====== Optical Setup For Wave Particle Duality ====== ===== The photon source ===== The left side of the optical table contains our photon source. (See Fig. 3.) Light from a 20 mW 405 nm laser diode (called the //pump beam//) is reflected off of two mirrors so that it passes through a beta barium borate (BBO) crystal which is mounted on a rotation stage. Most of the photons in the pump beam pass through the BBO crystal unaffected. However, about 1 out of 106 pump photons will excite the BBO crystal in such a way that when the crystal de-excites, a pair of //entangled// photons are emitted. This process is referred to as //[[https://en.wikipedia.org/wiki/Spontaneous_parametric_down-conversion|spontaneous parametric down-conversion]];// the photons emitted have correlated polarizations and together have a total momentum and energy equal to that of the incident photon (as required by conservation laws). Some of these down-converted photon pairs will be of equal energy (meaning each photon has //half// the energy of the pump photon) and will therefore emerge with identical angles relative to the pump beam (as required by momentum conservation). We will refer to one of these down-converted photons as the //idler photon// and the other as the //signal photon//. (Because the photons are always produced in pairs, we can, for example, use the detection of an idler photon as an indication that a signal photon was simultaneously created... or vice versa.) In our experimental configuration, the pump beam and BBO crystal have been oriented to create down-converted photons that are vertically-polarized with wavelengths of 810 nm (i.e., half the energy of the 405 nm pump photons).
| {{ :phylabs:lab_courses:phys-211-wiki-home:wave-particle-duality:wpd_layout_p1.png?400 |}} | | **Figure 3:** The photon source |
The **pump laser** is controlled by the ILX Lightwave **laser diode controller.** The controller has two functions – it maintains the diode at a constant temperature (TEC Mode) and it controls the current which operates the laser (Laser Mode). Both the operating temperature (24 ºC) and operating current (50 mA) have been preset. The controller should already be on and the TEC should be running when you enter the lab. If not, turn on the ILX using the key switch on the lower left corner of the operating panel and then press the button indicated by the GREEN dot in the //TEC Mode// section of the control panel. The temperature of the diode is indicated by the TEC LED display and should reach 24 ºC in a few minutes. When it is time to turn on the pump laser, press the button indicated by the //Orange// dot on the //Laser Mode// section of the control panel. The diode current, as indicated on the LED display, should read 50mA when the laser is on.
| {{ :phylabs:lab_courses:phys-211-wiki-home:wave-particle-duality:ilx.png?400 |}} |
**WARNING:** The pump laser beam **WILL** cause permanent eye damage – including possible blindness – if used without protection. Laser safety goggles **MUST** be worn at all times and the door **MUST** be closed when the laser is on, even when the table is covered. Always check your goggles to verify that they are the correct ones for the wavelength of the laser. The lenses of the goggles will indicate the wavelength ranges and their optical density (OD) ratings as indicated in the image below. If the goggles are not correct for the wavelength of the laser, or if they appear damaged in any manner, bring them to the attention of an instructor before turning on the laser.
| {{ :phylabs:lab_courses:phys-211-wiki-home:wave-particle-duality:goggles.png?400 |}} |
===== The Mach-Zehnder interferometer ===== The signal photon is reflected by a mirror into the Mach-Zehnder interferometer. (See Fig. 5.) The interferometer consists of two 50/50 beamsplitters and two mirrors.  Within each path, there is a half-waveplate (used to change the polarization of the light along that path, if desired) and an iris to restrict the field of view of the detectors. When the rotation mounts of the half-waveplates are both set to the 0º mark, they will have no effect on the polarization of the light in either path. Recall that when an electromagnetic wave reflects off a medium with a //higher// index of refraction than the medium in which the incident wave was propagating, a phase change of π can occur (or, equivalently, the wave is //inverted//). When the wave reflects off a medium with a lower index of refraction, no phase shift (inversion) occurs. (For more details see https://en.wikipedia.org/wiki/Reflection_phase_change.)
| {{ :phylabs:lab_courses:phys-211-wiki-home:wave-particle-duality:wpd_layout_p3.png?400 |}} (a) | {{ :phylabs:lab_courses:phys-211-wiki-home:wave-particle-duality:mzi.png?400 |}} (b) | | **Figure 5:** The Mach-Zehnder interferometer: (a) schematically, and (b) zoomed-in photo. ||
===== Optical Components ===== ----
| {{ :phylabs:lab_courses:phys-211-wiki-home:wave-particle-duality:avalanche_photodiode.png?400| }} | The photon detectors we use are **avalanche photodiodes (APDs)**. Ordinary photodiodes are semiconductor devices which generate an electric current via the photoelectric effect when struck by visible light photons. The magnitude of the current is proportional to the intensity of the light striking the diode. APDs operate analogously to a photomultliplier tube in that they use an avalanche process to produce a large number of liberated electrons for each photon captured. The APD modules we use are designed for single photon counting and produce a TTL output pulse for each photon detected. APDs can be damaged if directly exposed to too much light, such as room lights. Our APDs are protected from the room lights by coupling them to a fiber optic cable with a collimating lens (shown right) which only allows photons coming straight into the lens to reach the APD. Further, we place a narrow bandpass interference filter in front of the fiber optic collimating lens which passes only photons with wavelength 810 ± 5 nm. The bandpass data for the filters is posted in the lab. **CAUTION**: Power to the avalanche photodiodes (APDs) should never be turned on if the APDs are not connected to the fiber optic cables and with the narrow bandpass filters in place. Check that this is the case before turning on the APDs. | {{ :phylabs:lab_courses:phys-211-wiki-home:wave-particle-duality:photodiode_collimator.png?400 |}} |
| {{ :phylabs:lab_courses:phys-211-wiki-home:wave-particle-duality:goldmirror.jpg?400 |}} | We use front surface **gold mirrors** because of their high reflectivity in the near-infrared part of the spectrum which corresponds to the ~810 nm wavelength of our down-converted photons. When light reflects off of one of these mirrors, it is reflecting off of a surface with a //higher// index of refraction (gold) then the medium through which the light is traveling (air). Therefore, there is always a phase shift of π when light reflects off of a mirror in the interferometer. |
The **50/50 cube beamsplitters** are a bit more complicated than a simple mirror. These beamsplitters are constructed by attaching two prisims where the diagonal face of one of the prisims has a dielectric coating applied to it. (For more a more detailed description see https://en.wikipedia.org/wiki/Beam_splitter.) This means that whether or not a phase shift occurs (i.e. whether the light is traveling from low to high index of refraction or vice versa) depends on which side of the beamsplitter the light enters. We have marked our beamsplitters with a "+" sign to indicate the side with the higher index of refraction and a "-" sign to indicate the side with the lower index of refraction. Using this information you can determine whether or not a reflection in the beamsplitter will cause a phase shift in the reflected light. {{ :phylabs:lab_courses:phys-211-wiki-home:wave-particle-duality:5050bs.png?400 |}} {{ :phylabs:lab_courses:phys-211-wiki-home:wave-particle-duality:piezo.png?400 |}} One of the corner mirrors is mounted to a **piezoelectric crystal**. A piezoelectric crystal (piezo) has the property that it expands when a voltage is applied to it. This expansion is highly repeatable and can be controlled at the level of fractions of a micron. By applying a voltage to the piezo we can displace the attached mirror, and therefore change the path length of this arm of the interferometer by a fraction of the wavelength of a down-converted photon. The piezo voltage is controlled by the Y AXIS module of the //3-Axis Piezo Controller//. According to the manufacturers specifications the piezo expansion is 61±15nm/V.  Also note that the piezo displacement is along a line which makes a 45º angle with respect to the incoming and reflected photon paths. {{ :phylabs:lab_courses:phys-211-wiki-home:wave-particle-duality:piezo_controller.png?400 |}} The other corner mirror is mounted on a **micrometer stage** which allows the length of the two arms of the interferometer to be precisely matched. The micrometer stage has already been adjusted so that when the piezo voltage on the opposite mirror mount is set to 50 V, both arms of the interferometer are (approximately) the same length. {{ :phylabs:lab_courses:phys-211-wiki-home:wave-particle-duality:mic_mirror.png?400 |}} {{ :phylabs:lab_courses:phys-211-wiki-home:wave-particle-duality:mzi_output.png?400 |}} Photons leaving the second beamsplitter are detected by **Port A APD** (also called APD #2) or **Port B APD** (also colled APD #3) depending on which output path they took.