String processor

A receiving unit for a quantum communications system, the receiving unit comprising:

• a string processor;
• at least one detector comprising logic for detecting received quantum bits;
• and
• a phase modulator comprising logic arranged to modulate a phase of a portion of the received quantum bits for measurement, wherein
• wherein the phase modulator receives a biased output string from the string processor configured to output the biased output string comprising a first output value and a second output value given an unbiased input string of at least two input values,
• the string processor comprises a processing unit and a memory device,
• the memory device stores a code-word set, the code-word set comprising a plurality of code-words and each code-word comprising at least one input value,
• each output value has at least one corresponding code-word,
• the processing unit comprises logic configured to:
• compare a comparison string to the code-words, the comparison string comprising an input from the input string;
• and
• assign an output value to the output string when the comparison string matches a code-word, the assigned output value being that to which the matched code-word corresponds,
• an output probability of an output value is defined by a number and length of the code-words corresponding to that output value,
• the output probability of the first output value is larger than the output probability of the second output value, and
• the processing unit comprises logic further configured to, when the comparison string does not match a code-word:
• read a next available input in the input string;
• and
• add the next available input to the comparison string, an available input being an input that is not part of the comparison string.

A quantum communication system comprising:

• a string processor;
• a sending unit comprising:
• a quantum bit source comprising logic configured to generate a plurality of quantum bits for sending to a receiving unit;
• an intensity modulator comprising logic arranged to modulate an intensity of a portion of the quantum bits;
• and
• a phase modulator comprising logic arranged to modulate a phase of a portion of the quantum bits, and
• a receiving unit comprising:
• at least one detector comprising logic for detecting received quantum bits;
• and
• a phase modulator comprising logic arranged to modulate a phase of a portion of the received quantum bits for measurement, wherein
• at least one of the intensity modulator and the phase modulator in the sending unit and the phase modulator in the receiving unit receives a biased output string from the string processor configured to output the biased output string comprising a first output value and a second output value given an unbiased input string of at least two input values,
• the string processor comprises a processing unit and a memory device,
• the memory device stores a code-word set, the code-word set comprising a plurality of code-words and each code-word comprising at least one input value,
• each output value has at least one corresponding code-word,
• the processing unit comprises logic configured to:
• compare a comparison string to the code-words, the comparison string comprising an input from the input string;
• and
• assign an output value to the output string when the comparison string matches a code-word, the assigned output value being that to which the matched code-word corresponds,
• an output probability of an output value is defined by a number and length of the code-words corresponding to that output value,
• the output probability of the first output value is larger than the output probability of the second output value, and
• the processing unit comprises logic further configured to, when the comparison string does not match a code-word:
• read a next available input in the input string;
• and
• add the next available input to the comparison string, an available input being an input that is not part of the comparison string.