# Periodic single-photon source and quantum memory

## Summary (1 min read)

### 2. PERIODIC SINGLE-PHOTON SOURCE

- It should be noted that the heralding efficiencies in parametric down-conversion can be quite high.
- The authors current experimental efforts involve constructing two of these periodic sources, and demonstrating higherorder interference effects among the single-photons emitted from them.
- The required mode-matching of these independent photon sources is being accomplished through the use of single-mode optical fibers for spatial mode-matching, as well as temporal mode-matching through the use of ultra-short pulsed-pump down-conversion and narrow-band spectral filtering as used, for example, in quantum teleportation experiments.
- 16 Even with the additional parametric down-conversion phase-matching complications arising from the use of femtosecond pumping pulses, and the restrictions of coupling light into single-mode fibers, the authors have recently achieved heralding efficiencies higher than 50% using variations on the coupling techniques described by Weinfurter's group.

### 3. CYCLICAL QUANTUM MEMORY DEVICE

- In analogy with the periodic single-photon source results of Figure 4 , the upper row of Figure 6 demonstrates the ability to store and switch out the photons after some chosen number of cycles.
- For each of the plots in the upper row, the corresponding plot in the lower row shows the results of measuring the polarization state (eg. qubit value) with a polarization analyzer.
- The results clearly showed the expected Cosine-squared signature of the 30 o linear polarization state for photons stored an odd number of cycles, and the expected 60 o linear polarization state (eg. bit-flipped value) for photons stored an even number of cycles.

### 4. PERIODIC ENTANGLED PHOTON-PAIR SOURCE

- Whereas loss in either of the CQM's will cause an obvious reduction in the quality of the stored and released entangled state, the effects of phase shifts between the two devices are not as detrimental.
- Overall phase shifts between the two devices essentially factor out of the two-photon state in such a way that phase-locking of the two memory loops is not required to maintain the coherence of the stored Bell state.
- Fortunately, as described in Section 3, the CQM's are (in principle) immune to these phase shifts for pairs stored an even number of cycles.

### 5. SUMMARY

- The authors recent demonstrations of a periodic single-photon source 11 and cyclical quantum memory for singlephoton qubits 12 were of a proof-of-principle nature, and suffered from relatively large sources of errors.
- Both of these periodic devices suffered from losses on the order of 20% per round trip, which is clearly insufficient for a practical realization of applications such as the proposed method of producing a periodic source of entangled photon pairs.

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### Cites background from "Periodic single-photon source and q..."

...Temporal has been proposed for single-photon [7, 8] and entangled state generation [7, 9, 14, 20], as well as for photon memories [9] and boson sampling schemes [21]....

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### "Periodic single-photon source and q..." refers methods in this paper

...For example, the enlarged view of the CNOT gate within the dashed box of Figure 7 shows the use of our previously proposed probabilistic CNOT gate which uses two polarizing beam splitters and an ancilliary pair of entangled photons.(7) The use of a probabilistic linear optics CNOT gate (as opposed to a deterministic one) in this protocol will reduce the rate at which entangled pairs are heralded but, at least in principle, will not affect their quality....

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...In this scenario the CNOT gate can be probabilistic, and the expanded view of the dashed box shows the use of our proposed linear optics CNOT gate based on polarizing beam splitters.(7) The required values of the detected qubits are described in the text....

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