Two-photon Interference Measurements Research Articles

Bounding the outcome of a two-photon interference measurement using weak coherent states.

The interference of two photons at a beam splitter is at the core of many quantum photonic technologies, such as quantum key distribution or linear-optics quantum computing. Observing high-visibility interference is challenging because of the difficulty of realizing indistinguishable single-photon sources. Here, we perform a two-photon interference experiment using phase-randomized weak coherent states with different mean photon numbers. We place a tight upper bound on the expected coincidences for the case when the incident wavepackets contain single photons, allowing us to observe the Hong-Ou-Mandel effect. We find that the interference visibility is at least as large as 0.995-0.013+0.005.

Generation of frequency sidebands on single photons with indistinguishability from quantum dots

Generation and manipulation of the quantum state of a single photon is at the heart of many quantum information protocols. There has been growing interest in using phase modulators as quantum optics devices that preserve coherence. In this Letter, we have used an electro-optic phase modulator to shape the state vector of single photons emitted by a quantum dot to generate new frequency components (modes) and explicitly demonstrate that the phase modulation process agrees with the theoretical prediction at a single photon level. Through two-photon interference measurements we show that for an output consisting of three modes (the original mode and two sidebands), the indistinguishability of the mode engineered photon, measured through the secondorder intensity correlation (g2(0)) is preserved. This work demonstrates a robust means to generate a photonic qubit or more complex state (e.g., a qutrit) for quantum communication applications by encoding information in the sidebands without the loss of coherence.

50-GHz-spaced comb of high-dimensional frequency-bin entangled photons from an on-chip silicon nitride microresonator.

Quantum frequency combs from chip-scale integrated sources are promising candidates for scalable and robust quantum information processing (QIP). However, to use these quantum combs for frequency domain QIP, demonstration of entanglement in the frequency basis, showing that the entangled photons are in a coherent superposition of multiple frequency bins, is required. We present a verification of qubit and qutrit frequency-bin entanglement using an on-chip quantum frequency comb with 40 mode pairs, through a two-photon interference measurement that is based on electro-optic phase modulation. Our demonstrations provide an important contribution in establishing integrated optical microresonators as a source for high-dimensional frequency-bin encoded quantum computing, as well as dense quantum key distribution.

Two-photon interference from two blinking quantum emitters

We investigate the effect of blinking on the two-photon interference measurement from two independent quantum emitters. We find that blinking significantly alters the statistics in the second-order intensity correlation function g$^{(2)}(\tau)$ and the outcome of two-photon interference measurements performed with independent quantum emitters. We theoretically demonstrate that the presence of blinking can be experimentally recognized by a deviation from the g$^{(2)}_{D}(0)=0.5$ value when distinguishable photons impinge on a beam splitter. Our results show that blinking imposes a mandatory cross-check measurement to correctly estimate the degree of indistinguishablility of photons emitted by independent quantum emitters.

Two-photon interference from a bright single-photon source at telecom wavelengths

Long-distance quantum communication relies on the ability to efficiently generate and prepare single photons at telecom wavelengths. In many applications these photons must also be indistinguishable such that they exhibit interference on a beamsplitter, which implements effective photon-photon interactions. However, deterministic generation of indistinguishable single photons with high brightness remains a challenging problem. We demonstrate two-photon interference at telecom wavelengths using an InAs/InP quantum dot in a nanophotonic cavity. The cavity enhances the quantum dot emission, resulting in a nearly Gaussian transverse mode profile with high out-coupling efficiency exceeding 36% after multi-photon correction. We also observe Purcell enhanced spontaneous emission rate up to 4. Using this source, we generate linearly polarized, high purity single photons at 1.3 micron wavelength and demonstrate the indistinguishable nature of the emission using a two-photon interference measurement, which exhibits indistinguishable visibilities of 18% without post-selection and 67% with post-selection. Our results provide a promising approach to generate bright, deterministic single photons at telecom wavelength for applications in quantum networking and quantum communication.

Measuring the biphoton temporal wave function with polarization-dependent and time-resolved two-photon interference.

We propose and demonstrate an approach to measuring the biphoton temporal wave function with polarization-dependent and time-resolved two-photon interference. Through six sets of two-photon interference measurements projected onto different polarization subspaces, we can reconstruct the amplitude and phase functions of the biphoton temporal waveform. For the first time, we apply this technique to experimentally determine the temporal quantum states of the narrow-band biphotons generated from the spontaneous four-wave mixing in cold atoms.

Two-photon interference of weak coherent laser pulses recalled from separate solid-state quantum memories

Quantum memories allowing reversible transfer of quantum states between light and matter are central to quantum repeaters, quantum networks and linear optics quantum computing. Significant progress regarding the faithful transfer of quantum information has been reported in recent years. However, none of these demonstrations confirm that the re-emitted photons remain suitable for two-photon interference measurements, such as C-NOT gates and Bell-state measurements, which constitute another key ingredient for all aforementioned applications. Here, using pairs of laser pulses at the single-photon level, we demonstrate two-photon interference and Bell-state measurements after either none, one or both pulses have been reversibly mapped to separate thulium-doped lithium niobate waveguides. As the interference is always near the theoretical maximum, we conclude that our solid-state quantum memories, in addition to faithfully mapping quantum information, also preserve the entire photonic wavefunction. Hence, our memories are generally suitable for future applications of quantum information processing that require two-photon interference.

Postselected indistinguishable single-photon emission from the Mollow triplet sidebands of a resonantly excited quantum dot

Applying high power continuous-wave resonant s-shell excitation to a single self-assembled In- GaAs quantum dot, we demonstrate the generation of post-selected single-indistinguishable photons from the Mollow triplet sidebands. A sophisticated spatial filtering technique based on a double Michelson interferometer enabled us to separate the spectrally close lying individual Mollow components and perform almost background free two-photon interference measurements. Showing high consistency with the results of an independent determination of the emission coherence of 250 +/- 30 ps, our analysis reveals a close to ideal visibility contrast of up to 97 %. Due to their easy spectral tunability and their distinct advantage of cascaded photon emission between the individual Mollow sidebands, they resemble a versatile tool for quantum information applications.

Fast Measurements of Entangled Photons

We report on a system for the generation and measurement of polarization entangled photons. High speed quantum state tomographies with a raw fidelity of >; 91 % with respect to an ideal entangled state are recorded in <; 2 seconds, with longer-term accidental-count subtracted fidelities exceeding 99%. Two-photon interference measurements are recorded using an automated alignment and measurement procedure with the signal distributed over 20 km of fiber. These high speed measurement methods are useful for high rate monitoring of entangled states.

Two-Photon Interference Using Background-Free Quantum Frequency Conversion of Single Photons Emitted by an InAs Quantum Dot

We show that quantum frequency conversion (QFC) can overcome the spectral distinguishability common to inhomogeneously broadened solid-state quantum emitters. QFC is implemented by combining single photons from an InAs/GaAs quantum dot (QD) at 980 nm with a 1550 nm pump laser in a periodically poled lithium niobate (PPLN) waveguide to generate photons at 600 nm with a signal-to-background ratio exceeding 100:1. Photon correlation and two-photon interference measurements confirm that both the single photon character and wave packet interference of individual QD states are preserved during frequency conversion. Finally, we convert two spectrally separate QD transitions to the same wavelength in a single PPLN waveguide and show that the resulting field exhibits nonclassical two-photon interference.

Post-Selected Indistinguishable Photons from the Resonance Fluorescence of a Single Quantum Dot in a Microcavity

Applying continuous-wave pure resonant s-shell optical excitation of individual quantum dots in a high-quality micropillar cavity, we demonstrate the generation of post-selected indistinguishable photons in resonance fluorescence. Close to ideal visibility contrast of 90% is verified by polarization-dependent Hong-Ou-Mandel two-photon interference measurements. Furthermore, a strictly resonant continuous-wave excitation together with controlling the spontaneous emission lifetime of the single quantum dots via tunable emitter-mode coupling (Purcell) is proven as a versatile scheme to generate close to Fourier transform-limited (T2/(2T1)=0.91) single photons even at 80% of the emission saturation level.

Phase shaping of single-photon wave packets

Although the phase of a coherent light field can be precisely known, this is not true for the phase of the individual photons that create the field, considered individually1. Phase changes within single-photon wave packets, however, have observable effects. In fact, actively controlling the phase of individual photons has been identified as a powerful resource for quantum communication protocols2,3. Here we demonstrate arbitrary phase control of a single photon. The phase modulation is applied without affecting the photon's amplitude profile and is verified by means of a two-photon quantum interference measurement4,5, demonstrating fermionic spatial behaviour of photon pairs. Combined with previously demonstrated control of a single photon's amplitude6,7,8,9,10, frequency11, and polarization12, the fully deterministic phase shaping presented here allows for the complete control of single-photon wave packets. Arbitrary phase control within a single photon wave packet is demonstrated and verified by two-photon quantum interference measurements. Combined with the previously demonstrated ability to control a single photon's amplitude, frequency and polarization, the phase shaping presented here allows for the complete control of single-photon wave packets.

Quantum dots as single-photon sources for quantum information processing

Semiconductor pillar microcavities containing quantum dots have shown promise as efficient sources of single and correlated pairs of photons, which may find applications in quantum information processing. In this paper we discuss the use of these sources to generate single photons and the use of pillars with elliptical cross-section to enhance and select a particular photon state. Single-photon interference measurements are also performed and show coherence times of up to 180 ps for quasi-resonantly pumped dots. Hong–Ou–Mandel-type two-photon interference measurements using a fibre interferometer indicate that individually created photons display a large degree of indistinguishability.

Measurement of scattered light pulse by two photon interference

We have proposed a new scheme of two-photon interference measurement, which provides the intensity correlation of the envelope function of ultrashort light pulses. With this scheme, we have measured the pulse width and the pulse envelope function of the scattered light of picosecond laser pulses. Our experimental results are in good agreement with the theoretical calculation.

Two-photon interference of imperfectly mode-locked pulses

The Gaussian noise burst model and the domain model for imperfectly mode-locked laser pulses are examined through comparison with two types of two-photon interference measurements of a synchronously pumped cw mode-locked dye laser. The Gaussian noise burst model does not give correct results for the pulse-cycle-averaged, normalized second-order correlation function at zero time-delay, and the domain model fails to explain the two-component dip in coincidence count rate curves obtained with the interferometer configuration.