We propose a new coherent state quantum key distribution protocol that eliminates the need to randomly switch between measurement bases. This protocol provides significantly higher secret key rates with increased bandwidths than previous schemes that only make single quadrature measurements. It also offers the further advantage of simplicity compared to all previous protocols which, to date, have relied on switching. DOI: 10.1103/PhysRevLett.93.170504 PACS numbers: 03.67.Dd, 42.50.Dv, 89.70.+c Quantum cryptography is the science of sending secret messages via a quantum channel. It uses properties of quantum mechanics [1,2] to establish a secure key, a process known as quantum key distribution (QKD) [3] . This key can then be used to send encrypted information. In a generic QKD protocol, a sender (Alice) prepares quantum states which are sent to a receiver (Bob) through a potentially noisy channel. Alice and Bob agree on a set of noncommuting bases with which to measure the states. Using various reconciliation [4] and privacy amplification [5] procedures, the results of measurements in these bases are used to construct a secret key, known only to Alice and Bob. Switching randomly between a pair of noncommuting measurement bases ensures security: in a direct attack, an eavesdropper (Eve) will only choose the correct basis half the time; alternatively, if Eve uses quantum memory and performs her measurements after Bob declares his basis, she is unable to manipulate what Bob measures. It is commonly assumed that randomly switching between measurement bases is crucial to the success of QKD protocols. In this Letter, we show that this is not the case, and in fact greater secret key rates can be achieved by simultaneously measuring both bases. The original QKD schemes in the discrete variable regime were based on the transmission and measurement of random polarizations of single photon states [2] . Other discrete variable QKD protocols have been proposed [6] and experimentally demonstrated [7] using Bell states. However, the bandwidth of such schemes is experimentally limited by single photon generation and detection techniques. Consequently, in the last few years there has been considerable interest in the field of continuous variable quantum cryptography [8] , which provides an alternative to the discrete approach and promises higher key rates. Continuous variable QKD protocols have been proposed for squeezed and Einstein-Podolsky-Rosen entangled states [9] . However, these protocols require significant quantum resources and are susceptible to decoherence due to losses. QKD protocols using coherent states were proposed to overcome these limitations. Originally, such schemes were only secure for line losses less than 50% or 3 dB [10] . This apparent limitation was overcome using the secret key distillation techniques of post-selection [11] and reverse reconciliation [12] . In general, security in discrete variable cryptography protocols is ensured via random switching between measurement bases [2] or random switching of state manipulation [13] . The random switching between measurement bases can be achieved simply via a 50=50 beam splitter, where the selection of the measurement basis is chosen through the random photon transmission and reflection statistics. To date, all continuous variable cryptography protocols have also relied on randomly switching between noncommuting bases. In the continuous variable regime, switching requires precise control of the phase of a local oscillator beam, which is difficult to achieve in practice. This local oscillator switching currently places a serious technical limitation on the bandwidth of cryptography protocols. In this Letter, we introduce a new coherent state protocol that does not require switching. In this
Christian et al. REF Propose a new coherent state QKD protocol that does away with the need to randomly switch between measurement bases.
Quantum cryptography without switching.
Published, Phys. Rev. Lett. 93, 170504 (2004)
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