The possibility of performing quantum erasure using a single form of quantum erasure in an experiment is explored. This contrasts with the two possibilities for quantum erasure usually employed (e.g., whether a photon shed by an atom is eliminated or is instead just not subject to specification regarding its location in either one or the other of two micromaser cavities). Where a single form of erasure is employed, when the physical existent reaches the detection screen it should show a typical Young-type interference pattern like that found for classic two-slit gedankenexperiments in quantum mechanics where there is no manipulation of the "particle" in its travel to the double-slit. The specific form of single-form quantum erasure explored here allows for the possibility of transmitting information to locations remote from photons shed into one of a pair of micromaser cavities. The form of this information corresponds to the "dots" and "dashes" of Morse code or, in general, the binary bits of digital data. The velocity of light does not appear to be a limiting factor for the transmission of this information. The apparent possibility of faster-than-light communication depends on the separation of the measurement process that usually occurs in one step into two steps in the quantum eraser scenario employed. Usually a measurement occurs and is made available to the environment in one step. In the quantum eraser, these are two distinct steps. We have essentially a "measurement in process" after the time that a photon is shed in one of the micromaser cavities and before we definitely know into which cavity the photon has been shed by an atom passing through the cavity or before it is definitely known that the photon's location in one or the other cavity can no longer be determined. While the "measurement in process" exists, the conventional notion of time may be suspended as concerns this measurement and any other entangled existent that is affected by the measurement of the photon's location. Once it is known whether the photon is indeed in only one cavity or is instead now in the combined cavity that occupies the space formerly occupied by both cavities that were separated when the shutter was closed, the atom assumes its corresponding role in a suitable distribution when it is measured that is in line with its being entangled with the photon and the corresponding measurement result concerning the photon's location.
The possibility of performing quantum erasure using a single form of quantum erasure in an experiment is explored. This contrasts with the two possibilities for quantum erasure usually employed (e.g., whether a photon shed by an atom is eliminated or is instead just not subject to specification regarding its location in either one or the other of two micromaser cavities). Where a single form of erasure is employed, when the physical existent reaches the detection screen it should show a typical Young-type interference pattern like that found for classic two-slit gedankenexperiments in quantum mechanics where there is no manipulation of the "particle" in its travel to the double-slit. The specific form of single-form quantum erasure explored here allows for the possibility of transmitting information to locations remote from photons shed into one of a pair of micromaser cavities. The form of this information corresponds to the "dots" and "dashes" of Morse code or, in general, the binary bits of digital data. The velocity of light does not appear to be a limiting factor for the transmission of this information. The apparent possibility of faster-than-light communication depends on the separation of the measurement process that usually occurs in one step into two steps in the quantum eraser scenario employed. Usually a measurement occurs and is made available to the environment in one step. In the quantum eraser, these are two distinct steps. We have essentially a "measurement in process" after the time that a photon is shed in one of the micromaser cavities and before we definitely know into which cavity the photon has been shed by an atom passing through the cavity or before it is definitely known that the photon's location in one or the other cavity can no longer be determined. While the "measurement in process" exists, the conventional notion of time may be suspended as concerns this measurement and any other entangled existent that is affected by the measurement of the photon's location. Once it is known whether the photon is indeed in only one cavity or is instead now in the combined cavity that occupies the space formerly occupied by both cavities that were separated when the shutter was closed, the atom assumes its corresponding role in a suitable distribution when it is measured that is in line with its being entangled with the photon and the corresponding measurement result concerning the photon's location.
After they have interacted, quantum particles generally behave as a single nonseparable entangled system. The concept of entanglement plays an essential role in quantum physics. We have performed entanglement experiments with Rydberg atoms and microwave photons in a cavity and tested quantum mechanics in situations of increasing complexity. Entanglement resulted either from a resonant exchange of energy between atoms and the cavity field or from dispersive energy shifts affecting atoms and photons when they were not resonant. With two entangled particles (two atoms or one atom and a photon), we have realized new versions of the Einstein-Podolsky-Rosen situation. The detection of one particle projected the other, at a distance, in a correlated state. This process could be viewed as an elementary measurement, one particle being a ``meter'' measuring the other. We have performed a ``quantum nondemolition'' measurement of a single photon, which we detected repeatedly without destroying it. Entanglement is also essential to understand decoherence, the process accounting for the classical appearance of the macroscopic world. A mesoscopic superposition of states (``Schr\"odinger cat'') gets rapidly entangled with its environment, losing its quantum coherence. We have prepared a Schr\"odinger cat made of a few photons and studied the dynamics of its decoherence, in an experiment which constitutes a glimpse at the quantum/classical boundary. We have also investigated entanglement as a resource for the processing of quantum information. By using quantum two-state systems (qubits) instead of classical bits of information, one can perform logical operations exploiting quantum interferences and taking advantage of the properties of entanglement. Manipulating as qubits atoms and photons in a cavity, we have operated a quantum gate and applied it to the generation of a complex three-particle entangled state. We finally discuss the perspectives opened by these experiments for further fundamental studies.
We report a quantum eraser experiment which actually uses a Young double slit to create interference. The experiment can be considered an optical analogy of an experiment proposed by Scully, Englert, and Walther [Nature (London) 351, 111 (1991)]. One photon of an entangled pair is incident on a Young double slit of appropriate dimensions to create an interference pattern in a distant detection region. Quarter-wave plates, oriented so that their fast axes are orthogonal, are placed in front of each slit to serve as which-path markers. The quarter-wave plates mark the polarization of the interfering photon and thus destroy the interference pattern. To recover interference, we measure the polarization of the other entangled photon. In addition, we perform the experiment under ``delayed erasure'' circumstances.
The quantum eraser involves a two-slit experiment in which an individual chooses, after the particles have passed through the slits, whether to do an interference experiment or a which-way experiment. If the experimenter chooses to erase the which-way information, the interference is restored. In one design, an excited atom enters one or the other cavity and emits a photon. By choosing to observe the photon’s position or to let it be absorbed by a detector atom, the investigator can pick the type of experiment to be done. The analysis uses the change-of-basis method, in which the form of wave function used depends on the experiment performed. A similar experiment can be performed by using polarizers at the slits and at the screen.
The counterintuitive features of quantum physics challenge many common-sense assumptions. In an interferometric quantum eraser experiment, one can actively choose whether or not to erase which-path information (a particle feature) of one quantum system and thus observe its wave feature via interference or not by performing a suitable measurement on a distant quantum system entangled with it. In all experiments performed to date, this choice took place either in the past or, in some delayed-choice arrangements, in the future of the interference. Thus, in principle, physical communications between choice and interference were not excluded. Here, we report a quantum eraser experiment in which, by enforcing Einstein locality, no such communication is possible. This is achieved by independent active choices, which are space-like separated from the interference. Our setup employs hybrid path-polarization entangled photon pairs, which are distributed over an optical fiber link of 55 m in one experiment, or over a free-space link of 144 km in another. No naive realistic picture is compatible with our results because whether a quantum could be seen as showing particle- or wave-like behavior would depend on a causally disconnected choice. It is therefore suggestive to abandon such pictures altogether.
Single-photon interference is a beautiful manifestation of the wave–particle duality of light and the double-slit Gedankenexperiment is a standard lecture example for introducing quantum mechanical reality. Interference arises only if each photon can follow several (classical) paths from the source to the detector, and if one does not have the possibility to determine which specific path the photon has taken. Attaching a specific label to the photon traveling along a specific path destroys the interference. However, in some cases those labels can be erased from the photon between leaving the apparatus and being detected, by which interference can be restored, a phenomenon called quantum erasing. We present lecture demonstration experiments that illustrate the wave–particle duality of light and the phenomenon of quantum erasing. Both experiments are first shown with strong light and, in a second step, on a photon-by-photon basis. The smooth transition from the quantum to the classical case can be shown in real time by varying the incident light intensity.
After they have interacted, quantum particles generally behave as a single nonseparable entangled system. The concept of entanglement plays an essential role in quantum physics. We have performed entanglement experiments with Rydberg atoms and microwave photons in a cavity and tested quantum mechanics in situations of increasing complexity. Entanglement resulted either from a resonant exchange of energy between atoms and the cavity field or from dispersive energy shifts affecting atoms and photons when they were not resonant. With two entangled particles (two atoms or one atom and a photon), we have realized new versions of the Einstein-Podolsky-Rosen situation. The detection of one particle projected the other, at a distance, in a correlated state. This process could be viewed as an elementary measurement, one particle being a ‘‘meter’’ measuring the other. We have performed a ‘‘quantum nondemolition’’ measurement of a single photon, which we detected repeatedly without destroying it. Entanglement is also essential to understand decoherence, the process accounting for the classical appearance of the macroscopic world. A mesoscopic superposition of states (‘‘Schrodinger cat’’) gets rapidly entangled with its environment, losing its quantum coherence. We have prepared a Schrodinger cat made of a few photons and studied the dynamics of its decoherence, in an experiment which constitutes a glimpse at the quantum/classical boundary. We have also investigated entanglement as a resource for the processing of quantum information. By using quantum two-state systems (qubits) instead of classical bits of information, one can perform logical operations exploiting quantum interferences and taking advantage of the properties of entanglement. Manipulating as qubits atoms and photons in a cavity, we have operated a quantum gate and applied it to the generation of a complex three-particle entangled state. We finally discuss the perspectives opened by these experiments for further fundamental studies.