Nearly a century after Einstein first predicted the existence of gravitational waves, a global network of Earth-based gravitational wave observatories1, 2, 3, 4 is seeking to directly detect this faint radiation using precision laser interferometry. Photon shot noise, due to the quantum nature of light, imposes a fundamental limit on the attometre-level sensitivity of the kilometre-scale Michelson interferometers deployed for this task. Here, we inject squeezed states to improve the performance of one of the detectors of the Laser Interferometer Gravitational-Wave Observatory (LIGO) beyond the quantum noise limit, most notably in the frequency region down to 150 Hz, critically important for several astrophysical sources, with no deterioration of performance observed at any frequency. With the injection of squeezed states, this LIGO detector demonstrated the best broadband sensitivity to gravitational waves ever achieved, with important implications for observing the gravitational-wave Universe with unprecedented sensitivity.

Enhanced sensitivity of the LIGO gravitational wave detector by using squeezed states of light

The continuous-variable version of quantum key distribution (QKD) offers the advantages (over discrete-variable systems) of higher secret key rates in metropolitan areas as well as the use of standard telecom components that can operate at room temperature. An important step in the real-world adoption of continuous-variable QKD is the deployment of field tests over commercial fibers. Here we report two different field tests of a continuous-variable QKD system through commercial fiber networks in Xi'an and Guangzhou over distances of 30.02 km (12.48 dB) and 49.85 km (11.62 dB), respectively. We achieve secure key rates two orders-of-magnitude higher than previous field test demonstrations. This is achieved by developing a fully automatic control system to create stable excess noise and by applying a rate-adaptive reconciliation protocol to achieve a high reconciliation efficiency with high success probability. Our results pave the way to achieving continuous-variable QKD in a metropolitan setting.

https://iopscience.iop.org/article/10.1088/2058-9565/ab19d1/pdf

Continuous-variable QKD over 50 km commercial fiber

Quantum teleportation, which is the transfer of an unknown quantum state from one station to another over a certain distance with the help of nonlocal entanglement shared by a sender and a receiver, has been widely used as a fundamental element in quantum communication and quantum computation. Optical fibers are crucial information channels, but teleportation of continuous variable optical modes through fibers has not been realized so far. Here, we experimentally demonstrate deterministic quantum teleportation of an optical coherent state through fiber channels. Two sub-modes of an Einstein-Podolsky-Rosen entangled state are distributed to a sender and a receiver through a 3.0-km fiber, which acts as a quantum resource. The deterministic teleportation of optical modes over a fiber channel of 6.0 km is realized. A fidelity of 0.62 ± 0.03 is achieved for the retrieved quantum state, which breaks through the classical limit of 1/2. Our work provides a feasible scheme to implement deterministic quantum teleportation in communication networks.

Deterministic quantum teleportation through fiber channels

Continuous-variable QKD over 50km commercial fiber

Photonic quantum computing is one of the leading approaches to universal quantum computation. However, large-scale implementation of photonic quantum computing has been hindered by its intrinsic difficulties, such as probabilistic entangling gates for photonic qubits and lack of scalable ways to build photonic circuits. Here, we discuss how to overcome these limitations by taking advantage of two key ideas which have recently emerged. One is a hybrid qubit-continuous variable approach for realizing a deterministic universal gate set for photonic qubits. The other is the time-domain multiplexing technique to perform arbitrarily large-scale quantum computing without changing the configuration of photonic circuits. These ideas together will enable scalable implementation of universal photonic quantum computers in which hardware-efficient error correcting codes can be incorporated. Furthermore, all-optical implementation of such systems can increase the operational bandwidth beyond terahertz in principle, ultimately enabling large-scale fault-tolerant universal quantum computers with ultrahigh operation frequency.

Toward large-scale fault-tolerant universal photonic quantum computing

As important members of nonclassical states of light, squeezed states and entangled states are basic resources for realizing quantum measurements and constructing quantum information networks. We experimentally demonstrate that the two types of nonclassical optical states can be generated from an optical parametric oscillator (OPO) involving a periodically poled KTiOPO4 crystal with a domain-inversion period of 51.7 μm, by changing the polarization of the pump laser. When a vertically polarized 671 nm laser is used to pump the OPO, the intra-cavity frequency-down-conversion with type-0 quasi-phase matching is realized and the output optical beam is a quadrature amplitude squeezed state of light at the wavelength of 1342 nm with the fluctuation of quadrature component of 3.17 dB below the quantum noise limit (QNL). If the pump laser is horizontally polarized, the condition of the type-II quasi-phase matching is satisfied and the output optical beam becomes Einstein-Podolsky-Rosen entangled state of light w...

Generation of two types of nonclassical optical states using an optical parametric oscillator with a PPKTP crystal

A low-noise photodetector is a basic tool for the research of
quantum information processing. We present a specially designed low-noise
photoelectric detector with a bandwidth of 130 MHz, using a transimpedance
amplification circuit. Based on the detailed calculation of the dependence
on each parameter of the detector, a useful method of how to design a
low-noise and broadband photodetector is provided. When the optical power
is between 1.0 and 16 mW, the photodetector has a good linear response to
the injected light. Its electronics noise power is below −77 dBm, which is
within the whole bandwidth. When the incident light power is 2 mW, the
output noise powers are 10.0, 8.0, and 6.0 dB higher than the
corresponding electronics noise within the bandwidth of 1–50, 50–90, and
90–130 MHz, respectively, which is in good agreement with the theoretical
prediction. Thus, this photoelectric detector could have good application
prospects in quantum communication and an optical cavity locking
system.

Design of low-noise photodetector with a bandwidth of 130 MHz based on transimpedance amplification circuit

The invention provides an automatic locking system for a relative phase between an optical resonant cavity and two light beams, belongs to the field of photoelectric technology, and aims to solve the technical problem of easiness in causing complicated operations since an existing locking system for a relative phase between an optical resonant cavity and two light beams has the defects of high cost, large size, poor oscillation resistance and need of manual relocking after lock loss. A signal input end of a proportional amplifying and integrating circuit module is connected with a signal output end of a signal detector; a signal output end of the proportional amplifying and integrating circuit module is connected with a signal input end of an AD conversion module; the signal detector is connected with a system to be locked; a signal input end of the control module is connected with a signal output end of the AD conversion module; a signal output end of the control module is connected with a signal input end of a DA conversion module; a signal output end of the DA conversion module is connected with a signal input end of a high-voltage amplifier; and a signal output end of the high-voltage amplifier is connected with a signal input end of the system to be locked.

Meiru Huo

Papers

Deterministic quantum teleportation through fiber channels

Generation of two types of nonclassical optical states using an optical parametric oscillator with a PPKTP crystal

Design of low-noise photodetector with a bandwidth of 130 MHz based on transimpedance amplification circuit

Automatic locking system for relative phase between optical resonant cavity and two light beams