Monday 11 September 2023

Experimental quantum imaging distillation with undetected light

 


It is possible to image an object with an induced coherence effect by making use of photon pairs to gain information on the item of interest—without detecting the light probing it. While one photon illuminates the object, its partner alone is detected, thereby preventing the measurements of coincidence events to reveal information of the sought after object. This method can be made resilient to noise, as well.

In a new report published in Science Advances, Jorge Fuenzalida and a team in applied optics, precision engineering and theory communications in Germany experimentally showed how the method can be made resilient to noise. They introduced an imaging-distilled approach based on the interferometric modulation of the signal of interest to generate a high-quality image of an object regardless of the extreme noise levels surpassing the actual signal of interest.

Quantum imaging

Quantum imaging is a promising field that is emerging with valid advantages when compared to classical protocols. Researchers have demonstrated this method across different scenarios to work in the low-photon flux regime by making use of undetected probing photons for super-resolution imaging.

Scientists can also develop protocols in quantum imaging without a classical counterpart based on quantum interference and entanglement. Quantum imaging protocols can, however, be made resilient to noise. For instance, distillation or purification can remove decoherence introduced by the environment in a quantum system.

It is also possible to implement quantum imaging distillation with one and several photon pair degrees of freedom. In this work, Fuenzalida and team introduced and experimentally verified a quantum imaging distillation method to detect single photons only.

The method of quantum imaging with undetected light (abbreviated as QIUL) offers a two-photon wide-field interferometric imaging method. During this process, one photon can illuminate an object, while only its partner photon is detected on the camera. Incidentally, the photon illuminating the object remains undetected.

The method offers a unique discovery method to probe samples. The scientists then introduced a source of noise in the quantum imaging scheme to study its resilience to show good performance even for noise intensities above 250 times the quantum signal intensity.

Cleaning a quantum image and generating a photon pair

Quantum imaging distillation is a method in use to clean a quantum image from noise. The team illustrated the distillation method while defining a noise image as an unwanted signal superimposed over a quantum image on the camera. To distill an image, Fuenzalida and team used quantum holography with undetected light (abbreviated QHUL), where the object information was carried into a single-photon interference pattern. If the intensity difference of the method is bigger than the intensity variance of noise, the team could distill the quantum image.

To generate photon pairs mediated by the interaction of an intense pump beam with the atoms of a nonlinear crystal, the team used spontaneous parametric down-conversion. The imaging scheme used an interferometer to generate a pair of signal-idler photons in the forward and backward propagation modes. The noise variance in the setup contributed to the signal intensity variance; where a difference of signal intensity higher than the noise variance can distill the quantum image.
Resilience to different noise intensities. In the top row, the superpositions of quantum (IOF letters) and classical (square shape) images are shown. The ratio between their mean intensities is stated on top of each image. In the middle row, the experimental results for our distillation technique through QHUL are presented. In the last row, a transverse cut of the distilled images is presented. We observe that, while the noise intensity increases, the phase estimation diminishes. Credit: Science Advances, DOI: 10.1126/sciadv.adg9573
Experimental nonlinearity and noise sources

Fuenzalida and colleagues implemented an experimental setup using a nonlinear interferometer in a Michelson configuration and pumped a crystal with a continuous wave laser. Due to the strong nonlinearity of the experiment, the team generated a photon pair via spontaneous parametric down-conversion along the paths, although never simultaneously. They separated the signal, idler, and pump beams in the forward propagation direction by using dichroic mirrors and reflected into the crystal with a series of mirrors.

The camera in the experimental framework showed an interference pattern of signal photons, which the team noted as the transfer of object information obtained by the idler photon to the signal photon interference pattern. The team used a continuous wave diode laser with a variable pump power to introduce noise into the system, and varied the properties of classical illumination, intensity and variance to examine the effects of noise and the distillation performance.
Distillation performance across diverse noise intensities

The scientists superimposed classical and quantum images to perform quantum holography with undetected light to distill or clean the quantum image under diverse intensities of noise. For the quantum image, they used signal photons generated in a single pass through a crystal, where the signal intensity did not change during the experiments. They characterized different noise intensities by superimposing quantum and classical images on the camera, and as the noise intensity increased, they measured the accuracy of the experimental results.

The researchers conducted a second experiment to quantify the effects of varying noise on the phase accuracy of distilled images by using similar configurations of noise intensities. The experimental behavior was in good agreement to theory, and compared well to existing methods.
Distillation phase variance affected by noise variance. A light diffuser with four different rotation speeds is used to change the properties of the noise illumination; see supplementary text D. The different noise configurations are represented by different colors and symbols; see inset. Data points represent the experimental phase values obtained for different noise variances, and dashed lines represent their fits. A theoretical black solid line representing a Poissonian noise is also included. In all configurations, we observed that a higher noise variance increases the phase inaccuracy in QHUL. We also corroborate that the phase sensitivity is linearly dependent with the noise variance. Credit: Science Advances, DOI: 10.1126/sciadv.adg9573
Outlook

In this way, Jorge Fuenzalida, and colleagues investigated quantum imaging with undetected light (QIUL) in a two-photon wide-field interferometric imaging method. While one photon illuminated the object of interest and its partner remained on the camera, the illuminating photon remained undetected. The scientists distilled or cleaned the image by using quantum holography with undetected light (QHUL). They proved the imaging method by superimposing partially or completely a classical source of noise on top of the quantum image on the camera. The method worked every time, even with noise intensities higher than the signal intensity.

The team explored the limits of the method by presenting simulations of quantum holography under extreme noise scenarios. The experimental outcomes provide a step forward for quantum imaging in open systems to even examine the limits of innovative versions of quantum-based light detection and ranging (LIDAR), by using undetected light.

#QuantumLight #QuantumOptics   #QuantumFuture  #QuantumInnovation
#QuantumPhotonic #QuantumEntanglement #QuantumTechnology #QuantumPhysics
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Saturday 9 September 2023

Promising quantum state found during error correction research

 




Window glass, at the microscopic level, shows a strange mix of properties. Like a liquid, its atoms are disordered, but like a solid, its atom are rigid, so a force applied to one atom causes all of them to move.

It's an analogy physicists use to describe a quantum state called a "quantum spin-glass," in which quantum mechanical bits (qubits) in a quantum computer demonstrate both disorder (taking on seemingly random values) and rigidity (when one qubit flips, so do all the others). A team of Cornell researchers unexpectedly discovered the presence of this quantum state while conducting a research project designed to learn more about quantum algorithms and, relatedly, new strategies for error correction in quantum computing.

"Measuring the position of a quantum particle changes its momentum and vice versa. Similarly, for qubits there are quantities which change one another when they are measured. We find that certain random sequences of these incompatible measurements lead to the formation of a quantum spin-glass," said Erich Mueller, professor of physics in the College of Arts and Sciences (A&S). "One implication of our work is that some types of information are automatically protected in quantum algorithms which share the features of our model."

"Subsystem Symmetry, Spin-glass Order, and Criticality From Random Measurements in a Two-dimensional Bacon-Shor Circuit" published on July 31 in Physical Review B. The lead author is Vaibhav Sharma, a doctoral student in physics.

Assistant professor of physics Chao-Ming Jian (A&S) is a co-author along with Mueller. All three conduct their research at Cornell's Laboratory of Atomic and Solid State Physics (LASSP).

"We are trying to understand generic features of quantum algorithms—features which transcend any particular algorithm," Sharma said. "Our strategy for revealing these universal features was to study random algorithms. We discovered that certain classes of algorithms lead to hidden 'spin-glass' order. We are now searching for other forms of hidden order and think that this will lead us to a new taxonomy of quantum states."

Random algorithms are those that incorporate a degree of randomness as part of the algorithm—e.g., random numbers to decide what to do next.

Mueller's proposal for the 2021 New Frontier Grant "Autonomous Quantum Subsystem Error Correction" aimed to simplify quantum computer architectures by developing a new strategy to correct for quantum processor errors caused by environmental noise—that is, any factor, such as cosmic rays or magnetic fields, that would interfere with a quantum computer's qubits, corrupting information.

The bits of classical computer systems are protected by error-correcting codes, Mueller said; information is replicated so that if one bit "flips," you can detect it and fix the error. "For quantum computing to be workable now and in the future, we need to come up with ways to protect qubits in the same way."

"The key to error correction is redundancy," Mueller said. "If I send three copies of a bit, you can tell if there is an error by comparing the bits with one another. We borrow language from cryptography for talking about such strategies and refer to the repeated set of bits as a 'codeword.'"

When they made their discovery about spin-glass order, Mueller and his team were looking into a generalization, where multiple codewords are used to represent the same information. For example, in a subsystem code, the bit "1" might be stored in 4 different ways: 111; 100; 101; and 001.

"The extra freedom that one has in quantum subsystem codes simplifies the process of detecting and correcting errors," Mueller said.

The researchers emphasized that they weren't simply trying to generate a better error protection scheme when they began this research. Rather, they were studying random algorithms to learn general properties of all such algorithms.

"Interestingly, we found nontrivial structure," Mueller said. "The most dramatic was the existence of this spin-glass order, which points toward there being some extra hidden information floating around, which should be useable in some way for computing, though we don't know how yet."



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Experimental quantum imaging distillation with undetected light

  It is possible to image an object with an induced coherence effect by making use of photon pairs to gain information on the item of intere...