 100 times faster than previous equivalent implementations. This intrinsic source superiority places BosonSampling with larger photon numbers within near reach.
Figure: a) Second-order autocorrelation function in log scale. An ideal single-photon Fock state has a g2(0) = 0, here the authors measure g2(0) = 0.010. b) Experimental setup of BosonSampling with a solid-state single photon source. Laser pulses centred at 905 nm excite a quantum-dot embedded in a micropillar cavity, which itself is housed in an optically accessable cryostat (Cryo) system at 13 K. A dichroic mirror (DM), and band-pass filter (BP) with 0.85 nm FWHM are used to isolate the emitted single-photons at 932 nm collected in a single-mode fibre (SMF). A passive demultiplexer composed of beam-splitters with tunable transmittances—half-wave plates (HWP), and polarising beam-splitters (PBS)—and compensating delay lines of 12.5 ns probabilistically converts three consecutive single photons into separate spatial modes, where they are directed into the BosonSampling circuit. The scattering linear network is composed of polarisers (Pol), half-wave plates, a 3×3 non-polarising fibre beam-splitter (FBS), and polarising fibre beam-splitters (PFBS), which in combination form a 6×6 mode network. Six avalanche photo-diodes (APDs) are used at the output to detect up to 3-fold coincidence events.
PHYSICAL REVIEW B
covering condensed matter and materials physics
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.
PHYSICAL REVIEW X
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.