The Signal
Spin Injection Linked to Two-Photon Fluctuations in Quantum Dots
A study of electrically driven quantum dots found that two-photon emission statistics fluctuated during excitation pulses when spin injection was active, while single-photon counts followed a smoother profile. The authors say cooperative em
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A quantum-dot sample showed an average degree of photon polarization of about 21%, alongside fluctuations in two-photon emission statistics during electrical excitation. In a study posted to arXiv on October 29, researchers report that two-photon events fluctuated at several points during an electrical pulse, while single-photon events followed a smoother curve. The fluctuations disappeared after the spin-injection layer was demagnetized, linking the effect to spin-polarized carriers without identifying the microscopic process behind it.
Two-photon events depart from the pulse
The experiment used electrically contacted InGaAs/GaAs quantum dots in a spin light-emitting diode. A ferromagnetic CoFeB layer, separated from the semiconductor by an MgO tunnel barrier, injects spin-polarized electrons; its perpendicular magnetic anisotropy supports spin injection without an applied magnetic field. The researchers drove the device with voltage pulses about 20 nanoseconds long and collected the emitted light.
To separate photon events by number and timing, the team used an extended Hanbury Brown–Twiss interferometer with four single-photon detectors. The detectors can, in principle, resolve events up to three photons, but the study analyzed one- and two-photon events because measuring the three-photon state would require impractically long acquisition. A 500-picosecond coincidence window identified two-photon events, and each data set took eight hours to collect.
In the spin-polarized sample, the time-resolved single-photon counts followed a smooth profile, but the two-photon counts showed pronounced deviations. The pattern did not appear in measurements after the sample was demagnetized, a procedure that randomized magnetic domains and removed the net spin-injection bias while leaving the device and excitation unchanged.
The comparison isolates spin injection
The raw event totals differed between the two conditions: the polarized sample produced 3.8 × 108 single-photon events and 7 × 104 two-photon events; the unpolarized sample produced 5.1 × 108 and 3 × 105, respectively. The result is not a simple increase in the number of two-photon events when spins align. The reported distinction is in their time-dependent profile: fluctuations appear with spin injection and subside after demagnetization.
That difference matters because a single average statistic can hide dynamics within an excitation pulse. The authors note that conventional zero-delay intensity correlation, g(2)(0), averages over the pulse and may not capture the detailed behavior of an emitter ensemble. Their time-resolved, photon-number-sensitive measurement instead reveals that single- and two-photon events can have distinct temporal patterns.
A cooperative-emission explanation, not a mechanism
The researchers propose that spin alignment could make cooperative emission among multiple dots more likely. Their interpretation rests on the disappearance of the two-photon fluctuations when net spin injection is removed, alongside a model that reproduces the measured profiles. The sample’s average degree of photon polarization was about 21%; the authors estimate that fewer than 10 emitters are likely to participate in cooperative emission at a given time.
That model is deliberately phenomenological. It represents two-photon events with joint collapse operators that make pairs of emitters decay together, but the paper says such operators do not follow from a standard treatment of the system and are not a known decay channel. The simulation uses three spin-aligned emitters and a tunable parameter chosen to match the measured ratio of single- to two-photon events.
“The microscopic mechanism that couples the emitters is not identified here, however the joint collapse operators in Eqn. (3) capture its effect phenomenologically.”
Chiran Wijesundara et al., paper authors
What the result means for electrical sources
Electrical excitation offers a route to quantum-dot light sources that does not rely on an external pump laser, while allowing control over pulse duration and shape. The findings suggest that spin injection may influence more than the circular polarization of emitted photons: it may also shape higher-order photon statistics, a consideration for devices intended to suppress multi-photon events or generate higher photon-number states.
The evidence is an ensemble measurement, not a demonstration of a controlled two-photon source from a single dot. The paper also reports operation below 100 kelvin, and its proposed cooperative pathway still needs a microscopic explanation. Can the same fluctuations be measured and controlled in a single emitter, and what physical interaction couples the dots when their spins align?