A Direct Current Superconducting Quantrum Interference Device (SQUID) magnetometer configured to operate at a bandwidth of 200 MHz and higher, comprising: a DC SQUID circuit, a source of low-frequency modulation flux Φ m sin(ω m t +φ m) inductively coupled to an input of said DC SQUID circuit, wherein Φ m is the amplitude of the modulation flux, ω m is the frequency of the modulation flux, and φ m is the phase of the modulation flux, a source of Radio Frequency flux ω RF ( t)sin(ω RF t +φ RF) inductively coupled to said input of said DC SQUID circuit, wherein Φ RF ( t) is an amplitude of said RF flux, ω RF is a frequency of the RF flux, and φ RF is the phase of the RF flux, wherein, responsive to said RF flux and low-frequency modulation flux coupled thereto, said DC SQUID produces an output RF voltage binary phase modulated at said frequency ω m between 0° and 180°, a demultiplexing circuit coupled to an output of said DC SQUID to separate said output RF voltage into an RF signal component and a low-frequency signal component, an RF demodulation circuit receiving said RF signal component of said binary phase modulated output RF voltage and configured to double-lock said RF signal component at said ω m and ω RF to produce an output signal representative of said RF flux, said RF demodulation circuit including a first demodulation unit referenced to said ω RF and a second demodulation unit referenced to said ω m and coupled to an output of said first demodulation unit; and a flux-locked loop circuit coupled between said input and output of said DC SQUID circuit to inductively couple a feedback flux n Φ 0, n = 0,1,2,... to said input of said DC SQUID circuit, where Φ 0 is the magnetic flux quantum, wherein said FLL circuit is coupled to an output of said demultiplexing circuit to receive said low-frequency signal component therefrom, said low-frequency signal component being processed in said FLL circuit to generate said feedback flux n Φ 0, and wherein said second demodulation unit is coupled to said FLL circuit to control said low-frequency modulation flux. a DC SQUID circuit, a source of low-frequency modulation flux Φ m sin(ω m t +φ m) inductively coupled to an input of said DC SQUID circuit, wherein Φ m is the amplitude of the modulation flux, ω m is the frequency of the modulation flux, and φ m is the phase of the modulation flux, a source of Radio Frequency flux ω RF ( t)sin(ω RF t +φ RF) inductively coupled to said input of said DC SQUID circuit, wherein Φ RF ( t) is an amplitude of said RF flux, ω RF is a frequency of the RF flux, and φ RF is the phase of the RF flux, wherein, responsive to said RF flux and low-frequency modulation flux coupled thereto, said DC SQUID produces an output RF voltage binary phase modulated at said frequency ω m between 0° and 180°, a demultiplexing circuit coupled to an output of said DC SQUID to separate said output RF voltage into an RF signal component and a low-frequency signal component, an RF demodulation circuit receiving said RF signal component of said binary phase modulated output RF voltage and configured to double-lock said RF signal component at said ω m and ω RF to produce an output signal representative of said RF flux, said RF demodulation circuit including a first demodulation unit referenced to said ω RF and a second demodulation unit referenced to said ω m and coupled to an output of said first demodulation unit; and a flux-locked loop circuit coupled between said input and output of said DC SQUID circuit to inductively couple a feedback flux n Φ 0, n = 0,1,2,... to said input of said DC SQUID circuit, where Φ 0 is the magnetic flux quantum, wherein said FLL circuit is coupled to an output of said demultiplexing circuit to receive said low-frequency signal component therefrom, said low-frequency signal component being processed in said FLL circuit to generate said feedback flux n Φ 0, and wherein said second demodulation unit is coupled to said FLL circuit to control said low-frequency modulation flux.