Single-photon Polarization Research Articles

Dependence of the decoherence of polarization states in phase-damping channels on the frequency spectrum envelope of photons

We consider the decoherence of photons suffering in phase-damping channels. By exploring the evolutions of single-photon polarization states and two-photon polarization-entangled states, we find that different frequency spectrum envelopes of photons induce different decoherence processes. A white frequency spectrum can lead the decoherence to an ideal Markovian process. Some color frequency spectrums can induce asymptotical decoherence, while, some other color frequency spectrums can make coherence vanish periodically with variable revival amplitudes. These behaviors result from the non-Markovian effects on the decoherence process, which may give rise to a revival of coherence after complete decoherence.

Verification of the quantum-domain process using two nonorthogonal states

If a quantum channel or process cannot be described by any measure-and-prepare scheme, we may say the channel is in the quantum domain (QD) since it can transmit quantum correlations. The concept of the QD clarifies the role of the quantum channel in quantum-information theory based on the local operation and classical communication (LOCC) paradigm: The quantum channel is useful only if it cannot be simulated by LOCC. We construct a simple scheme to verify that a given physical process or channel is in the QD by using two nonorthogonal states. We also consider its application for experiments such as the transmission or storage of quantum optical coherent states, single-photon polarization states, and squeezed vacuum states.

Polarization-switchable single photon source using the Stark effect

A polarization-switchable single photon source is demonstrated by embedding a self-assembled quantum dot in a high-quality, electrically gated, oxide-apertured micropillar cavity. Due to the noncircular aperture, the polarization degeneracy of the fundamental cavity mode is lifted, leaving two linearly polarized Q≈20 000 modes separated by 194 μeV. An intracavity electric field generated by an applied bias enables Stark shift tuning of the quantum dot emission over a frequency range containing both polarization modes, switching the dominant single photon polarization through the Purcell effect. We measure polarization switching up to 300 kHz, limited by the RC time constant of the device.

Entanglement Purification Through Cavity Input-Output Process

We propose a scheme to purify entanglement of two atoms from not-too-impure entangled states by checking the parity of the two atoms through the cavity input-output process. As the parity check is made by measurement on single-photon polarization, which would not affect the entanglement of the two atoms, our scheme has the successful probability double of that in a previous well-known scheme with linear optical elements [Nature (London) 410 (2001) 1067], and is insensitive to the photon loss and the detection inefficiency. Experimental feasibility of our scheme with current technology is discussed.

Minimum-disturbance measurement without postselection

We propose and demonstrate a linear optical device which deterministically performs optimal quantum measurement or minimum disturbance measurement on a single-photon polarization qubit with the help of an ancillary path qubit introduced to the same photon. We show theoretically and experimentally that this device satisfies the minimum disturbance measurement condition by investigating the relation between the information gain (estimation fidelity) and the state disturbance due to measurement (operation fidelity). Our implementation of minimum disturbance measurement is postselection-free in the sense that all detection events are counted toward evaluation of the estimation fidelity and the operation fidelity, i.e., there is no need for coincidence postselection of the detection events.

Positive-operator-valued-measure view of the ensemble approach to polarization optics

The statistical ensemble formalism of Kim et al [J. Opt. Soc. Am. A4, 433 (1987)] offers a realistic model for characterizing the effect of stochastic nonimage-forming optical media on the state of polarization of transmitted light. With suitable choice of the Jones ensemble, various Mueller transformations-some of which are hitherto unknown-are deduced. It is observed that the ensemble approach is formally identical to the positive-operator-valued measures (POVMs) on the quantum density matrix. This observation, in combination with the recent suggestion by Ahnert and Payne [Phys. Rev. A71, 012330-1 (2005)]-in the context of generalized quantum measurement on single photon polarization states-that linear optics elements can be employed in setting up all possible POVMs enables us to propose a way of realizing different types of Mueller devices.

Quantum Cryptography in Spin Networks

In this paper we propose a new scheme of long-distancequantum cryptography based on spin networks with qubits stored inelectron spins of quantum dots. By conditional Faraday rotation,single photon polarization measurement, and quantum state transfer,maximal-entangled Bell states for quantum cryptography between twolong-distance parties are created. Meanwhile, efficient quantumstate transfer over arbitrary distances is obtained in a spin chainby a proper choice of coupling strengths and using spin memorytechnique improved. We also analyse the security of the schemeagainst the cloning-based attack which can be also implemented inspin network and discover that this spin network cloning coincideswith the optimal fidelity achieved by an eavesdropper forentanglement-based cryptography.

Universal discriminator for completely unknown optical qubits

We propose an experimental setup that is capable of unambiguously discriminating any pair of linearly independent single photon polarization qubits, about which we do not have any knowledge except that an extra pair of these unknown states are provided as the reference. This setup, which is constructed with optical controlled-NOT (CNOT) gates, weak cross Kerr nonlinearities, Bell state analyzers, and other linear optical elements, transforms the unknown triple photon input states to the corresponding single photon states to be deterministically processed by a linear optics circuit. The optimal discrimination of the unknown states is achieved by this setup.

Implementation of optimal phase-covariant cloning machines

The optimal phase-covariant quantum cloning machine (PQCM) broadcasts the information associated to an input qubit into a multiqubit system, exploiting a partial a priori knowledge of the input state. This additional a priori information leads to a higher fidelity than for the universal cloning. The present article first analyzes different innovative schemes to implement the $1\ensuremath{\rightarrow}3$ PQCM. The method is then generalized to any $1\ensuremath{\rightarrow}M$ machine for an odd value of $M$ by a theoretical approach based on the general angular momentum formalism. Finally different experimental schemes based either on linear or nonlinear methods and valid for single photon polarization encoded qubits are discussed.

Active polarization stabilization in optical fibers suitable for quantum key distribution

Polarization feedback control of single-photon pulses has been achieved in long-distance fibers for more than 10 hours, which facilitated "one-way" polarization-encoded quantum key distribution with long-term stabilities. Experimental test of polarization encoding in 75 km fibers demonstrated that the single-photon polarization transformation in long-distance fibers could be controlled to provide a typical QBER of (3.9+/-1.5)% within a long-term operation of 620 minutes.

Optimal cloning of single-photon polarization by coherent feedback of beam splitter losses

Light fields can be amplified by measuring the field amplitude reflected at a beam splitter of reflectivity R and adding a coherent amplitude proportional to the measurement result to the transmitted field. By applying the quantum optical realization of this amplification scheme to single-photon inputs, it is possible to clone the polarization states of photons. We show that optimal cloning of single-photon polarization is possible when the gain factor of the amplification is equal to .

Fault-tolerant quantum computing with coded spins using the conditional Faraday rotation in quantum dots

We propose a scalable fault-tolerant scheme for deterministic quantum computing with spins that is based on a three-particle entanglement produced by the conditional Faraday rotation of the polarization of single photons due to the nonresonant interaction with spins of quantum dots, embedded in microcavities inside a photonic crystal. The resulting conditional phase gate yields switching times of $50\phantom{\rule{0.3em}{0ex}}\mathrm{ps}$. We show that it acts fault-tolerantly not only on the Calderbank-Shor-Steane quantum error correction codes, but also on Shor's code in a single shot. Single-qubit operations on Shor's logical qubits can be implemented by means of the optical Stark effect combined with the optical Ruderman-Kittel-Kasuya-Yosida interaction in a single shot.

Generation of Atomic Cluster States through the Cavity Input-Output Process

We propose a scheme to implement a two-qubit controlled-phase gate for single atomic qubits, which works in principle with nearly ideal success probability and fidelity. Our scheme is based on the cavity input-output process and the single-photon polarization measurement. We show that, even with the practical imperfections such as atomic spontaneous emission, weak atom-cavity coupling, violation of the Lamb-Dicke condition, cavity photon loss, and detection inefficiency, the proposed gate is feasible for generation of a cluster state in that it meets the scalability criterion and it operates in a conclusive manner. We demonstrate a simple and efficient process to generate a cluster state with our high probabilistic entangling gate.

Entangled eavesdropping in quantum key distribution

Recent designs are summarized for some optimized quantum cryptographic entangling probes to be used in attacking the BB84 protocol of quantum key distribution. They are all based on alternative optimum unitary transformations, each yielding the same maximum information on the pre-privacy amplified key. One of the optimum unitary transformations, not previously addressed in terms of implementation, is shown, for a certain choice of free parameters, to again yield one of the recent designs. In each of the designs, the main quantum circuit consists of a single CNOT gate which produces the optimum entanglement between the BB84 signal states and the correlated probe states. For each design a different well-defined single-photon polarization state serves as the initial state of the probe, and in each case the probe is projectively measured.

Robust multiparty quantum secret key sharing over two collective-noise channels

Based on a polarization-based quantum key distribution protocol over a collective-noise channel [Phys. Rev. Lett. 92 (2004) 017901], a robust ( n , n ) -threshold scheme of multiparty quantum secret sharing of key over two collective-noise channels (i.e., the collective dephasing channel and the collective rotating channel) is proposed. In this scheme the sharer entirety can establish a joint key with the message sender only if all the sharers collaborate together. Since Bell singlets are enough for use and only single-photon polarization needs to be identified, this scheme is feasible according to the present-day technique.

Comment on “Measuring a Photonic Qubit without Destroying It”

Recently, Pryde et al reported the demonstration of a quantum non-demolition scheme for single-photon polarization states with linear optics and projective measurements [Phys. Rev. Lett. 92, 190402 (2004)]. Here, we argue that their interpretation of the experiment is inconsistent with the fidelity measure they use.

Measurement of Quantum Weak Values of Photon Polarization

We experimentally determine weak values for a single photon's polarization, obtained via a weak measurement that employs a two-photon entangling operation, and postselection. The weak values cannot be explained by a semiclassical wave theory, due to the two-photon entanglement. We observe the variation in the size of the weak value with measurement strength, obtaining an average measurement of the S1 Stokes parameter more than an order of magnitude outside of the operator's spectrum for the smallest measurement strengths.

General implementation of all possible positive-operator-value measurements of single-photon polarization states

Positive Operator Value Measures (POVMs) are the most general class of quantum measurements. We propose a setup in which all possible POVMs of a single photon polarization state (corresponding to all possible sets of two-dimensional Kraus operators) can be implemented easily using linear optics elements. This method makes it possible to experimentally realize any projective orthogonal, projective non-orthogonal or non-projective sets of any number of POVM operators. Furthermore our implementation only requires vacuum ancillas, and is deterministic rather than probabilistic. Thus it realizes every POVM with the correct set of output states. We give the settings required to implement two different well-known non-orthogonal projective POVMs.

Realization of the optimal universal quantum entangler

We present the first experimental demonstration of the ''optimal'' and ''universal'' quantum entangling process involving qubits encoded in the polarization of single photons. The structure of the ''quantum entangling machine'' consists of the quantum injected optical parametric amplifier by which the contextual realization of the 1->2 universal quantum cloning and of the universal NOT (U-NOT) gate has also been achieved.

Nonorthogonal projective positive-operator-value measurement of photon polarization states with unit probability of success

In this paper we describe a scheme for performing a nonorthogonal projective positive-operator-value measurement of any arbitrary single-photon polarization input state with unit probability of success. While this probability is reached in the limit of infinite cycles of states through the apparatus, only one actual physical setup is required for a feasible implementation. Specifically, our setup implements a set of three nonorthogonal measurement operators at angles of 120\ifmmode^\circ\else\textdegree\fi{} to each other.