Photon Detectors Rewrite the Rules of Quantum Computing
scitechdaily.com
scitechdaily.com
> [...] A pertinent example of continuous variables comprise the quadrature representation of the optical quantum state, as opposed to the discrete-variable photon-number representation. The wave-like nature of the field results from the coherence between different photon number components of a field with uncertain photon number. Therefore, to study the wave-like nature of the field, one must be able to measure superpositions of photon number states.
> To do so, the field of a weak optical quantum state (signal) comprising a few photons interferes with the field of a bright coherent state (i.e., a local oscillator, LO) on a balanced beam splitter. The two output modes of the beam splitter are then measured with two photodetectors. By calculating the difference between the detector count rates, which is proportional to the field quadrature at a reference phase provided by the local oscillator, the optical quantum state can be characterized [3–5]. The ability to characterize optical quantum states in their phase-space representation [1] makes homodyne detection an essential tool for quantum information processing with continuous variables [6].
> Typically, conventional semiconductor photodiodes are used as the detector in balanced homodyne detection. The high optical flux arising from the LO lifts the optical signal above the electronic noise floor, which is typically in the pW√Hz range at telecommunication wavelengths. As a result, the generated carriers in the photodiode can be integrated over a characteristic response time, resulting in a photocurrent which is proportional to the incident optical flux [7]. For BHD, it is essential that the photodetector output is directly proportional to the intensity (photon flux) of the input light. In this case, we refer to the detector output as linear.
> As well as NV centres and molecules, quantum dots (QDs),[14] quantum dots trapped in optical antenna,[15] functionalized carbon nanotubes,[16][17] and two-dimensional materials[18][19][20][21][22][23][24] can also emit single photons and can be constructed from the same semiconductor materials as the light-confining structures. It is noted that the single photon sources at telecom wavelength of 1,550 nm are very important in fiber-optic communication and they are mostly indium arsenide QDs.[25] [26] However, by creating downconversion quantum interface from visible single photon sources, one still can create single photon at 1,550 nm with preserved antibunching. [27]
"A physical [photonic] qubit with built-in error correction" (2024) https://news.ycombinator.com/item?id=39243929 :
> "Logical states for fault-tolerant quantum computation with propagating light" (2024)
From "Physicists use a 350-year-old theorem to reveal new properties of light waves" (2023) https://news.ycombinator.com/item?id=37226121#37226160 :
>> "This means that hard-to-measure optical properties such as amplitudes, phases and correlations—perhaps even these of quantum wave systems—can be deduced from something a lot easier to measure: light intensity."