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Summary of Contents for qutools quED
- Page 1 Entanglement Demonstrator Manual V4.0...
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Page 2: Table Of Contents
4.2.1 There is polarization contrast in both arms ................23 There is no polarization contrast in at least one arm ..............23 Experiments with the quED ..........................24 Measurement of correlation curves ......................25 Bell state discrimination ..........................26 Bell experiment ..............................26 Tomographic reconstruction of the two-photon density matrix .......... -
Page 3: Introduction And General System Description
The quED is a complete system for the generation and analysis of polarization-entangled photon pairs. In its basic version the quED delivers the coincidence counting rate of above 1 kHz with at least 90% visibility of correlation curves in two complementary bases. These parameters guarantee that the genuine two-particle entanglement can be detected and demonstrated, e.g. -
Page 4: Basic Principles Of Operation
To generate entangled photon pairs, a second-order nonlinear process, usually referred to as spontaneous parametric down-conversion (SPDC), is used in the quED. In the SPDC process photons of an intense laser pump beam spontaneously convert in a nonlinear crystal with a −11... - Page 5 To obtain polarization entanglement from SPDC, the quED utilizes a well-known method of coherent spatial overlap of the emissions from two adjacent type-I crystals. Consider two non- linear crystals, both operated in type-I phase-matching configuration and pumped with linearly polarized light. The otherwise identical crystals are oriented such that their optical axes lie in mutually perpendicular planes.
- Page 6 Since a (spectrally broadband) free- running blue laser diode is used as the pump in the quED, a special birefringent crystal has to be included in the path of the pump beam. It introduces a proper temporal retardation between its horizontally and vertically polarized components and thus effectively pre- compensates the effect.
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Page 7: Description
filtering of generated photons is applied in the quED. Description Referring to Fig. 3, there is shown the source for the generation of polarization-entangled photon pairs. - Page 8 Fiber coupler PM fiber Figure 3: Architecture of the quED optical setup for the generation and collection of polarization-entangled photon pairs. The pump laser and the actual crystal source is hidden under the white cover for laser safety reasons. More details are displayed in Figure 4.
- Page 9 filter optionally motorized, a long-pass filter, an adjustable optical coupler and a polarization maintaining (PM) single-mode fiber (12). Long-pass filters are inserted into the paths of down-conversion photons to block the residual laser-diode light and stray light. quED Manual www.qutools.com...
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Page 10: Unpacking And Installation
2 long pass filter caps in front of the couplers. alignment plate (only to be installed during a full alignment procedure (see Figure 8 and section 3.4.3). Optical fiber checker Cable with with SUB-D9 connectors (to connect down-conversion source with control unit) quED Manual www.qutools.com... -
Page 11: Installation
2. Place the control unit next to the breadboard and connect it to a power socket. 3. Connect the quED with the control unit using the SUB-D9 connector cable. 4. After removing the protective caps, plug and fasten the FC/PC connectors of the single mode fibers into the optical inputs of the control unit (see also Figure 5). -
Page 12: Operation
Therefore, it is absolutely necessary to take overall safety measures when operating the source. The quED should be operated in a restricted area equipped with post warning signs to alert those present. To ensure adequate warning of hazards associated with the accessible laser radiation, relevant warning signs and labels are conspicuously affixed to the quED. -
Page 13: Additional Warnings
ESD, putting on work gloves to protect against static electricity, maintaining environmental humidity 40–50% or more etc. • Opening or removing the housing of the laser head might expose the user to the danger of laser radiation and might cause the laser diode failure. quED Manual www.qutools.com... -
Page 14: Switching On
If the displayed single and coincidence count rates reach approximately the values specified in the inspection data sheet, the quED is still perfectly aligned and it is ready to be used for entanglement tests and other experiments. b) If the displayed coincidence count rate is well above zero and single count rates are well above the level of dark counts, but all of them do not reach the regime specified in the... -
Page 15: Improving The Coupling
Pointing alignment: This first type of adjustment aims to correct for a simple mispointing. In the quED case, this means that one (or both) of the fiber couplers does not point exactly into the pump center within the BBO crystal while it is assumed, that the angle of the coupler axis relative to the pump beam is fine. -
Page 16: Realignment Of The Setup
Angle alignment, aka walking: This kind of adjustment aims to manipulate the angle under which a target is hit. For the quED this usually means the angle between the coupler mode and the pump beam axis while the overlap with the target – the pump center – is already established. -
Page 17: Full Alignment Procedure
This alignment can only be accomplished iteratively: first, align for the engraving on the helper plate using the two screws of the coupler mount, then remove the plate and align for the small cross using the screws of the mirror mount. Repeat this until both marks are hit simultaneously. quED Manual www.qutools.com... - Page 18 3.4.2 with diagonal polarizers. You might want to check the HH and VV coincidence rates to be fairly equal for a better entanglement. 17. Check the entanglement as described in section 3.5. quED Manual www.qutools.com...
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Page 19: Checking The Entanglement Quality (Quick Procedure)
Figure 7: Alignment of the coupling modes using the alignment screen with the pinhole and the engraved pinpoints. Figure 8: Alignment helper plate. To be used only for a full alignment procedure. Remove for normal quED operation. quED Manual www.qutools.com... -
Page 20: Troubleshooting
BBO crystals are preadjusted to overlap maximally. Due to the spectral width, however, one can get reasonable coincidence rates from asymmetric collection modes (see Figure 10) The procedure in Figure 11 aims to correct this error. quED Manual www.qutools.com... - Page 21 ). From the pre-alignment you might arrive with a situation as indicated with the red lines. While the absolute value of �� is not crucial for the entanglement, follow the instructions in section 4.1 to make the coupling symmetric. quED Manual www.qutools.com...
- Page 22 “d” (not so far as to lose the coincidence rate completely) and compensate with the horizontal adjuster of the coupler mount. For fine tuning, this procedure can also be applied to the vertical direction. quED Manual www.qutools.com...
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Page 23: High Single Count Rates But Very View Coincidences
3.4.3. There is no polarization contrast in at least one arm This means, the rates are due to fluorescence in the long pass filter. The situation for both arms is sketched in Figure 12. quED Manual www.qutools.com... -
Page 24: Experiments With The Qued
Figure 12 Experiments with the quED We suggest and describe a few experiments to be accomplished with the quED, providing the basis for an undergraduate laboratory course on quantum mechanics and quantum optics. In the following the measurement procedures and the interpretation of results are discussed in short for each of the experiments. -
Page 25: Measurement Of Correlation Curves
Measurement of correlation curves The simplest test to verify entanglement of photon pairs generated in the quED involves a measurement of correlation curves in two non-orthogonal complementary bases. This is accomplished by fixing the orientation �� of one polarizing filter and continuously changing the orientation ��... -
Page 26: Bell State Discrimination
| �� ⟩ √ 2 √ 2 can be directly prepared in the quED by rotating the pre-compensation crystal, please see sections for more details. They both show correlations in the horizontal/vertical basis, i.e. maximum coincidence count rates are observed for the polarizer combinations �� = 0°/ �� = 0°, ��... - Page 27 ��(��, ��) in the definition of the value ��. Each of the terms corresponding to the four terms ��(��, ��) is calculated from four numbers of coincidences, making together 16 count rates which have to be experimentally taken. quED Manual www.qutools.com...
- Page 28 _____________________________________________________________________________________________________________________ When the individual events are assumed to be independent of each other, the resulting statistics becomes Poissonian. The variance of a Poisson distribution with mean �� is also ��. √ ��. Hence the standard deviation is quED Manual www.qutools.com...
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Page 29: Tomographic Reconstruction Of The Two-Photon Density Matrix
Tomographic reconstruction of the two-photon density matrix The output from real-life entanglement sources like the quED is not truly pure as described by the Bell states in section 5.2. In reality we always have to deal with mixed polarization states, which are completely characterized using density matrices.
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