WAVEPLATES AND BEAMSPLITTERS FOR QUANTUM COMPUTING

Waveplates for quantum computing
The primary function of a waveplate in quantum computing is to manipulate the polarization state of photons, which is crucial in encoding, processing and reading quantum information.
Entangled pair generation - waveplates for polarization adjustment
During Spontaneous Parametric Down Conversion (SPDC) in non-linear crystals a photon pair is generated – single pump photon is converted into two lower-energy photons (signal and idler). These can be entangled in polarization - if one photon is measured as having horizontal polarization, its pair will be polarized vertically (for this particular Bell state).
A waveplate can be very handy here, because it can be used to adjust the polarization state of the pump photon, which can influence the polarization correlation of the generated entangled pair. And even signal and idler photons can be further manipulated with waveplates to fine tune the entanglement (e.g. if signal and idler photons go through additional waveplates, their polarization states can be adjusted to specific angles, this process enables the creation of various entangled states).
Entangled photons are the main component in quantum communication protocols (e.g. QKD – quantum key distribution).
Qubit manipulation using waveplates
Quantum gates in photonic systems often require the change of polarization state, waveplate is used to rotate the polarization thus enabling the quantum logic gate. As photons are used as qubits, the polarization state represents the quantum information (|0⟩ or |1⟩), waveplates can be used to prepare the initial polarization state of photons – You can adjust the angle of the waveplate to precisely set the photon’s polarization and effectively represent the specific quantum state.
Readout and measurements using waveplates
Combination of waveplate and polarizer can be used to convert the photon polarization state into measurable signal:
- Photon enters the waveplate (with unknown quantum state)
- Waveplate rotates the polarization to specific angle (based on initial state and desired measurement basis)
- Photon goes through polarizer (polarizer is aligned in specific direction, e.g. |Horizontally⟩)
- Measurement (if photon passed it would be registered as |Horizontally⟩) e.g. |1⟩, if not - registered as |0⟩)
- Photon reaches the photodetector, which registers the photon (or no-photon) providing binary outcome of the quantum measurement. Waveplate can alter and maintain the state of photon without measuring it, e.g. half-wave plate can rotate polarization by certain angle and thus changing the state of a qubit. In this configuration polarizers will allow only certain polarization photons to pass, while reflecting the others (e.g. will transmit |Horizontally⟩ and reflect |Vertically⟩)
Real experiment application guidelines:
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Start: Photon is polarized at 45 ° (an equal superposition of |H⟩ and |V⟩).
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Rotate: Pass it through a half-wave plate whose fast axis is set at 22.5 °. Result: the linear polarization turns by 45 °, becoming vertical (|V⟩).
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Select: A vertical polarizer lets the |V⟩ photon through and blocks |H⟩.
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Alignment: Even a small mis-angle in the wave-plate or polarizer can flip the outcome.
What characteristics to consider when choosing waveplate for quantum applications?
When choosing the waveplate consider a high precision retardation half waveplate (λ/2) and quarter waveplates (λ/4) available at Alien Photonics. You should also keep in mind variety of choices regarding waveplate order: True Zero Order, Zero Order, Low & multi order, … Do not hesitate to contact us for consultation if you are not entirely sure which option would fit you the best. Also - custom waveplates are available. Although 800 nm or 1550 nm waveplates are most popular in photonic quantum computing research and applications, other wavelength options are available. During your quantum experiments, please keep in mind environmental factors such as temperature fluctuations and humidity, as these factors might affect the polarization.
Beamsplitters for quantum computing
Entangled pair generation using beamsplitters
Ideal 50/50 beamsplitter, e.g. Alien Photonics Partial 50% Reflector, creates equal probability for the photon to take either path so when single photon reaches this type of beamsplitter it has a 50% probability to be transmitted and 50% probability to reflected, effectively this puts a photon in superposition (transmitted and reflected). Hong-Ou-Mandel Effect (two photon interference) which occurs when two indistinguishable photons enter beamsplitter with 50/50 split ratio from different sides. This interaction results in a such way that they both exit together from the same side (both reflected or both transmitted).
Beamsplitters for photonics quantum Gates
Beamsplitters are used in fundamental logic quantum gates, e.g. CNOT (Controlled NOT) and Hadamard. Contact Alien Photonics - we will help you to choose the best beamsplitter.
Readout and measurements using beamsplitters
Beamsplitters not just help with creating quantum states, but can also be used in measurement, placed just before the photodetector to help with the quantum state detection.
- Photon enters beamsplitter (still in superposition)
- Beamsplitter randomly directs the photon in one of two possible paths, effectively creating a path superposition.
- As the photon travels along the path(s), in some designs/setups goes to recombination in another beamsplitter, causing quantum interference.
- Photodetector at the end of the path detects the photon revealing its quantum state
- Measurement data further goes to computation.
Real application example
In LOQC (Linear Optical Quantum Computing) beamsplitter can be used to entangle photons (employing quantum interference) and (or) implementing quantum gates. During Quantum teleportation experiments beamsplitters are used to interfere a photon whose state is to be teleported with one of the entangled photons. Beamsplitters are also used in QKD (Quantum Key Distribution), to randomly choose the photons’ paths, effectively determining measurement basis.
What characteristics to consider when choosing beamsplitter for quantum computing experiment?
Go for smaller deviation when choosing optical coating technology! Sputtering coating technologies like magnetron sputtering and ion beam sputtering are more attractive option, because they can be more precisely controlled, the splitting values would be way (~5-10 times) closer to ideal 50%/50% split ratio, when comparing to alternative, less expensive coating options such as e-beam (EBE), where the deviation is larger and can reach few percents. Absorption and scattering can also play important role, e.g. in e-beam the coating is porous in comparison to sputtering, thus over the time and in not-controlled environment the characteristics might degrade. Final recommendation from Alien Photonics – choose sputtered beamsplitter coating on high-purity glass substrate.
Comparison of key roles of Alien Photonics waveplates and beamsplitters in photonic quantum computers
Feature | Waveplate | Beamsplitter |
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Fundamental functions | Altering polarization of photons by using birefringence. | Splitting light beam into two separate paths by reflecting and transmitting photons. |
Key roles in quantum computing | Quantum state preparation, Control/rotate polarization states. Encoding/Decoding quantum information in photon’s polarization | Entanglement generation by creating superposition and quantum interference. Constructing, manipulating quantum logic gates. |
What to look for in specifications | Phase control, retardation, wavelength dependency. | High precision in splitting ratio (low deviation). |
Critical points when choosing | Precise alignment and controlled environmental conditions. | Precise alignment, challenging integration in photonics chips at large scales. |