Generating universal resource states for measurement-based quantum computation out of laser light

Of the many ways of making a quantum computer, the measurement-based approach is perhaps the best suited to quantum optics platforms. In this paradigm, the task of implementing a quantum algorithm is broken into two steps:

  1. Generate a fixed large-scale quantum state with a specific entanglement structure, known as a “cluster state”
  2. Implement the desired algorithm by a specific adaptive sequence of single-site measurements on the individual subsystems that make up the state

An international team of researchers, including Dr. Rafael Alexander from CQuIC, have demonstrated the first experimental generation of a cluster state (specifically, a continuous-variable cluster state) that is both large scale and has a two-dimensional entanglement structure. Both of these are requirements for large-scale measurement-based quantum computation.

This breakthrough, which appeared in Science, was achieved by a team of experimentalists at the University of Tokyo, lead by Professor Akira Furusawa.

The full experimental team involved (lead author) Warit Asavanant, Yu Shiozawa, Hiroki Emura, Baramee Charoensombutamon, Dr. Shota Yokoyama, Dr. Shuntaro Takeda, Dr. Jun-ichi Yoshikawa, and Prof. Akira Furusawa. Design of this experiment developed through collaboration between Dr. Alexander, Dr. Nicolas Menicucci at RMIT, Dr. Shota Yokoyama and Dr. Hidehiro Yonezawa at UNSW-Canberra, and Warit Asavanant, and Prof. Furusawa at the University of Tokyo.

Optimized communication strategies with binary coherent states over phase noise channels

Optimized communication graphic illustration

The total amount of information that two parties can share is determined by the physical properties of the channel they are communicating over. Typically in optical communication, lasers are used to produce coherent states of light, and information can be encoded into either the phase, amplitude, or both. While the limits of information transfer over typical channels such as lossy optical fiber have been extensively studied, such limits in channels that also induce phase noise are not well understood, even though they are more realistic models for certain situations. When information is encoded into the phase of light, these channels severely degrade the amount of information that can be transmitted, especially when using conventional techniques of encoding and decoding, i.e. modulation and measurement.

This restriction of conventional approaches to optical communication lead us to propose and demonstrate methods for optimized communication strategies over such a phase-noise channel. The two key ingredients are finding particular physical states of light that can help shield information from the noise while keeping it easily extractable, and a novel coherent measurement based on counting single photons. These ingredients are optimized together for a specific noise channel in order to maximize the total amount of information transfer. This approach of a joint optimization allows for an increase in the amount of information that can be transmitted compared to conventional methods, even though the fundamental limits for the channel are not well known.

Reference information:

Optimized communication strategies with binary coherent states over phase noise channels

  1. T. DiMario, L. Kunz, K. Banaszek & F. E. Becerra

npj Quantum Information 5, 65 (2019)

Optimal Pure-State Qubit Tomography via Sequential Weak Measurements


A sequence of weak isotropic measurements can yield the optimal measurement of a collection of identical qubits — a POVM over the continuous set of spin-coherent states. We show here a numerical simulation for an independent sequence of collective isotropic weak measurements on an ensemble of 50 qubits. The vertical axis is a measure how close a POVM element is to a spin-coherent state projector with the value the “coherency” C=1 corresponding to a spin-coherent state. The olive region consists of the coherency for 50 samples of the measurement record as a function of time. The spheres are Husimi distributions of the POVM element (left) and the post-measurement state (right) for various times of the black sample trajectory. The POVM converges asymptotically, extracting the estimated direction of the unknown spin, whilst the state does not change drastically, because the measurements are weak.

 

It’s been known since the early days of quantum information science that the optimal way to gain information about a quantum state given many copies is to do a joint measurement of the whole ensemble. An example is seen in quantum state tomography. In 1995, Massar and Popescu proved that the optimal measurement for estimating an unknown direction of N copies of a pure qubit is a collective measurement. The realization for every N is the so-called spin-coherent-state POVM. After various attempts, by 2002 many researchers had come to the conclusion that this measurement was not practically realizable for large N. However, in recent work published in Physical Review Letters (give link) researchers at CQuIC showed that an implementation of the spin-coherent-state POVM could indeed be implemented via a sequence of collective and isotropic weak measurements without feedback or adaptivity.

Spin-coherent states are quantum states that transform in the same way as a vector under 3-dimensional rotations.This vector can be a literal direction in 3D real space but can also be an abstract direction such as in the Bloch sphere of a qubit. For N copies of a qubit, these directions are represented by states which sit in an irreducible subspace that spans N+1 of the 2^N dimensions of the total Hilbert space. This subspace is also known as the spin-J=N/2 representation of SU(2). The spin-coherent-state measurement of a spin-J system is a measurement where the outcomes are the continuum of directions which is represented by a POVM with elements that are projectors onto the spin-coherent states. The weak measurement record eventually converges on the estimated direction.

This protocol could be implemented on currently existing platforms, such can be done using the Faraday interaction to measure the collective spin projection of an atomic ensemble with a continuous laser probe.

With this new breakthrough, CQuIC graduate student Ezad Shojaee and postdoc Chris Jackson, together with Prof. Ivan Deutsch are working on applications for more generalized coherent-state measurements such as those with SU(1,1) symmetry and other semi-simple Lie group symmetries.

This work was published in the September 26, 2018 issue of Physical Review Letters:

Optimal Pure-State Qubit Tomography via Sequential Weak Measurements
Ezad Shojaee, Christopher S. Jackson, Carlos A. Riofrío, Amir Kalev, and Ivan H. Deutsch
Phys. Rev. Lett. 121, 130404

The full article can be found online.

 

Becerra’s group publishes article on robust measurements at low powers by counting photons

Dr. Elohim Becerra Chavez’s research group recently published Robust Measurement for the Discrimination of Binary Coherent States in Physical Review Letters.

Optical communication uses light to encode and transmit information over long distances, such as in optical fibers with losses, and requires reliable detection schemes to read out information from low levels of light. However, noise and imperfections in real-world devices and detectors severely affect the measurement fidelity and ultimately limits the amount of information that we can communicate. Recent work in the quantum-optics group led by Elohim Becerra at UNM demonstrated an optimized measurement capable of overcoming these imperfections by counting the number of photons in pulses of light carrying information in their optical phase. This measurement reads off the information contained in the light by first combining the light pulse with a reference pulse to compare their relative phase, and then counting the number of photons in the combined pulse. Previous work in ultra-sensitive measurements has focused instead on detecting no light vs. any amount of light in the combined pulse, and their sensitivities have been limited by noise and imperfections. This novel measurement scheme allows for a high degree of robustness to these imperfections while reaching high measurement sensitivities. Moreover, due to its simplicity, this measurement is inherently compatible with high-bandwidth communication technologies to accommodate the high rates of information transfer in today’s optical communication networks.

This work was published in the July 2 issue of Physical Review Letters:

Robust Measurement for the Discrimination of Binary Coherent States
M. T. DiMario and F. E. Becerra
Phys. Rev. Lett. 121, 023603 (2018)

The full article can be found online.