Abstract Adaptive quantum state estimation (AQSE) provides the best possible estimation of unknown parameters of quantum states. In implementations of AQSE, half wave plates (HWPs) driven by mechanical rotators are used to change the measurement basis, but these significantly limit the overall speed of the system. Here we report an AQSE system using a liquid crystal retarder (LCR). With precise calibration, the phase accuracy of an LCR can be below 3.6 × 10 −3 radians and as low as 1.2 × 10 −3 radians. Using an LCR, the time required to change the measurement basis is approximately 30 times smaller than for a system using an HWP, and the total time of AQSE is reduced by a factor of 5.
Abstract Adaptive quantum state estimation (AQSE) provides the best possible estimation of unknown parameters of quantum states. In implementations of AQSE, half wave plates (HWPs) driven by mechanical rotators are used to change the measurement basis, but these significantly limit the overall speed of the system. Here we report an AQSE system using a liquid crystal retarder (LCR). With precise calibration, the phase accuracy of an LCR can be below 3.6 × 10 −3 radians and as low as 1.2 × 10 −3 radians. Using an LCR, the time required to change the measurement basis is approximately 30 times smaller than for a system using an HWP, and the total time of AQSE is reduced by a factor of 5.
Accurate estimation of unknown quantum states using limited number of samples is very important but difficult task using fixed measurement basis states since the best local unbiased estimator (LUE) for estimating the parameter depends, in general, on the unknown parameter itself. In order to solve this problem, Nagaoka advocated an adaptive quantum state estimation (AQSE) procedure. Later, Fujiwara proved the strong consistency and asymptotic efficiency for AQSE. AQSE has been demonstrated by us for the linear polarizations of photons, and recently for a single polarization qubit. However, in order to apply AQSE to actual measurement including application to life science, we need to improve the speed of the system which is limited by the response time of the mechanical rotation stage used in the previous experiment [Phys. Rev. Lett. 109, 130404 (2012)]. To solve this problem, we realized high-speed AQSE experiment using liquid crystal retarder (LCR). We demonstrate the adaptive quantum state estimation using a LCR instead of a half wave plate on a mechanical rotator in previous research. We realized a substantial speed-up of estimation time, keeping the accuracy of estimation at the same level. In fact, it took 0.40 second per photon in the previous study, whereas 0.073 second per photon in the present. Specifically, the time to determine the polarization measurement basis was greatly decreased, from 0.34 second per photon in the previous study to 0.010 second per photon in the present.
Summary form only given. Quantum theory is inherently statistical. This entails repetition of experiments over a number of identically prepared quantum objects, for example, quantum states, if one wants to know the “true state” or the “true value” of the parameter that specifies the quantum state. Such an estimation procedure is particularly important for quantum communication and quantum computation, and is also indispensable to quantum metrology [1,2]. In applications, one needs to design the estimation procedure in such a way that the estimated value of the parameter should be close to the true value (consistency), and that the uncertainty of the estimated value should be as small as possible (efficiency) for a given limited number of samples. In order to realize these requirements, Nagaoka advocated an adaptive quantum state estimation (AQSE) procedure [3], and recently Fujiwara proved the strong consistency and asymptotic efficiency for AQSE [4]. In this paper, we report the first experimental demonstration of AQSE using photons[5]. The angle of a half wave plate (HWP) that initializes the linear polarization of input photons is estimated using AQSE (Fig. 1). sequence of AQSE is carried out with 300 input photons, and the sequence is repeated 500 times for four different settings of HWP. The statistical analysis of these results verifies the strong consistency and asymptotic efficiency of AQSE. Recently, it has been mathematically proven that the precision of AQSE outperforms the conventional state tomography [6]. It is thus expected that AQSE will provide a useful methodology in the broad area of quantum information processing, communication, and metrology. This work was supported in part by JSPS Quantum Cybernetics, JSPS-Kakenhi, JST-CREST, FIRST Program, Special Coordination Funds for Promoting Science and Technology, Research Foundation for Opto-Science and Technology.
Accurate estimation of unknown quantum states using limited number of samples is very important but difficult task using fixed measurement basis states since the best local unbiased estimator (LUE) for estimating the parameter depends, in general, on the unknown parameter itself. In order to solve this problem, Nagaoka advocated an adaptive quantum state estimation (AQSE) procedure [1], where the measurement basis states are adaptively selected for each measurement result. Fujiwara proved the strong consistency and asymptotic efficiency for AQSE [2]. Recently, AQSE has been demonstrated by us for the linear polarizations of photons [3]. However, AQSE was designed for a static quantum state, and was not applicable to dynamic states where quantum states change in time. Here, we propose a new protocol, sequential adaptive quantum state estimation (SAQSE) for dynamic quantum states. Our numerical simulation (shown below) clearly show that the estimated value given by SAQSE can follow the true value changing in time whereas AQSE fails. We believe this new protocol will open up the use of quantum state estimation for various applications including biology, e.g. monitoring the motion of fluorescent molecules.
The first experimental demonstration of an adaptive quantum state estimation (AQSE) is reported. The strong consistency and asymptotic efficiency of AQSE have been mathematically proven [A. Fujiwara, J. Phys. A 39, 12489 (2006)]. In this Letter, the angle of linear polarization of single photons, the phase parameter between the right and the left circularly polarization, is estimated using AQSE, and the strong consistency and asymptotic efficiency are experimentally verified. AQSE will provide a general useful method in both quantum information processing and metrology.
Quantum science and technology have seen rapid progress in recent decades, finding applications in computation, information processing, communication, and sensing. Accurate characterization and manipulation of quantum states is essential to the successful implementation of these applications. Moreover, optimal quantum state estimation is closely related to fundamental questions, such as limits on how fast information can be extracted about quantum systems. In this thesis, we present several contributions to efficiently estimating and transforming quantum states. We discuss optimal quantum state estimation, both in the regime of few available quantum systems as well as the asymptotic regime. We numerically investigate how assuming a pure-state can affect the performance of tomographic methods, and where adaptive quantum state tomography is in fact advantageous. While collective measurements have been formally considered as optimal solutions to various problems in quantum information, their implementation has been largely neglected due to experimental challenges. We present a proof-of-concept experiment realizing Massar and Popescu's optimal collective measurement for quantum state estimation, utilizing two-photon interference and polarization-dependent loss to achieve non-maximally entangled state projections. We demonstrate that collective strategies can outperform the best separable measurements. Furthermore, we introduce and experimentally implement a fast and automated optical polarization compensation scheme using liquid crystal variable retarders and rotating quarter-wave plate tomography. Desired accuracy thresholds can be achieved within a few compensation steps. Finally, we use a matrix-model approach to investigate the realization of spatial unitary transformations via sequences of thin phase gratings and free-space propagation. We numerically demonstrate that the grating parameters and propagation distances can be optimized to achieve various two-beam and three-beam unitary transformations, such as Pauli gates and the discrete Fourier transform. Through experimental and numerical approaches, this thesis offers new perspectives on methods for quantum state estimation and unitary transformation.