We present a systematic comparison of different methods of fidelity estimation of a linear optica... more We present a systematic comparison of different methods of fidelity estimation of a linear optical quantum controlled-Z gate implemented by two-photon interference on a partially polarizing beam splitter. We have utilized a linear fidelity estimator based on the Monte Carlo sampling technique as well as a non-linear estimator based on maximum likelihood reconstruction of a full quantum process matrix. In addition, we have also evaluated lower bound on quantum gate fidelity determined by average quantum state fidelities for two mutually unbiased bases. In order to probe various regimes of operation of the gate we have introduced a tunable delay line between the two photons. This allowed us to move from high-fidelity operation to a regime where the photons become distinguishable and the success probability of the scheme significantly depends on input state. We discuss in detail possible systematic effects that could influence the gate fidelity estimation.
We report on experimental verification of quantum non-Gaussianity of a heralded single photon sta... more We report on experimental verification of quantum non-Gaussianity of a heralded single photon state with positive Wigner function. We unambiguously demonstrate that the generated state cannot be expressed as a mixture of Gaussian states. A sufficient information to witness the quantum non-Gaussianity is obtained from a standard photon anti-correlation measurement.
We have studied how the quality of transfer of a qubit state depends on distinguishability of int... more We have studied how the quality of transfer of a qubit state depends on distinguishability of internal states of the particles carrying qubits. The transfer is implemented without any direct interaction, just by a partial exchange of photons, measurement on one of them, and conditional feed-forward correction. It appears that the quality of the transfer is only influenced by the level of distinguishability of the states of unaccessible internal degrees of freedom not used for information encoding. We have found a directly measurable parameter quantifying this level of distinguishability and proved it usefulness experimentally.
We propose and experimentally verify a cooling limit for a quantum channel going through an incoh... more We propose and experimentally verify a cooling limit for a quantum channel going through an incoherent environment. The environment consists of a large number of independent non-interacting and non-interfering elementary quantum systems--qubits. The qubits travelling through the channel can only be randomly replaced by environmental qubits. We investigate a conditional cooling limit that exploits an additional probing output. The limit specifies when the single-qubit channel is quantum, i.e. it preserves entanglement. It is a fundamental condition for entanglement-based quantum technology.
We investigate optimal discrimination between two projective single-qubit measurements in a scena... more We investigate optimal discrimination between two projective single-qubit measurements in a scenario where the measurement can be performed only once. We consider general setting involving a tunable fraction of inconclusive outcomes and we prove that the optimal discrimination strategy requires an entangled probe state for any nonzero rate of inconclusive outcomes. We experimentally implement this optimal discrimination strategy for projective measurements on polarization states of single photons. Our setup involves a real-time electrooptical feed-forward loop which allows us to fully harness the benefits of entanglement in discrimination of quantum measurements. The experimental data clearly demonstrate the advantage of entanglement-based discrimination strategy as compared to unentangled single-qubit probes.
A Mach-Zehnder interferometer for the detection and noninvasive optical amplification of polariza... more A Mach-Zehnder interferometer for the detection and noninvasive optical amplification of polarization rotation Rev. Sci. Instrum. 79, 063106 (2008);
Abstract The simultaneous inseparable reconstruction of quantum process and states is presented. ... more Abstract The simultaneous inseparable reconstruction of quantum process and states is presented. The method is based on constrained extremization of the likelihood functional. This statistical approach is ingenuous in quantum mechanical frame and ensures physically correct results. Adopted scheme predicts the input density operators and the operator of quantum process all at once with the help of noisy measured data only. Attention is paid to the comparison of posed method with sequential measurement of the states and the ...
Acrobat PDF (28 KB) Open Access * * Note that full-text PDFs from conferences typically contain 1... more Acrobat PDF (28 KB) Open Access * * Note that full-text PDFs from conferences typically contain 1-3 pages of content, some or all of which might be an abstract, summary, or miscellaneous items. ... Please enable Javascript in your browser to view the abstract as intended. ... By means of a hybrid detector we suggest and implement two protocols. We theoretically propose a feasible loophole-free violation of Bell's inequality and we experimentally realize a Hadamard transform of coherent superposition states. ... References are not available for this paper.
Various approaches for quantum communication and computation are being intensely explored. The mo... more Various approaches for quantum communication and computation are being intensely explored. The most studied optical approach for the implementation of quantum communication and computing has relied on single photon states described in a finite dimensional (mostly two-dimensional) Hilbert space where information is encoded as qubits containing a discrete degree of freedom. A much less studied alternative to quantum communication and computation is the approach where information is encoded in multi-photonic states such as coherent ...
We propose an efficiently measurable lower bound on quantum process fidelity of N-qubit controlle... more We propose an efficiently measurable lower bound on quantum process fidelity of N-qubit controlled-Z gates. This bound is determined by average output state fidelities for N partially conjugate product bases. A distinct advantage of our approach is that only fidelities with product states need to be measured while keeping the total number of measurements much smaller than what is necessary for full quantum process tomography. As an application, we use this method to experimentally estimate quantum process fidelity F of a three-qubit linear optical quantum Toffoli gate and we find that F ! 0:83. We also demonstrate the entangling capability of the gate by preparing Greenberger-Horne-Zeilinger-type three-qubit entangled states from input product states.
We present a systematic comparison of different methods of fidelity estimation of a linear optica... more We present a systematic comparison of different methods of fidelity estimation of a linear optical quantum controlled-Z gate implemented by two-photon interference on a partially polarizing beam splitter. We have utilized a linear fidelity estimator based on the Monte Carlo sampling technique as well as a non-linear estimator based on maximum likelihood reconstruction of a full quantum process matrix. In addition, we have also evaluated lower bound on quantum gate fidelity determined by average quantum state fidelities for two mutually unbiased bases. In order to probe various regimes of operation of the gate we have introduced a tunable delay line between the two photons. This allowed us to move from high-fidelity operation to a regime where the photons become distinguishable and the success probability of the scheme significantly depends on input state. We discuss in detail possible systematic effects that could influence the gate fidelity estimation.
We derive sampling functions for estimation of quantum-state fidelity with Schrödinger-cat-like s... more We derive sampling functions for estimation of quantum-state fidelity with Schrödinger-cat-like states, which are defined as superpositions of two coherent states with opposite amplitudes. We also provide sampling functions for fidelity with squeezed Fock states that can approximate the catlike states and can be generated from Gaussian squeezed states by conditional photon subtraction. The fidelities can be determined by averaging the sampling functions over quadrature statistics measured by homodyne detection. The sampling functions are designed such that they can compensate for losses and inefficient homodyning provided that the overall efficiency exceeds a certain threshold. The fidelity with an odd coherent state and the fidelity with a squeezed odd Fock state provide convenient witnesses of negativity of the Wigner function of the measured state. The negativity of the Wigner function at the origin of phase space is certified if any of these fidelities exceeds 0.5. Finally, we discuss the possibility of reducing the statistical uncertainty of the fidelity estimates by a suitable choice of the dependence of the number of quadrature samples on the relative phase shift between the local oscillator and the signal beam.
We experimentally demonstrate a programmable single-qubit quantum gate. This device applies a uni... more We experimentally demonstrate a programmable single-qubit quantum gate. This device applies a unitary phase shift operation to a data qubit with the value of the phase shift being fully determined by the state of a program qubit. Our linear optical implementation is based on the encoding of qubits into polarization states of single photons, two-photon interference on a polarizing beam splitter, and measurement on the output program qubit. We fully characterize the programmable gate by quantum process tomography. The achieved average quantum process fidelity exceeding 97% illustrates very good performance of the gate for all values of the encoded phase shift. We also show that by using a different set of program states the device can operate as a programmable partial polarization filter.
A quantum analog of the fundamental classical NOT gate is a quantum gate that would transform any... more A quantum analog of the fundamental classical NOT gate is a quantum gate that would transform any input qubit state onto an orthogonal state. Intriguingly, this universal NOT gate is forbidden by the laws of quantum physics. This striking phenomenon has far-reaching implications concerning quantum information processing and encoding information about directions and reference frames into quantum states. It also triggers the question of under what conditions the preparation of quantum states orthogonal to input states becomes possible. Here we report on experimental demonstration of orthogonalization of partly unknown single-and two-qubit quantum states. A state orthogonal to an input state is conditionally prepared by quantum filtering, and the only required information about the input state is a mean value of a single arbitrary operator. We show that perfect orthogonalization of partly unknown two-qubit entangled states can be performed by applying the quantum filter to one of the qubits only.
We present a systematic comparison of different methods of fidelity estimation of a linear optica... more We present a systematic comparison of different methods of fidelity estimation of a linear optical quantum controlled-Z gate implemented by two-photon interference on a partially polarizing beam splitter. We have utilized a linear fidelity estimator based on the Monte Carlo sampling technique as well as a non-linear estimator based on maximum likelihood reconstruction of a full quantum process matrix. In addition, we have also evaluated lower bound on quantum gate fidelity determined by average quantum state fidelities for two mutually unbiased bases. In order to probe various regimes of operation of the gate we have introduced a tunable delay line between the two photons. This allowed us to move from high-fidelity operation to a regime where the photons become distinguishable and the success probability of the scheme significantly depends on input state. We discuss in detail possible systematic effects that could influence the gate fidelity estimation.
We report on experimental verification of quantum non-Gaussianity of a heralded single photon sta... more We report on experimental verification of quantum non-Gaussianity of a heralded single photon state with positive Wigner function. We unambiguously demonstrate that the generated state cannot be expressed as a mixture of Gaussian states. A sufficient information to witness the quantum non-Gaussianity is obtained from a standard photon anti-correlation measurement.
We have studied how the quality of transfer of a qubit state depends on distinguishability of int... more We have studied how the quality of transfer of a qubit state depends on distinguishability of internal states of the particles carrying qubits. The transfer is implemented without any direct interaction, just by a partial exchange of photons, measurement on one of them, and conditional feed-forward correction. It appears that the quality of the transfer is only influenced by the level of distinguishability of the states of unaccessible internal degrees of freedom not used for information encoding. We have found a directly measurable parameter quantifying this level of distinguishability and proved it usefulness experimentally.
We propose and experimentally verify a cooling limit for a quantum channel going through an incoh... more We propose and experimentally verify a cooling limit for a quantum channel going through an incoherent environment. The environment consists of a large number of independent non-interacting and non-interfering elementary quantum systems--qubits. The qubits travelling through the channel can only be randomly replaced by environmental qubits. We investigate a conditional cooling limit that exploits an additional probing output. The limit specifies when the single-qubit channel is quantum, i.e. it preserves entanglement. It is a fundamental condition for entanglement-based quantum technology.
We investigate optimal discrimination between two projective single-qubit measurements in a scena... more We investigate optimal discrimination between two projective single-qubit measurements in a scenario where the measurement can be performed only once. We consider general setting involving a tunable fraction of inconclusive outcomes and we prove that the optimal discrimination strategy requires an entangled probe state for any nonzero rate of inconclusive outcomes. We experimentally implement this optimal discrimination strategy for projective measurements on polarization states of single photons. Our setup involves a real-time electrooptical feed-forward loop which allows us to fully harness the benefits of entanglement in discrimination of quantum measurements. The experimental data clearly demonstrate the advantage of entanglement-based discrimination strategy as compared to unentangled single-qubit probes.
A Mach-Zehnder interferometer for the detection and noninvasive optical amplification of polariza... more A Mach-Zehnder interferometer for the detection and noninvasive optical amplification of polarization rotation Rev. Sci. Instrum. 79, 063106 (2008);
Abstract The simultaneous inseparable reconstruction of quantum process and states is presented. ... more Abstract The simultaneous inseparable reconstruction of quantum process and states is presented. The method is based on constrained extremization of the likelihood functional. This statistical approach is ingenuous in quantum mechanical frame and ensures physically correct results. Adopted scheme predicts the input density operators and the operator of quantum process all at once with the help of noisy measured data only. Attention is paid to the comparison of posed method with sequential measurement of the states and the ...
Acrobat PDF (28 KB) Open Access * * Note that full-text PDFs from conferences typically contain 1... more Acrobat PDF (28 KB) Open Access * * Note that full-text PDFs from conferences typically contain 1-3 pages of content, some or all of which might be an abstract, summary, or miscellaneous items. ... Please enable Javascript in your browser to view the abstract as intended. ... By means of a hybrid detector we suggest and implement two protocols. We theoretically propose a feasible loophole-free violation of Bell's inequality and we experimentally realize a Hadamard transform of coherent superposition states. ... References are not available for this paper.
Various approaches for quantum communication and computation are being intensely explored. The mo... more Various approaches for quantum communication and computation are being intensely explored. The most studied optical approach for the implementation of quantum communication and computing has relied on single photon states described in a finite dimensional (mostly two-dimensional) Hilbert space where information is encoded as qubits containing a discrete degree of freedom. A much less studied alternative to quantum communication and computation is the approach where information is encoded in multi-photonic states such as coherent ...
We propose an efficiently measurable lower bound on quantum process fidelity of N-qubit controlle... more We propose an efficiently measurable lower bound on quantum process fidelity of N-qubit controlled-Z gates. This bound is determined by average output state fidelities for N partially conjugate product bases. A distinct advantage of our approach is that only fidelities with product states need to be measured while keeping the total number of measurements much smaller than what is necessary for full quantum process tomography. As an application, we use this method to experimentally estimate quantum process fidelity F of a three-qubit linear optical quantum Toffoli gate and we find that F ! 0:83. We also demonstrate the entangling capability of the gate by preparing Greenberger-Horne-Zeilinger-type three-qubit entangled states from input product states.
We present a systematic comparison of different methods of fidelity estimation of a linear optica... more We present a systematic comparison of different methods of fidelity estimation of a linear optical quantum controlled-Z gate implemented by two-photon interference on a partially polarizing beam splitter. We have utilized a linear fidelity estimator based on the Monte Carlo sampling technique as well as a non-linear estimator based on maximum likelihood reconstruction of a full quantum process matrix. In addition, we have also evaluated lower bound on quantum gate fidelity determined by average quantum state fidelities for two mutually unbiased bases. In order to probe various regimes of operation of the gate we have introduced a tunable delay line between the two photons. This allowed us to move from high-fidelity operation to a regime where the photons become distinguishable and the success probability of the scheme significantly depends on input state. We discuss in detail possible systematic effects that could influence the gate fidelity estimation.
We derive sampling functions for estimation of quantum-state fidelity with Schrödinger-cat-like s... more We derive sampling functions for estimation of quantum-state fidelity with Schrödinger-cat-like states, which are defined as superpositions of two coherent states with opposite amplitudes. We also provide sampling functions for fidelity with squeezed Fock states that can approximate the catlike states and can be generated from Gaussian squeezed states by conditional photon subtraction. The fidelities can be determined by averaging the sampling functions over quadrature statistics measured by homodyne detection. The sampling functions are designed such that they can compensate for losses and inefficient homodyning provided that the overall efficiency exceeds a certain threshold. The fidelity with an odd coherent state and the fidelity with a squeezed odd Fock state provide convenient witnesses of negativity of the Wigner function of the measured state. The negativity of the Wigner function at the origin of phase space is certified if any of these fidelities exceeds 0.5. Finally, we discuss the possibility of reducing the statistical uncertainty of the fidelity estimates by a suitable choice of the dependence of the number of quadrature samples on the relative phase shift between the local oscillator and the signal beam.
We experimentally demonstrate a programmable single-qubit quantum gate. This device applies a uni... more We experimentally demonstrate a programmable single-qubit quantum gate. This device applies a unitary phase shift operation to a data qubit with the value of the phase shift being fully determined by the state of a program qubit. Our linear optical implementation is based on the encoding of qubits into polarization states of single photons, two-photon interference on a polarizing beam splitter, and measurement on the output program qubit. We fully characterize the programmable gate by quantum process tomography. The achieved average quantum process fidelity exceeding 97% illustrates very good performance of the gate for all values of the encoded phase shift. We also show that by using a different set of program states the device can operate as a programmable partial polarization filter.
A quantum analog of the fundamental classical NOT gate is a quantum gate that would transform any... more A quantum analog of the fundamental classical NOT gate is a quantum gate that would transform any input qubit state onto an orthogonal state. Intriguingly, this universal NOT gate is forbidden by the laws of quantum physics. This striking phenomenon has far-reaching implications concerning quantum information processing and encoding information about directions and reference frames into quantum states. It also triggers the question of under what conditions the preparation of quantum states orthogonal to input states becomes possible. Here we report on experimental demonstration of orthogonalization of partly unknown single-and two-qubit quantum states. A state orthogonal to an input state is conditionally prepared by quantum filtering, and the only required information about the input state is a mean value of a single arbitrary operator. We show that perfect orthogonalization of partly unknown two-qubit entangled states can be performed by applying the quantum filter to one of the qubits only.
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Papers by Miroslav Jezek