PhD Defense: Hui Chen
Monday, March 24, 2014 · 1:30 - 3:30 PM
TITLE:
The High-Order Quantum Coherence of Thermal Light
ABSTRACT:
A practical quantum computing requires a large number of quantum bits (qubits), i.e. a large number N-photon entanglement. However, people still have difficulty in preparing more than three entangled photons, due to low efficiency of non-linear effect such as spontaneous parametric down-conversion. Besides technical improvements of producing high-order entangled photons, some scientists turn to another sources instead of light, such as entangled electrons in solid state. However, thermal light had not been considered a feasible source for quantum computing, until recently non-local effects were found in thermal light. Since it is relative easily to prepare a thermal N-photon state and to perform a N-fold correlation measurement, these discoveries suggest thermal light as a feasible and practical source for quantum computing and quantum information. Nevertheless, the new discoveries also raise a lot of debates and arguments, since conventionally it is believed that, as a classical source, thermal light can only produce classical effects. We should recall that, historically it is not quit true: Plank’s research on black-body radiation pointed out an important but most overlooked truth that thermal light’s radiation could not be fully understood until quantum mechanics was introduced. This implies that thermal light possesses quantum effects we need to reveal.
Contained herein are our recent discoveries and several new experiments. Firstly, an anti-correlation was observed from two incoherent and independent thermal radiations. The visibility of the anti-correlation is 100%, which clearly shows us a a similar quantum effect that was thought existing only in entangled states. Secondly, with manipulating polarization, a thermal Bell-state was build from two incoherent and orthogonally polarized thermal radiations. The interferometer is similar to Shih-Alley Bell-state experiment in 1986, and its nontrivial effects with 100% visibility are also similar to their results. The experiment not only confirms the existence of non-local interferences of two incoherent thermal photons, but also demonstrates a thermal 2-qubit system, which consequently paves a ways for us to use high-order thermal N-photon to build a N-qubit. Based on the thermal Bell-state experiment, a new type of Franson interferometer with thermal light has been created to prepare N-photon qubits. We has also applied a novel detection scheme, Positive-Negative Fluctuation Correlation(PNFC) protocol, with which we can observe Frason-type correlations from the two-photon qubits with visibilities more than 71%. The success of two-photon qubits offers a perspective for producing a large number of N-photon qubits for practical quantum computing.
Importantly, the novel detection scheme, PNFC protocl, was invented to measure the intensity fluctuation correlation, in which the intensity fluctuations are separated to positive and negative parts (higher and lower than their mean values). Interestingly and surprisingly, a coexisting anti-correlation is observed in a typical Hanbury Brown and Twiss(HBT) interferometry. Above all, this scheme easily brings us 100% visibility for HBT correlation, anti-correlation, thermal Bell-type correlation and Franson-type interferometry, which breaks the limit of 50% visibility for thermal light correlations. Therefore, PNFC protocol has a great significant for thermal N-qubits and practical quantum computing as well. We believe, with correlations of 100% visibility by using PNFC protocol, thermal N-qubits would give us a practical quantum computer soon.
Location: Physics, Room 401
The High-Order Quantum Coherence of Thermal Light
ABSTRACT:
A practical quantum computing requires a large number of quantum bits (qubits), i.e. a large number N-photon entanglement. However, people still have difficulty in preparing more than three entangled photons, due to low efficiency of non-linear effect such as spontaneous parametric down-conversion. Besides technical improvements of producing high-order entangled photons, some scientists turn to another sources instead of light, such as entangled electrons in solid state. However, thermal light had not been considered a feasible source for quantum computing, until recently non-local effects were found in thermal light. Since it is relative easily to prepare a thermal N-photon state and to perform a N-fold correlation measurement, these discoveries suggest thermal light as a feasible and practical source for quantum computing and quantum information. Nevertheless, the new discoveries also raise a lot of debates and arguments, since conventionally it is believed that, as a classical source, thermal light can only produce classical effects. We should recall that, historically it is not quit true: Plank’s research on black-body radiation pointed out an important but most overlooked truth that thermal light’s radiation could not be fully understood until quantum mechanics was introduced. This implies that thermal light possesses quantum effects we need to reveal.
Contained herein are our recent discoveries and several new experiments. Firstly, an anti-correlation was observed from two incoherent and independent thermal radiations. The visibility of the anti-correlation is 100%, which clearly shows us a a similar quantum effect that was thought existing only in entangled states. Secondly, with manipulating polarization, a thermal Bell-state was build from two incoherent and orthogonally polarized thermal radiations. The interferometer is similar to Shih-Alley Bell-state experiment in 1986, and its nontrivial effects with 100% visibility are also similar to their results. The experiment not only confirms the existence of non-local interferences of two incoherent thermal photons, but also demonstrates a thermal 2-qubit system, which consequently paves a ways for us to use high-order thermal N-photon to build a N-qubit. Based on the thermal Bell-state experiment, a new type of Franson interferometer with thermal light has been created to prepare N-photon qubits. We has also applied a novel detection scheme, Positive-Negative Fluctuation Correlation(PNFC) protocol, with which we can observe Frason-type correlations from the two-photon qubits with visibilities more than 71%. The success of two-photon qubits offers a perspective for producing a large number of N-photon qubits for practical quantum computing.
Importantly, the novel detection scheme, PNFC protocl, was invented to measure the intensity fluctuation correlation, in which the intensity fluctuations are separated to positive and negative parts (higher and lower than their mean values). Interestingly and surprisingly, a coexisting anti-correlation is observed in a typical Hanbury Brown and Twiss(HBT) interferometry. Above all, this scheme easily brings us 100% visibility for HBT correlation, anti-correlation, thermal Bell-type correlation and Franson-type interferometry, which breaks the limit of 50% visibility for thermal light correlations. Therefore, PNFC protocol has a great significant for thermal N-qubits and practical quantum computing as well. We believe, with correlations of 100% visibility by using PNFC protocol, thermal N-qubits would give us a practical quantum computer soon.
Location: Physics, Room 401