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arXiv:1912.11362

Quantum Noise in Balanced Differential Measurements in Optics: Implication to the Wave Modes of Quantum Vacuum

C. S. Unnikrishnan

Published Dec 23, 2019• 1 citation
arXivHTMLPDF

Abstract

Experimental tests for assessing the physical reality of the hypothetical wave modes of quantum vacuum with zero-point energy are of fundamental importance for quantum field theories and cosmology. Physical effects like the Casimir effect have alternate description in terms of retarded interaction between charged matter, due to quantum fluctuations of material dipoles. However, there are simple quantum optical configurations where the hypothetical quantum vacuum modes seem to assume an essential real role in the observable quantum noise of optical signals. I present the logical and theoretical basis of a decisive test that relies on the comparisons of balanced homodyne detection with a novel differential scheme of balanced wave-front division detection, when the two real optical beams at the detectors are derived from one coherent beam as input. Both ideal and practical configurations of my experimental test are discussed. Results from the experiments on balanced detection, beam localization of optical beams, and atomic Bose-Einstein condensates are used to reach definite conclusions against the reality of the wave modes of quantum vacuum. It is shown that the entire quantum noise follows consistently from the state reduction of quantum superpositions of particle-number states at the point of detection, where the quantum measurement is completed. This is consistent with the demonstrated applications of squeezed light in interferometry and quantum metrology. This result achieves consistency between quantum noise in quantum optics and observational cosmology based on general relativity, by avoiding the wave modes of quantum vacuum with divergent zero-point energy density. Generalization from the limited sphere of quantum optics to general quantum field theories promises the complete solution to the problem of a divergent cosmological constant.

DOI
10.48550/arxiv.1912.11362

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Experimental tests for assessing the physical reality of the hypothetical wave modes of quantum vacuum with zero-point energy are of fundamental importance for quantum field theories and cosmology. Physical effects like the Casimir effect have alternate description in terms of retarded interaction between charged matter, due to quantum fluctuations of material dipoles. However, there are simple quantum optical configurations where the hypothetical quantum vacuum modes seem to assume an essential real role in the observable quantum noise of optical signals. I present the logical and theoretical basis of a decisive test that relies on the comparisons of balanced homodyne detection with a novel differential scheme of balanced wave-front division detection, when the two real optical beams at the detectors are derived from one coherent beam as input. Both ideal and practical configurations of my experimental test are discussed. Results from the experiments on balanced detection, beam localization of optical beams, and atomic Bose-Einstein condensates are used to reach definite conclusions against the reality of the wave modes of quantum vacuum. It is shown that the entire quantum noise follows consistently from the state reduction of quantum superpositions of particle-number states at the point of detection, where the quantum measurement is completed. This is consistent with the demonstrated applications of squeezed light in interferometry and quantum metrology. This result achieves consistency between quantum noise in quantum optics and observational cosmology based on general relativity, by avoiding the wave modes of quantum vacuum with divergent zero-point energy density. Generalization from the limited sphere of quantum optics to general quantum field theories promises the complete solution to the problem of a divergent cosmological constant.
Abstract The balanced homodyne detection as a readout scheme of gravitational-wave detectors is carefully examined from the quantum field theoretical point of view. The readout scheme in gravitational-wave detectors specifies the directly measured quantum operator in the detection. This specification is necessary when we apply the recently developed quantum measurement theory to gravitational-wave detections. We examine the two models of measurement. One is the model in which the directly measured quantum operator at the photodetector is Glauber’s photon number operator, and the other is the model in which the power operator of the optical field is directly measured. These two are regarded as ideal models of photodetectors. We first show these two models yield the same expectation value of the measurement. Since there is consensus in the gravitational-wave community that vacuum fluctuations contribute to the noises in the detectors, we also clarify the contributions of vacuum fluctuations to the quantum noise spectral density without using the two-photon formulation which is used in the gravitational-wave community. We found that the conventional noise spectral density in the two-photon formulation includes vacuum fluctuations from the main interferometer but does not include those from the local oscillator. Although the contribution of vacuum fluctuations from the local oscillator theoretically yields the difference between the above two models in the noise spectral densities, this difference is negligible in realistic situations.
The balanced homodyne detection as a readout scheme of gravitational-wave detectors is carefully examined, which specifies the directly measured quantum operator in the detection. This specification is necessary to apply the quantum measurement theory to gravitational-wave detections. We clarify the contribution of vacuum fluctuations to the noise spectral density without using the two-photon formulation. We found that the noise spectral density in the two-photon formulation includes vacuum fluctuations from the main interferometer but does not includes those from the local oscillator which depends on the directly measured operators.
This chapter is a short summary of recent developments concerning quantum noise. A quantum state is said to be squeezed when the fluctuations of one observable have been reduced below that of the vacuum state, that is, the ‘shot-noise’ level of the coherent state. Squeezed states are detected using the technique known as balanced homodyne detection. The chapter reviews the reductions in phase and amplitude fluctuations of a squeezed state and discusses their use in interferometry and spectroscopy. The phase-dependent nature of the squeezed field results in line-narrowing of the incoherent spectrum for particular phase choices while other choices of phase result in line-broadening. Interferometry with squeezed light as well as quantum non-demolition measurements are attempting to assess quantum measurement limits in order to further extend the experimental manipulation of light.
Vacuum fluctuation at audio frequencies is very important and interesting in many research fields, such as the gravitational wave detection, ultra-weak magnetic field measurement, and the research of quantum metrology, etc. Since the generation of squeezed light in 1985, most of the squeezed light have been generated and measured at radio frequencies (～MHz) as there has not been much technical noise at higher frequencies. In the Michelson-interferometer-based gravitational wave detection, the detection band has frequencies from a few to tens of thousands Hz. Measuring vacuum noise at such low frequencies is a challenge since we have to stabilize and control all the audio noises and the interferences from a variety of mechanical and electronic noises, therefore a very high classical noise suppression is needed when the measurement time increases. In order to measure the squeezed light of low frequencies, the standard vacuum noise at audio frequencies must be measured. In this paper, a balanced homodyne detection system for measuring the low-frequency quantum vacuum noises is reported. It is not trivial to extend the detected frequency to very low analysis frequencies. Through a self-made self-subtraction balanced homodyne configuration, which can eliminate the DC component of each photocurrent from the photodiode and the classical common-mode technical noise, the standard vacuum noise has been detected. The linearity of the vacuum noise power has been validated by varying the local oscillator power, showing that the saturation power of light incidence is about 3.2 mW. When the incident-light power is 400 W, the standard vacuum noise is 11 dB higher than the electronic noise at 80 Hz. In the regime of about 80 Hz to 400 kHz, the linearity of the standard noise power as a function of incident laser power is verified. However, when the measurement is carried out at even lower frequencies, for example, 50 Hz, we may encounter some excess and non-stationary noises and find that the measured noise power is not proportional to the incident light power any more. These non-stationary noises are the main technical obstacle at low frequencies. The average common mode rejection ratio in the test frequency range from 10 Hz to 400 kHz is 55 dB and its maximum 63 dB at 100 Hz is obtained, implying a high suppression of the technical noise. This self-made homodyne vacuum noise detector can be widely used for precision measurement in quantum metrology and quantum optics.
The nature of phase noise in quantum optics is analyzed. In an experiment involving the measurement of the electromagnetic field the two quantities of interest are the energy and phase of the field. However, measurements of the quantities produce quantum fluctuations. The quantum fluctuations are regarded as noise in the treatment presented here. The quantum system is represented by a probability distribution, the Wigner function, and the quantum fluctuations are treated as stochastic noise associated with the quantity being measured. The difficulties of associating a quantum operator with the phase of the system are reviewed and the related energy-phase uncertainty relation is discussed. The alternate interpretation of the phase noise of a quantum system as being the classical phase noise of the Wigner function is presented. In particular the energy and phase noise of the vacuum state, the coherent state, the squeezed state and the squeezed vacuum are discussed in this way. The squeezed states of light are minimum uncertainty states with respect to the quadrature operators and exhibit noise of one quadrature below the noise level associated with the vacuum. The reduced noise level in one quadrature of the field underlies the importance of squeezed states in many practical applications where there is a need to reduce the quantum noise of one quadrature of coherent light. The periodic phase operator eliminates the difficulties associated with the multivalued nature of phase. The analysis of the vacuum and intense coherent state of Carruthers and Nieto by employing periodic phase operators is reviewed, particularly with respect to the energy-phase uncertainty relations and we generalize the approach to develop a phase operator analysis of the squeezed state in the intense field and vacuum limits. We demonstrate here for the first time that the phase operator is simply related to the phase of the squeezed state in the intense field limit and that the squeezed state is approximately an energy-phase minimum uncertainty state in the low-squeezing limit. Also we enlarge on previous work to demonstrate that the phase operator corresponds simply and unambiguously to the phase of the squeeze parameter for the strongly squeezed vacuum and the intensely squeezed vacuum is an energy-phase minimum uncertainty state for some values of phase. The occurrence of squeezing for the case of two coupled quantum oscillators is presented. The system consisting of one mode of the electromagnetic field coupled to a spinless nonrelativistic electron subjected to an harmonic potential is represented by two coupled harmonic oscillators. The dynamics are compared for the case that the rotating wave approximation is employed and for the case that the counter-rotating terms are included. These calculations have not been performed before. The parametric amplifier Hamiltonian with a nonresonant coupling is also studied in order to provide insight into the effects of the counter-rotating terms. Squeezing of the field produced by the electron is a consequence of the inclusion of the counter-rotating terms. The case of a spinless nonrelativistic electron subject to an harmonic potential and coupled to a continuum of electromagnetic field modes is also considered. The case of two coupled oscillators discussed above is generalized by replacing the oscillator which represents the single-mode field by a bath of oscillators. The effects of including counter-rotating terms and of ignoring the counter - rotating terms in the Hamiltonian are compared. The interaction is assumed to produce a frequency shift and an exponential damping term for the oscillating electron. The frequency shift is assumed to be small in either case and so the Wigner-Weisskopff approximation is employed to solve the equations of motion. We demonstrate the new results that dissipation-induced phase-dependent noise is a consequence of including the counter-rotating terms and that the noise is phase-independent for the case that the counterrotating terms are excluded. The relation between these results and recent work on quantum tunnelling in superconducting quantum interference devices is discussed. We conclude by suggesting further research related to the work in this thesis.
Measurement of the squeezed fluctuations of an optical field by the method of conditional homodyne detection is discussed. It is shown that conditional homodyne detection is able to distinguish qualitatively between vacuum-state squeezing and squeezed classical noise. Whereas for conventional squeezed-light detection, only a quantitative distinction can be made, based on the setting of the shot-noise level, under conditional detection, the presence of classical noise changes the actual shape of the measured correlation function. The correlations show a positive peak due to the unsqeezed classical noise frequencies, set inside the negative dip associated with the squeezed fluctuations. The width in time of the positive peak is the larger of the detector response time and the inverse of the classical noise bandwidth. The fundamental distinction between vacuum-state squeezing and squeezed classical noise is that there is no positive peak, even in the limiting form of a delta-function, when the unsqueezed frequencies correspond to vacuum state modes. Implications for the literal interpretation of vacuum fluctuations, such as is adopted in stochastic electrodynamics, are discussed. The ideas are presented in general terms and illustrated by an example which treats the generation and detection of broadband squeezed light, including finite-bandwidth classical noise, within the framework of the quantum trajectory theory of cascaded open systems.