FIELD OF THE INVENTION

The present disclosure relates to generating shared random information, quantum communication, quantum cryptography, and quantum key distribution. The following publications by the inventors are each hereby incorporated by reference in their entirety:Passive state preparation in the Gaussian-modulated coherent-states quantum key distribution, B. Qi, P. Evans and W. Grice, Physical Review A 97, 012317, published Jan. 16, 2018; andPassive state preparation in continuous-variable quantum key distribution, B. Qi, P. Evans and W. Grice, in Conference on Lasers and Electro-Optics, OSA Technical Digest [online] (Optical Society of America, 2018), paper JTh2A.9.

BACKGROUND

Quantum key distribution (QKD) has drawn attention for its proven security against adversaries with unlimited computing power. QKD is a secure communication method that implements a cryptographic scheme (sometimes referred to as a protocol) involving quantum mechanics. In QKD, two remote legitimate clients (typically referred to as Alice and Bob or transmitter and receiver) can establish a secure key by transmitting quantum states through an insecure channel controlled by an adversary (typically referred to as Eve or eavesdropper). The security of the key is based on features of quantum physics, rather than assumptions regarding computationally difficult problems. QKD exploits quantum phenomena to enable communications that can only be intercepted by violating known laws of physics. Accordingly, any attacks by Eve will, with a high probability, disturb the transmitted quantum state, and thus can be detected.

Many practical QKD systems are based on a prepare-and-measure scheme, where Alice prepares quantum states and transmits them to Bob, who in turn performs measurements. The measurement results can act as a key or be used as inputs to a key generation algorithm that can enable secure communication between Alice and Bob. The quantum state preparation step is conventionally implemented in an active manner: Alice first generates truly random numbers using a quantum random number generator, which she uses to prepare a corresponding quantum state by performing modulations on the output of a single source, or switching among multiple sources. One well-known example is the decoy state BB84 QKD using phase randomized weak coherent sources, where for each transmission, Alice randomly prepares one of the four BB84 states, randomly changes the average photon number (to generate either the signal state or one of the decoy states), and (in certain implementations) randomizes the global phase of the weak coherent state. As the transmission rate in QKD has been growing dramatically over the years, it is becoming more and more challenging to prepare quantum state precisely at the corresponding speed.

More recently, passive state preparation schemes have been proposed in QKD as an alternative approach. In this scheme, Alice uses intrinsic fluctuations of the source, or intentionally designs the source in a way such that certain parameters (for example, intensity) will present unpredictable fluctuations. Typically, two optical modes with correlated fluctuations are output from the source. By measuring one mode locally, Alice can determine the random noise carried by the other mode, which will be transmitted to Bob. This idea was initially proposed as a simple way to generate random intensity fluctuations in the decoy-state QKD schemes. Later on, it was also applied in preparing the four BB84 states approximately. So far, the passive state preparation scheme has only been developed in a discrete-variable (DV) QKD based on single photon detection.

Two families of QKD schemes are discrete-variable (DV) and continuous variable (CV). In essence, CV-QKD is “analog”, while discrete-variable (DV) QKD is “digital.” One CV-QKD scheme is the Gaussian-modulated coherent-states (GMCS) QKD scheme, which is illustrated inFIG. 1and has been demonstrated over practical distances. Known implementations of the GMCS QKD are based on an active state preparation scheme: for each transmission, Alice first generates a pair of Gaussian-distributed random numbers104, encodes them on a weak coherent state102using optical amplitude and phase modulators106, and then transmits the Gaussian-modulated weak coherent pulse to Bob over a quantum channel108. Since the modulation format is relatively complicated and the tolerable modulation error is small, high extinction ratio modulators with good stability are commonly required in the GMCS QKD scheme.

CV-QKD based on coherent detection is especially appealing for a number of applications because of its compatibility with standard telecom technologies. Nevertheless, high-speed Gaussian random number generators and high performance optical modulators are currently required to implement CV-QKD, impeding its practical application.

SUMMARY

A system and method are provided to yield a passive continuous variable quantum key distribution (CV-QKD) scheme using a thermal source. In one embodiment, a CV-QKD scheme is provided based on coherent detection using a broadband thermal source, such as a continuous wave or pulsed source. The CV-QKD scheme is passive because it operates without a random number generator and without an optical modulator, which simplifies the implementation of CV-QKD and makes it more practical relative to known active CV-QKD schemes and more practical relative to known passive DV-QKD schemes.

In one embodiment of the CV-QKD scheme with a passive state preparation scheme using a thermal source, the transmitter client splits the output of a thermal source into two spatial modes using a beam splitter. The transmitter client measures one mode locally using optical homodyne detectors, and transmits the other mode to the receiver client after applying optical attenuation. A secure key can be established based on the correlation between the transmitter client's measurement results and the receiver client's measurement results. Given the initial thermal state generated by the source being strong enough, this scheme can tolerate high detector noise at the transmitter side. Furthermore, the output of the source does not need to be single mode, since an optical homodyne detector can selectively measure a single mode determined by the local oscillator.

In summary, the present invention provides a simple passive state preparation scheme in CV-QKD, which can be implemented using off-the-shelf amplified spontaneous emission (ASE) sources.

Before the embodiments of the invention are explained in detail, it is to be understood that the invention is not limited to the details of operation or to the details of construction and the arrangement of the components set forth in the following description or illustrated in the drawings. The invention may be implemented in various other embodiments and of being practiced or being carried out in alternative ways not expressly disclosed herein. Also, it is to be understood that the phraseology and terminology used herein are for the purpose of description and should not be regarded as limiting. The use of “including” and “comprising” and variations thereof is meant to encompass the items listed thereafter and equivalents thereof as well as additional items and equivalents thereof. Further, enumeration may be used in the description of various embodiments. Unless otherwise expressly stated, the use of enumeration should not be construed as limiting the invention to any specific order or number of components. Nor should the use of enumeration be construed as excluding from the scope of the invention any additional steps or components that might be combined with or into the enumerated steps or components.

