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Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 19, Iss. 27 — Dec. 19, 2011
  • pp: 26011–26016
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Entangled photon polarimetry

Joseph B. Altepeter, Neal N. Oza, Milja Medić, Evan R. Jeffrey, and Prem Kumar  »View Author Affiliations


Optics Express, Vol. 19, Issue 27, pp. 26011-26016 (2011)
http://dx.doi.org/10.1364/OE.19.026011


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Abstract

We construct an entangled photon polarimeter capable of monitoring a two-qubit quantum state in real time. Using this polarimeter, we record a nine frames-per-second video of a two-photon state’s transition from separability to entanglement.

© 2011 OSA

1. Introduction

Photonic entanglement is a fundamental resource for quantum information processing and quantum communications [1

1. M. Nielsen and I. Chuang, Quantum computation and quantum information (Cambridge Univ. Press2000).

]. Engineering suitable entanglement sources for a particular application, or integrating those sources into a larger system, however, can be a challenging experimental task. Generating high-quality entanglement requires protecting against or compensating for decoherence, single-qubit rotations, and partial projections. For both free-space [2

2. P. G. Kwiat, K. Mattle, H. Weinfurter, A. Zeilinger, A. V. Sergienko, and Yanhua Shih, “New high-intensity source of polarization-entangled photon pairs,” Phys. Rev. Lett. 75, 4337–4341 (1995). [CrossRef] [PubMed]

5

5. M. Aspelmeyer, H. R. Böhm, T. Gyatso, T. Jennewein, R. Kaltenbaek, M. Lindenthal, G. Molina-Terriza, A. Poppe, K. Resch, M. Taraba, R. Ursin, P. Walther, and A. Zeilinger, “Long-distance free-space distribution of quantum entanglement,” Science 301, 5633 (2003). [CrossRef]

] and fiber/waveguide-based entanglement sources [6

6. X. Li, P. L. Voss, J. E. Sharping, and P. Kumar, “Optical-fiber source of polarization-entangled photons in the 1550 nm telecom band,” Phys. Rev. Lett. 94, 053601 (2005). [CrossRef] [PubMed]

10

10. M. Medic, J. B. Altepeter, M. A. Hall, M. Patel, and P. Kumar, “Fiber-based telecommunication-band source of degenerate entangled photons,” Opt. Lett. 35, 802–804 (2010). [CrossRef] [PubMed]

], this means compensating for any polarization rotations or decohering effects which may occur in transit to a destined application. In addition to the aforementioned static effects, it is necessary to test the source’s stability in the face of real-time system perturbations such as atmospheric turbulence or fiber breathing owing to environmental fluctuations. At present, the best available technique for measuring two-qubit entangled states is quantum state tomography [11

11. U. Leonhardt, “Quantum-state tomography and discrete Wigner function,” Phys. Rev. Lett. 74, 4101–4105 (1995). [CrossRef] [PubMed]

16

16. M. S. Kaznady and D. F. V. James, “Numerical strategies for quantum tomography: Alternatives to full optimization,” Phys. Rev. A 79, 022109 (2009). [CrossRef]

], a procedure which can provide a precise reconstruction of the quantum state, but which generally requires 5–30 minutes to complete. This long measurement time can make debugging systematic experimental problems—particularly those with short timescales—challenging, if not impossible.

The field of classical optical communications has faced similar problems when transmitting polarized light over long distances. A polarimeter is a common tool which is used to debug unwanted polarization rotations or depolarization effects, which provides an experimenter with a real-time picture of the optical field’s polarization state. An entangled photon polarimeter—a measurement device capable of performing quantum tomographies and displaying the reconstructed two-qubit states in real time—would be a valuable tool for optimizing and deploying entangled photon sources.

In this paper we present the first experimental implementation of an entangled photon polarimeter, which is capable of displaying nine reconstructed density matrices per second via complete quantum state tomographies. This represents a speed improvement of 2–3 orders of magnitude over the best quantum state tomography systems currently in use in laboratories around the world. Using this new tool, we record the first live video—at 9 frames-per-second (fps)—of a two-photon quantum state’s transition from separability to entanglement.

2. Two-qubit polarimetry

Two-qubit polarimetry is a specific example of two-qubit quantum state tomography, a procedure for reconstructing an unknown quantum state from a series of measurements (generally either 9 or 36 coincidence measurements performed using two single-photon detectors per qubit [14

14. R. T. Thew, K. Nemoto, A. G. White, and W. J. Munro, “Qudit quantum-state tomography,” Phys. Rev. A 66, 012303 (2002). [CrossRef]

]), each performed on an ensemble of identical copies of the unknown state. Three key parameters can be used to characterize any experimental apparatus for quantum state tomography: the time required to complete the state reconstruction procedure and the accuracy and precision with which the reconstructed density matrix represents the unknown quantum state.

The time required to complete a tomography, T, is given by TM × (τm + τs) + τa, where M is the number of two-qubit measurement settings taken per reconstruction, τm is the time per measurement setting, τs is the the time to switch between measurement settings, and τa is the time to numerically reconstruct the unknown density matrix from an analysis of the measurement results.

The accuracy and precision of a tomography are closely related, both indicating how closely the reconstructed density matrix, ρ, matches the “true” unknown density matrix, ρideal. The “accuracy” of a tomographic reconstruction measures error due to systematic effects, such as improperly performed projective measurements, uncharacterized drifts in the detectors’ efficiency, or a non-identical ensemble of unknown quantum states. The “precision” of a tomographic reconstruction measures the statistical error in ρ, and is strongly dependent on the total number of measurable states N in the identical ensemble (which is in turn dependent on the entanglement source’s pair production rate, R, and the total single-qubit measurement efficiency, η). In general, the tomographic precision decreases as T (and therefore N) decreases [16

16. M. S. Kaznady and D. F. V. James, “Numerical strategies for quantum tomography: Alternatives to full optimization,” Phys. Rev. A 79, 022109 (2009). [CrossRef]

]. For sufficiently small T we can neglect systematic effects and quantify tomographic precision (as a function of N and of ρideal) to be the average fidelity between ρ and ρideal:
Fp(N,ρideal)F(ρ,ρideal)¯=(Tr{ρρidealρ})2¯.
(1)
Note that the equation above uses the usual definition for fidelity between two mixed states [17

17. R. Jozsa, “Fidelity for mixed quantum states,” J. of Mod. Opt. 41, 2315–2323 (1994). [CrossRef]

], which for a pure ρideal ≡ |ψ〉〈ψ|, simplifies to the more familiar F (ρ,ρideal) ≡ Tr{ρρideal} = 〈ψ|ρ|ψ〉. Figure 1(a) shows Fp(N, |ϕ+〉〈ϕ+|) with |ϕ+12(|HH+|VV), where each data point represents a Monte Carlo simulation of the average fidelity between a reconstructed density matrix and the ideal unknown state.

Fig. 1 (a) Tomographic precision Fp(N,ρideal) for ρideal = |ϕ+〉〈ϕ+| with |ϕ+=12(|HH+|VV). Each data point represents a 2000-tomography numerical Monte Carlo simulation of the average fidelity between the reconstructed density matrix and ρideal, under realistic assumptions about the system noise (a coincidence-to-accidental ratio of 3). Each simulated tomography utilizes four detectors and nine coincidence measurements, such that each unknown quantum state in the N-state ensemble is projected onto one of nine four-element orthonormal bases (e.g., HH, HV, VH, VV). Results for both the maximum likelihood technique and the truncated-eigenvalue, linear-least-squares-fit technique are shown. For a given N, the maximum likelihood technique is slightly more precise [16]. (b) Using the same simulated data, Fp is shown as a function of total tomography time T for two different experimental systems: a traditional free-space tomography system with η = 0.1, τs = 5 s, τa = 5 s and an entangled photon polarimeter with η = 0.07, τs = 0.02 s, τa = 0.001 s. In both systems R = 106 pairs/second and M = 9.

Two-qubit polarimetry is an application of two-qubit polarization tomography which maximizes precision for very short T (≤ 1s), allowing an experimenter to manipulate an entangled photon source using real-time tomographic feedback (by updating after every measurement, the time between updates can be reduced to T/9). (In this paper, entangled photon polarimetery refers to the application of two-qubit polarimetry to entangled photon states.) Because maximizing precision requires maximizing N, the ideal entangled photon polarimeter will minimize both the time between measurements (τs) and the time for numerical analysis (τa):
N=Rη2Mτm=Rη2(TMτsτa).
(2)

3. Experimental details

Below, we briefly discuss the differences between these two techniques after reviewing the entangled photon source used to test the tomography apparatuses. Figure 1(b) highlights the differences between the two techniques, showing the expected tomographic precision Fp as a function of total tomography time T.

3.1. Entangled photon source

To test the entangled photon polarimeter, we utilize a fiber-based, frequency-degenerate, 1550-nm, polarization entangled photon-pair source [10

10. M. Medic, J. B. Altepeter, M. A. Hall, M. Patel, and P. Kumar, “Fiber-based telecommunication-band source of degenerate entangled photons,” Opt. Lett. 35, 802–804 (2010). [CrossRef] [PubMed]

]. The source utilizes spontaneous four-wave-mixing in dispersion-shifted fiber and is pumped by 50-MHz repetition rate dual-frequency pulses spectrally carved from the output of a femtosecond pulsed laser. Because the output photons are identical, reverse Hong-Ou-Mandel interference in a Sagnac loop is used to deterministically split the output photons into separate output single-mode fibers. See Fig. 2(a).

Fig. 2 (a) The entangled photon source used to test the entangled photon polarimeter. (b) The entangled photon polarimeter, composed of fast electro-optic modulators, in-fiber polarizers, and a four-detector array.

The same source is used to test two separate tomography systems, the automated wave-plate-based apparatus first described in [10

10. M. Medic, J. B. Altepeter, M. A. Hall, M. Patel, and P. Kumar, “Fiber-based telecommunication-band source of degenerate entangled photons,” Opt. Lett. 35, 802–804 (2010). [CrossRef] [PubMed]

] and the entangled photon polarimeter presented here.

3.2. Polarization measurements

Traditionally, two-qubit polarization tomography is performed using bulk, free-space, birefringent crystals (i.e., wave plates). A quarter- and a half-wave plate followed by a polarizer on each of the two qubits can implement an arbitrary projective measurement [14

14. R. T. Thew, K. Nemoto, A. G. White, and W. J. Munro, “Qudit quantum-state tomography,” Phys. Rev. A 66, 012303 (2002). [CrossRef]

]. By collecting photons from both the transmitted and the reflected ports of each qubit’s polarizer, one can project an unknown photon pair into one of four orthonormal basis states, defined by the wave plates. If well characterized, this can lead to a very accurate tomography, though the measurement-to-measurement transition time τs will in general be very large (≈ 5s). For the fiber-based source above, this type of polarization analyzer will lead to a single-qubit loss of ≈ 1.5 dB (including the fiber to free-space to fiber coupling losses).

To decrease τs, we have constructed an all-fiber/waveguide polarization analyzer based on electro-optic modulators (EOMs). These LiNbO3 EOMs (EOSpace, model PC-B4-00-SFU-SFU-UL) allow precise control of both the retardance and optic axis of a birefringent crystalline waveguide using the fringe fields from three electrodes. In general, this process has an extremely short response time leading to EOM switching rates of up to 10 MHz. In practice, we are able to implement arbitrary polarization measurements at 125 kHz, which is a limit set by the speed of our computer-controlled voltage sources.

Although high-speed, EOMs are more difficult to precisely characterize than bulk wave plates; using a standard polarimeter we have characterized the six transformations performed by each EOM-based analyzer (corresponding to projections onto the H, V, D(H+V)/2, A(HV)/2, R(H+iV)/2, and L(HiV)/2 basis states). EOM projections deviated from an ideal measurement by an average of 2.1 degrees on the Poincaré sphere. The single-qubit losses of the EOM-based analyzers varied between 3.0–3.4 dB.

3.3. Single-photon detection

3.4. Tomographic reconstruction

By using a simpler analysis technique based on a linear least-squares fit, we are able to increase the state reconstruction speed by more than three orders of magnitude [16

16. M. S. Kaznady and D. F. V. James, “Numerical strategies for quantum tomography: Alternatives to full optimization,” Phys. Rev. A 79, 022109 (2009). [CrossRef]

]. We use the 2-qubit Stokes vector as a linear model, and solve the least-squares problem wM · S = wC. Here, M is the set of measurements, which can be arbitrary POVMs; C is the measured counts, and S is the Stokes vector we solve for; w is a weight vector representing the distribution width for each measurement. We assume the counting process to be Poissonian, and use the large-N limit where the Poisson distribution is approximated as a Gaussian with width N. To guarantee a legal density matrix, we post-process the least-squares fit by truncating the negative eigenvalues [16

16. M. S. Kaznady and D. F. V. James, “Numerical strategies for quantum tomography: Alternatives to full optimization,” Phys. Rev. A 79, 022109 (2009). [CrossRef]

]. We have found that this type of linear fit provides results identical to those obtained via the maximum likelihood method with a negligible drop in precision (see Fig. 1(a)), only much faster (∼ 1.3 ms per tomography using Matlab on a 2.4-GHz CPU).

4. Entangled photon polarimeter performance

By utilizing fast EOM-based analyzers, a four-detector array triggered at 50 MHz, and a linear least-squares algorithm for tomographic reconstruction, the entangled photon polarimeter is capable of performing nine tomographies per second. Operated at this speed, τm = 80 ms, τs = 20 ms, and τa = 1 ms. Total single-qubit insertion loss is measured to be η = 3–3.4 dB (not including detector inefficiency). The tomographic precision is estimated using a Monte Carlo simulation of this polarimeter’s application to the entanglement source pictured in Fig. 2 (≈ 1000 coincidences / second). For nine-measurement tomographies (T ≈ 1s), Fp(N = 1000,ρideal) ≈ 92%. For 36-measurement tomographies (T ≈ 4s), Fp(N = 4000,ρideal) ≈ 96%. Because long (30-minute) tomographies have previously verified the source’s fidelity to a maximally entangled state to be 99.7% ± 0.4% [10

10. M. Medic, J. B. Altepeter, M. A. Hall, M. Patel, and P. Kumar, “Fiber-based telecommunication-band source of degenerate entangled photons,” Opt. Lett. 35, 802–804 (2010). [CrossRef] [PubMed]

], we use |ϕ+〉 as an approximation to ρideal.

To experimentally verify this performance, we recorded three 9-fps live videos of a two-photon polarization state using the 36-measurement configuration. First we recorded two videos where the measured state is not changed during the course of the measurement run, for a totally separable pure state, |DV〉 (see Fig. 3( Media 2)), and a maximally entangled state, |ϕ+〉 (see Fig. 3( Media 3)). By analyzing each frame and comparing it to the target state, we directly measured the system precision to be 98% ± 1% (for |DV〉) and 95% ± 2% (for |ϕ+〉). Note that this experimentally measured system precision for |ϕ+〉 is in good agreement with the theoretically predicted 96% (see the prediction for a 4-s, 36-measurement tomography above). Finally, we recorded a video of a two-photon state’s transition from separability to entanglement (the transition is physically implemented by rotating wave plate HWP in the entangled photon source setup—see Fig. 2). Selected frames from this video are shown in Fig. 3( Media 1).

Fig. 3 Selected frames from the nine fps video of a two-qubit photon state’s transition from separability to entanglement ( Media 1). Each frame shows a density matrix reconstructed using the previous 36 measurements (≈ 4 s of data). Similar videos show non-transitioning separable ( Media 2) and maximally entangled ( Media 3) states. (All videos at 3x speed).

This research was supported in part by the DARPA ZOE program (Grant No. W31P4Q-09-1-0014) and the NSF IGERT Fellowship (Grant No. DGE-0801685).

References and links

1.

M. Nielsen and I. Chuang, Quantum computation and quantum information (Cambridge Univ. Press2000).

2.

P. G. Kwiat, K. Mattle, H. Weinfurter, A. Zeilinger, A. V. Sergienko, and Yanhua Shih, “New high-intensity source of polarization-entangled photon pairs,” Phys. Rev. Lett. 75, 4337–4341 (1995). [CrossRef] [PubMed]

3.

J. B. Altepeter, E. R. Jeffrey, and P. G. Kwiat, “Phase-compensated ultra-bright source of entangled photons,” Opt. Exp. 13, 8951–8959 (2005). [CrossRef]

4.

C.-Z. Peng, T. Yang, X.-H. Bao, J. Zhang, X.-M. Jin, F.-Y. Feng, B. Yang, J. Yang, J. Yin, Q. Zhang, N. Li, B.-L. Tian, and J.-W. Pan, “Experimental free-space distribution of entangled photon pairs over 13 km: towards satellite-based global quantum communication,” Phys. Rev. Lett. 94, 150501 (2005). [CrossRef] [PubMed]

5.

M. Aspelmeyer, H. R. Böhm, T. Gyatso, T. Jennewein, R. Kaltenbaek, M. Lindenthal, G. Molina-Terriza, A. Poppe, K. Resch, M. Taraba, R. Ursin, P. Walther, and A. Zeilinger, “Long-distance free-space distribution of quantum entanglement,” Science 301, 5633 (2003). [CrossRef]

6.

X. Li, P. L. Voss, J. E. Sharping, and P. Kumar, “Optical-fiber source of polarization-entangled photons in the 1550 nm telecom band,” Phys. Rev. Lett. 94, 053601 (2005). [CrossRef] [PubMed]

7.

J. Fan, M. D. Eisaman, and A. Migdall, “Bright phase-stable broadband fiber-based source of polarization-entangled photon pairs,” Phys. Rev. A 76, 043836 (2007). [CrossRef]

8.

H. Takesue, H. Fukuda, T. Tsuchizawa, T. Watanabe, K. Yamada, Y. Tokura, and S. Itabashi, “Generation of polarization entangled photon pairs using silicon wire waveguide,” Opt. Exp. 165721–5727 (2008). [CrossRef]

9.

M. A. Hall, J. B. Altepeter, and P. Kumar, “Drop-in compatible entanglement for optical-fiber networks,” Opt. Exp. 17, 14558–14566 (2009). [CrossRef]

10.

M. Medic, J. B. Altepeter, M. A. Hall, M. Patel, and P. Kumar, “Fiber-based telecommunication-band source of degenerate entangled photons,” Opt. Lett. 35, 802–804 (2010). [CrossRef] [PubMed]

11.

U. Leonhardt, “Quantum-state tomography and discrete Wigner function,” Phys. Rev. Lett. 74, 4101–4105 (1995). [CrossRef] [PubMed]

12.

K. Banaszek, G. M. DAriano, M. G. A. Paris, and M. F. Sacchi, “Maximum-likelihood estimation of the density matrix,” Phys. Rev. A 61, 010304(R) (1999). [CrossRef]

13.

D. F. V. James, P. G. Kwiat, W. J. Munro, and A. G. White, “Measurement of qubits,” Phys. Rev. A 64, 052312 (2001). [CrossRef]

14.

R. T. Thew, K. Nemoto, A. G. White, and W. J. Munro, “Qudit quantum-state tomography,” Phys. Rev. A 66, 012303 (2002). [CrossRef]

15.

J. B. Altepeter, E. R. Jeffrey, and P. G. Kwiat, “Photonic state tomography,” Adv. At., Mol., Opt. Phys. 52, 105–159 (2005).

16.

M. S. Kaznady and D. F. V. James, “Numerical strategies for quantum tomography: Alternatives to full optimization,” Phys. Rev. A 79, 022109 (2009). [CrossRef]

17.

R. Jozsa, “Fidelity for mixed quantum states,” J. of Mod. Opt. 41, 2315–2323 (1994). [CrossRef]

OCIS Codes
(120.2130) Instrumentation, measurement, and metrology : Ellipsometry and polarimetry
(270.5565) Quantum optics : Quantum communications

ToC Category:
Quantum Optics

History
Original Manuscript: August 31, 2011
Revised Manuscript: November 13, 2011
Manuscript Accepted: November 14, 2011
Published: December 6, 2011

Citation
Joseph B. Altepeter, Neal N. Oza, Milja Medić, Evan R. Jeffrey, and Prem Kumar, "Entangled photon polarimetry," Opt. Express 19, 26011-26016 (2011)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-19-27-26011


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References

  1. M. Nielsen and I. Chuang, Quantum computation and quantum information (Cambridge Univ. Press2000).
  2. P. G. Kwiat, K. Mattle, H. Weinfurter, A. Zeilinger, A. V. Sergienko, and Yanhua Shih, “New high-intensity source of polarization-entangled photon pairs,” Phys. Rev. Lett.75, 4337–4341 (1995). [CrossRef] [PubMed]
  3. J. B. Altepeter, E. R. Jeffrey, and P. G. Kwiat, “Phase-compensated ultra-bright source of entangled photons,” Opt. Exp.13, 8951–8959 (2005). [CrossRef]
  4. C.-Z. Peng, T. Yang, X.-H. Bao, J. Zhang, X.-M. Jin, F.-Y. Feng, B. Yang, J. Yang, J. Yin, Q. Zhang, N. Li, B.-L. Tian, and J.-W. Pan, “Experimental free-space distribution of entangled photon pairs over 13 km: towards satellite-based global quantum communication,” Phys. Rev. Lett.94, 150501 (2005). [CrossRef] [PubMed]
  5. M. Aspelmeyer, H. R. Böhm, T. Gyatso, T. Jennewein, R. Kaltenbaek, M. Lindenthal, G. Molina-Terriza, A. Poppe, K. Resch, M. Taraba, R. Ursin, P. Walther, and A. Zeilinger, “Long-distance free-space distribution of quantum entanglement,” Science301, 5633 (2003). [CrossRef]
  6. X. Li, P. L. Voss, J. E. Sharping, and P. Kumar, “Optical-fiber source of polarization-entangled photons in the 1550 nm telecom band,” Phys. Rev. Lett.94, 053601 (2005). [CrossRef] [PubMed]
  7. J. Fan, M. D. Eisaman, and A. Migdall, “Bright phase-stable broadband fiber-based source of polarization-entangled photon pairs,” Phys. Rev. A76, 043836 (2007). [CrossRef]
  8. H. Takesue, H. Fukuda, T. Tsuchizawa, T. Watanabe, K. Yamada, Y. Tokura, and S. Itabashi, “Generation of polarization entangled photon pairs using silicon wire waveguide,” Opt. Exp.165721–5727 (2008). [CrossRef]
  9. M. A. Hall, J. B. Altepeter, and P. Kumar, “Drop-in compatible entanglement for optical-fiber networks,” Opt. Exp.17, 14558–14566 (2009). [CrossRef]
  10. M. Medic, J. B. Altepeter, M. A. Hall, M. Patel, and P. Kumar, “Fiber-based telecommunication-band source of degenerate entangled photons,” Opt. Lett.35, 802–804 (2010). [CrossRef] [PubMed]
  11. U. Leonhardt, “Quantum-state tomography and discrete Wigner function,” Phys. Rev. Lett.74, 4101–4105 (1995). [CrossRef] [PubMed]
  12. K. Banaszek, G. M. DAriano, M. G. A. Paris, and M. F. Sacchi, “Maximum-likelihood estimation of the density matrix,” Phys. Rev. A61, 010304(R) (1999). [CrossRef]
  13. D. F. V. James, P. G. Kwiat, W. J. Munro, and A. G. White, “Measurement of qubits,” Phys. Rev. A64, 052312 (2001). [CrossRef]
  14. R. T. Thew, K. Nemoto, A. G. White, and W. J. Munro, “Qudit quantum-state tomography,” Phys. Rev. A66, 012303 (2002). [CrossRef]
  15. J. B. Altepeter, E. R. Jeffrey, and P. G. Kwiat, “Photonic state tomography,” Adv. At., Mol., Opt. Phys.52, 105–159 (2005).
  16. M. S. Kaznady and D. F. V. James, “Numerical strategies for quantum tomography: Alternatives to full optimization,” Phys. Rev. A79, 022109 (2009). [CrossRef]
  17. R. Jozsa, “Fidelity for mixed quantum states,” J. of Mod. Opt.41, 2315–2323 (1994). [CrossRef]

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